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Chapter 3 - Neuroscience Video Lecture Psych 1XX3

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McMaster University
Joe Kim

Video Lecture Psych 1XX3 Chapter 3: Neuroscience Neuroscience I  The Brain o Made up of approximately 100 billion neurons o A typical cortical neuron can have between 1 to 10,000 connections with other neurons o The number of different combinations of activity between brain cells that is theoretically possible is larger than the number of elementary particles in the universe  Rene Descartes – 17 century scientist and philosopher o Separate the mental processes of the mind from the physical processes of the brain o Dualist Framework – the mind was seen as a separate entity existing outside of our biology, yet in control of our actions and thoughts o The physical brain serves in part as a connection between mind and body  Neuroscience o Some neuroscientists work in the space where psychology and neuroscience intersect  They conduct their work at a number of different levels, studying the molecules, cells, and systems of the brain to elucidate the mechanisms that underlie your sophisticated mental abilities o The function of the neuron – the fundamental building block of the nervous system The Neuron  The cells of your body are specialized for different functions o Some secrete hormones o Some join to form protective barriers such as skin o Some contract and form the muscles in the body o Some are specialized for communication – neurons  Each of the 100 billion neurons are organized into signaling pathways to communicate via synaptic transmission  A typical neuron contains two distinct zones o Receptive Zone – designed to pass on signals to other cells  Made up of dendrites branching out from the cell body  Begins with the cell body; cell body contains most of the vital organelles which keep the cell functioning  Branching from the cell body are a number of projections called dendrites which look a lot like the long, stretching branches of a tree  Dendrites reach out to other neurons and receive signals to be relayed through the dendritic branch to the cell body, where some signals will go on to be conveyed down the axon  Once a signal is received in the receptive zone of the neuron, it is passed down a long fiber called the axon (can vary in length) Video Lecture Psych 1XX3 o Transmission Zone –  Made up of the axon and terminal boutons  At the end of the axon, approaching the transmission zone, is a cluster of branches called end-feet/terminal boutons/terminal ends  Terminal boutons reach out and make connections with receptive zone of nearby neurons to transmit the signal further  Each neuron can receive inputs from thousands of other neurons through their dendrites and terminal boutons to form an astoundingly complex network of information transfer  Glial Cells o Provides the structural support, nourishment and insulation needed by the neurons  The glial cells and neurons work together, resulting in a bath of ions, chemicals and blood vasculature make up the entirety of your brain The Action Potential  A neurons cell membranes separate the intracellular fluid, which fills the neuron, and the extracellular fluid which surrounds it o Each contains different concentrations of important ions; including sodium, potassium and chloride Ions in the Brain  Neurons cell membrane separates the intracellular and extracellular fluid environments  Cell membrane o Is selectively permeable, preferentially allowing different ions to pass through it with various levels of ease o Contains a number of protein channels which acts as passageways for ions to pass through  Important channels to consider include the potassium channel and the sodium channel Video Lecture Psych 1XX3  The selective movement of ions across the cell membrane into and out of the neuron is critical for neuron communication  Each of the ions in the intracellular and extracellular fluid contains a positive or negative charge  Resting Potential of the Neuron – the electrical imbalance between the outside and inside of the neuron produced by the starting baseline for the differing concentrations of ions if you add up the charges o The inside of a typical neuron starts off at -70mv relative to the outside of the cell o Controlled by two forces; diffusion and electrostatic force  Diffusion – the tendency for molecules to distribute themselves evenly in a medium o Interacts with the electrostatic force between charged ions  Electrostatic Force – the repulsion between ions with the same charge  Potassium Channel o Leaky Channel – the freedom it affords the potassium ions (K ) is a major contributor to maintaining the resting potential of the neuron  Always allows K to pass through the cell membrane out of the neuron – some potassium ions pass through, some remain inside the cell at rest o Voltage