Neuroanatomy refined Notes.docx

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University of Toronto Created By: Lindsay Arathoon 1 Neuroanatomy Chapter 1: Neuroscience – Past, Present, and Future Neuroscience: the study of the brain, related to disciplines of medicine, biology, psychology, physics, chemistry, mathematics, etc. where the brain can best be examined through the combination of these disciplines. Trepanation: producing a hole in the skull to produce therapeutic, curing effects in ancient times. People during prehistoric time appreciated that the brain was vital to life. The procedure showed evidence of healing afterwards, indicating it was done on live subjects, possibly to treat headaches or mental disorders. However the heart was considered the origin of the soul and consciousness until Hippocrates. Hippocrates: 460-379 BC, the father of Western medicine who believed that the brain was not only involved in sensation but also intelligence. This is known as the cephalocentric or brain hypothesis. Aristotle: 384-322 BC, Greek philosopher who believed that the heart was the centre of sensation and intelligence and that the brain was responsible for cooling the blood coming from the warm heart. This is known as the cardiac or cardiocentric hypothesis. Galen: 130-200 AD, Greek physician who agreed with Hippocrates view of the brain. He was involved with many gladiator injuries and animal dissections to try to discover the functions of the cerebrum and cerebellum, proposing that sensations were received by the cerebrum and muscle movements by the cerebellum, as the cerebrum was soft enough to be imprinted on, and the cerebellum was harder and stronger. He also discovered that the brain was hollow (ventricles) and filled with fluid which matches the 4 vital fluid (humors) model as the fluid moved through the nerves to interpret sensations and initiate movements. The right conclusions were made for the wrong reasons. Andreas Vesalius: 1514-1564, renaissance anatomist who added more details to the structure of the brain except in the case of the ventricles. This brought on the idea that the CSF was forced out into the muscles during a muscle contraction. René Descartes: 1596-1650, French mathematician and philosopher who believed in the fluid-mechanical theory of brain function except in the case of humans. This was because he believed only humans had a soul, that behaviour was controlled by brain mechanisms, and that mental capabilities existed outside of the brain with the mind, known as dualism. The mind can only communicate with the brain through the pineal gland since it lacks a left and right component. His theory could not account for voluntary behaviour or the variability in behaviour. 18 t/19 thCentury Discoveries: during this time, white and gray matter were discovered, along with the CNS, the PNS, and that identical gyri and sulci existed in all humans which allowed for distinguishing lobes for cerebral localization (different parts have different functions). Also, it was examined that injury to the brain disrupted sensation, movements and thoughts and even caused death. Also, nerves were discovered to allow the brain to communicate with the body, and that the brain functions as a machine according to the laws of nature. Luigi Galvani and Emil du Bois-Reymond: Galvani discovered that the brain, muscle, and nerve cells produce electricity and Reymond used the galvanometer to measure the current in muscles to link it to movement when nerves were electrically stimulated. University of Toronto Created By: Lindsay Arathoon 2 Together they dismissed the idea of fluid-based neural communication, as the nerves are ‘wires’ to conduct electrical signals to and from the brain. They did not find out the same nerves for mechanoreceptors were also used in muscle contraction. Bidirectional Communication: suggested as the ability for a neural impulse to move to and from the brain in the same nerve, since when a nerve is cut the sensation and movement is lost. At the same time it was known that within each nerve existed nerve fibres that carried it’s own information in their own directions. François Magendie and Charles Bell: discovered that the dorsal roots from the back of the spine has sensory functions and the ventral roots from the front of the spine had motor functions, which suggested that the brain could also be divided for functional as well as anatomical reasons. This was tested by destroying the dorsal and ventral nerve roots one by one. The nerve fibres of each root were either able to carry information into the brain or out to the muscles. This solved the problem of bidirectional communication. Bell proposed the origin of muscle fibres is the cerebellum and destination of sensory fibres is the cerebrum. Experimental Ablation Method: used to test for localization in the brain, by destroying parts of the brain one by one to determine the sensory and motor deficits. Marie-Jean-Pierre Flourens: Used the experimental ablation method was in 1823 in animals, which provided strong experimental support for Bell and Galen’s ideas about the cerebellum and cerebral functions and that the medulla was important to vital processes. Flourens was also a strong critic of phrenology and did experiments with it. For example if one who was good at music had a bump in the music centre of the head, and another good at music didn’t, it was assumed that the second person was better at a different aspect of music and therefore had the bump elsewhere showing that traits are not isolated to one region and that skull shape ≠ brain shape. Also, he found that regions with injury might restore function, not to the one area but to all areas, as he believed the brain was equipotent and functioned as a whole because he resented Gall. It was later determined by Goltz that the size of the lesion, not the location affected behaviour. Franz Joseph Gall: states that the brain was divided into 27 different areas called faculties, which could be found on the cortex of the brain. He also believed the cortex acted as a muscle, where a larger area was associated with a larger function. This increased size would cause a bump, which could be examined by cranioscopy. This measurement along with personality became known as phrenology. Paul Broca: French neurologist, examined a patient with a problem in producing speech but not in understanding it. After death of the patient, he discovered a lesion or soft tissue on the anterior left hemisphere. He discovered the deficit was in articulating speech and comprehension, and that the left hemisphere in the frontal cortex of the brain was responsible for speech in most people. Gustav Fritsch and Eduard Hitzig: in 1870 showed that small electrical currents to a region of the brain of dogs could elicit discrete movements. David Ferrier replicated the experiment on monkeys, and showed that removal of that part of the brain caused paralysis of the particular muscles. Hermann Monk used experimental ablation to discover that the occipital lobe’s main function is vision. Charles Darwin: assisted by Wallace, published the Origin of Species, which presents the theory of evolution. He sailed on the HMS Beagle and travelled to the Galapagos University of Toronto Created By: Lindsay Arathoon 3 Islands for 5 years examining the finches there for isolation, different beak size and function. Using Linnaeus’ classification supported ideas of evolution, William Smith’s idea that the Earth was older than believed and that some species have changed or gone extinct, Lyell’s idea that geological processes are still continuing, and Thomas Malthus’s idea that populations grow exponentially until it reaches carrying capacity they could propose the idea of natural selection. Organisms with traits that are favourable to their survival will pass them on Ex. the response to fear in mammals is almost identical which indicates evolution from a common ancestor with favourable behaviour retained while other behaviours are adapted to allow the organism to survive in their environment. Theodore Schwann: German zoologist, proposed the cell theory, where all tissues are composed of microscopic units called cells. This led to the recognition of the neuron as the basic unit of the nervous system. Levels of Neuroscience: there are 5 main levels involved with neuroscience. Breaking down a problem into smaller levels of analysis is known as the reductionist approach: • Molecular Neuroscience: brain matter consists