The cell body of the second neuron, or postganglionic neuron, is located within the autonomic ganglion and it extends an axon from the ganglion to the visceral effector. It extends an axon from the CNS to an autonomic ganglion. The cell body of the first neuron, or preganglionic neuron, is located within the brain or spinal cord. Unlike the somatic nervous system, in which a single motor neuron extends from the CNS to a skeletal muscle, the ANS uses two motor neurons in sequence to carry motor nerve impulses to an effector.

Smooth muscle, cardiac muscle, adipose tissue, and glands One motor neuron extends an axon from the CNS to an effectorĪ preganglionic neuron extends an axon from the CNS to an autonomic ganglion and synapses with a postganglionic neuron that extends an axon to an effector Comparison of Somatic And Autonomic Nervous Systems The parasympathetic preganglionic nerve fibers usually travel almost all the way to the target before making the synapse with the postganglionic fibers.

The parasympathetic nerves exit the CNS through cranial nerves III, VII, IX, and X and through the S2 through S4 sacral spinal cord segments. The preganglionic nerve fibers synapse in either the paravertebral sympathetic chain ganglia or the prevertebral ganglia before the postganglionic nerve fibers run to the target tissue. The sympathetic nerves originate in the intermediolateral horn of the spinal cord and exit at the T1 through L2 spinal cord segments. Cells express different receptor subtypes, and the receptor subtype mediates the action of SNS or PNS on that cell. The PNS stimulates nicotinic or muscarinic cholinergic receptors on the target tissues. The SNS stimulates a- and/or P-adrenergic receptors on target tissues. The pharmacologic division of the ANS is based on the receptor subtype activated. The parasympathetic nerves are active during quiescent periods, such as after ingestion of a meal and during sleep.

The SNS is activated by multiple stimuli, including perceived threat, pain, hypotension, or hypoglycemia. The adaptive component of the ANS characterizes the SNS as mediating “fight or flight” and the PNS as mediating “rest and digest.” This classification provides a logical structure for the diverse actions of the sympathetic and parasympathetic nerves on various target tissues. The ANS, along with the endocrine system, regulates most body functions through a standard negative feedback process. The physiologic perspective of the ANS is based on both homeostatic control and adaptive responses. There is not an endocrine arm to the PNS. The parasympathetic nerves use acetylcholine as the neurotransmitter for both the preganglionic and postganglionic nerves. They synapse in ganglia close to the target tissue and have short postganglionic nerves. The parasympathetic nerves arise from the cranial and sacral portions of the spinal cord and have a long preganglionic nerve. Plasma epinephrine originates primarily from the adrenal medulla. Circulating plasma norepinephrine levels come from both overflow from the sympathetic nerve terminals and from the adrenal medulla. There is an endocrine component of the SNS. Acetylcholine is the preganglionic nerve neurotransmitter, and norepinephrine is the postganglionic neurotransmitter, except for the sweat glands, which have a sympathetic cholinergic innervation. The preganglionic nerves synapse in the sympathetic chain, and long postganglionic nerves innervate the final target. The sympathetic nerves arise from the thoracolumbar spinal cord and have short preganglionic neurons. The anatomic perspective separates the ANS into a sympathetic and a parasympathetic branch, based in part on the origin and length of the nerves.

Autonomic nervous system lens