Gated Channel – important player for the action potential  Voltage Gates Na Channel  The protein molecules (-) keep the chloride ions (Cl ) primarily on the outside of the cell  Channels are closed in the resting state of the neuron; so low concentrations of sodium ions (Na ) flow into the cell – most of the Na remain resting on the outside of the cell  The flow of Na is far less important to the resting state of the neuron than K +  The Threshold o The resting voltage of the neuron is constantly fluctuating somewhere around -70 mv o Under the influence of nearby neurons and random ion flow, sometimes a large enough change in the resting charge will occur to reach an important threshold level  Eg/ -50 mv  action potential is triggered  Sodium-Potassium Pump – role of removing Na from the cell and + replacing K o Expels 3 Na+ from the intracellular fluid and replaces the with 2 K+ o Moves slowly and utilizes extensive energy – plays little role in the action potential o Important part of maintaining the ion balance of the neuron and recovering from action potential cascades Video Lecture Psych 1XX3  Action Potential – fundamental unit of communication for neurons o When the -50mv threshold is reached o Na channels along the cell membrane begin to open; most Na have piled up on the outside of the cell before this o The force of diffusion causes the Na to begin rushing into the neuron, rapidly causing the charge on the inside of the cell to become more positive relative to the outside o As the Na rushes into the cell, the electrostatic force begins to push some of the K out of the cell through the leaky potassium channels  Net Effect – increase in positive charge building up inside the cell to the point where the voltage gated K channels open which allow more K to rush out of the cell o After reaching a peak charge of +40mv on the inside of the cell, the sodium channels close – + + Na stops entering the cell, but the K continues to rush outward through the still-open voltage gated potassium channels o Inside of the cell begins losing positive charge and overshoots the baseline of -70mv o The voltage gated potassium channels have completely closed at this point o Cell slowly returns to -70mv and a short refractory period occurs, where the neuron cannot fire another action potential until it settles and recovers from the previous cascade  Action Potential – how a signal is sent down the neuron o Action potential begins in the receptive zone of the neuron, where the cell body connects to the axon o Rapid changes that occur here causes changes in the ion concentrations surrounding nearby channels, leading to an action potential in the adjacent location o Action potentials cascade along the axon toward the terminal boutons  Speed of Action Potential o The process of cascading action potentials along the axon maintains the signal but can be slow and ineffective o Glial cells coat the axons with myelin (a fatty insulating tissue) o Myelin  Oligodendrocytes in Central Nervous System  Schwann Cells in Peripheral Nervous System  Allows the action potential to travel down the axon much faster Video Lecture Psych 1XX3 o When an action potential reaches a myelin sheath it jumps across it through salutatory conduction o Nodes of Ranvier – open regions on axon between segments of myelin o A signal can travel through a long axon very rapidly without any loss of strength  As the electrical signal jumps through the myelin sheath, it weakens  The signal can be strengthened again through ion channel cascades before continuing along and jumping through the next myelin sheath  Sending A Signal o All action potentials produced by a given neuron are roughly identical in strength and duration and proceed in an all or none fashion  Once a threshold is reached, the action potential proceeds to completion without fail o Messages are encoded by frequency, not relative strength o A strong signal will lead to many sequential action potentials, while a weak signal will lead to fewer action potentials in the same period of time  Synapse o Once an action potential travels along an axon, it reaches a terminal bouton where it can connect to nearby neurons o Synapse – area of connection between the terminal bouton of neuron A and the receptive zone of neuron B The Synapse  Not a direct physical connection  Special mechanisms exist to transmit a signal from the presynaptic neuron to the receiving post synaptic neuron  Neurotransmitters – a variety of chemicals within the terminal bouton of the presynaptic neuron o Found within a small intracellular vesicle o As the action potential reaches the terminal boutons, some of the vesicles move toward the cell membrane of the presynaptic neuron o The vesicle fuses with the membrane of the presynaptic neuron and opens, spilling the neurotransmitters into the extracellular fluid o Different Neurotransmitters – depends on the