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.

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.

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):

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.




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).

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.

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

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

Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Autonomic nervous system
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