Sympathetic nervous systemSympathetic system is the larger of the two parts of ANS and is widely distributed throughout the body.It innervates heart and two lungsIt innervates the smooth muscles of blood vessels and viscera in abdomen and pelvisIt innervates glands of the body including all the sweat glands of skin and arrector muscles of hair folliclesSympathetic system prepares the body for an emergencyHeart rate is increased.It is interesting to note that arterioles of skin and intestines are constricted and those of skeletal muscle are dilated.Blood pressure is raised.There is a redistribution of blood. The blood supply to skin and gastrointestinal tract is decreased and the blood supply to brain, heart, and skeletal muscle is increased.Pupils dilate, smooth muscles of bronchi are inhibited, peristalsis in intestines decreases, and urinary bladder is relaxed. The sphincters are closed.The hair is made to stand on end and sweating occurs.Sympathetic nervous system consists of1. Lateral horns in spinal cord2. Efferent nerve fibers3. Rami communicantes4. Two sympathetic trunks5. Nerve branches6. Nerve plexuses7. Regional ganglia8. Afferent nerve fibersThe lateral gray columns (horns) of the spinal cord from the first thoracic segment to the second lumbar segment (sometimes third lumbar segment) possess the cell bodies of sympathetic connector neurons
Cross section of spinal cord at the level of fifth thoracic segment.The H-shaped grey matter shows three horns. Look at the lateral horn. This is the site for the cell bodies of sympathetic connector neurons. The myelinated axons of the cell bodies of sympathetic connector neurons leave spinal cord through anterior nerve roots and pass via the white rami communicantes to paravertebral ganglia of sympathetic trunk. These are called preganglionic fibers.
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Cross section of spinal cord at the level of fifth thoracic segmentOn left side general arrangement of somatic system is shownOn right side general arrangement of sympathetic system is shownGray ramus is shown in gray color and white ramus is shown in white color. The ganglion of sympathetic trunk is shown in blue colorNote the postganglionic fiber entering into anterior or ventral ramus through gray ramus. Ventral or anterior rami of spinal nerves are connected to ganglia of sympathetic trunk by two bundles of nerve fibers. The connecting links between the ganglia of sympathetic trunk and anterior rami are called rami communicantes. There are two rami communicantes that join an anterior ramus of a spinal nerve with associated paravertebral ganglion of sympathetic trunk. White rami communicantesThey are white because they contain myelinated preganglionic fibers that leave anterior rami and pass to paravertebral ganglia of sympathetic trunk. White rami also contain afferent sympathetic fibers. These fibers are also myelinated. White ramus is always the distal one.Within the trunk, the fibers of the white rami communicantes run longitudinally. They end on the nerve cells in the ganglia throughout the length of the sympathetic trunk.Through these nerve fibers the central nervous system controls the activity of all the nerve cells in the sympathetic trunk. Thus it can alter the secretion of sweat, the amount of blood flowing through the various tissues, and the erection of hairs (goose-flesh) throughout the body by way of the processes of the sympathetic nerve cells that are distributed through the spinal (and cranial) nerves. It is important to note that the nerve fibers which connect the central nervous system to the sympathetic nervous system run only in the first thoracic to second or third lumber spinal nerves. If all these nerves or the white rami communicants arising from them were cut, the sympathetic nervous system would be separated from the control of the central nervous system. This would result in the loss of a number of responses which arise from afferent impulses discharging directly into the central nervous system over the dorsal roots, e.g. the sweating and dilatation of skin vessels on exposure to heat and the contraction of skin vessels with goose-flesh in response to cold or fear.