of a variety of molecules, which are mostly unique to the nervous system and which play different roles in communication, sentries controlling movement of materials in and out of neurons, conductors controlling neuronal growth, and archive past experiences. • Cellular Neuroscience: how the molecules in the brain work together to give the neuron its properties. Types of neurons, differences in function, communication etc is examined. • Systems Neuroscience: neurons forming complex circuits to carry out a larger motor or sensory function and how these different circuits analyze sensory information, form perceptions of the world, make decisions, and execute movements. • Behavioural Neuroscience: how neural systems work together to produce larger behaviours, where different drugs act, how the different systems contribute to mood and behaviour, where dreams come from. • Cognitive Neuroscience: looks at the mechanisms involved with selfawareness, mental imagery, and language. How brain activity creates the mind. Neuroscientists: those that study the brain, where clinical neuroscience is conducted by physicians, and experimental neuroscience is conducted by those with a M.D or PhD. Clinical neuroscientists include neurologists, psychiatrists, neurosurgeons, and neuropathologists where experimental neuroscience is much broader including computational, developmental, neuroanatomists, neurophysiologists, neuropharmacologists, molecular neurobiologists, neurochemists, neuroethologists, neuropsychologists etc. Neuroscientists try to associate damage with behaviours and come up with possible treatments. Scientific Process: there are 4 major components to this process which allow scientists to establish facts about the nervous system regardless of the level of analysis: • Observation: made during experiments to test a hypothesis and by watching the world around us, for example a human clinical case such as Broca observing his patients speech problems. University of Toronto Created By: Lindsay Arathoon 4 • Replication: can be experimental or clinical, experiments must be replicated on different subjects to ensure the results are consistent and valid and didn’t occur due to chance. • Interpretation: if the observation is correct it can then be interpreted based on the knowledge of those carrying it out during the time period. Ex. Flouren’s testing on birds gave different results than what was found in humans in terms of localization and his beliefs were altered by his hate for Gall. • Verification: when the results are very strong and the same experiment could be done by others and produce the same results and therefore the observations can be accepted as a fact. Inaccuracies and insufficient replication due to slight changes in variables such as time of day can prevent verification. Animal Research: experiments on animals is crucial to what neuroscientists know today. Less than 1% of animals used for food are also used for research and even less for neuroscience. They are used due to low cost, accessibility, and imilarity to humans. The animal used depends on what is being tested, the level of analysis, and how much relatedness to humans there is. More basic processes can be observed in animals with less evolutionary relation to humans. Animals must be treated well, are only used for worthwhile experiments that promise advances in neuroscience, pain and distress is minimized, and animal alternatives are considered. The Institutional Animal Case and Use Committee with a vet, scientists, and others, then expert neuroscientists, then a higher committee must review and evaluate the proposals to see if it is relevant. Economic Costs of Brain Disorders: the costs are very large, as there are more people suffering from these than from heart disease or cancer and billions are spent on helping people with these disorders, on research, and producing effective treatments. This requires neuroscientists to have a proper understanding of normal brain function, though there is still much that is unknown. Chapter 7: The Structure of the Nervous System Anterior: also known as rostral, referring to locations towards the nose of the organism. Posterior: also known as caudal, referring to locations towards the tail of the organism. Dorsal: also known as superior, referring to locations towards the top/back. Ventral: also known as inferior or basal, referring to locations the bottom/belly. Bilateral Symmetry: referring to the fact that the brain has one axis of symmetry right down the mid-sagittal line or midline. This separates the right and left hemispheres of the brain. Most structures found in the brain come with a left and right component with some exceptions. Medial: referring to structures of the body that are closest to the midline. Lateral: referring to structures of the body that are furthest from the midline. In addition, structures that are found on the same side of the body are ipsilateral and those found on opposite sides are contralateral. Section: slicing the brain into parts for examination purposes. Anatomical Planes of Section: there are three planes of the body which are perpendicular to each other in by which sections can be cut: • Sagittal Plane: refers to the plane that cuts the body into left and right halves. This can be along the midline, which is midsagittal, or off-centered which is parasagittal. University of Toronto Created By: Lindsay Arathoon 5 • Frontal Plane: also known as the coronal plane, refers to the plane that cuts the body into anterior and posterior halves and is perpendicular to the ground. • Horizontal Plane: also known as the transverse plane, refers to the plane that cuts the body into superior and inferior halves and is parallel to the ground. Central Nervous System (CNS): encased by bone, consists of the brain and spinal cord. The cerebrum, cerebellum, and brain stem are common in both rats and humans. The brain has 3 major sections: • Forebrain (prosencephalon): makes up most of the brain, the diencephalon and telencephalon. • Midbrain (mesencephalon): has 2 subdivisions, tectum and tegmentum. • Hindbrain (rhombencephalon): the area between the brain and the spinal cord, divided into the metencephalon and the myelenchephalon. Cerebrum: the most rostral and largest part of the brain, consists of 2 cerebral hemispheres separated by the mid-sagittal fissure and held together by the corpus callosum. Usually, these hemispheres receive information from and control contralateral parts of the body. Cerebellum: part of the hindbrain or metencephalon, behind the cerebrum responsible for coordinating and initiating movement, also involved in learning and language processing. The cerebrum contains as many nerves as both cerebral hemispheres combined. Information it receives and where it controls is ipsilateral. It has 3 zones: lateral (in each hemisphere, multijoint movements), intermediate (in each hemisphere, guides limb movement), and vermis (centre, for posture/body movement). Within these zones there are 3 nuclei called deep cerebellar nuclei: fastigial receives from vermis, interpositus receives from intermediate, and dentate received from lateral. Fissure: deep sulci that reach far into the cortex. The longitudinal fissure runs sagittal separating hemispheres, the central fissure runs coronal separating the frontal and parietal lobes, and the lateral or Sylvian fissure separates the temporal lobe in humans. Brain Stem: the remaining part of the brain, forming a stalk where the cerebellum and cerebrum branch out. It is made of a nexus of fibres and cells to relay information from the spinal cord to the cerebrum and cerebellum and vice versa. This is where vital information is processed such as breathing, heart beat, heart rate, consciousness etc therefore although primitive, is vital to life. There are 2 main sections: • Medulla Oblongata: lowest part of the hindbrain, helps regulate involuntary processes such as sleep, breathing, heart rate, etc. • Pons: a bulge above the medulla above the metencephalon and myelencephalon, relays sensory information from the spinal cord to the cerebellum and other brain structures, through the thalamus. Spinal Cord: encased in bone and attached to the brain stem. This relays information from the PNS to and from the brain and all over the body. The spinal cord has 31 sections with 2 spinal nerve projections each (left/right). This is divided into cervical, thoracic, lumbar, sacral, and coccygeal. If one of these is damaged that section and below (caudal) loses sensation and control (paralysis). The muscles are functional but the brain cannot control them after this. Spinal Nerves: how the spinal cord communicates with the body. These are a part of the PNS and exit the vertebrae through notches between each vertebrae. The spinal University of Toronto Created By: Lindsay Arathoon 6 nerves connect to the spinal cord through the dorsal and ventral roots carrying sensory and motor information respectively. Peripheral Nervous System (PNS): efferent systems, the portion of the nervous system existing outside of bone (skull and vertebrae). It has 2 major divisions, the ANS (visceral PNS) and the sPNS. Autonomic Nervous System (ANS): also known as the visceral PNS, part of the PNS that is involved with regulating neurons which innervate blood vessels, organs, and glands to control internal states (ex. Temperature, blood pressure, heart rate etc). It conveys information from the body’s organs to the CNS. There are 2 types of ANS efferent nerves, which are: • Sympathetic: form a network that prepares the body for vigorous activity (ex. Fight/flight, excitement/adrenaline). • Parasympathetic: form a network to sustain non-emergency behaviour, opposes the sympathetic system to create balance. Somatic Nervous System (sPNS): contains all spinal nerves, which innervate the skin, joints, and muscles under voluntary control. Somatic motor axons derive from motor neurons in the ventral spinal cord and where the cell bodies lie in the CNS and somatic sensory axons derive from sensory neurons in the dorsal roots where cell bodies lie outside of the spinal cord in dorsal root ganglia. There is one dorsal root ganglia per spinal nerve. The visceral nervous system projects to smooth and cardiac muscle. Cranial Nerves: there are 12 pairs of nerves that are visible on the cranial surface, many of which attach at the medulla and innervate the head. Some are part of the CNS and others are part of the sPNS or ANS: ooottafvgvsh ssmmbmbsbbmm I olfactory: special sensory, smell II optic: special sensory, vision III oculomotor: somatic/visceral motor, eye motion, pupil dilation IV trochlear: somatic motor, eye movement. V trigeminal: somatic sensory/somatic motor, somatosensory info (face), muscle movement during mastication (chewing). VI abductens: somatic motor, eye movement. VII facial: somatic sensory/special sensory, movement of muscles of facial expression, sensation of anterior tongue. VIII acoustic (vestibulocochlear): special sensory, hearing and balance. IX glossopharyngeal: somatic motor,/visceral motor/special sensory/visceral sensory, movement of throat muscles, parasymp control of salivary glands, post. tongue, blood pressure change detection. X vagus: visceral motor/visceral sensory/somatic motor, control of viscera XI spinal accessory: somatic motor, head/neck/throat movement. XII hypoglossal: somatic motor, tongue muscles/movement. NOTE: throat = oropharynx Cranial Nerve Damage: can result in Parry Romberg Syndrome where the trigeminal nerve is damaged and severe pain in facial tissues. Vestibular neuritis can also occur when the vestibular nerve is inflamed causing vertigo and dizziness. Meninges: Greek for hard covering, doesn’t allow the CNS to come into direct contact with the bone. This provides protection and support for the brain. This is composed of 3 layers: University of Toronto Created By: Lindsay Arathoon 7 • Dura Matter: Latin for hard mother, the layer closest to the skull, a thick fibrous, inelastic layer. There is no space between this and the arachnoid layer. • Arachnoid Matter: Greek for spider, the middle layer, web-like, spongy matter where blood vessels run from the outer brain. • Pia Matter: ‘gentle mother’, very thin layer closest to the brain. Many blood vessels run along this layer. Subarachnoid Space: below the arachnoid matter (between the arachnoid and pia matter) contains cerebrospinal fluid, which helps excrete wastes, maintaining the CNS environment. The CSF can be absorbed into blood vessels by arachnoid villi. Subdural Hematoma: when a blood vessel passing through the dura breaks causing a buildup between the dura and arachnoid matter and can compress on the CNS causing damage therefore must be drained. Ventricles: four hollow areas in the brain filled with cerebrospinal fluid, the lateral (1 st and 2 nd) and 3rdventricle (in midline of brain) are connected by the interventricular foramen, and the 3 rdand 4 thare connected by the cerebral aqueduct (aqueduct of Sylvius) and central canal connects to the spinal cord. They are lined with CSFproducing ependymal cells. The massa intermedia passes through the centre of the 3 rd ventricle. The cerebrospinal fluid is able to flow from the ventricular system to the subarachnoid space through small openings near where the cerebellum attaches to the brainstem. If the flow of CSF is disrupted it can be harmful. Cerebral Spinal Fluid: CSF functions to maintain the ion concentration/pH balance of the brain, protection, cleans the brain/excretion of waste, lubricates it/buoyancy, endocrine medium. Choroid Plexuses: a network of blood vessels in the floor of the lateral ventricles and roof of the 3rd/4thventricles produce the cerebrospinal fluid. This fluid is absorbed by arachnoid granulations in the sagittal sinus. Interventricular Foramen: the foramen of Monroe, connects the lateral and 3 rd ventricles. Hydrocephalus: water on the brain, a developmental disorder. Causes an enlarged skull and ventricles causing the brain to be compressed. Alzheimer’s Disease: neurodegenerative disease where neurons die and shrink so ventricles fill up to take up the extra space, causing the sulci to be enlarged as well. Schizophrenia and hydrocephalus also causes enlarged ventricles. This is caused by a disruption of the cytoskeleton in neurons in the cerebral cortex and the formation of neurofibrillary tangles involving microtubule-associated protein tau. Microtubules usually run parallel to each other, but when tau detaches and accumulates in the soma, the axons wither since materials aren’t being transported properly, impeding the flow of information. The cause is uncertain but can be linked to amyloid plaques. Structural Imaging: a test that provides an image of the brain structure, which helps clinicians find the location of an injury or abnormality. • Computed Axial Tomography (CAT/CT scans): developed by Hounsfields and Cormack, give a non-invasive view of the brain involving computing and x-rays. It involves sending X-rays into the body at different angles then computed into a 3- D image and brain slices where dense areas are bright and the rest is dark, and where ventricles can be seen. The problem is this technology cannot differentiate between white and grey matter. The axial version scans in one plane. These University of Toronto Created By: Lindsay Arathoon 8 scans help identify abnormalities in the brain shown by changes in density on the scan (except for tumors similar in density to normal cells) so they could be related to behavioural defects. • Magnetic Resonance Imaging (MRI): produced an image using nuclear magnetic precision measurement, similar to CT scans, however they have better resolution, don’t use X-ray, and produces images in any plane. Since hydrogen is a common element in the body with poles facing random directions, this scan places a strong magnetic field near the brain so the atoms poles become aligned and polarized. The machine measures the relaxation time (time it takes for the atoms to return to their low-energy normal positions) therefore measures H density. The image shows brain, bone, air and water as dark and with fluid as bright. The receiver coil (measures intensity) and gradient field produce the 3-D image. The limitations are that any internal metal in the body will be attracted to the magnet, which is dangerous ex bits of needle from tattoos or metal salts in the ink. Functional Imaging: collects brain information without measuring electrical currents or magnetic fields. These tests measure changes in blood flow. No oxygen and little glucose is stored in the brain therefore the brain works on blood flow. • Positron Emission Tomography: invasive test capable of blood flow scans with higher resolution, as well as testing utilization of substances such as dopamine. A compound must be radioactively labeled with a substance that emits positrons and gamma rays and injected into the person while a machine scans for positrons emitting in the brain and creates