location of type of neuron; each has own function depending on the receptor on the postsynaptic neuron that it binds to  Glutamate  GABA  Serotonin  Dopamine Video Lecture Psych 1XX3  Synaptic Cleft o Once neurotransmitter molecules are released, they enter the synaptic cleft (space between two neurons) o Neurotransmitter molecules float freely in the cleft along with a number of other molecules which can have direct effects on the neurotransmitter  Some may remove particular neurotransmitters from the cleft  Postsynaptic Neuron o Along the membrane of the receiving postsynaptic neuron are a number of receptors designed to receive specific types of neurotransmitter molecules o The free neurotransmitter molecules in the cleft can bind to their specific receptors to continue the process of signal transmission by a number of possible actions  Eg/ Excitatory Postsynaptic Potential (EPSP) – modify the ion channels nearby  During an excitatory post-synaptic potential (EPSP), Na channels open, allowing some + positively charges Na to flow into the cell  This depolarizes the cell moving it away from the -70 mv resting potential and bringing it closer to the -50mv threshold to fire  A number of EPSP’s must occur to reach the -50mv threshold o Temporal Summation – occur one after another from the same presynaptic connection causing a slow climb towards threshold o Spatial Summation – multiple EPSPs can occur simultaneously from several different presynaptic connections with the receptive zone of the postsynaptic neuron Video Lecture Psych 1XX3  If every receptor binding event led to EPSP – neurons would fire uncontrollably with more noise than signal leading to little relevant information being communicated  Eg/ Inhibitory Postsynaptic Potential (IPSP)  Cl channels on the cell membrane open, allowing some Cl to enter the cell  Neuron is said to be hyperpolarized as the action brings the resting potential of neuron to be even more negative and further away from its threshold to fire Neural Development  The Developing Brain o Must grow from a handful of cells into 100 billion neurons, organized in structural unity with highly specific connections  Neurogenesis – birth of neurons o Begins as early as 18 days after conception, when the outer layer at the back of the embryo begins to thicken, forming a plate o Neural Tube – formed when the edges of this thick plate then curl upwards and begin to fuse together by day 21  Completely closed by day 28  Will eventually become the central nervous system, with the brain at the top of the tube and spinal cord making up the bottom  By week 20, this mass of cells actually starts looking like a brain  Ventricular Zone is lined with founder cells that begin dividing as soon as the tube is closed at day 28  From day 28 to 42, cell division is said to be symmetrical o The division of each founder cell leads to two identical founder cells  From around day 42 to 125, cell division is asymmetrical o The dividing founder cell produces one founder cell that stays put, along with a cell that will become a neuron or a glial cell which migrates outward from the ventricular zone Video Lecture Psych 1XX3  Migration – the travelling of neurons to their correct locations o Beings almost immediately after the first neurons are born at day 42 and continues for about 6 weeks after the last neuron is born o Neurons are almost always produced before glial cells, which support the neurons  One Exception – the radial glia cells o Radial Glial Cells – fibers that extend outward from the ventricular zone like a form of scaffolding and they end at the outer layer of the cortex  The neurons use the radial glia cells to migrate from the ventricular zone to the surface of the cortex – the brain grows from the inside out, with the deepest layers of the brain being formed before the outer most layers  As the brain increases in thickness with the addition of more and more neurons, the radial glia cells grow as well so that they always end at the outermost surface o Neurons that are born later have to travel a lot farther and push their way through the other neurons to reach their final resting place; a journey that took a day for the first neurons but could take as long as two weeks for the last neurons  Differentiation – the transformation of cells into the correct type of cell according to their function o Genetics – neural differentiation is partly determined by the location in the ventricular zone where a founder cell originated  Eg/ Some founder cells may be pre-wired to become part of the visual cortex because of where these cells came from in the ventricular zone, whereas others may be set to become part of the frontal cortex Video Lecture Psych 1XX3  Would be a disadvantage for the brain to be completely pre-wired because it wouldn’t allow the brain to change with experience o Environment – input from other cells affects neural