The nervous system is divided into two anatomical divisions:
1- The central
nervous system (CNS), which is composed of the brain and spinal cord,
and the peripheral nervous system, which includes neurons located outside
the brain and spinal cord, that is, any nerves that enter or leave the CNS.
2- The peripheral nervous system is subdivided into the efferent
and afferent divisions.
The efferent neurons carry signals away from the
brain and spinal cord to the peripheral tissues, and the afferent neurons
bring information from the periphery to the CNS. Afferent neurons provide
sensory input to modulate the function of the efferent division through
reflex arcs or neural pathways that mediate a reflex action.
Types of neurotransmitters:
- norepinephrine (and the closely related epinephrine).
- acetylcholine.
- dopamine.
- serotonin.
- histamine.
- glutamate.
- γ-aminobutyric acid.
Each of these chemical signals binds to a specific family of receptors. Acetylcholine and norepinephrine are the primary chemical signals in the ANS,
whereas a wide variety of neurotransmitters function in the CNS.
Acetylcholine mediates the transmission of nerve impulses across autonomic ganglia in both the sympathetic and
parasympathetic nervous systems. It is the neurotransmitter at the adrenal medulla. Transmission from the autonomic postganglionic nerves to the effector organs in the
parasympathetic system, and a few sympathetic system organs, also involves the release of acetylcholine. In the
somatic nervous system, transmission at the neuromuscular junction (the junction of nerve fibers and voluntary muscles) is also cholinergic.
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