a 3-D image. • Functional Magnetic Resonance Imaging: an MRI scan that detects changes in the positions of H atoms since they are found in water and blood, which is constantly moving in the brain by alternating magnetic gradients very quickly. Since oxygenated and deoxygenated blood have different magnetic properties the MRI shows that oxygenated blood flow increases in part of the brain being used. It gives better photos and resolution however has the same limitations as an MRI. Gray Matter: collection of neuronal cells bodies in the CNS, seen in a freshly dissected brain. The brain is grey on the outside and the spinal cord is grey on the inside. This is mostly composed of neuron cell bodies and blood vessels. Cortex: Latin for bark, a collection of neurons that form a thin sheet at the brain’s surface, just under the surface of the cerebrum. Nucleus: Latin for nut, a clearly distinguishable mass of neurons, usually deep in the brain. ex. the lateral geniculate nucleus, which receives signals from the visual system before being relayed to the occipital lobe. Substantia: a group of related neurons within the brain, but less distinct borders as in a nucleus ex. substantia nigra, involved with control of voluntary movements. Locus: a small, well-defined group of cells ex. locus coeruleus, involved with wakefulness and arousal. Ganglion: Greek for knot, a collection of neurons in the PNS, ex. dorsal root ganglia, which contains cell bodies of axons entering the spinal cord. The only one found in the CNS is the basal ganglia, which is part of the telencephalon and involved with movement. University of Toronto Created By: Lindsay Arathoon 9 Nerve: a bundle of axons in the PNS, one collection of CNS axons (optic nerve). White Matter: a collection of CNS axons, appear white in a freshly dissected brain. The brain is white on the inside and the spinal cord is white on the outside. This is mostly composed of myelinated axons. Tract: a collection of CNS axons with a common origin and destination, ex. corticospinal tract. Bundle: collection of axons that run together, but do not have the same origin and destination as a tract does. Capsule: collection of axons that connect the cerebrum with the brain stem. Commissure: collection of axons that connect one side of the brain with the other, ex. corpus callosum is the largest commissure. Lemniscus: a tract that exists through the brain as a ribbon, ex. medial lemniscus, which bring touch information to the brain from the spinal cord. Hypothalamus: composed of 22 nuclei, part of the diencephalon, involved with feeding, hormone regulation, satiety, homeostasis, and sex. Thalamus: a miniature brain with gyri (bumps) and sulci (grooves), relays sensory information between the cortex and sensory organs. Forms a heart shaped structure in the mid-thalamus junction and is dorsal to the hypothalamus and contains 2 nuclei known as the ventral posterior nucleus of the somatic sensory system, which projects to the postcentral gyrus and the ventral lateral nucleus, which is related to the ventral anterior nucleus, which relays to the motor system. The different nuclei relay information to different areas of the cortex. Pituitary Gland: Involved with hormone secretion, attached to the hypophysis, close to the cranial nerves. Infundibulum stalk is left after the pituitary is removed. Corpus Callosum: the largest commissure in the brain, consisting of white matter. It contains homotopic and hetertopic connections (connections between similar and dissimilar cortical areas respectively). Posterior to anterior is the splenium, then genu, then rostrum. Tectum: part of the midbrain that contains a number of motor nuclei, including the red nucleus and substantia nigra, relays visual and auditory sensory information. There are 4 small bumps on the dorsal midbrain (2 inferior colliculi and 2 superior colliculi): • Superior Colliculus: also called the optic tectum, receives direct input from the eye to control eye movements with motor neurons that innervate the muscles of the eye. • Inferior Colliculus: receives sensory information from the ear to relay it to the thalamus. Tegmentum: located ventral to the tectum, contains the substantia nigra (black matter), red nucleus (motor nuclei) to control voluntary movement, the periaqueductal gray for pain perception. Frontal Lobe: the most anterior part of the brain, containing many important gyri and sulci, involved in control of movement, memory, thinking, awareness, regulating social behaviour, and personality. Temporal Lobe: separated by the Sylvian fissure and parietooccipital sulci, containing many important gyri and sulci. It is involved with language processing, memory, object recognition, and emotion. University of Toronto Created By: Lindsay Arathoon 10 Parietal Lobe: bordered by the central, lateral, and parietooccipital fissures, again contains many important gyri and sulci. It plays a role in sensory and somatosensory function, spatial cognition, and sensory integration. Occipital Lobe: bordered by the parietooccipital sulci, and contains the calcarine fissure, which is surrounded by the primary visual cortex. The most posterior portion is the occipital pole. Cerebral Cortex: the main part of the brain consisting of the frontal, parietal, occipital, and temporal lobes. This structure has 7 different layers, where the neurons closest to the surface are separated by a layer with no neurons (layer I). Deeper layers have cells with large dendrites which penetrate layer I. The layer covering the first layer is the pia matter and the darker regions are denser. Layer 5 has large pyramidal cells compared to layer 3. Layer 6 has a mixture of cells with stellate, Golgi, spindle, pyramidal etc. Below this layer is the corpus callosum. These are anatomically and can be functionally different. These layers can range in depth depending on where in the brain they are. Hippocampus: medial to the lateral ventricle, a structure in the temporal lobe involved with the limbic system, learning, and memory but not storage of memories. Olfactory Cortex: connected ventrally to the hippocampus and continuous with the olfactory bulb. This is separated from the neocortex by a sulcus known as the rhinal fissure. Neocortex: only found in mammals, since the brain has expanded over time by evolution, however its structure and shape has remained constant. This is separated from the hippocampus and olfactory bulb by the rhinal fissure. This has 6 layers and the most amount of laminae Cytoarchitectural Map: can be used to divide the brain and its parts into different zones, similar to how Korbinian Brodmann did, where each area of cortex is labeled by a number. He was unable to show that cortical areas that looked different performed different functions but there were 52 areas. Chapter 7: Appendix Gross Features: gross features of the brain seen lateral include, the cerebrum (which includes the frontal, parietal, occipital, and temporal lobes), the brain stem (pons and medulla) and the cerebellum. The olfactory bulb of the cerebrum can also be seen. Gyri, Sulci, and Fissures: where gyri are bumps, sulci are grooves, and fissures are deep grooves, which form patterns common to all humans with slight differences. The postcentral gyrus lies posterior the central sulcus and is involved with somatic sensation and the precentral gyrus is located anterior to the central sulcus and is involved with voluntary movement. The superior temporal gyrus is located ventral to the Sylvian/lateral fissure and contains the primary auditory cortex. Gyrencephalic refers to organisms with gyrated brains and lissencephalic are organisms with smooth brains ex. rats. Insula: a buried piece of cerebral cortex, can be seen if the Sylvian/lateral fissure is pulled apart. This borders and separates the frontal and temporal lobes. Cortical Sensory Areas: organized by Brodmann into areas, where visual areas 17, 18, and 19 are in the occipital lobe, sensory areas 1, 2, and 3 are in the parietal lobe, University of Toronto Created By: Lindsay Arathoon 11 and auditory areas 41 and 42 are in the superior temporal gyrus of the temporal lobe. On the inferior parietal lobe (operculum) on the insula is gustatory area 43 for taste. Cortical Motor Areas: also organized by Brodmann into areas, where primary motor cortex area 4 and anterior to that, premotor (lateral) and supplementary motor (medial) area 6 lie in the frontal lobe anterior to the central sulcus. Association Areas: areas 5, 7, 20, 