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Nervous system anatomy and physiology block 8 action

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Nervous SystemANATOMY AND PHYSIOLOGYBLOCK 8Action potentials in myelinated axonsMore rapidlyan action potential at one node of Ranvier causes a localcurrent to flow through the surrounding extracellular fluidand through the cytoplasm of the axon to the next node,stimulating an action potential at that node of Ranvier.By this means, action potentials “jump” from one node ofRanvier to the next along the length of the axon. This typeof action potential conduction is calledsaltatory(to leap)conductionSaltatory conductionGreatly increases the conduction velocity because the nodes ofRanvier make it unnecessary for action potentials to travel along theentire cell membrane. Action potential conduction in a myelinatedfiber is like a child skipping across the floor, whereas in anunmyelinated 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 organMuscleGlandPresynaptic terminalEnd of the axonPostsynaptic membraneThe membrane of the dendrite or effector cellSynaptic cleftspace separating the presynaptic and postsynapticmembranesNeurotransmitterssubstances (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.Trans # 1Group # 1: Surname, Surname, Surname | STH: Surname | TH: Surname9 of 22
Nervous SystemANATOMY 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.

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