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Neuroscience 1 Neuron.docx

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Joe Kim

Neuroscience 1 (neuron) - Module 1: Intro to Neuroscience o The Brain  ~100 billion neurons, typical neuron can have 1-10000 connections, Therefore number of different combinations of activity b/t brain cells that is theoretically possible is larger than the number of elementary particles in the universe. o René Descartes (1600s):  Approach to understanding by separate the mental processes of the mind from the physical processes of the brain  In his Dualist frame work, 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 thought to serve, in part, as a connection b/t mind and body o Some neuroscientists work in levels of molecules, cell sand systems of the brain - Module 2: The neuron o Neurons = cells specialized for communication o Each of 100 billion neurons are organized into signaling pathways to communicate via synaptic transmission. Their structure makes them especially food at communication. o 2 typical zones:  Receptive zone designed to receive signals from other neurons, dendrites  Begins w/ the cell body which contain most of the vital organelles which keep the cell functioning  Branching from cell are dendrites – reach out to other neurons and receive signals to be relayed thought the dendritic branch to the cell body, where some signals will go on to be conveyed down the axon  Transmission zone designed to pass on signals to other cells, axons and terminal boutons  Signal is passed down a usually long fibre called the axon. Small – 1m length.  End of Axons are more branches called terminal boutons or terminal ends. Reach out and make connections with the receptive zone of nearby neurons to transmit the signal further. o Glial cells provide structural support, nourishment and insulation to neuron cells - Module 3: The Action Potential o A neuron’s cell membrane separates intra and extracellular fluid which contain different concentrations of important ions including sodium, potassium and chloride. o The cell membrane is selectively permeable and contain several protein channels (potassium and sodium channels) o Adding up positive and negative charges, the starting baseline for the differing concentrations of ions produces an electrical imbalance such that the inside of the cell =~-70 mv relative to the outside of the cell. i.e. resting potential o The resting potential of a neuron is controlled by two forces:  Diffusion  The tendency for molecules to distribute themselves evenly in a medium  Electrostatic force  The repulsion b/t ions w/ the same charge, and attraction b/t ions of opposite charge o Ions:  Large protein molecules (negative) are too large to pass through membrane and so remain trapped inside. The K+, Na+, and Cl- ions are mobile  Two types of Potassium channels:  Leaky Potassium Channel o Is like a tap that is always open, allowing +vely charged K to pass through the cell membrane out of the neuron. (Much of the K+ remains inside cell at rest.) Overall, this leaky K channel and the freedom it affords K+ ions is the major contributor to maintain the resting potential of the neuron.  Voltage Gated Channel  The negatively charged Cl- ions are also mobile and the electrostatic force of the negatively charged protein molecules keeps the Cl- ions on the outside of the cell.  Voltage Gated Na+ channels are closed in the resting state of the neuron and so the +vely charged Na+ ions flow in only very low concentrations in to the cell (to the extent that most Na+ remains outside cell and is far less important to the resting state as is potassium) o In reality, the resting voltage is constantly fluctuating around -70 mv. Each neuron is connected to many other neurons. Under the influence of nearby neurons and random ion flow, sometimes, a large enough change in the resting charge will occur to reach the important threshold level of -50 mV. o Action Potential  The fundamental unit of communication for the neurons  When -50mV reached, cascade of events triggered.  Na+ channels open and force of diffusion causes the +vely charged sodium ions to being rushing into the neuron (rapidly causing inside charge to become more +ve relative to outside)  Electrostatic force begins to push some of the K+ ions out of the cell through the leaky K channels (but overall still increasing +ve charge)  Voltage gated K+ channels open which all more +vely charged K+ ions to rush out of the cell  After reaching peak of +40 mV on inside of cell, Na+ channels close. Therefore, Na+ stops entering but K+ ions continue to leave, through the still-open voltage gated K channels. The inside of the cell beings losing +ve charge and continues to fall and actually overshoots the baseline -70 mV resting potential  . The voltage gated K+ channels have completely closed.  The cell returns to -70 mV occurs and a short refractory period occurs, where the neuron cannot fire another potential until it settles and recovers from the cascade.    Sodium Potassium Pump:  Acts throughout the action potential and after it is complete.  Has the role of removing 3 sodium from the intracellular fluid and replacing them w/ 2 potassium, returning concentrations to baseline  Na+/K+ pump Moves slowly and utilizes extensive energy, therefore playing little role in the action potential itself.  Important part of maintain the ion balance and recovering from cascades  Action Potential begins in receptive zone and cascades towards terminal boutons.  Cascading action potentials along axons maintains signals but can be too slow for efficient communication.  Special Glial cells goat many axons w/ type of fatty insulating tissue called myelin. Special Cells are: o Oligodendrocytes in the Central Nervous System o Schwann cells in the Peripheral nervous system  When an action potential reached a myelin sheath, it jumps across it in a process known as salutatory conduction. Open regions between segments of myelin sheath are the nodes of Ranvier (important to strength signal again through ion channel cascades before continuing along and jumping through the rest of myelin sheath) Thus signal can travel through a long axon very rapidly w/out any loss of strength. o All action Potentials by a given neuron are roughly identical in strength and duration and process in an all or none fashion. That, once threshold reached, action potential proceeds to completion without fail – no half action potential etc. o Instead of encoding messages based on relative strength, they are encoded by frequency – the rate of action potentials it is only limited by the refractory period.  Strong signal = many action potentials  Weak signal = fewer action potentials in the same period of time. o The area of connection b/t the terminal bouton of neuron A and the receptive zone of neuron B is called the synapse - Module 4: The Synapse o Within the terminal end of the presynaptic neuron are a variety of chemicals known as neurotransmitters. They are w/in vesicles. As the action potential reaches the terminal ends, some of the vesicles move towards the cell membrane of the presynaptic neuron. o The vesicle fuses w/ the membrane of the presynaptic neuron and open, spilling neurotransmitter molecules into the extracellular fluid. There are a variety of different neurotransmitters depending on location or type of neuron and include: Glutamate, GABA, Serotonin, and Dopamine. A single neurotransmitter can also have multiple functions, depending on the receptor on the postsynaptic neuron that it binds to. o They enter the space b/t the two neurons, called the synaptic effects. Floating in cleft, there are also other molecules which interact with neurotransmitters (removing, modifying etc.) o Along membrane
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