An Introduction to Peripheral Nervous System.

 

The peripheral nervous system consists of 

1) Spinal nerves

• Thirty one pairs of spinal nerves arising from the spinal cord. 

• Spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge. 

• 31 pairs of spinal nerves are distributed as.

1. 8 pairs of cervical nerves (C1- C8)

2. 12 pairs of thoracic nerves (T1-T12)

3. 5 pairs of lumbar nerves (L1-L5)

4. 5 pairs of sacral nerves (S1-S5)

5. 1 pair of coccygeal nerve (Co1)

Nerve plexuses

• Axons of the spinal nerves (except for thoracic nerves T2-T12) do not go directly to the body structures. 

• Instead, they join with various numbers of axons of adjacent nerves and form the networks on both sides of the body. Such a network of axons is called a plexus.

• The principal plexuses are

1. The cervical plexus is formed by the first four cervical nerves (C1-C4). It supplies to the skin and muscles of the head, neck and superior part of the shoulder and the chest.

2. The brachial plexus is formed by the 5th, 6th, 7th and 8th (C5-C8) cervical nerves and 1st thoracic nerve (T1). It is supplied to the shoulder and upper limb.

3. The lumbar plexus is formed by the first four lumbar nerves (L1-L4). It supplies the abdominal wall, external genitals and parts of lower limbs.

4. The sacral plexus is formed by 4th, 5th lumbar nerves (L4-L5) and 1st, 2nd, 3rd, 4th sacral nerves (S1-S4). It supplies the lower limbs.

Cranial nerves

• Twelve pairs of cranial nerves arising from the brain. 

Sr.No.	Name	Type	Origin	Function
	Olfactory nerve	Sensory	Olfactory mucosa (superior part of nasal cavity)	Sense  of smell
	Optic nerve	Sensory	Retina of eyes	Sense of vision
	Oculomotor nerve	Motor	Midbrain	Movement of upper eyelid and eyeball, constriction of pupil.
	Trochlear nerve	Motor	Midbrain	Movement of the eyeball
	Trigeminal nerve	Mixed	Motor nerves originate in the pons	Chewing
			Sensory nerves originate in the eyeball, eyelid, nasal cavity, upper and lower teeth, cheek.	Conveys impulses for touch, pain and temperature.
	Abducens nerve	Motor	Pons	Movement of the eyeball
	Facial nerve	Mixed	Motor nerve originate in pons	Facial expression
			Sensory nerves originate in tongue.	Taste
	Vestibulocochlear nerve	Sensory	Semicircular canals, saccule and utricle of inner ear	Sense of hearing
	Glossopharyngeal nerve	Mixed	Motor nerve originate in medulla oblongata	Movement of pharynx and secretion of saliva
			Sensory nerve originates from the posterior part of the tongue.	Taste
	Vagus nerve	Mixed	Motor nerve originate in medulla oblongata	Swallowing, coughing, voice production, contraction and relaxation of smooth muscle
			Sensory nerve originates from pharynx, epiglottis, thoracic and abdominal cavities.	Taste and sensation like touch, pain, temperature.
	Accessory nerve	Motor	Medulla oblongata	Movement of swallowing, movement of head and shoulder.
	Hypoglossal nerve	Motor	Medulla oblongata	Movement of tongue

AUTONOMIC NERVOUS SYSTEM

• Autonomic nervous system controls the “autonomic” functions of the body. 

• ANS reflexes are regulated by centers in the hypothalamus and brainstem. 

• ANS stimulation does not occur voluntarily. 

• ANS operates without conscious control.

e.g.- An increase in the heart rate.

• The autonomic nervous system consists of two divisions.

• 1) Sympathetic nervous system

• 2) Parasympathetic nervous system

• These two divisions have structural and functional differences.

• They normally work in an opposing manner.

Sympathetic nervous system

• Sympathetic nervous system stimulates during physical or emotional stress. 

• Activation of the sympathetic nervous system shows a series of physiological responses collectively called the fight-or-flight response. 

• It originates in the thoracic and lumbar region of the spinal cord.

• Also called “Thoracolumbar Nervous System”.

• Shorter length of neuronal pathway.

• Neurotransmitter: Adrenaline / Noradrenaline.

Parasympathetic nervous system

• Parasympathetic responses conserve and restore body energy during time of rest and recovery. 

• It allows energy supplying food to be digested and absorbed. 

• Hence the parasympathetic division enhances “rest and digest” activity. 

• It originates in the cranial and sacral region of the spinal cord.

• Also called “Craniosacral Flow or Craniosacral Nervous System”

• Longer length of neuronal pathway.

• Neurotransmitter: AcetylCholine (Ach).

Both these divisions consists of 

i) Preganglionic nerve- It is myelinated nerve fibre arising from the lateral horn cell of the spinal cord.

ii) Ganglion- It is a small mass of nervous tissue located outside the spinal cord.

iii) Postganglionic fibre- It extends from ganglion. It passes the impulses from ganglion to effector organs (muscles or glands).

Physiology of ANS

Sr. No.	Organ	Sympathetic stimulation (Adrenergic effect)	Parasympathetic stimulation (Cholinergic effect)
	Iris muscle	Dilation of pupil (mydriasis)	Constriction of pupil (miosis)
	Salivary glands	Decrease saliva secretion	Increase saliva secretion
	Ciliary muscles	Relaxation of distant vision	Contraction of near vision
	Bronchi	Dilation of bronchi	Constriction of bronchi
	Heart	Rate and force of contraction increases	Rate and force of contraction decreases
	Coronary arteries	Vasodilation	Vasoconstriction
	Stomach	Decrease motility and tone, Contraction of sphincter	Increase motility and tone, Relaxation of sphincter
	Skeletal blood vessel	Vasodilation	Vasoconstriction
	Adrenal gland	Secretion of adrenaline and noradrenaline	No known effect
	Kidney	Decrease urine volume	Increase urine volume
	Urinary bladder	Relaxation of muscular wall, Constriction of sphincter	Contraction of muscular wall, relaxation of sphincter
	Sex organs	Contraction of smooth muscles of vas deferens and seminal vesicle	Vasodilation, erection of penis (males), and clitoris (females)

Commonly Asked Questions:

1. Write a short note on Spinal Nerves.

2. Write in short about the Autonomous Nervous System.

3. Write a short note on,

1. Sympathetic Nervous System.

2. Parasympathetic Nervous System.