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Psych 1XX3 Neuroscience I Lecture Notes.pdf

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Joe Kim

th Psych 1XX3 –Notes on Neuroscience 1 – January 26 , 2010 ▯ Descartes- Mind and brain are separate entities (separate the mental processes of the brain and the physical processes of the brain as formalized)=> dualist framework; the mind was seen as a separate entity existing outside of our biology, yet in control of our actions and thoughts. ▯ The physical brain was serve, in part, as a connection between mind and body. ▯ Challenge in modern times for neuroscience is to understand how the biological brain produces the mental processes of the minds ▯ Some neuroscientist work in the space where psychology and neuroscience intersect: conduct their work at number of different levels, studying molecules, cells, systems of the brain to elucidate the mechanisms that underlie your sophisticated mental abilities The Neuron ▯ Cells are specialized for different functions: some secrete hormones, others join to form protective barriers such as your skin, and still others contract and form the muscles in your body. Neurons fall in the category for communications ▯ Each of your 100 billion neurons is organized into signaling pathways to communicate via synaptic transmission. What makes neurons good at communication is their unique structure. ▯ A typical neuron contains two distinct zones to receive signals from other neurons, and transmission zone designed to pass on signals to other cells. The receptive zone is made up of the dendrites branching out from the cell body, while transmission zone is made up of the axon and terminal boutons. ▯ The receptive zone of the neuron begins with the cell body. The cell body contains most of the vital organelles, which keep the cell functioning. Branching form the cell body are number of projections called dendrites which look a lot like the long, stretching branches of a tree. The dendrites reach out to other neurons and receive signals to be relayed through the dendrite branch to the cell body, where some signals will go on to be conveyed down the axon. ▯ 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. ▯ 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. ▯ At the end of the axon, approaching the transmission zone of the neuron, is another cluster of branches; these branches at the end of the neuron look like little feet and are called end-feet or terminal boutons or terminal ends. The terminal boutons reach out and make connections with receptive zone of nearby neurons to transmit the signal further. ▯ A network is built. Each neuron can receive inputs from thousands of other neurons through their dendrites and terminal boutons to form complex network of information to transfer. ▯ Glial cells provide the structural support, nourishment, and insulation needed by the high profile neurons. The glial cells and neurons work together, resting in a bath of ions, chemicals and blood vasculature make up the entity 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 act 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. ▯ If you add up all the charges, the starting baseline for the differing concentration of ions produces an electrical imbalance between the outside and inside of the neuron. In fact, the inside of a typical neuron starts off at -70mv relative to the outside of the cell. This baseline imbalance is called the resting potential of the neuron. The Resting Potential: Def’n  of  Diffusion:  The  tendency  for  molecules  to  distribute  themselves  evenly  in  a  medium. Def’n  of  Electostatic  Force:  The  repulsion  between  ions  with  the  same  charge. ▯ Why does the inside of the neuron start off at -70 mv compared to the outside? The resting potential of a neuron is controlled by two forces – diffusion and electrostatic force. This 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.  It’s  the  net  result  of  the  diffusion  and  electrostatic  forces  that  leads  to  an   overall resting potential of -70mv outside the cell compared to the inside. ▯ The negatively charged large protein molecules w/I the neuron are so large they cannot pass though the cell membrane so they remain trapped inside. The K, Na and Cl ions are mobile. ▯ Two different types of K channels – the so-called leaky channel and the voltage gated channel. ▯ The leaky channel is like a tap that is always open, allowing positively charged potassium to pass through the cell membrane out of the neuron. ▯ is the major contributor to maintaining the restinndpotential of the neuron. ▯ The 2 type of K channel is the voltage gated channel which is an important player for the action potential. ▯ The negatively charged Cl ions are also mobile and the electrostatic force of the negatively charged protein molecules keeps them primarily on the outside of the cell. ▯ Voltage gated Na channels are closed in the resting state of the neuron and so the positively charged Na ions flow in only very low concentrations into the cell. 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 K. The Threshold: ▯ 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 -70 mv. This is shown in the figure as a squiggly line fluctuating around the -70 mark. ▯ 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. The threshold of -50mv is reached, and the action potential is triggered. The Action Potential: ▯ The action potential is the fundamental unit of communication for neurons. When the -50 mv threshold is reached, a cascade of events is triggered. It starts with the Na channels along the cell membrane beginning to open. ▯ With the Na channels now open, the force of diffusion causes the positively charged Na ions to begin rushing into the neuron, rapidly causing the charge on the inside of the cell to become more positive relative to the outside. ▯ As the positively charged Na rushes into the cell, the electrostatic force begins to push some of the positively charged K ions out of the cell through the leaky K channels. ▯ Overall, the net effect is to still increase the positive net charge building up inside the cell to the point where the voltage gated K channels open which allow more positively charged K ions to rush out of the cell. ▯ After reaching a peak charge of about +40 mv on the inside of the cell, the Na channels close. This means that Na stops entering the cell, but the K continues to rush outward though the still- open voltage gated K channel. ▯ The inside of the cell begins losing positive charge and continues to fall and actually overshoots the baseline -70mv resting potential. At this point, the voltage gated K channels have completely closed. ▯ With the rush of ions complete, the cell slowly returns to -70 ma and a short refractory period occurs where the neuron cannot fire another action potential until it settles and recovers from a previous cascade/ ▯ Sodium potassium pump: has the role of removing sodium from the cell and replacing potassium. ▯ 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. ▯ The action potential begins in the receptive zone of the neuron ▯ The rapid changes that occur here cause changes in the ion concentrations surrounding nearby channels, leading to an action potential in the adjacent location ▯ action potentials cascade along the axon toward the terminal boutons. ▯ This process of cascading action potentials along the axon maintains the signal, but it can be too slow for efficient communication ▯ Solution: special glial cells coat 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 ▯ they are very important because as the electrical signal jumps 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. ▯ TF: a signal can travel through long axon very rapidly without any loss of strength. ▯ (See image on next page.) Sending a Signal: ▯ All action potentials produced by a given neuron are roughly identical in strength and duration and proceed in an all or none fashion. (Once the threshold is reached, the action potential proceeds to completion without  fail;;  there‘s  no  such  thing as a half action potential) ▯ This is shown on the oscilloscope record here; each red line represents an action potential spike of the same magnitude. ▯ How are different types of messages encoded? Messages are encoded by frequency (how often an action potential fires). Recall that immediately following an action potential is refractory period during which another action potential cannot begin; however shortly thereafter the neuron can potentially fire again. triggering another action potential cascade. ▯ 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. This frequency and pattern encodes the message to be passed on to the neighboring cell. ▯ Once an action potential travels along an axon, it reaches a terminal bouton, where it can connect to nearby neurons. ▯ This area of connection between the terminal bouton of neuron A, and the receptive zone of neuron, B, is called the synapse. (See image on next page.) The Synapse: ▯ The synapse is not a direct physical connection▯ special mechanisms exist to transmit a signal from the presynaptic neuron to the receiving postsynaptic neuron. Neurotransmitters: ▯ Within the terminal b
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