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Lecture 6

Lecture 6 Neuroscience.docx

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Joe Kim

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Lecture 6: Neuroscience II The Structure of the Brain  In humans, the nervous system axis or “neuraxis” curves  Dorsal always refers to the back of the axis and ventral means to the front of the axis or “to the belly”  Because of the curve in the neuraxis, at the level of the head, dorsal is up, but at the level of the spinal cord, dorsal is to the back  Rostral means towards the top of the axis and caudal means towards the bottom of the axis  Locations in the brain that are more central or towards the midline of the brain are medial and regions towards the outside of the brain are lateral  These terms can be combined to locate a very specific brain region  Ex. The medulla is a region in the hindbrain, which can be further divided into several subregions o One subregion is called the rostral ventral medial medulla o This means that it is towards the top, in front of the neuraxis and towards the midline of the brain Studying the Brain  Lesion studies o In 1848, Phineas Gage was a foreman of a railway construction crew, in charge of using explosives to remove large sections of rock from the path of the railroad o He was athletic, intelligent and full of life, by all means respected by his crew o One day, Gage was a victim of an accident, resulting in the blasting of a 3 foot iron rod completely though his left cheekbone and through the top of his skull o Remarkably, he survived and except for some loss of vision and facial disfigurement, he recovered completely o However, his friends barely recognized him as they Gage they once knew o He now become prone to selfish behaviour and bursts of profanity o He became erratic and unreliable and had trouble forming and following through on plans o Gage’s case provided support for the view that the brain has specialized structures for complex behaviours o An advantage of case studies such as Gage’s is that it gives scientists a direct measure of a brain’s structure and function o A disadvantage is that it is hard to selectively target particular regions and draw conclusions o This problem can be overcome by studying specific brain lesions induced in animal models  In such scenarios, a researcher destroys, removes or inactivates a defined brain region and observes the result on behaviour  The accuracy of this emerging understanding of structure and function can depend on the precision of the lesion  An alternative approach to lesioning is to electrically stimulate an area of the brain and observe the result on behaviour to build an anatomical map related to function o This technique was used extensively by neurologist Penfield as he performed brain surgery to treat patients with severe epileptic seizures o Penfield revolutionized techniques in brain surgery as he perfected his “Montreal Procedure” to treat patients experiencing severe seizures o In doing so, he had to be sure that critical areas of the brain were left intact o Because the brain itself does not have pain receptors, a patient undergoing surgery could be under local anaesthetic and fully conscious, working with Penfield to probe the exposed brain to locate and removed scarred tissue that caused the seizures o Penfield used a thin wire carrying a small electric charge to stimulate the cortex o This stimulation leads individual neurons to fire and thus Penfield could accurately map perceptual processes and behaviours to specific brain regions o Ex. If an area of the visual cortex was stimulated, a patient reported seeing flashes of light and if an area of the motor cortex was stimulated, a patient would experience a muscle twitch  Single cell recording o Electrodes can also be used to record ongoing electrical activity in the brain through single cell recording techniques o A small electrode is inserted into the nervous tissue of a live animal model with its tip held just outside the cell body of an individual neuron o From this electrode, neural activity is recorded while the animal performs a task or a stimulus is presented o The pattern of firing reveals a particular neuron’s functional role o In a typical experiment, cats were presented with specific visual stimuli while recording from single cells in the visual cortex o In this way, individual cell types were identified that responded to specific categories of visual stimuli  Structural neuroimaging o To study large-scale structure and function of brain regions, neuroscientists use structural and functioning neuroimaging techniques o The first structural neuroimaging technique developed was computed tomography (CT)  During a CT scan, a series of X-ray slices of the brain are taken and pieced together to produce a relatively quick and inexpensive picture of the brain  These scans are often helpful to diagnose brain injuries  A major limitation with the CT scan by today’s standards is its relatively low resolution  It is very difficult to examine fine brain anatomy with a CT scan and as such it is not often used in neuroscience research o For a more detailed structural image of the brain, neuroscientists use MRI (magnetic resonance imaging)  In an MRI machine, powerful magnetic fields are generated, which align the hydrogen atoms found throughout the brain  While these atoms are aligned, an MRI can be used to localize tissue very precisely throughout the brain  Functional neuroimaging o Cognitive neuroscientists can use a functional imaging technique such as the positron emission tomography (PET scan) to learn how brain function relates to cognitive tasks such as language and memory o In a PET scan, a radioactive tracer of glucose or oxygen is injected into the bloodstream  The radioactive molecules make their way to the brain and are used in metabolic processes, which are detected by the PET scan  The logic is that more active brain areas will use more metabolic resources and so an image of the brain’s relative pattern of activity can be constructed  A disadvantage of the PET scan is that it requires a radioactive tracer to be injected, a relatively invasive procedure o Functional magnetic resonance image (fMRI) is often preferred because it can produce a relatively clear image of the brain’s activity without the need for a radioactive tracer  fMRI works by measured the blood oxygen dependent signal and uses many of the same principles as the MRI  It is able to measure the relative use of oxygen throughout the brain and operates under the same basic assumption as the PET scan (more active areas of the brain require more metabolic resources)  A limitation of fMRI is that it provides a very rough image of brain activation  Oxygen use by the brain often spikes a few seconds later than the spikes of activity in the brain, which can be a very long time in terms of brain function  As such, fMRI is not the best method to use if a researcher is interested in precise timing of brain activation and function o A final neuroimaging method is the electroencephalogram or EEG  The electrical activity of the brain can be recorded through the scalp by wearing a cap of very sensitive electrodes  The EEG provides only a very rough image of the brain’s overall activity, from populations of neurons  However, with a few modifications, the EEG can