The autonomic nervous system is a system of efferent (motor) nerves that connect the CNS with internal organs, blood vessels and glands, but unlike the motoneurones that connect directly with skeletal muscles, the pathway between the spinal cord and a target organ contains a synapse within an autonomic ganglion.
Autonomic nerves are a motor system that regulates the activity of internal organs, including the heart, stomach and intestines. These nerves differ in structure from the somatic nerves innervating skeletal muscle. The autonomic nerves arise in the lateral horns of the spinal cord and are finely myelinated; they synapse on other, mainly unmyelinated, neurones in autonomic ganglia.
The sympathetic nerves originate from segments T1 to L2 of the human spinal cord ('the thoraco-lumbar outflow') and synapse within sympathetic ganglia; the sympathetic chain occupies a para-vertebral position, alongside the vertebral column.
The parasympathetic nerves arise for the lateral horns of the sacral spinal cord (S2-4) and synapse in parasympathetic ganglia situated close to the organs they innervate (the lower third of the intestine and the pelvic viscera). In addition parasympathetic nerves travel in the cranial nerves, so the parasympathetic is sometimes called the 'cranio-sacral outflow'.
In some internal organs the effects of the sympathetic and parasypathetic are opposed, e.e.in the intesting the parasympathetic causes smooth muscle to contract amd the sympatheitc nerves caus it to relax. Blood vessels do not receive a parasympathetic nerve supply.
Key Words: sympathetic and parasympathetic nerves; thoraco-lumbar and cranio-sacral outflows; autonomic ganglia; innervation of internal organs, glands and blood vessels; homeostasis
The two divisions of the ANS have different functions
Image source: Merck Manuals
Autonomic Nerves are divided into two systems - sympathetic and parasympathetic
Autonomic nerves are divided into two systems (the sympathetic and parasympathetic nerves) and are two independent motor systems thatinnervate the organs of the body to varying degrees. Their pathways contain autonomic ganglia which contain a synapse between pre-ganglionic axons and post-ganglionic cell bodies.
The position of the autonomic ganglia in sympathetic and parasympathetic nerves is different: the sympathetic ganglia are located in a chain of ganglia that run on each side of the vertebral bodies (sometimes called the para-vertebral ganglia). The parasympathetic ganglia are situated in or close to the organs that they innervate.
Consequently, sympathetic preganlionic neurones have relatively short axons and long post-ganglionic pathways. In contrast, parasympathetic pre-ganglionic axons are long, and the postganglionic axons are short.
In the spinal cord sympathetic and parasympathetic preganglionic neurones all originate in the lateral horn of the spinal grey matter.
However the two systems originate from different segments of the spinal cord. The sympathetic preganglionic nerones are only found in segments T1 to L2 of the cord, sometimes called the 'thoraco-lumbar outflow'. The parasypatetic preganglionic neurones are found in the sacral segments of the cord, and in the cranial nerves, so it is sometimes called the carnio-sacral outflow.
Some authorities include the enteric nervous system (a system of nerves within the gut wall) as part of the autonomic nervous system.
What happens in Autonomic Ganglia?
Image source: faculty.pasadena.edu
Structural differences between Autonomic and Somatic Nerves.
A major structural difference between autonomic and somatic nerves is the presence of autonomic ganglia.
Autonomic ganglia contain the cell bodies of post-ganglionic autonomic axons as well as the axon terminals of pre-ganglionic axons that synapse there.
Somatic (alpha-) motoneurones have large diameter myelinated axons. In contrast, autonomic postganglionic axns are unmyelinated, and the preganglionic axons are small diameter finely myelinated fibres.
Sympathetic Preganglionic Neurones originate in the lateral horn of the spinal cord between the first thoracic and second lumbar segments (T1-L2) - the 'thoraco-lumbar outflow'.
Sympathetic ganglia are found mainly near the vertebral column, whereas the parasympathetic ganglia are placed near or within the organs that they innervate.
Because of this, sympathetic post-ganglionic axons are usually long, and parasympathetic post-ganglionic axons are short.
The sympathetic ganglia are located in two chains, one on each side of the vertebral bodies, and sometimes called the paravertebral ganglia.
The eye and glandular tissues within the head and neck are innervated by the cervical sympathetic ganglia - the rostral extension of the sympathetic chain into the neck.
Postganglionic sympathetic nerves travel within spinal nerves to targets such as blood vessels and sweat glands: the innervate structures throughout the body - from the top of the head to the soles of the feet.
Internal organs have separate, pre-vertebral, sympathetic ganglia situated in the thorax or abdomen, at a distance from the viscera they innervate; these receive preganglionic fibres that traverse the sympathetic chain, and examples include the stellate (inferior cardiac) ganglion, the coeliac ganglion and the inferior mesenteric (hypogastric) ganglion.
Also included along with the sympathetic system is the adrenal medulla - an endocrine gland situated above the kidney that consists of modified post-ganglionic sympathetic cells that secrete the hormone, adrenaline, into the blood stream.
Pre-ganglionic parasympathetic neurones originate in the lateral horn of the sacral cord and the nuclei of the oculomotor (III), facial (VII), glossopharyngeal (IX) and vagus (X) nerves. It is sometimes called the 'cranio-sacral outflow'
Parasympathetic ganglia receive synapses from long axons of pre-ganglionic neurones, and have short post-ganglionic axons because these ganglia are often within the target organ or close to it.
Most of the parasympathetic postganglionic neurones in the gastrointestinal tract are found within the intrinsic plexuses.
Sacral parasympathetic pathways innervate the pelvic organs and the lower third of the gastro-intestinal tract.
The eye and glandular tissues within the head and neck are innervated by paraympathetic neurones the third, seventh and ninth cranial nerves. The ciliary ganglion is a parasympathetic ganglion close to and behind the eyeball.
The vagus nerve deserves a special mention - it's name suggest that it is a wanderer - and it innervates the larynx, heart, trachea and lungs, and the upper two thirds of the gastro-intestinal tract, including the pancreas and liver.