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Biomedical Sciences
BIOM 2000
Dan Meegan

Unit 2: the nervous system th September 14 , 2012 Part A: Neurons and Nerve impulses Definitions Neuron – basic cellular unit; an electrically excitable cell that processes and transmits information by electrical and chemical signalling Dendrite – receive; the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body or the soma of the neuron from which the dendrites project Axon – message sent, action potential; long slender projection of a nerve cell, or neuron that typically conducts electrical impulses away from the neuron’s cell body Synapse – releases message; a structure that permits a neuron to pass an electrical or chemical signal to another cell Resting membrane potential – to send sodium out of cell and potassium in; (resting voltage); relatively static Depolarization of the axon membrane – start of the sodium channels being opened Action potential – short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Occur in excitable cells; include neurons, muscle cells, and endocrine cells (play important role in cell to cell communication in neurons) NERVE IMPULSES Salutatory conduction – jumping over: propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials without needing to increase the diameter of an axon The structure of a neuron Have four parts: - Cell body – has a nucleus - Dendrites – “docking” site of synapses of other neurons - Axon – carries nerve impulses (action potentials) - Synapses – at the end of the axon, which contain the neurotransmitter substance used to activate other cells Structure of Neurons: Review D Multipolar AX SY (myelinated) D Bipolar AX SY (non-myelinated) D Unipolar AX SY (myelinated) Review: - In unit 1, we examined one type of neuron, the multipolar neuron - Two other neuron types, bipolar and unipolar, are found - Each of these neuron types has a cell body (arrow), dendrites (D), an axon (AX) and synapses (SY) look at slide for diagram In all cases, the nerve impulses move from the dendrites toward the synapses multipolar & unipolar neurons are myelinated; the bipolar neurons are non-myelinated The myelin sheath is formed by Schwann cells which wrap around the axon - It insulates the axon and facilitates the rapid transmission of nerve impulses by salutatory conduction The action potential: review - Resting axon membrane has a small electrical charge (-70 to -90 mV) called the resting membrane potential (RMP) - The RMP is largely due to an unequal distribution of Na+ and K+ on the inside and outside of the membrane - This charge difference is maintained by the activity of the sodium-potassium pump Protein Pumps – proteins have multiple functions - sodium binds to cytoplasm which stimulates the ATP which causes the pump protein to change its shape - expels sodium to the outside. Extracellular potassium binds and causes release of this phosphate group - loss of phosphate restores the original conformation of the pump protein. Potassium is released and sodium sites are ready to bind sodium again; cycle repeats (look at diagram) Action potentials are momentary discharges (depolarization’s) of the resting membrane potential caused by a rapid influx of Na+ caused by the opening of Sodium ion channel. Once initiated they move along the axon membrane toward the synapse. This is illustrated in the picture below; the yellow disk represents the part of the axon membrane that is depolarized (look at slide 15) Sodium channel opens and sodium rushes in not enough time to let whole action potential in and then restore Sodium opens and charge is changed; constantly restoring the resting potential - Want to have it restored so that immediately after another can be fired off - Potassium restores Each depolarization is identical; the signal being transmitted is in the frequency of action potentials traveling along the axon The frequency of action potentials arriving at the synapse at any time determines the amount of neurotransmitter substance released from the synapse. This, in turn, determines the strength of the stimulus (signal) passed along to the next cell. The higher the frequency, the more neurotransmitter substance released - In the non-myelinated axons, the action potential passes smoothly along the axon, and all parts of the membrane are depolarized - In myelinated axons, the action potential jumps between the non-insulated nodes (of Ranvier) by salutatory conduction - Salutatory conduction allows more rapid movement of the action potentials, and needs less energy to restore the membrane after the action potential has been transmitted The synapsis and post-synaptic events: Review - The synapse may be associated with another neuron (at a dendrite), a muscle cell or a glandular cell - If the targ
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