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neurons 2.docx

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John Mitterer

Refractory Period  Represents a time during which the membrane cannot be depolarised again o During repolarisation and hyperpolarisation o Membrane is impermeable to Na+ ions / sodium ion channels closed o Sodium ions cannot enter axon o K+ ions move out as membrane is more permeable to K+ ions o Membrane becomes more negative than resting potential  Nerve impulses can only travel in one direction o Action potential can only depolarise the membrane in front o Membrane behind is recovering from refractory period (previous action potential)  Limits frequency with which neurones can transmit impulses Factors Affecting the Speed of Conductance: Myelin, Axon Diameter, Temperature  Impulses travel faster in myelinated neurones → SALTATORY CONDUCTION o Schwann cells prevent diffusion of ions o Flow of current between adjacent nodes of Ranvier o Thus, depolarisation only at nodes of Ranvier o Action potential jumps from node to node  Temp affects speed of conduction of impulses o Higher temp increases rate of diffusion of ions  Impulses faster in an axon with larger diameter o Small cells / large surface area:volume ratio / ion leakage weakens membrane o Myelin stops ion leakage \ diameter only important for unmyelinated neurones Synaptic Transmission  Synaptic cleft (gap) of 20μm separates two neurones at a synapse (junction of 2 neurones) o Presynaptic membrane is at the end of a neurone o Postsynaptic membrane is at the next neurone in the chain  Synaptic knob of a presynaptic neurone contains o Neurotransmitters in small vesicles o Mitochondria to produce ATP needed for neurotransmitter synthesis Aspects Of Synaptic Transmission  Unidirectional o Neurotransmitter always travels from pre- to p
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