become more informative  In an event related potential (ERP) experiment, a specific stimulus is presented to the subject repeatedly, while the EEG is recording  Although the EEG will generally produce very noisy waves, the specific stimulus presented can have a small and consistent effect on the readout  By averaging the EEG signal across many trials, the noise can be balanced out and what remains is a characteristic signal  These ERP signals can still be difficult to interpret, but there are a number of reliable signs reported throughout the literature that serves as markers for different types of neural processes  Ex. One marker is called the N170 wave, which is thought to correspond to face processing  When combined with a behavioural measure, EEG and ERP signals can be highly informative markers, with precise temporal resolution, on the order of milliseconds The Brain Regions – Hindbrain  All information into and out of the brain travels through cranial nerves or through the spinal cord, which connects to the hindbrain at the base of the brain  The hindbrain consists of the medulla, pons, reticular formation and the cerebellum  These structures are evolutionarily the oldest parts of the brain and found in some form in nearly every vertebrate species  Primarily involved in the regulation of vital bodily functions  Medulla o Most caudal part of hindbrain and lies directly above spinal cord o Structurally, looks like an extension of the spinal cord and plays an important role in vital functions such as breathing, digestion and regulation of heart rate  Pons o The pons is a small structure that is rostral to the medulla o Relays information about movement from the cerebral hemispheres to the cerebellum o The pons also contains a number of nuclei that are generally part of the reticular formation o Pons also processes some auditory information and is thought to be involved in some aspects of emotional processing  Reticular formation o A set of interconnected nuclei found throughout the hindbrain (excluding cerebellum) o Two main components  The ascending reticular formation (also called reticular activating system or RAS) is primarily involved in arousal and motivation and may be a part of the network responsible for conscious experience  The RAS plays an important role in circadian rhythms  Damage to the RAS leads to devastating losses in brain function and in extreme cases, permanent coma  The descending reticular formation is involved in posture and equilibrium and plays a role in motor movement  Cerebellum o Resembles a miniature version of the entire brain o Motor commands pass through the cerebellum as they signal muscles to contract and during the production of movement, sensory signals return to the cerebellum for immediate error correction o Patients with damage to the cerebellum display exaggerated jerky movements, overshooting or missing targets completely The Brain Region – Midbrain  A relatively small region that lies between the hindbrain and the forebrain  Generally the midbrain contains two major subdivisions, the tectum and the tegmentum  Within these regions are a number of structures involved in a variety of functions, including perception, arousal and motor control  The tectum o Located in the dorsal portion of the midbrain and contains two primary structures, the superior and inferior colliculi o These two structures are involved in functions related to perception and action o The superior colliculus is thought to be involved in eye movements and visual reflexes o The inferior colliculus is thought to be involved in auditory integration  The tegmentum o Contains important structures, including nuclei of the reticular formation, the red nucleus and the substantia nigra o The red nucleus is an important structure involved in the production of movement  In vertebrates with less complex brains, the red nucleus is one of the most important structures for the regulation and production of movement, as it projects directly to the cerebellum and spinal cord  In humans, with their relatively advanced forebrain structures, the red nucleus plays a lesser role in the production of movement and instead serves primarily as a relay station for information from higher motor areas to and from the cerebellum and spinal cord  However, in the still developing brain of young infants, many motor behaviours may still be controlled by the red nucleus o The substantia nigra is another important and interconnected region of the midbrain, with projections into a variety of forebrain regions  The substantia nigra is involved in tasks such as motor planning and learning and reward seeking  The substantia nigra contains neurons that produce the neurotransmitter dopamine, which is released in high concentrations during a variety of rewarding behaviours  Artificial rewards such as drugs of abuse act to increase the amount of dopamine in the synaptic cleft to unnatural levels, which may contribute to the addictive properties of these drugs  At the other extreme, damage to dopaminergic neurons in the substantia nigra leads to reduced levels of dopamine  This has been directly implicated in motor tremors that are characteristic of Parkinson’s Disease The Brain Region – Forebrain  Contains structures involved in complex functions such as emotion, memory, perception and thought  Is the largest region of the brain  Subcortical structures of the limbic system o Hypothalamus  Controls several integrative functions including directing stress responses, regulating energy metabolism by influencing feeding, digestion and metabolic rate and regulating reproduction through hormonal control of mating, pregnancy and lactation (Fight, Flight, Feeding, Reproduction)  Hypothalamus exhibits these regulatory roles through neurons that are capable of producing a variety of regulatory hormones and via connections with the pituitary gland and key subcortical structures that lie below the surface of the cortex o Pituitary  The pituitary gland lies inferior to the hypothalamus  Because of the variety of vital hormones it regulates and releases, it is often called the master gland of the endocrine system  Contains two subregions, the anterior and the posterior  The anterior pituitary receives signals from the brain, usually via the hypothalamus and releases stimulating hormones to regulate other important endocrine glands such as the thyroid, testes, ovaries and adrenals  The posterior pituitary is an extension of the hypothalamus and releases two hormones called oxytocin and vasopressin  Oxytocin is involved in basic physiological functions such as lactation and uterine contractions in women and may also play a role in bonding, love and trust  Vasopressin is a vital blood hormone that regulates your levels of thirst by interacting with your kidneys to regulate glucose levels o Thalamus  A large structure near the centre of the brain  Axons from every sensory modality synapse in the thalamus, which processes and relays the information selectively to areas of the cerebral cortex  Output from the cerebellum and limbic system also first relay through the thalamus on its way to the cortex o Amygdala  The amygdala are two symmetrical almond shaped structures located below the surface of each temporal lobe  Receives sensory information from the thalamus and contains nuclei,
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