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.



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