Autonomic nervous system

Autonomic Nervous System

The autonomic (self-controlling) nervous system is that part of the nervous system which regulates most of the involuntary activities of the body, such as the activities of smooth muscles of bronchial tree, gut, genitourinary system, pupil, arrector pili muscles of the hair, cardiac muscle and secretion of the glands. Thus it represents the visceral component of the nervous system, hence sometimes also called ‘visceral nervous system’.

The autonomic nervous system, like the somatic nervous system is made up of afferent and efferent pathways.

The visceral afferent pathways resemble somatic afferent pathways. The cell bodies of first order sensory neurons (pseudo-unipolar) are located in the cranial and dorsal root ganglia. There peripheral processes are distributed through autonomic ganglia or plexuses or through somatic nerves. The central processes accompany somatic afferent fibres through cranial nerves or dorsal spinal roots into the central nervous system when they establish connections to mediate autonomic reflexes and visceral sensation.

Thus the afferent fibres of the autonomic nervous system are identical to the afferent fibres of the somatic nervous system and form part of general afferent component of the entire nervous system. Visceral efferent pathways, however, differ from those of somatic efferent pathways (i.e. the efferent fibres, differ in the somatic and autonomic nervous systems).

Somatic efferent fibres pass directly from the CNS to the skeletal muscle, whereas autonomic (visceral) efferent fibres do not pass directly to the visceral effector organs from the CNS, instead they first relay in the autonomic ganglia outside the CNS, and then the postganglionic fibres supply the effector organs. The only exception is the innervation of the adrenal medulla.

Thus the efferent pathway of autonomic nervous system is made up of two neurons, preganglionic and postganglionic.

For all practical purposes, the autonomic nervous system (ANS) is a general visceral efferent motor system which controls and regulates smooth muscle, cardiac muscle and glands. These three divisions of autonomic nervous system differ in their organization and structure but they are closely related functionally.

N.B. All the efferent peripheral nerve fibres belong to the autonomic nervous system except those to the skeletal muscles.

Divisions of the Autonomic Nervous System

The autonomic nervous system is divided into three divisions, the sympathetic nervous system and parasympathetic nervous system, and the enteric nervous system.

1. Sympathetic nervous system: The preganglionic sympathetic fibres arise from lateral horn cells of the thoracic and upper two lumbar segments (T1 to L2) of the spinal cord, hence sympathetic nervous system constitutes the thoracolumbar outflow (Fig. 20.1). The axons of preganglionic neurons project to the autonomic ganglia (the sympathetic ganglia). The ganglia are connected to each other and form a beaded chain.

Fig. 20.1 Schematic diagram to show the craniosacral outflow (parasympathetic) and thoracolumbar outflow (sympathetic). Note that postganglionic parasympathetic fibres are shorter in length as compared to postganglionic sympathetic fibres.

2. Parasympathetic nervous system: The preganglionic parasympathetic fibres arise from general visceral efferent (GVE) nuclei of brainstem and lateral horn cells of the second, third and fourth sacral segments of the spinal cord, hence parasympathetic nervous system constitutes the craniosacral outflow (Fig. 20.1). The axons of preganglionic fibres project to the autonomic ganglia (the parasympathetic ganglia) which are located near or embedded in the wall of viscera/glands.

The sympathetic ganglia are located near the CNS along the sympathetic chain in the paravertebral region or in front of vertebral column, hence the postganglionic sympathetic fibres are longer in length, on the other hand the parasympathetic ganglia are located near effector organ, hence the postganglionic fibres are shorter in length (Fig. 20.1).

The effects of sympathetic and parasympathetic control are mainly stimulation or inhibition of glandular secretion and contraction or relaxation of smooth/cardiac muscle.

3. Enteric nervous system: It is a network of neurons in the wall of gastrointestinal tract.

N.B. In general the sympathetic and parasympathetic nervous systems produce opposite effects, viz. sympathetic stimulation causes dilation of pupil, whereas parasympathetic stimulation causes constriction of the pupil. In tissues or organs innervated by both parasympathetic and sympathetic systems, the two systems function in an integrated reciprocal manner to produce a balanced action.

Sympathetic Nervous System

The activities (functions) of the sympathetic nervous system are such that as if it prepares the body to deal with the emergency (exciting and stressful) situations. The heart rate is increased, arterioles of the skin and intestine are constricted, those of skeletal muscle are dilated, and the blood pressure is raised. There is redistribution of the blood so that it leaves the skin and GIT and pass to the brain, heart and skeletal muscles. In addition there is dilatation of pupils, inhibition of the smooth muscle of the bronchi, intestine, and urinary bladder, and closure of sphincters.

It is generally said that the sympathetic stimulation mobilizes the body energy for flight or fight.

