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Joe Kim (962)
Lecture 5

Lecture 5 Neuroscience.docx

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McMaster University
Joe Kim

Lecture 5: Neuroscience I Introduction to Neuroscience  Descartes o Tried to understand the mental processes that allow a person to learn, feel and act and relate it all to the brain o His approach was to separate the mental processes of the mind from the physical processes of the brain o In his 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 was thought to serve as a connection between the mind and body o In modern times, the challenge for neuroscience is to understand how the biological brain produces the mental processes of the mind The Neuron  Neurons are cells that are specialized for communication  Each of your 100 billion neurons are organized into signalling pathways to communicate via synaptic transmission  What makes neurons good at communicating is their unique structure o A typical neuron contains two distinct zones  A receptive zone designed to receive signals from other neurons  Made up of dendrites branching out from the cell body  A transmission zone designed to pass on signals to other cells  Made up of the axon and terminal boutons  The receptive zone o Begins with the cell body o The cell body contains most of the vital organelles, which keep the cell functioning o Branching from the cell body are a number of projections called dendrites o These 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  The axon o Once a neuron receives a signal in the receptive zone, it is passed down a long fiber called the axon, which can vary in length o Some neurons have very short axons, while others have axons that can be 1m in length as they extend from your spine to the bottom of your feet o At the end of the axon, approaching the transmission zone of the neuron, is another cluster of branches o These branches at the end of the neuron are called end-feet or terminal boutons or terminal ends o The terminal ends reach out and make connections with receptive zones of nearby neurons to transmit a signal further  A neural network o Each neuron can receive inputs from thousands of other neurons through their dendrites and terminal boutons to form a complex network of information transfer o The glial cells are the hardworking, co-stars of the nervous system  They provide structural support, nourishment and insulation needed by the high profile neurons o The glial cells and neurons that work together, resting in a bath of ion chemicals and blood vasculature make up the entirety of your brain The Action Potential  A neuron’s cell membrane separates the intracellular fluid, which fills the neuron and the extracellular fluid, which surrounds it  Each contains different concentrations of important ions, including sodium, potassium and chloride  The cell membrane is selectively permeable, preferentially allowing different ions to pass through it with various levels of ease  The cell membrane also 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  The selective movement of ions across the cell membrane into and out of the neuron is critical for neural communication  The resting potential o The inside of a typical neuron starts of at -70mv relative to the outside of the cell o This baseline imbalance is called the resting potential of the neuron o The resting potential of a neuron is controlled by two forces, diffusion and electrostatic force o Diffusion is the force that distributes molecules evenly throughout a medium o The diffusion force interacts with the electrostatic force between charged ions  When two similarly charged ions meet, they repel each other and when two oppositely charged ions meet, they attract o The net result of the diffusion and electrostatic forces leads to an overall resting potential of -70mv inside the cell compared to the outside o At the start of the resting potential, the negatively charged large protein molecules within the neuron are so large that they cannot pass through the cell membrane and so they remain trapped inside o On the other hand, potassium, sodium and chloride ions are mobile o Two different types of potassium channels  The leaky potassium channel is like a tap that’s always open  It allows positively charged potassium to pass through the cell membrane out of the neuron  However, most of the potassium remains inside the cell at rest  Overall the leaky potassium channel is a major contributor to maintaining the resting potential of the neuron  Voltage gated channel  Important for the action potential o The negatively charged chloride ions are also mobile and the electrostatic force of the negatively charged protein molecules keep them primarily on the outside of the cell o Voltage gated sodium channels are closed in the resting state of the neuron and so the positively charged sodium ions flow in only very low concentrations into the cell o Despite this subtle inward flow, most of the sodium ions remain resting on the outside of the cell and the flow of sodium is far less important to the resting state of the neuron than potassium  The threshold o The forces governing the distribution of ions are not rigidly stuck in place and in reality, the resting voltage of the neuron is constantly fluctuating somewhere around -70mv o Under the influence of nearby neurons and random ion flow, a large enough change in the resting charge will occur to reach an important threshold level o When the threshold of -50mv is reached, the action potential is triggered  The action potential o The fundamental unit of communication for neurons o When