21, 37, Involved in higher order processing for single separate sensory/motor modalities (unimodal) ex. smelling a muffin, as well as the integration of multiple modalities (multi/heteromodal) ex. smelling/tasting/seeing a muffin. Association areas are more wide spread, lots of regions are involved with this activity including the prefrontal cortex, posterior parietal cortex, and inferotemporal cortex. Midsagittal Structures: midsagittally, the parts of the brain stem can be seen, as well as the diencephalon thalamus and hypothalamus, the midbrain’s mesencephalon tectum and tegmentum, and the hindbrain’s mylencephalon pons and medulla and metencephalon the cerebellum. Forebrain: • Corpus Callosum: the largest commissure in the brain, consisting of white matter. It contains homotopic and hetertopic connections (connections between similar and dissimilar cortical areas respectively). Posterior to anterior is the splenium, then genu, then rostrum. From a thalamus-telencephalon junction, it is seen connecting the 2 hemispheres in white. • Fornix: can also be seen, which connects the hippocampus on each side with the hypothalamus and mammillary bodies and functions to regulate memory storage. This shows as a stalk between the lateral ventricles and on each side lateral to where the lateral and 3 rdventricles meet and dorsal to the thalamus, ventral to the septal area, which connects it to the corpus callosum. • Mammillary Bodies: nuclei, small protrusions along the ventral brain surface, which play a role in memory as they receive information from the fornix. These can be seen in the mid-thalamus junction and exist ventral to the subthalamus structure. • Amygdala: a small almond-shaped structure in the temporal lobe, part of the limbic system involved with emotions and memory. Cannot be see midsagittally because it lies deep within the cortex. This is located near the ventral surface in each hemisphere and can be seen in the mid-thalamus junction. • Hippocampus: a structure in the temporal lobe involved with the limbic system, learning, and memory but not storage of memories. Cannot be see midsagittally because it lies deep within the cortex. Ventricles: the lateral walls of the 3rdand 4 thventricles, as well as the cerebral aqueduct and spinal canal can be seen from a midsagittal view. The thalamus and hypothalamus lie next to the 3 rdventricle, the midbrain is next to the aqueduct, the pons, medulla, and cerebellum are next to the 4 thventricle, and the spinal cord forms the walls of the spinal canal. The lateral ventricles (1 and 2) extend from the 3 rdventricle but can’t be seen midsagittally. A thalamus-midbrain coronal cut will intersect the horns of the lateral ventricles twice in each hemisphere. • Lateral Ventricle: associated with the cerebral cortex and telencephalon • Third Ventricle: associated with the thalamus and hypothalamus University of Toronto Created By: Lindsay Arathoon 12 • Cerebral Aqueduct: associated with the tectum and tegmentum • Fourth Ventricle: associated with the cerebellum, pons, medulla Ventral Surface: from this view, the 12 cranial nerves of the brain can be seen, as well as the optic chiasm where optic nerves from the eyes may cross over anterior to the hypothalamus. Posterior to the chiasm is there the optic tract begins and disappears into the thalamus. The mammillary bodies, 2 nuclei of the hypothalamus, are visible anterior to the pituitary and infundibulum and are a target of the axons of the fornix. The olfactory bulbs and tracts, pons, medulla, and midbrain can also be seen. Dorsal Surface: from here, the frontal and parietal lobes can be seen, as well as the corpus callosum deep within the medial latitudinal fissure between the two hemispheres. If the cerebrum is removed, the cerebellum dominates the dorsal view and controls movements and balance, with 2 lateral structures and a midline called the vermis. If the cerebellum is also removed, the brain stem is exposed, including the pineal body on top of the thalamus, which secretes melatonin to regulate circadian rhythm and sexual behaviour. Also, the superior and inferior colliculi, for eye movements and audition respectively can be seen posterior to the pineal body. The cerebellar peduncles on either side can be seen and used to connect the cerebellum to the brain stem. Cross Sections: usually best when perpendicular to the neuraxis, can be made with a knife or with brain imaging techniques such as MRI or CT scans. Thalamus-Telencephalon Junction: the telencephalon surrounds the lateral ventricles, and the thalamus surrounds the 3 rdventricle. From a slice, the third ventricle appears as a slit and the lateral ventricles branch out dorsally from it. The hypothalamus forms the floor beneath the 3 rdventricle. The insula is found at the base of the Sylvian fissure separating the frontal and temporal lobe. The basal forebrain lies in the telencephalon lateral to the 3 rdventricle and thalamus and medial to the insula. Internal Capsule: a large collection of axons connecting the cortical white matter with the thalamus. Septal Area: the neurons associated with this contribute axons to the fornix and are involved with memory storage. Basal Ganglia: a division of the telencephalon, important to initiating movements and maintaining muscle tone. This is made of 3 parts: • Caudate Nucleus: along with the putamen, is called the striatum, extends from putamen. This is part of the basal ganglia and is located lateral to the lateral ventricles. • Putamen: along with the globus pallidus, is called the lentiform nucleus, encase the globus pallidus. Part of the basal ganglia and is located lateral to the globus pallidus. • Globus Pallidus: part of the basal ganglia, forms the lentiform nucleus with the putamen and is located medial to the putamen. Mid-Thalamus Junction: more caudal than the Thalamus-Telencephalon Junction, where the thalamus is heart-shaped in the centre of the brain surrounding the end of the 3rdventricle and dorsal to the hypothalamus. The lateral fissure separates the parietal and temporal lobe. Subthalamus: part of the motor system, can be seen in the mid-thalamus junction just dorsal to the mammillary bodies and the hypothalamus. University of Toronto Created By: Lindsay Arathoon 13 Substantia Nigra: means black substance, part of the tectum of the midbrain, can be seen in the mid-thalamus junction. This is located near the base of the brain lateral to the mammillary bodies and is involved with voluntary movements. Parkinson’s results when this structure is degenerated. Thalamus-Midbrain Junction: at this junction the third ventricle connects to the cerebral aqueduct. The thalamus is surrounding this 3 rdventricle and the midbrain surrounds the aqueduct. The lateral ventricles exist here still but are now separated and the 2 horns in each hemisphere can be seen. Medial Geniculate Nuclei: a nucleus of the thalamus, which relays information to the auditory cortex. This is medial to the ventral horn of the lateral ventricles. Lateral Geniculate Nuclei: a nucleus of the thalamus, which relays information to the visual cortex. This is dorsal to the ventral horns of the lateral ventricles and lateral to the medial geniculate nucleus. Hippocampus: medial to the lateral ventricle, a structure in the temporal lobe involved with the limbic system, learning, and memory but not storage of memories. This is located lateral to the ventral horn of the lateral ventricle and beneath the lateral geniculate nucleus. Rostral Midbrain: cuts across the midbrain perpendicular to the neuraxis. This includes the cerebral aqueduct and the roof of the midbrain (tectum) contains the superior colliculus and the substantia nigra, as well as the red nucleus and the periaqueductal gray. Red Nucleus: part of the periaqueductal gray and controls somatic pain sensations. Both are located ventral to the superior colliculi and lateral to the cerebral aqueduct, but the red nucleus is more ventral. Caudal Midbrain: this is a cut below the rostral midbrain and contains the inferior colliculus, cerebral aqueduct, the periaqueductal gray, and the substantia nigra. The roof is formed by the inferior colliculus instead of the superior colliculus as in the rostral midbrain. Pons and Cerebellum: cuts right through the cerebellum and pons and contains the 4 th ventricle, the deep cerebellar nuclei which receive output from the cerebellum, and pontine reticular formation, the pontine nuclei which sends information to the cerebellum, and the