Functions of the sympathetic nervous system

• Dilatation of pupil

• Widening of the palpebral fissure

• Secretion of sweat glands

• Motor for arrector pili muscles

• Vasoconstriction of blood vessels all over the body, except those in skeletal muscles

• Stimulation of heart

• Bronchodilatation and inhibition of secretion from bronchial glands

• Inhibition of gastrointestinal motility and secretions

• Sensory for pain from most of the viscera

• Contraction of sphincters of the bladder and bowel

• Motor for ejaculation.

Efferent nerve fibres (thoracolumbar outflow)

The preganglionic sympathetic fibres arise in the lateral grey column of the spinal cord from segments T1 to L2. From each of these segments they emerge as small myelinated axons into the corresponding anterior primary ramus and pass via white ramus communicans into the ganglion of a sympathetic trunk.

Once preganglionic fibres reach the paravertebral ganglia (ganglia in the sympathetic chain) they are distributed as follows (Figs 20.2, 20.3):

Fig. 20.2 Sympathetic efferent and afferent fibres. The preganglionic sympathetic fibres are shown by solid red lines and postganglionic sympathetic fibres by interrupted red lines. The sympathetic afferent fibres are shown by green lines. Note the fate of preganglionic sympathetic fibres. These may (a) relay in their corresponding ganglion and pass to their corresponding spinal nerve for distribution, (b) ascend or descend in the sympathetic chain and relay in higher or lower ganglia, or (c) pass without synapse to a peripheral (prevertebral) ganglion for relay.

Fig. 20.3 Sympathetic nervous system (thoracolumbar outflow). The postganglionic sympathetic fibres are shown by interrupted lines.

• Synapse with the cells in the ganglion. The postganglionic fibres (non-myelinated axons) arising from ganglion cells rejoin the spinal nerves (grey rami communicantes) and distributed through its branches to the blood vessels, sweat glands and arrector pili muscles.

• Ascend in the sympathetic trunk to synapse in the cervical sympathetic ganglia.

• Descend in the sympathetic trunk to synapse in lumbar sympathetic ganglia.

• Leave the ganglia without synapsing as splanchnic nerves and relay in the prevertebral ganglia. Some of these fibres pass to the suprarenal gland where they synapse with the cells within the medulla. These medullary cells which may be regarded as modified postganglionic sympathetic neurons, secrete epinephrine and norepinephrine.

Sympathetic trunks

The sympathetic trunks are two ganglionated chains of nerve fibres lying one on either side of the vertebral column, extending along its whole length, i.e. from atlas vertebra above to the coccyx below. Each chain (trunk) bears a number of knot-like enlargements, the sympathetic ganglia. The number of ganglia is variable, but generally there are three in the cervical region, 11 in the thoracic region, four in the lumbar region, and four in the sacral region. Thus each sympathetic chain possesses 22 sympathetic ganglia. The ganglia associated with sympathetic chain are termed paravertebral ganglia.

N.B. The terminal ganglion, the ganglion impar is formed by fusion of two sympathetic trunks as they converge in front of the coccyx.

Cervical sympathetic ganglia

The cervical part of sympathetic trunk extends from base of skull to the neck of first rib. It consists of three large ganglia, superior, middle and inferior. They are believed to be formed during development by the fusion of smaller segmental ganglia.

• The superior cervical ganglion is largest and formed by the fusion of upper 4 cervical ganglia. It is spindle-shaped and lies in front of transverse processes of C2 and C3 vertebra.

– Grey rami communicantes to ventral rami of upper four cervical spinal nerves.

– Arterial branches to, both external and internal carotid arteries. These branches form plexuses around these arteries and are distributed along their branches. The fibres carried by external carotid artery are vasomotor, sudomotor, and pilomotor and supply areas of skin mainly supplied by trigeminal nerve.

The fibres carried by internal carotid artery (internal carotid nerve) are (motor) to the dilator pupillae and smooth muscle of levator palpebrae superioris (Muller’s muscle).

– Cardiac branch (superior cervical cardiac branch) to the heart. It is motor (visceral) to cardiac muscle.

• The middle cervical ganglion is small and formed by the fusion of 5th and 6th cervical ganglia. It lies in close relationship to inferior thyroid artery at the level of cricoid cartilage (C6).

– Grey rami communicantes to 5th and 6th cervical spinal nerves.

– Arterial branch to inferior thyroid artery for distribution to pharynx, larynx, and upper parts of the trachea and oesophagus, thyroid and parathyroid.

– Cardiac branch (middle cervical cardiac branch) to the heart.

• The inferior cervical ganglion is formed by the fusion of 7th and 8th cervical ganglia. It fuses with the first thoracic ganglion to form the cervicothoracic ganglion (also called stellate ganglion because its numerous branches give it a star-shaped appearance).