differentiation  Neuronal differentiation is sensitive to the input a neuron receives from its connections with other neurons – if an emerging neuron is connected with a neuron from the visual cortex then that neuron will end up doing something related to processing vision  Maturation – the growth of neurons by establishing connections with other neurons o Grow dendrites, axons and synapses o Begins as soon as the neurons reach their final destination after migration and continues into adulthood o Ventricular zone produces many more neurons than are needed – neurons that fail to make connections are pruned away o All neurons receive Neurotropic Factors from other neurons – if you take away neurotropic factors, the neuron will die o Limited among of Neurotropic Factors – neurons are competing with each other for it – only the neurons that make connections will survive; 20-80% of available neurons are eventually trimmed away o Neural connections are also pruned o Many more synaptic connections are formed during this stage than are present later on in development  Eg/ Visual Cortex – the number of synapses doubles between 2 and 4 months of age, and then continues to increase until it reaches its peak at around 1 year of age  After that, the number of synapses begins to decline for the rest of the life span  This increases the processing efficiency of the brain and retains only the most useful connections Case Study: Depression  When brain chemistry is altered, thoughts and behaviours can be dramatically affected  Depression is characterized by a number of symptoms, including feelings of intense sadness, loss of motivation and trouble sleeping  Two neurotransmitters in particular have become implicated in our understanding depression and gained prominence with the advent of tricyclic antidepressants in the 1950s o Serotonin o Norepinephrine  These drugs inhibit the reuptake of serotonin and norepinephrine back to the presynaptic neuron o Reuptake – a normal process of recovering neurotransmitter o Inhibiting reuptake increases the availability of serotonin and norepinephrine in the brain and can alleviate the symptoms of depression  Tricyclic antidepressants were the most popular treatment for depression until the arrival of monoamine oxidase inhibitors (MAO-Is) o Monoamine oxidase – normally found in the synapse to break down serotonin o MAO-I – inhibit the action of monoamine oxidase, preventing the breakdown of serotonin and making it more available  Drawbacks – severe side effects Video Lecture Psych 1XX3  Serotonin Reuptake Inhibitors SSRIs) – more specific to the reuptake of serotonin and seem to have less side effects o Eg/ Prozac  Neurogenesis usually occurs only in early development, but in some areas of the brain, new neurons continue to grow throughout your lifetime o In severely depressed individuals, this neurogenesis seems to be stunted and the organization of neurons in some of these areas is disrupted o May be due to a compound known as brain-derived a neurotropic factor (BDNF), which are vital for the growth and survival of neurons Neuroscience II Terminology  Nervous System Axis (Neuraxis) o Dorsal – to the back of the axis o Ventral – to the front of the axis o Rostral – towards the top of the axis o Caudal – towards the bottom of the axis  Locations in the brain o Medial – locations that are more central or towards the midline of the brain o Lateral – regions toward the outside of the brain Studying the Brain  Most invasive research in neuroscience is not done with humans but rather with animals, tissues or more reduced preparations  Lesion Studies o Eg/ Phineas Gage; upbeat and friendly, had a brain injury in a mining explosion, became short tempered and erratic o Advantage – a direct measure of a brain structures function o Disadvantage – hard to selectively target particular regions and draw conclusions o Limitation to case studies of human brain lesions – rarely isolated to specific brain structures; makes a difficult task of assigning impaired function to specific brain areas o Can study specific brain lesions induced in animal models  Researcher destroys, removes or inactivates a defined brain region and observes the result on behaviour  A variety of behaviours can be affected by a single lesion because the brain is so highly interconnected  Stimulation and Single Cell Recording – lesioning o Electrically stimulate an area of the brain and observe the result on behaviours to build an anatomical map related to function o Used by Wilder Penfield when he performed brain surgery using the techniques “Montreal Procedure” to treat patients with severe epileptic seizures o The brain itself doesn’t have pain receptors – a patient undergoing surgery can be under local anesthetic and fully conscious Video Lecture Psych 1XX3 o Stimulation leads individual neurons to fire and specific brain regions can accurately