Background information Organization of efferent innervation
Efferent innervation includes:
• autonomic nervous system, which regulates involuntary responses of the heart, blood vessels, other smooth muscle organs and glandular tissue;
• somatic (motor) nerves innervating skeletal muscles and thus regulating voluntary movement. The autonomic nervous system includes: parasympathetic and sympathetic divisions. Autonomic nerve fibers interact with their target organs by a two-neuron pathway.
The first neuron originates in the CNS and synapses outside the spinal cord with the second neuron lying in the ganglia, postganglionic fibers of which innervate the target organs.
The parasympathetic nervous system arises from the brainstem and sacral region of the spinal cord. Nearly all of the parasympathetic ganglia lie in or near the innervated organs.
The sympathetic nervous system arises from thoracic and lumbar regions of the spinal cord. Sympathetic ganglia are located outside innervated organs and tissues and most of them lie in the sympathetic chains on either side of the vertebral column.
The neurotransmitter released by preganglionic nerves in both parasympathetic and sympathetic gan-glionic synapses is acetylcholine. Acetylcholine is also released by postganglionic parasympathetic nerves and operates as a neurotransmitter of parasympathetic neuroeffector synapses. Somatic nerves release acetylcholine as a neurotransmitter at neuromuscular junctions. Synapses, in which acetylcholine operates as a neurotransmitter, are termed cholinergic synapses.
Norepinephrine is released by postganglionic sympathetic nerves and operates as a neurotransmitter in sympathetic neuroeffector synapses. These synapses are termed adrenergic synapses.