– Grey rami communicantes to C7 and C8 (in most cases T1 also) spinal nerves.

– Arterial branches to both subclavian and vertebral arteries.

– Cardiac branch (inferior cardiac branch) to the heart.

N.B. The cervical ganglia receive their preganglionic fibres from first and second thoracic (T1 and T2) spinal segments by the white rami communicantes of first and second thoracic spinal nerves.

Thoracic ganglia

The thoracic part of sympathetic trunk usually consists of 11 ganglia (because first thoracic ganglion gets fused with the inferior cervical ganglion).

• They are connected by grey and white rami communicantes to the thoracic spinal nerves.

• The first five thoracic ganglia (T1-T5) give postganglionic fibres to the heart, lung and oesophagus (through cardiac, pulmonary and oesophageal plexuses).

• The lower eight ganglia give preganglionic fibres, which are grouped to form the three splanchnic nerves:

• Greater splanchnic nerve arises from 5th to 9th thoracic ganglia.

• Lesser splanchnic nerve arises from 10th and 11th thoracic ganglia.

• Least splanchnic nerve arises from 12th (or last thoracic) ganglion.

The three splanchnic nerves pierce the crus of diaphragm to end in coeliac and superior mesenteric plexuses and ganglia on abdominal aorta. From there postganglionic fibres arise and supply the abdominal viscera.

Lumbar ganglia

The abdominal part of sympathetic trunk consists of four ganglia:

• Only upper two ganglia receive white rami communicantes from first and second lumbar spinal nerves.

• All ganglia give grey rami communicantes to corresponding lumbar spinal nerves.

• Visceral branches and lumbar splanchnic nerves from ganglia pass downwards, to form a plexus in front of the 5th lumbar and upper sacral vertebrae, called superior hypogastric plexus. It lies between the common iliac vessels. It divides into two inferior hypogastric plexuses, one on each side of the rectum.

Sacral ganglia

The pelvic part of sympathetic trunk consists of four ganglia. They give grey rami communicantes to sacral and coccy-geal nerves to supply lower limbs, buttocks and perineum; and visceral branches to the pelvic plexuses.

Prevertebral ganglia

There are three prevertebral ganglia, which are situated in front of vertebral column in the abdominal cavity. They are located close to the origins of superior, middle and inferior mesenteric arteries and are named after them, viz.

• Coeliac ganglion, at the origin of coeliac artery

• Superior mesenteric ganglion, at the origin of superior mesenteric artery

• Inferior mesenteric ganglion, at the origin of inferior mesenteric artery.

Parasympathetic Nervous System

The activities of parasympathetic nervous system are directed towards conserving and restoring energy. The heart rate is slowed, the pupils are constricted, the peristalsis and glandular activity is increased, the sphincters are relaxed (opened), and the bladder wall is contracted.

In general the parasympathetic stimulation slows down the body processes except that of GIT and genitourinary system, and allows restoration processes to occur quietly and peacefully.

Functions of the parasympathetic nervous system

• Constriction of pupil.

• Increase in lens curvature (antero-posterior) for accommodation.

• Secretion of respiratory and digestive glands.

• Inhibition of cardiac excitation, conduction and contraction.

• Bronchoconstriction and increased secretion from bronchial glands.

• Increased gastrointestinal motility and inhibition of pyloric and anal sphincters.

• Sensory differentiation of faeces and flatus.

• Contraction of muscular wall of urinary bladder (detrusor muscle) and inhibition of the internal urethral sphincter (sphincter vesicae).

• Motor for initiation and maintenance of erection of penis.

The functions of sympathetic and parasympathetic nervous system are compared in Table 20.1.

Table 20.1

Comparison of sympathetic and parasympathetic functions

Structure	Sympathetic function	Parasympathetic function
Eye	Dilatation of pupil	Constriction of pupil
Lacrimal gland and Salivary glands	Viscous secretion	Watery secretion
Bronchial smooth muscle	Relaxation	Contraction
Heart	Increases heart rate	Decreases heart rate
GIT	Decreases peristalsis and constricts the sphincters	Increases peristalsis and relaxes sphincters
Genitourinary tract
• Bladder wall and sphincter	Relaxes bladder wall and constricts the sphincter	Contracts the bladder wall and relaxes the sphincter
• Penis	Ejaculation	Erection
Skin
• Sweat glands	Produce sweating	No effect
• Arrector pili	Contraction of arrector pili leading erection of hair	No effect
Blood vessels
• Skin, splanchnic vessels	Contraction	No effect
• Skeletal muscles	Relaxation	No effect

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Tags: Textbook of Clinical Neuroanatomy

Jan 2, 2017 | Posted by admin in NEUROLOGY | Comments Off

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