map perceptual processes and behaviours  Technique:  Make sure that critical areas of the brain are left intact  Probe the exposed brain to locate and remove the scarred tissue that causes seizures  Tools:  Thin, wire carrying a small electric charge to stimulate the cortex o Can 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 body of an individual neuron  Neuron activity is recorded while the animal performs a task or a stimulus is presented  The pattern of firing reveals a particular neurons functional role  Structural Neuroimaging o Used to study large-scale structures and function of brain regions o Computed Tomography (CT) Scan – first structural neuroimaging technique  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  Helpful to diagnose brain injuries  Limitations  Relatively Slow Resolution  Difficult to examine fine brain anatomy o Magnetic Resonance Imaging (MRI)  Powerful magnetic fields are generated, which align the hydrogen atoms found throughout the brain  While the atoms are aligned, the MRI can be used to localize tissue very precisely throughout the brain  Functional Neuroimaging o Positron Emission Tomography (PET) Scan  Used to learn how brain function related to cognitive tasks such as language and memory  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 more active brain areas will use more metabolic resources, so an image of the brains relative pattern of activity can be constructed  Disadvantage – requires a radioactive tracer to be injected, a relatively invasive procedure o Functional Magnetic Resonance Image (fMRI)  Produces a relatively clear image of the brains activity without the need for a radioactive tracer  Measures the blood oxygen dependent signal and uses many of the same principles as the MRI Video Lecture Psych 1XX3  Able to measure the relative use of oxygen throughout the brain and operates under the same basic assumptions as the PET scan – more active areas of the brain require more metabolic resources  Limitations  Provides a rough image of brain activation  Oxygen use by the brain often spikes a few seconds later than the spikes of activity in the brain – few seconds can be a long time in terms of brain function  Not best method if interested in the precise timing of brain activation and function o Electroencephalogram (EEG)  The electrical activity of the brain can be recorded through the scalp by wearing a cap of very sensitive electrodes  Provides a very rough image of the brains overall activity from populations of neurons  Event Related Potential (ERP) Experiment – a specific stimulus is presented to the subject repeatedly while the EEG is recording  EEG will 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 – what remains is a characteristic signal  ERP signals can 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  Eg/ N170 Wave (Marker) – 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  Challenges o Sorting through and interpreting this array of data can be challenging o Finding an area of the brain region active can mean  That this area is responsible for processing the stimulus presented  That this area is a part of a long processing pathway  That its activation is related to some other variable you’re not considering The Brain Regions Video Lecture Psych 1XX3 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 very base of the brain  Consists of the medulla, pons, reticular formation and the cerebellum o Evolutionary the oldest parts of the brain o Found in some form in nearly every vertebrate species o Primarily involved in the regulation of vital bodily functions  Medulla o Lies directly above the spinal cord; looks like an extension of the spinal cord o Important role in vital functions such as breathing, digestion and regulation of heart rate Video Lecture Psych 1XX3  The Pons o A small structure that is rostral to the medulla o Relays information about movement from the cerebral hemispheres to the cerebellum o Contains a number of nuclei that are generally part of the reticular formation o Processes some auditory information o Thought to be involved in some aspects of emotional processing  Reticular Formation o A set of interconnected nuclei found throughout the hindbrain (excluding the cerebellum) o Two main components 1. Ascending Reticular Formation (Reticular Activating System, RAS)  Primarily involved in arousal and motivation  May be a part of a large network responsible for conscious experience  Plays an important role in circadian rhythms  Damage to the RAS leads to devastating losses in brain function, and can lead to permanent coma 2. Descending Reticular Formation  Involved in posture and equilibrium  Plays a role in motor movement  Cerebellum o Translates to “little brain”, resembles a miniature version of the entire brain o Motor commands pass through the cerebellum as t
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