cerebellar cortex. Reticular Formation: means net, runs from the midbrain to the medulla at the core of the brain stem under the cerebral aqueduct and 4 thventricle, regulates sleep and wakefulness. Pontine Reticular Formation: located anterior to the 4 thventricle on each side, is involved with controlling body posture. Rostral Medulla: a cut located right below the pons, where the brain surrounding the 4thventricle is the medulla. This also contains the dorsal and ventral cochlear nuclei lateral to the 4thventricle and the superior and inferior olive for motor control, and the raphe nucleus for the modulation of pain, mood, and wakefulness. Medullary Pyramids: lie at the floor of the medulla, where huge bundles of axons from the forebrain meet the spinal cord. These pyramids have corticospinal tracts involved in voluntary movement. Mid-Medulla: cuts below the rostral medulla, with the same structures as the rostral medulla but also includes the medial lemniscus. University of Toronto Created By: Lindsay Arathoon 14 Medial Lemniscus: located along the ventral median fissure to bring information about somatic sensation to the thalamus. Gustatory Nucleus: serves for sense of taste, part of the larger nucleus of the solitary tract, which regulates visceral function, located ventral to the vestibular nucleus and dorsal to the medullary reticular formation. Vestibular Nuclei: serves for sense of balance, located dorsal to the other structures. Medulla-Spinal Cord Junction: the cut right above the brainstem, where the 4 th ventricle disappears as well as the medulla, where the spinal cord begins. This contains the dorsal column nuclei and medial lemniscus with the medullary pyramids. Dorsal Column Nuclei: receives somatic sensory information from the spinal cord. Axons from here cross to the other side of the brain (decussate) and ascend to the thalamus by the medial lemniscus. Spinal Nerves: part of the somatic PNS that communicate with the spinal cord through notches between the vertebrae. These are named according to the vertebrae directly above, where the first 7 are cervical, next 12 are thoracic, next 5 are lumbar, and the rest are sacral and coccygeal, however the cord ends at the 3 rdlumbar vertebrae since it doesn’t grow after birth as the vertebrae does. Cauda Equina: ‘horse’s tail’ the bundles of spinal nerves streaming down within the lumbar and sacral vertebral column. Ventral-Lateral Spinal Cord: when the nerve branches off of the spinal cord and through the vertebral notch, it splits into the dorsal root carrying sensory axons with cell bodies in he dorsal root ganglia and the ventral root which carries motor axons from the gray matter in the ventral spinal cord. Spinal Gray Matter: forms a butterfly shape, with neuronal cell bodies. There are dorsal, ventral, and lateral horns. The white matter with axons run up and down the cord is divided into 3 columns: dorsal between the dorsal horns, lateral between the dorsal and lateral horns, and ventral between the ventral horns. Spinothalamic Tract: carries information about painful stimuli and temperature, located surrounding the ventral ad lateral horns of the gray matter. This is part of the ascending sensory pathways leading to the brain. Ascending Sensory Pathway: includes the spinothalamic tract, and where the whole dorsal column consists of sensory axons to the brain. Descending Motor Pathways: contributes to 2 different pathways: lateral pathway for commands for voluntary movements and ventromedial for the maintenance of posture and reflexes. This includes the vestibulospinal tract, which originates in the vestibular nuclei of the medulla and ends in the spinal cord. Sympathetic Ganglia: appears as a chain of ganglia that runs along the vertebral column, communicating with spinal nerves, with each other, and with internal organs. Parasympathetic Fibres: arises from the vagus nerve to innervate viscera, and the sacral spinal nerves. Vertebral Arteries: 2 arteries that enter the back of the brain from the vertebral column. They supply the posterior and anterior inferior cerebellar arteries before joining the basilar artery. This branches into the posterior cerebral arteries and the superior cerebellar arteries, where the posterior cerebral artery branches into the posterior communicating artery, which connects to the internal carotid arteries. University of Toronto Created By: Lindsay Arathoon 15 Carotid Arteries: 2 arteries that supply blood to the anterior hemispheres of the brain. These separate into the anterior cerebral arteries. The 2 carotid arteries are joined by the anterior communicating artery of the circle of Willis and branch off from the posterior communicating artery. Basilar Artery: receives blood from the vertebral arteries, which give rise to the posterior cerebral arteries and the superior cerebellar arteries. Circle of Willis: a circular region where the arteries join in the brain. If one section becomes blocked, this compensates. This is made up of the anterior and posterior communicating arteries. Also equalizes blood pressure in the brain. Anterior Cerebral Arteries: supplies the anterior cerebral cortex, supplies the frontal lobe, and the medial wall of the cerebral hemisphere. Middle Cerebral Arteries: supplies the midsection of the cerebral cortex, frontal and parietal lobes, the lateral sides of the cortex, and the deep basal forebrain, branches off of the internal carotid arteries. Posterior Cerebral Arteries: branches off of the basilar artery, supplies the posterior cerebral cortex, the medial wall of the occipital lobe, and the inferior part of the temporal lobe. Stroke: neurons are very sensitive to oxygen and glucose and wont survive for long without it. And ischemic stroke (short disruption) this can cause permanent cell death. The severity depends on how long the brain region goes without blood. There are different strokes: • Thrombotic: ischemic, occlusion of cerebral blood vessels due to plaque buildup. • Embolic: ischemic, plugging of the cerebral artery with a dislodged embolus from heart/plaque buildup from vertebral arteries. • Hemorrhagic: rupturing of a cerebral blood vessel from hypertension, aneurysms etc. This can cause pressure build up, causes 20% of stroke but can damage tissues. Broca’s Aphasia: affects Broca’s area, where speech is highly impaired, including long pauses and anomia (difficulty finding words), mainly content words such as nouns, adjective, and verbs are used but not function/connecting words (no grammar structure or grammatically correct speech, agrammatism). Comprehension is still normal. Chapter 2: Neurons and Glia Neurons: cells in the nervous system that are responsible for communication of neural impulses and therefore behaviour. There are 100 billion with a total surface area of 25,000m 2. They sense changes in the environment and communicate these changes with other neurons to command a response. They learn and store information about their external environment. The neuron shape allows it to receive, conduct, and transmit signals. There are 3 types based on the number of neurites extending from the soma: • Unipolar: have only one process coming from the cell body. • Bipolar: have only 2 processes coming from the cell body. • Multipolar: have numerous processes coming from the cell body. These are the most common. University of Toronto Created By: Lindsay Arathoon 16 • Interneurons: most common, relay information within structures instead of between structures (ex. Connect the sensory and motor neurons of the reflex arc to connect the responses). • Afferent Neurons: also called primary sensory neurons, send information to the brain from sensory surfaces. • Efferent Neurons: also called motor neurons, send information away from the brain to muscles and release acetylcholine making them cholinergic. • Golgi type I Neurons: also called projection neurons, have long axons that extend from one part of the brain to another, usually are pyramidal. • Golgi type II Neurons: also called local circuit neurons, have short axons and do not extend beyond the vicinity of the cell body usually are stellate. Glia: means glue, cells in the nervous system that are responsible for support, maintenance, insulation, and nourishment of neurons. There are 3 types: • Astrocytes: large star shaped glia, which fills the space between neurons. There is a small gap between these and the neuron probably allowing the neurite to contract/grow. These are also involved in the blood-brain barrier