Cross section of spinal cord at the level of fifth thoracic segment. On left side general arrangement of somatic system is shown. On right side general arrangement of sympathetic system is shown. Gray ramus is shown in gray color and white ramus is shown in white color. Note the postganglionic fiber entering into dorsal ramus. Do remember that gray ramus is connected to anterior or ventral ramus. Gray rami communicantesSoon after its formation, each ventral ramus receives a slender bundle of non-myelinated nerve fibers from the corresponding ganglion of sympathetic trunk. This bundle of non-myelinated nerve fibers is given the name the gray ramus communicans. It is proximal to white ramus. They are gray colored because the nerve fibers in these rami are devoid of myelin. They bring postganglionic sympathetic fibers to ventral rami and are distributed through their branches. They also enter every branch of dorsal ramus. Gray ramus contains non-myelinated efferent postganglionic fibers that leave the paravertebral ganglion and pass to anterior (ventral) or posterior (dorsal) ramus of spinal nerve. These sympathetic fibers supply smooth muscles of blood vessels, smooth muscles of the hair (arrector pilorum) and the sweat glands. Thus the spinal nerve supplies involuntary and voluntary structures. The fibers in gray ramus are those that are distributed via the branches of spinal nerve to blood vessels (vasomotor fibers), sweat glands (sudomotor fibers) and arrector pili muscles (pilomotor fibers).Every spinal nerve receives a gray ramusAll thoracic and the upper two lumbar nerves have gray rami and white rami.All cervical, lower lumbar and all sacral nerves do not have gray rami, because there is no sympathetic outflow from these segments of spinal cord. The ganglia with which these nerves are connected by their white rami receive their preganglionic fibers from thoracolumbar lateral horn cells whose fibers after entering sympathetic trunk have gone up or down in it
The arrangement of sympathetic fibers in cervical regionNote thatThere is no gray ramusThe preganglionic fibers do not synapse in the sympathetic ganglion and ascend upwards Superior cervical ganglion gives off four gray rami.Middle cervical ganglion gives off two gray rami.Inferior cervical ganglion gives off two gray rami.Stellate ganglion which is formed by the fusion of inferior cervical ganglion and first thoracic ganglion, gives three gray rami and one white ramus communicans.Note that the gray rami contain only efferent postganglionic fibers but white rami contain efferent preganglionic and afferent fibers.
The arrangement of sympathetic fibers in lower lumbar and all sacral nervesNote thatThere is no gray ramusThe preganglionic fibers do not synapse in the sympathetic ganglion and descend downwards There are two sympathetic trunks one on each side of the vertebral column. Each trunk consists of a vertical chain of ganglia. These ganglia are united by nerve fibers. They extend the whole length of vertebral column. Each sympathetic trunk extends alongside the vertebral column from the base of the skull to the coccyx. In neck, the trunks lie anterior to the transverse processes of cervical vertebrae. There are three ganglia in cervical regionSuperior cervicalMiddle cervical andInferior cervical ganglia Occasionally middle cervical ganglion is absentInferior cervical ganglion and first thoracic ganglion often unite to form stellate ganglion. Sometimes second thoracic may also be included or even third and fourth thoracic may be In thorax, sympathetic trunks lie on the sides of vertebral bodies, anterior to the heads of ribs.There are eleven or twelve ganglia in thoracic region.They are named according to their number; first thoracic ganglion, second thoracic ganglion, etc.In abdomen, the trunks are anterolateral to the sides of the bodies of lumbar vertebrae.There are four or five ganglia in lumbar region.They are first lumber ganglion, second lumbar ganglion and so on. In pelvis, sympathetic trunks are anterior to sacrum.There are four or five ganglia in sacral region.They are first sacral ganglion, second sacral ganglion and so on. Superiorly the proximal ends of sympathetic trunks are separate but inferiorly the two trunks join each other at a single ganglion. This ganglion where the two trunks join is called ganglion impar. Theoretically there is a ganglion for each spinal nerve, but fusion occurs, especially in the cervical region.First four ganglia unite to form superior cervical ganglion.Fifth and sixth ganglia join to form middle cervical ganglion.Seventh and eighth cervical ganglia fuse and form inferior cervical ganglion. Elsewhere there is usually one ganglion less than the number of nervesThere are usually 11 thoracic ganglia (but there may be twelve also)There may be 4 lumbar ganglia (but may be 5also) andThere are 4 sacral ganglia
Diagrammatic representation of paravertebral sympathetic ganglia. The anterior aspect of vertebral column is shown. The paravertebral ganglia are linked together and form two (right and left) sympathetic chains. Splanchnic nervesThe preganglionic efferent sympathetic nerve fibers enter the sympathetic ganglia via white rami communicantesSome of them synapse theirSome ascend and some descend without synapsingSome pass through the ganglia without synapsing. They do not ascend or descend. These myelinated fibers leave the sympathetic trunk as greater splanchnic, lesser splanchnic and lowest or least splanchnic nerves.
