A neuron consists of a cell body, which contains a nucleus, and one or more extensions protruding from the cell body. Dendrites receive nerve impulses from other neurons or from sensory receptors. The axon carries the nerve impulses (action potential) away from the cell body to another neuron or to an effector organ such as a muscle.
A stimulus affects the axon by changing the permeability of the axon to positive ions. The influx of positive ions reduces the electrical potential across one segment of the membrane (depolarization). The change in electrical potential in the first part of the axon triggers a change in electrical potential in the adjacent segment of the axon such that the impulse travels along the axon as a self-generating chain reaction.
At most synapses, arrival of the impulse at the presynaptic terminal leads to release of neurotransmitter, which crosses the synapse to interact with receptors on the membrane of the postsynaptic cell. This interaction opens ion-specific channels in the postsynaptic membrane, changing the membrane’s permeability for positive ions.
The transient change in voltage induced by the action potential is determined by the concentration of ions on either side of the cell membrane. Maintaining these ionic gradients is an energy-consuming process that requires a constant supply of glucose and oxygen to the neuron.
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