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Psych 1XX3 Neuroscience II Lecture Notes.pdf

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Joe Kim

Psych 1XX3 – Notes on Neuroscience II – Jan 27th, 2010  In humans, the nervous system axis, or "neuraxis", curves as you can see in this diagram.  Dorsal always refers to the back of the axis, and ventral means to the front of the axis or "to or 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. The term rostral means towards the top of the axis, and caudal means towards the bottom of the axis.  Finally, 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. Studying the Brain:  Neuroscientists have long been interested in case studies of accidental brain injury which can link anatomy with associated cognitive and behavioural deficits that are observed. Lesion Studies:  Consider the famous case study of Phineas Gage. In 1848, Gage was the victim of a tragic accident, resulting in the blasting of a 3 foot iron rod completely through his left cheekbone and through the top of his skull. Remarkably; Gage survived, and he recovered completely. However, once an upbeat, polite and caring person, Gage became prone to selfish behaviour and bursts of profanity. He became erratic and unreliable, and had trouble forming and following through on plans. Gage's case provided support for the view that the brain has specialized structures for complex behaviours.  Case studies such as Phineas Gage have given neuroscientists tantalizing hints to the relationship between structure and function in the brain.  A limitation of most case studies of human brain lesions is that they are rarely isolated to specific brain structures. This certainly makes a more difficult task of assigning impaired function to specific brain areas.  This problem can be overcome by studying specific brain lesions induced in animal models. In such ablation studies, 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. Even so, because the brain is so highly interconnected, often a variety of behaviours are affected by a single lesion. Stimulation and Signal Cell Recording:  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.  This technique was used extensively by the Canadian neurologist Penfield as he performed brain surgery to treat patients with severe epileptic seizures. Single Cell Recording:  Penfield revolutionized techniques in brain surgery as he perfected his "Montreal Procedure" to treat patients experiencing severe seizures.  In doing so, he had to be sure that critical areas of the brain were left intact. 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 remove the scarred tissue that caused the seizures.  Penfield used a thin, wire carrying a small electric charge to stimulate the cortex. This stimulation leads individual neurons to fire, and thus Penfield could very accurately map perceptual processes and behaviours to specific brain regions.  For example, 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.  Penfield’s pioneering work revealed specific function to previously unmapped regions of the brain.  Electrodes can also be used to record ongoing electrical activity in the brain through single cell recording techniques.  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.  From this electrode, neural activity is recorded while the animal performs a task or a stimulus is presented. The pattern of firing reveals a particular  neuron’s   functional role.  For example, in your study of Vision, you will encounter the seminal work of Hubel and Wiesel. In a typical experiment, cats were presented with specific visual stimuli while recording from single cells in the visual cortex.  In this wary, individual cell types were identified that responded to specific categories of visual stimuli.  Limitation: it only provides information about a limited area in the brain. Structural Neuroimaging:  To study large-scale structure and function of brain regions, neuroscientists use structural and functional neuroimaging techniques.  The first structural neuroimaging technique developed was computed tomography (or 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.  Limitation: its relatively low resolution.  For a more detailed structural image of the brain, neuroscientists use MRI, or 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:  Cognitive neuroscientists can use a functional imaging technique such as positron emission tomography (or PET scan), to learn how brain function relates to cognitive tasks such as language and memory.  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.  Disadvantage: requires a radioactive tracer to be injected, a relatively invasive procedure.  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 measuring 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.  Popular, but has limitations: 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 - and a few seconds 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 the precise timing of brain activation and function.  A final neuroimaging method to consider is the electroencephalogram (EEG). The electrical activity of the brain can 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 clever modifications, the EEG can become more informative.  In an event related potential (or 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 signals reported throughout the literature that serve as markers for different types of neural processes.  Example: one such 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 very precise temporal resolution, on the order of milliseconds. The Brain Regions (See image below.) Introduction:  Your look at the brain will progress through three broad regions: the hindbrain, midbrain, and forebrain. The Hindbrain: (See image below.) Def’n:  Region  at  base  of  brain  that  connects  the  brain  to  the  spinal  cord.  All information into and out of the brain travels through cranial nerves or through the spinal cord, which connects to the hindbrain at the very 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. And so it's not surprising that they are primarily involved in the regulation of vital bodily functions. The Hindbrain: The Medulla   The medulla is the most caudal part of the hindbrain and lies directly above the spinal cord. Structurally, it 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. The Hindbrain: The Pons   The pons is a small structure that is rostral to the medulla. The pons relays information about movement from the cerebral hemispheres to the cerebellum.  The pons also contains a number of nuclei that are generally part of the reticular formation.  Additionally the pons processes some auditory information and is thought to be involved in some aspects of emotional processing. The Hindbrain: The Reticular Formation   The reticular formation is a set of interconnected nuclei found throughout the hindbrain (excluding the cerebellum).  The reticular formation has two main components: (1) The ascending reticular formation (also called reticular activating system or RAS) is primarily involved in arousal and motivation, and may be a part of a large network responsible for your conscious experience.  Beyond that, the RAS plays an important role in circadian rhythms. Damage to the RAS leads to devastating losses in brain function, and in the extreme case a permanent coma.  (2) The descending reticular formation is involved in posture and equilibrium, and plays a role in motor movement. The Hindbrain: The Cerebellum  The cerebellum translates to "little brain" and resembles a miniature version of the entire brain.  The cerebellum is the maestro of the orchestra that coordinates all movement. 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.  The importance of this structure is apparent in patients with damage to the cerebellum who display exaggerated, jerky movements overshooting or missing targets completely. The Midbrain:  The midbrain is a relatively small region that lies between the hindbrain and the forebrain.  Generally, the midbrain contains two major subdivisions: the tectum and the tegmentum. The Tectum: (Image shown on next page.)  Within these regions are a number of structures involved in a variety of functions, including perception, arousal, and motor control.  The tectum is located in the dorsal portion of the midbrain and contains two primary structures: the superior and inferior colliculi.  These two structures are involved in functions related to perception and action.  The superior colliculus is thought to be involved in eye movements and visual reflexes, while the inferior colliculus is thought to be involved in auditory integration. The Tegmentum:  The tegmentum contains important structures, including nuclei of the reticular formation, the red nucleus, and the substantia nigra. The Red Nucleus:  The red nucleus is an important structure involved in the production of movement. In vertebrates with less complex brains, it 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 fore brain 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. (See image below) The Substantia Nigra (See Image on Prev. Page):  The substantia nigra is another important and highly interconnected region of the midbrain, with projections into a variety of forebrain regions.  The substantia nigra is involved in such tasks as motor planning and learning
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