A part of sympathetic system showing preganglionic fibers which do not synapse in sympathetic chain ganglia and pass through these ganglia without synapsing and synapse in prevertebral ganglia with postganglionic neuronsPostganglionic fibers arise here in prevertebral ganglia Greater splanchnic nervesRight and left greater splanchnic nerves are formed from the branches from fifth to ninth thoracic ganglia on both sides. They descend obliquely on the sides of the bodies of thoracic vertebrae and pierce the crura of diaphragm and synapse with excitor cells in the ganglia of celiac plexus, renal plexus, and suprarenal medulla.
Sympathetic nervous system highlighting greater splanchnic nerve Lesser splanchnic nervesThere are also right and left lesser splanchnic nerves. They are formed from the branches of tenth and eleventh thoracic ganglia (sometimes twelfth also) on both sides. They descend with greater splanchnic nerves and pierce the diaphragm to synapse the excitor cells in the ganglia of the lower part of celiac plexus.
Sympathetic nervous systemConcentrate on lesser splanchnic nerve Lowest or Least splanchnic nervesWhen present they arise from right and left twelfth thoracic ganglia, pierce the diaphragm to synapse with excitor neurons in the ganglia of renal plexus. Nerve plexusesLarge collections of sympathetic and parasympathetic efferent nerve fibers and their associated ganglia, together with visceral afferent fibers, form autonomic nerve plexuses in the thorax, abdomen, and pelvis. Branches from these plexuses innervate the viscera.In the thorax there are the cardiac, pulmonary, and esophageal plexuses.In the abdomen there are preaortic plexuses that are associated with aorta and its branches. These autonomic plexuses are named according to the branch of aorta along which they are lying: celiac, superior mesenteric, and inferior mesenteric plexuses.In the pelvis there are the superior and inferior hypogastric plexuses. Regional gangliaThey are present in prevertebral or preaortic position. They are in the plexuses that surround the origins of ventral branches of abdominal aorta.They are called prevertebral ganglia or preaortic or subsidiary ganglia or collateral gangliaThere are three sets of prevertebral gangliaCeliac ganglia two in number present in celiac plexus that surround the origin of celiac artery.Superior mesenteric ganglion present in superior mesenteric plexus that surrounds the origin of superior mesenteric artery.Inferior mesenteric ganglion present in inferior mesenteric plexus that surrounds the origin of inferior mesenteric plexus. Structure of an autonomic ganglionAutonomic ganglia are often irregular in shape. They are the site where preganglionic nerve fibers synapse on postganglionic neurons.Ganglia are situated along the course of efferent nerve fibers of the autonomic nervous system.Sympathetic ganglia are part of sympathetic trunk and they are present around the roots of ventral branches of abdominal aorta.Parasympathetic ganglia, on the other hand, are situated close to or within the walls of the viscera. Majority of autonomic ganglia resemble sensory ganglia in having a similar connective tissue capsule and framework. But unlike sensory ganglia, autonomic ganglia contain synapses.An autonomic ganglion consists of a collection of multipolar neurons. There are numerous branched dendrites and an axon which forms unmyelinated postganglionic visceral efferent fibers.In larger ganglia, each neuron cell is surrounded by a layer of satellite cells as in spinal sensory ganglia. Nerve bundles are attached to these ganglia consist ofPreganglionic nerve fibers that enter the ganglion,Postganglionic nerve fibers that are leaving the ganglion,Afferent and efferent nerve fibers that pass through the ganglion without synapsing. The preganglionic fibers are myelinated. The postganglionic fibers are unmyelinated.Postganglionic fibers are much more numerous than preganglionic fibersPreganglionic axons may synapse with many postganglionic neurons for wide dissemination and amplification of sympathetic activityWhile an autonomic ganglion is the site where preganglionic fibers synapse on postganglionic neurons, the presence of small interneurons and collateral branches suggests that a ganglion may play a greater role than simply relaying information Preganglionic and postganglionic fibersThe myelinated axons of sympathetic connector neurons (the lateral horn cells) leave spinal cord through anterior nerve roots (with the axons of anterior horn cells) to reach the spinal nerve and its anterior ramus.Then they pass via the white rami communicantes to paravertebral ganglia of sympathetic trunk. These are preganglionic fibers. After reaching a paravertebral ganglion, the preganglionic fibers have one of the five possible synaptic alternatives * The commonest is for them to synapse with the