supply nutrients to neurons, regulate the chemicals and ions in extracellular space, keep neurotransmitters within the synaptic gap, regulate and store neurotransmitters (especially when in excess). These also have receptors where neurotransmitters can bind and create electrical activity. • Oligodendrocytes: glia and Schwann cells to make myelin. They wrap around axons of the CNS to insulate the axons. It is a sheath because it covers the whole axons with small interruption at the nodes of Ranvier. The glia are found in the CNS and the Schwann cells are found in the PNS. • Microglia: small glia made outside the CNS by microphages, which are phagocytes that remove debris from dead neurites and glia in the nervous system. Excessive microglia can cause neurodegenerative diseases. Ependymal Cells: provide the lining of the ventricles and direct cell migration during brain development. Blood-Brain Barrier: a specialized system of capillary endothelial cells connected by tight junctions, different than found in the peripheral tissue, separating the brain from the rest of the body. This creates a protective barrier against other molecules and toxins except those which are fat soluble such as glucose, oxygen, small fat-soluble molecules, ions involved with critical neuron activity, and few water-soluble molecules. This prevents most toxins from getting in except lead. It is also essential to homeostasis by preventing large fluctuations in concentration of ions. Ex by buffering K + ions into extracellular brain fluid preventing premature/sustained neuron depolarization. The barrier is not well developed in young children so lead can accumulate and impair learning and memory even in pregnant women it may not affect the mother but it can pass into the placenta. Drugs sometimes cannot get in and influence the CNS but researchers to modify drugs to have similar structures to molecules, which can enter the blood-brain barrier, such as L-DOPA for PD, which is similar to dopamine. Antiretroviral drugs for HIV can’t cross the barrier. Axon: there is usually one per neuron, extends from the cell body of the neuron at the axon hillock into the axon proper and transmits action potentials to other neurons over great distances. The diameter is uniform along its length and is proportional to the University of Toronto Created By: Lindsay Arathoon 17 speed of the signal it sends. Axons may be branched, with branches at right angles called axon collaterals. These may connect to the same neuron or neighbouring neurons and are called recurrent collaterals. No rough ER extends into the axon but few free ribosomes may exist but means there is no protein synthesis in the axon and all proteins must originate from the soma. These also contain cytoskeleton (except the terminal), mitochondria, and vesicles. Also, the proteins in the membrane of the axon may differ from those in the soma. Dendrites: means tree, only about 2mm in length, extend from the neuron cell body and are the component of the neuron that receives information from other neurons and transmit it to the cell body. These contain some major organelles. They are highly branched to increase the surface for receiving signals and to indicate how many axons it may be connected with. The diameter is not constant along its length, as these generally taper into a point at the tips. These have receptors on them to receive neurotransmitters from the synaptic gap and may contain polyribosomes. All the dendritic branches on a single neuron are known as the dendritic tree. There are 2 types: • Stellate Cells: neuron cells in which the dendritic trees are star-shaped. These neurons can be spiny or aspinous and are usually Golgi type II neurons. These usually respond to steady depolarizing current injected into the soma by firing action potentials at a steady frequency. • Pyramidal Cells: neuron cells in which the dendritic trees are pyramid-shaped. These neurons are spiny and are usually Golgi type I neurons. These can’t usually sustain a steady firing rate, as they fire rapidly at the beginning of the stimulus and then slow down even if the stimulus is steady. This is due to adaptation because of the many ion channels. Large pyramidal cells can also respond with bursts of fast firing with breaks in between. Dendritic Spines: growths on the outside of the dendritic branches that receive synaptic input. The number of spines is also related to the environment the organism was exposed to during development. Neurons with spines are known as spiny, those without are aspinous. New spines can form to store new knowledge and memories, or they can be modified to do so or change existing memories. Structural appearance and function may be modified by experience, learning and memory, environment, development, and stress. Change in number can occur with estrogen or hormones to make more synapses, they can decrease with age and not being used, a result of Down’s Syndrome, Schizophrenia, Huntington’s Disease, Fragile X (more immature spines such as filopodia). The outgrowths can be simple including long and thin (filopodium, immature), simple, stubby, or crook sessile, thin, mushroom, or gemmule pedunculated, or branched. Formaldehyde: a chemical used to ‘fix’ or harden tissues so that the brain could be cut into thin slices for examination. This is not possible otherwise because the consistency of the brain is similar to Jell-O and is not firm enough to be sliced. However, this technique stains all tissues the same colour making it hard to view different types of tissues. Histology: the microscopic study of the structure of tissues. This is difficult to do when the brain has been fixed in formaldehyde, therefore stains must be used to colour some but not all parts of the brain cells. University of Toronto Created By: Lindsay Arathoon 18 Nissl Stain: developed by Franz Nissl, a German neurologist. He showed that certain dyes could be used to stain nuclei of the brain and clumps of material surrounding the nuclei of neurons called Nissl bodies. The stain is purple/blue and doesn’t stain dendrites or axons like the Golgi stain. This is used to distinguish neurons and glia from each other and allows histologist to study the cytoarchitecture of neurons in different parts of the brain. This allowed for the realization that the brain has many specialized regions with different functions. Golgi Stain: developed by Camillo Golgi, an Italian histologist who stained brain tissue with silver chromate solution where some neurons (including dendrites and axons) become darkly coloured in their entirety, instead of just a lump around their nucleus as with the Nissl stain. This stain shows that the neuron has a central part with a nucleus known as the cell body, as well as thin tubes, which radiate away from it known as neurites, which includes a single axon and the dendrites. Golgi believed that neurites of different cells are fused together to form a continuous network, and that the brain is an exception to the cell theory. 1 in 700 or 1400 are stained, but the reason it is stained is unknown. Santiago Ramón Y Cajal: Spanish histologist, used Golgi stain to trace connections and circuitry to the brain, and examine dendrites. He proposed neurons are not continuous and must communicate by contact not continuity (neuron doctrine) and adheres to the cell theory, opposite to Golgi’s theory. He noted that neurons come in all shapes and sizes that correspond to certain parts of the brain known as cytoarchitecture. This was later proven when the electron microscope was invented. The Neuron Doctrine: states that the brain is composed of separate neurons and cells that are structurally, metabolically and functionally independent and that information is transmitted from cell to cell across tiny gaps called synapses. Soma: also known as the cell body or perikaryon, the spherical central part of the neuron, about 20μm in diameter containing cytosol rich in sodium and potassium all surrounded by the neuronal membrane. Like other cells the soma contains organelles. Nucleus: means ‘nut’, located within the neuron soma contained within the nuclear envelope, a perforated membrane which holds in the chromosomes which contain DNA identical to the DNA found in all cells of the body however different genes are expressed in each type of cell. Gene Expression: the reading of DNA, where the product is proteins of all shapes, sizes, and functions by protein synthesis in the cytoplasm. Transcription: occurs because DNA cannot