Many neurons of the peripheral nervous system end, however, not at synapses on other neurons but at neuroeffector junctions on muscle and gland cells. The neurotransmitters released by these efferent neurons’ terminals or varicosities provide the link by which electrical activity of the nervous system can regulate effector cell activity. The events that occur at neuroeffector junctions are similar to those at a synapse. The neurotransmitter is released from the efferent neuron upon the arrival of an action potential at the neuron’s axon terminals or varicosities. The neurotransmitter then diffuses to the surface of the effector cell, where it binds to receptors on that cell’s plasma membrane.
The receptors may be directly under the axon terminal or varicosity, or they may be some distance away so that the diffusion path followed by the neurotransmitter is tortuous and long. The receptors on the effector cell may be associated with ion channels that alter the membrane potential of the cell, or they may be coupled via a G protein to enzymes that result in the formation of second messengers in the effector cell. The response (altered muscle contraction or glandular secretion) of the effector cell to these changes will be described in later chapters. The major neurotransmitters released at neuroeffector junctions are acetylcholine and norepinephrine.
I. The junction between a neuron and an effector cell is
called a neuroeffector junction.
II. The events at a neuroeffector junction (release of
neurotransmitter into an extracellular space,
diffusion of neurotransmitter to the effector cell, and
binding with a receptor on the effector cell) are
similar to those at a synapse.