cell bodies of an excitor neuron. The gap between the two (connector and excitor) neurons is bridged by acetylcholine. Acetylcholine is a neurotransmitter. The postganglionic nonmyelinated axons leave the paravertebral ganglion and pass to the thoracic spinal nerves through gray rami communicantes. These postganglionic fibers are distributed in branches of the spinal nerves to smooth muscle in the walls of blood vessel, sweat glands, and arrector pili muscles of skin. * Most of the preganglionic fibers entering the upper part of sympathetic trunk from the upper thoracic segments of spinal cord travel superiorly to cervical sympathetic ganglia. They synapse in these ganglia in the cervical region. The postganglionic nerve fibers pass via gray rami communicantes to join the cervical spinal nerves. These postganglionic fibers are distributed in branches of the spinal nerves to smooth muscle in the blood vessel walls, sweat glands, and arrector pili muscles of skin like thoracic region. * Many of the preganglionic fibers entering the lower part of the sympathetic trunk from the lower thoracic and upper two lumbar segments of the spinal cord travel inferiorly to synapse in ganglia in the lower lumbar and sacral regions. Here again, the postganglionic nerve fibers pass via gray rami communicantes to join the lumbar, sacral, and coccygeal spinal nerves. Because there is no sympathetic outflow from the cervical part of the cord, nor from the lower lumbar and sacral parts, those preganglionic fibres which are destined to synapse with the cell bodies whose fibers are going to run with cervical nerves must ascend in the sympathetic trunk to cervical ganglia, and those for lower lumbar and sacral nerves must descend in the trunk to lower lumbar and sacral ganglia. * The fourth alternative is to leave paravertebral ganglion without synapsing and to pass to a prevertebral ganglion for synapse. These myelinated fibers leave sympathetic trunk as the greater, lesser, and lowest splanchnic nerves. These splanchnic nerves are formed by preganglionic fibers.Postganglionic fibers arise from the excitor cells in the peripheral plexuses and are distributed to the smooth muscle and glands of the viscera. * A few preganglionic fibers, traveling in the greater splanchnic nerve, end directly on the cells of suprarenal medulla. These medullary cells, which may be regarded as modified sympathetic excitor neurons, are responsible for the secretion of epinephrine and norepinephrine. Each sympathetic trunk ganglion has a collateral or visceral branch, usually called a splanchnic nerve in the thoracic, lumbar and sacral regions but in the cervical region called a cardiac branch because it proceeds to the cardiac plexus. The visceral branches generally arise high up and descend steeply to form plexuses for the viscera. Thus cardiac branches arise from the three cervical ganglia to descend into the mediastinum to the cardiac plexus, which is supplemented by fibers from upper thoracic ganglia. Mediastinum is the central part of thorax between the two lungs. From lower thoracic ganglia three splanchnic nerves pierce the diaphragm to reach celiac plexus. From upper lumbar ganglia the lumbar splanchnic nerves descend to superior hypogastric plexus and this divides to enter the left and right inferior hypogastric plexuses. The inferior hypogastric plexuses (collectively forming the pelvic plexus) are joined by visceral branches from all the sacral ganglia (sacral splanchnic nerves). Do not confuse sacral splanchnic nerves which are sympathetic with pelvic splanchnic nerves (S2, 3, 4), which are parasympathetic. Sympathetic visceral plexuses thus formed are joined by parasympathetic nerves (vagus nerve to celiac plexus and pelvic splanchnic nerves to inferior hypogastric plexuses). The mixed visceral plexuses reach the viscera by branches that hitch-hike along the relevant arteries. These visceral branches supply not only the smooth muscle and glands of viscera but also the blood vessels of these viscera. All sympathetic trunk ganglia give off vascular branches to adjacent large blood vessels.The cervical ganglia give branches to the carotid arteries and their branches, including the internal carotid plexus along the internal carotid arteries.The thoracic and lumbar ganglia give filaments to the various parts of the aortic plexus and its derivatives, including those along the common iliac and median sacral arteries.The sacral ganglia give branches to lateral and median sacral arteries. Note that the head and neck arteries receive direct branches from cervical trunk ganglia.On the other hand limb vessels get their sympathetic innervation by nerve fibers that run with the adjacent peripheral nerves before passing to the vessels.In limbs fibers do not run long distances along the vessels themselves. Thus the nerve filaments to the vessels of