the -50mv threshold is reached, a cascade of events is triggered  It starts with the sodium channels along the cell membrane beginning to open  Up to this point, most of the sodium ions are on the outside of the cell  With the sodium channels now open, the force of diffusion causes the positively charged sodium ions to being rushing into the neuron, causing the charge on the inside of the cell to rapidly become more positive relative to the outside  As the positively charged sodium rushes into the cell, the electrostatic force begins to push some of the positively charged potassium ions out of the cell through the leaky potassium channels  Overall, the net effect is to still increase the positive charge building up inside the cell to the point (0mv) where the voltage gated potassium channels open, which allows more positively charged potassium ions to rush out of the cell  After reaching a peak charge of about +40mv on the inside of the cell, the sodium channels close  This means that sodium stops entering the cell, but potassium continues to rush outward through the still-open voltage gated potassium channel  The inside of the cell begins to lose positive charge and continues to fall and actually overshoots the baseline -70mv resting potential (reaching - 100mv)  At this point, the voltage gated potassium channels have completely closed  With the rush of ions complete, the 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 o Throughout the action potential and after it is complete, another active player along the cell membrane is the sodium-potassium pump o This pump has the role of removing sodium from the cell and replacing potassium  To do so, it expels three sodium ions from the intracellular fluid and replaces them with two potassium ions  The sodium-potassium pump moves slowly and utilizes extensive energy, therefore playing little role in the action potential itself  It is however, an important part of maintaining the ion balance of the neuron and recovering from action potential cascades o The action potential begins in the receptive zone of the neuron, where the cell body connects to the axon o The rapid change that occurs here causes changes in ion concentrations surrounding nearby channels, leading to an action potential in the adjacent location o And thus, action potentials cascade along the axon toward the terminal boutons o This process of cascading action potentials along the axon maintains the signal, but it can be too slow for efficient communication  There’s a clever solution  Special glial cells coat many axons with a type of fatty, insulating tissue called myelin  These special cells are the Oligodendrocytes in the Central Nervous System and Schwann cells in the Peripheral Nervous System  The insulating layer of myelin allows the action potential to travel down the axon much faster  When an action potential reaches a myelin sheath, it jumps across it through a process called saltatory conduction  Between the segments of myelin are open regions on the axon called the Nodes of Ranvier  These nodes are very important because as the electrical signals jump through the myelin sheath, it weakens  At the nodes, the signal can be strengthened again through ion channel cascades before continuing along and jumping through the next myelin sheath  Through this process, a signal can travel through a long axon very rapidly without any loss of strength  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 o Once the threshold is reached, the action proceeds to completion without fail, there is no such thing as a half action potential o How then are different types of messages encoded  Instead of encoding messages by relative strength of an action potential, messages are encoded by frequency, how often an action potential fires o Immediately following an action potential is the refractory period during which another action potential cannot begin o However, shortly after, the neuron can potentially fire again, triggering another action potential cascade o In this way, 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 o The frequency and pattern encodes the message to be passed on to the neighbouring cell o Once an action potential travels along an axon, it reaches a terminal bouton, where it can connect to nearby neurons o This area of connection between the terminal bouton of neuron A and the receptive zone of neuron B, is called the synapse The Synapse  The synapse is not a direct physical connection and instead, special mechanisms exist to transmit a signal from the presynaptic neuron to the receiving postsynaptic neuron  Neurotransmitters o Within the terminal bouton of the presynaptic neuron are a variety of chemicals collectively known as neurotransmitters o These neurotransmitters are found within small intracellular containers called vesicles o As the action potential reaches the terminal bouton, 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 neurotransmitter molecules into the extracellular fluid o There are a variety of different neurotransmitters that may be released, depending on the location of type of neuron and include  Glutamate, GABA, serotonin, dopamine  Each perform a different function o A single neurotransmitter can also have multiple functions, depending on the receptor on the postsynaptic neuron that it binds to  The synaptic cleft o Once neurotransmitter molecules are released, they enter the space between two neurons, called the synaptic cleft o The neurotransmitter molecules float freely in the cleft along with a number of other molecules, which can have direct
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