Ranvier make it unnecessary for action potentials to

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Ranvier make it unnecessary for action potentials to travel along the entire cell membrane. Action potential conduction in a myelinated fiber is like a child skipping across the floor, whereas in an unmyelinated axon it is like a child walking heel to toe across the floor.Medium-diameter, lightly myelinated axons, characteristic ofautonomic neurons, conduct action potentials at the rate of about 3–15 meters per second (m/s), whereas large-diameter, heavilymyelinated axons conduct action potentials at the rate of 15–120 m/s.These rapidly conducted action potentials, carried by sensory andmotor neurons, allow for rapid responses to changes in the externalenvironment. In addition, several hundred times fewer ions cross thecell membrane during conduction in myelinated cells than inunmyelinated cells. Much less energy is therefore required for thesodium-potassium pump to maintain the ion distribution.C.THE SYNAPSESynapseJunction where axon of one neuron interacts with anotherneuron or with cells of an effector organ Muscle Gland Presynaptic terminalEnd of the axon Postsynaptic membraneThe membrane of the dendrite or effector cellSynaptic cleftspace separating the presynaptic and postsynapticmembranes Neurotransmitterssubstances (neuro-, nerve + transmitto, to send across) arestored in synaptic vesicles in the presynaptic terminalWhen an action potential reaches the presynapticterminal, voltage-gated Ca2+ channels openand Ca2+ moves into the cell. 9 of 21
Nervous System ANATOMY AND PHYSIOLOGYBLOCK 8This influx of Ca2+ causes the release ofneurotransmitters by exocytosis from thepresynaptic terminal. The neurotransmitters diffuse across the synapticcleft and bind to specific receptor molecules onthe postsynaptic membrane. The binding of neurotransmitters to thesemembrane receptors causes chemically gatedchannels for Na+, K+, or Cl− to open or close inthe postsynaptic membrane, depending on thetype of neurotransmitter in the presynapticterminal and the type of receptors on thepostsynaptic membrane. The response may be either stimulation orinhibition of an action potential in thepostsynaptic cell. For example, if Na+ channels open, thepostsynaptic cell becomes depolarized, and anaction potential will result if threshold is reached.If K+ or Cl− channels open, the inside of thepostsynaptic cell tends to become more negative,or hyperpolarized (h per-po lar- -zed), and anı̄′ı̄action potential is inhibited from occurring.Of the many neurotransmitters or suspectedneurotransmitter substances, the best known are acetylcholine(ACh) (as e-til- ko len) norepinephrine(nor ep-i-nef rin)Other neurotransmitters include serotonin (ser-o-to nin)dopamine (do pa-men)(gamma)-aminobutyric (gam a a-me no-bu-ter ik)acid (GABA), glycine, and endorphins (en dor-finz; endogenous morphine)(table 8.2).

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