the tip of a finger or toe run not with the digital arteries but with the digital nerves and only leave the nerves near the actual site of innervation. From the information given above, it should be clear that branches of nerves to skin (cutaneous branches) are not entirely sensory but also contain sympathetic efferent fibers. Similarly, branches to muscles are not entirely efferent but also contain sensory fibers and sympathetic fibers. Thus the signs of nerve injury are not simply paralysis of muscle and loss of sensation, but also loss of sweating, blood-vessel control, and goose-flesh. In addition to its gray rami communicantes to the spinal nerves and equivalent branches to the cranial nerves, the sympathetic trunk distributes postganglionic fibres through branches which pass on to the arteries of the body wall, limbs, head, and neck. Hence there is a dual route via nerves and arteries to these structures from sympathetic trunk ganglion cells. The internal organs (viscera), including the gut tube from mouth to anus, also receive postganglionic sympathetic nerve fibers. These arise from separate sympathetic ganglia closely associated with the arteries which supply these organs and pass to these organs along the arteries as periarterial plexuses of nerves fibers. These visceral or splanchnic ganglia are brought under the control of the central nervous system through preganglionic nerve fibres which emerge in the white rami communicantes, enter the sympathetic trunk, and pass through it to the splanchnic ganglia as splanchnic nerves. They appear as branches of the sympathetic trunk but merely pass through it, emerging at every level to run to the splanchnic ganglia which lie at that level in early development. Subsequently, the developing viscera and their splanchnic ganglia move caudally relative to the vertebral column and to the sympathetic trunks and the nerves to which they are attached by rami communicantes so that the splanchnic nerves come to run caudally from the sympathetic trunk in the adult.
Sympathetic nervous systemLook at the nerve supply to the viscera Afferent sympathetic nerve fibersThese are myelinated nerve fibers. They do not have their cell bodies in sympathetic ganglia. They have their cell bodies in the posterior root ganglia of spinal nerves.The peripheral processes travel from the viscera through some plexus or subsidiary ganglia with which the efferent fibers were involved. Then they enter sympathetic ganglia without synapsing. They pass to the spinal nerve via white rami communicantes and reach their cell bodies in the posterior root ganglion of the corresponding spinal nerve.
A part of sympathetic system shown
Look at the black colored afferent neuron between stomach and lateral horn of spinal cord The central axons (processes) then enter the spinal cord via the posterior nerve root (like any other afferent fibres) at approximately the same segmental level as the preganglionic cells.Here they may form the afferent component of a local reflex arc or ascend to higher centers, such as the hypothalamus. Visceral pain fibers enter the posterior horn of spinal cord, and thereafter the pain pathway is the same as that for spinal nerve pain fibers. Others concerned with reflex activities may synapse with interneurons in spinal cord or ascend to the hypothalamus and other higher centers.
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Many afferent nerve fibres which innervate sensory nerve endings in the viscera travel with the sympathetic nerve fibres (in rami communicantes, sympathetic trunk, splanchnic nerves, and periarterial plexuses) but have no functional connection with them, merely transmitting their sensory information directly to the central nervous system through the dorsal roots of the spinal nerves. They are, therefore, part of the general sensory system and not of the sympathetic nervous system
SympathectomyFor the control of excessive sweating and vasoconstriction in the limbs, parts of the sympathetic trunk with appropriate ganglia can be removed to abolish the normal sympathetic influence.In cervical sympathectomy for the upper limb (so called because it is usually carried out through the neck, not because any cervical part of the trunk is removed), the second and third thoracic ganglia with their rami and the intervening part of the trunk are resected. The first thoracic ganglion is not removed, since the preganglionic fibers for the upper limb do not usually arise above T2 level and its removal would result in Horner’s syndrome.For lumbar sympathectomy the third and fourth lumbar ganglia and the intervening trunk are removed; preganglionic fibers do not arise below L2. The first lumbar ganglion should be preserved otherwise ejaculation may be compromised; the exact positions of the ganglia vary, and they have no constant relation to the lumbar vertebrae.