leave the nucleus, therefore mRNA makes a copy so it can enter the cytoplasm through the nuclear pores and initiate protein synthesis. The sequence of nucleotides in the mRNA chain represents the information in the gene, the code for a specific protein to be made. The promoter exists at one end of the gene, where RNA-polymerase binds to initiate transcription, all of which is regulated by transcription factors. The terminator is a sequence, which the RNApolymerase recognizes to stop transcription. Introns: although some portions of DNA do not code for anything (junk DNA), there are also regions of genes that code for proteins. Introns, and they exist within the coding sequences called exons. Introns are spliced out during posttranscriptional modification in a process called RNA splicing. Parts of exons may also be spliced in some cases to University of Toronto Created By: Lindsay Arathoon 19 produce an mRNA strand coding for a different protein. This means transcription of one gene can code for many types of proteins and mRNA’s. Translation: when the mRNA enters the cytoplasm and binds to a ribosome to initiate protein synthesis, where amino acids are added on by tRNA molecules according to their anticodon sequence (sequence opposite to the codons found on the mRNA). Many ribosomes can act on a single mRNA molecule. NOTE: transcription and translation are a part of the central dogma, by Watson and Crick. Rough Endoplasmic Reticulum: outside of the nucleus, and dotted with ribosomes, mostly found in neurons compared to glia and other brain cells. These are also the Nissl bodies found as stained surrounding the stained nucleus. This is a major site of protein synthesis, along with free ribosomes, and polyribosomes, which are ribosomes attached to an mRNA strand. Free ribosomes make the proteins used in the cytosol and rough ER make the proteins destined to be inserted into the cell or organelle membranes. Smooth Endoplasmic Reticulum: fills the cytosol and performs different functions in different locations and in some cases is also connected to the rough ER. One function is to regulate internal concentrations of substances (ex. calcium). The one furthest from the nucleus is the Golgi apparatus for post-translational processing and protein sorting. Mitochondria: enclosed by 2 membranes and contain folds known as cristae with the inner space known as the matrix. This is the site of cellular respiration, taking in pyruvate and oxygen, which enter the Kreb’s cycle, to yield ATP (17 molecules per pyruvate entering). Neuronal Membrane: a barrier to enclose the cytoplasm inside the neuron from the fluids existing outside the neuron. This membrane, like other cells has embedded proteins for transport. Cytoskeleton: scaffolding inside the cell, giving the cell it’s characteristic shape. Microtubules, microfilaments, and neurofilaments make up this structure, though they are not stiff, but dynamic: • Microtubules: big, straight, hollow pipe, run longitudinally down neurites, walls are made of smaller braided strands made of globular tubulin monomer units to form a polymer (resembles pearls connected on a necklace). They are regulated by microtubule-associated protein (MAPs), which anchor them to each other and parts of the neuron. Changes (tau) are associated with Alzheimer’s. • Microfilaments: same thickness as the cell membrane, found all over the neuron but mainly in the neurites, a braid of 2 thin strands (polymers) of actin involved in changing the cell shape. These are also bound to the membrane of the neuron. • Neurofilaments: intermediate sized filaments of neurons, found in all other cells of the body as intermediate filaments. Consist of multiple subunits organized like sausage links, where each unit has 3 long protein strands woven together (not strands of monomers as in microtubules and filaments) making it very strong. Terminal Button: the end of the axon or axon terminal, from which information is sent to the synapse and dendrites of other neurons. This contains several vesicles containing neurotransmitters located near areas high in proteins called active zones. When the action potential is received, as Ca +ions channels open allowing Ca + ions to enter to stimulate proteins to fuse the vesicles to the cell membrane and release University of Toronto Created By: Lindsay Arathoon 20 (exocytose) the neurotransmitters into the synaptic gap at the active zones. When the stimulus stops, the Ca are pumped back out to prevent more neurotransmitters from entering the synaptic gap. Reuptake occurs for neurotransmitters remaining in the gap or are broken down. Innervation is when an axon terminal comes into contact with another cell. The cytoplasm here has no microtubules, and many mitochondria due to high-energy demand. Terminal Arbor: when axons branch out on the ends and connect to the same region on another neuron’s dendrites or soma. Boutons En Passant: when axons form synapses at swollen regions along their length and then continue on to terminate elsewhere. Synapse: the gap between the axon terminal of one neuron and the dendrites of another where information is passed. Presynaptic events are those that occur before the synapse in the axon terminal and postsynaptic are those that occur after the synapse in the dendrites or soma. There are 2 types: • Electrical Synapse: when the membranes the pre and post synaptic cells are touching and connected by junctions allowing electrical impulses to flow between cells uninterrupted for fast transmission. • Chemical Synapse: more common synapse, when the membranes of pre and postsynaptic cells are separated by a gap (synaptic gap/cleft). The axon terminal releases neurotransmitters when the electrical signal is received, which are sent to the dendrite receptors which stimulates a new electrical impulse equal in magnitude. Transfer of information across the gap is called synaptic transmission. Neurotransmitter: a chemical substance used in neuronal communication at synapses. It is released from the active zone of the terminal button and diffuses across the synapse to the dendrite where it binds to its specific receptor protein. There are 2 types: • Direct: bind directly to a ligand-gated ion channel in the postsynaptic membrane, which changes the flow of ions in the postsynaptic cell. • Indirect: work slowly as 1stmessengers, bind to G protein-coupled receptors which then activate receptors to trigger the generation of 2 ndmessengers such as cyclic AMP. The 2 ndmessenger then controls ion channels. Wallerian Degeneration: discovered by English physiologist Waller, that when the axon is separated from the parent cell body, the axon degenerates and cannot be sustained. This is because the proteins the axon needs are synthesized in the soma and shipped over which cannot occur if the connection is lost. This is a way to trace axonal connections in the brain using stains. Axoplasmic Transport: the movement of materials down the axon, as determined by Weiss, where tying a string around the axon interrupted the flow and caused a buildup of materials on the side proximal to the soma and removing the string allowed the materials to continue. It was found that the movement was slow until radioactive amino acids were injected, built into proteins and then observed to the rate it moved into the axon. Grafstein discovered that along with the slow axoplasmic transport there was also a fast axoplasmic transport. Anterograde Transport: movement of materials in the direction from the soma to the axon. Materials are moved inside of vesicles, which move along microtubules and kinesin using ATP. University of Toronto Created By: Lindsay Arathoon 21 Retrograde Transport: movement of materials in the direction from the axon to the soma, to provide signals to the soma about changes in the metabolic needs of the axon terminal. The movement of materials is also done on vesicles moving along microtubules using ATP but dynein is used instead of kinesin. Chapter 3: The Neuronal Membrane at Rest Long Distance Signaling: while telephone wires are copper (good conductor) suspended in air (bad conductor) and well insulated, electrons can be transferred extremely quickly, however in the axon, it is not as well insulated, it is surrounded by salty ions which can also conduct electricity, and the charge is carried by atoms instead of electrons making it less efficient. Excitable Membrane: found on cells capable of generating and
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