Clinical Anatomy And Physiology Of Peripheral Nervous System

Introduction:

Peripheral nerves lesions are common in clinical practice and can be caused by a wide variety of diseases like trauma, neoplasms, infections, metabolic disease and chemical toxins such as lead therefore it is of paramount importance for a physician to know the basic structure of peripheral nerves.

The peripheral nervous system (PNS) is considered a relatively simple part of the nervous system, compared to the central nervous system (CNS). It is composed of afferent sensory fibers and of efferent motor fibers that project to striated skeletal muscle innervating target tissues via the neuromuscular junction. interneurons located in the CNS (spinal cord, brainstem, or brain) modulate the information before efferent fiber activation in response to afferent stimulation. Thus, the PNS is considered to mediate the relationship between the environment and the self. The sensory spinal nerves, whose cell bodies are located in the dorsal root ganglia (DRG), and the motor spinal nerves formed by the motor axons, whose cell bodies are located in the anterior horn, constitute the so-called somatic PNS.

Spinal nerves enter the spinal cord by the dorsal root and exit the spinal cord by the ventral root through notches between each vertebra of the vertebral column. These roots merge together to form the spinal nerves. At the level of each vertebra, sensory and motor nerves come from or go to targets that are located at the same anatomical level.

2 Clinical Anatomy of PNS

2.1. Development of PNS: PNS motor and preganglionic autonomic neurons develop from the neural tube.

• Gastrulation: consist of 3 primordial germinal layers
• the primitive ectoderm
• the endoderm
• the mesoderm
• Neural induction: 3rd week of gestation in humans the primitive ectoderm divides into two parts.
• The central part will form the future nervous system
• The more lateral region gives rise to the surface ectoderm, which generates the epidermis

This phase of differentiation is called neural induction

• Neurulation: takes place during 4th week of gestation in humans during this process, the neural plate undergoes a series of morphological movements leading to the formation of a neural tube.

Neurulation leads to the formation of a neural tube that represents the primordial of CNS.

• Ventralizing the neural tube:

Formation of nervous system video = https://youtu.be/Cu4lQYbOzzY

• The peripheral nervous system: Peripheral nerves are composed of a large number of fascicles that are surrounded by connective tissue sheaths. Each nerve fascicle contains Schwann cells and fibrocytes, axons, myelin sheath, collagen fibrils of endoneurium and blood vessels.
• The diameter of peripheral nervous axons varies between 1 and 20 micrometres in humans. The nerve fibres are classified into unmyelinated and myelinated. the axons in the unmyelinated nerves merely indent the surface membrane of Schwann cells. the myelinated axons are wrapped around by Schwann cells. the connective tissue forms 3 sheaths in the peripheral nerves which are known as endoneurium, perineurium and epineurium from inside out. The endoneurium
• Nerve fibres: axon of the nerve cell is termed as nerve fibres. The bundles of nerve found in the peripheral nervous system are called peripheral nerves

Myelinated and non-myelinated nerve fibres

• Formation of myelin: the process of myelination begins before birth in the late fetal period but is not complete until a year or more later after the birth.

Myelination of the peripheral nerve fibres

The axons invaginate the side of a Schwann cell, as a result the plasma membrane of the Schwann cell forms a mesaxon, which suspends the axon within the Schwann cell. The layer of plasma membrane immediately around the axon is continuous with the remainder of the plasma membrane through a double-layered mesaxon.as the process continues, the cytoplasm is extruded from the spirals into the Schwann cell body. Thus, myelin sheath consists of many regular layers of plasma membrane material which is predominantly white lipid-protein, giving the myelinated axons a whitish appearance.

Functions of the myelin sheath

• Provides support to the nerve fibres
• Aids in the condition of the nerve impulses.
• Insulates an axon from the extracellular environment.
• Responsible for the colour of the white matter of the brain and spinal cord.
• Formation of non-myelinated fibres
• Surrounded by Schwann cells.
• Axons are surrounded by Schwann cells.
• The Schwann cell plasma membrane fuses along the opening of the groove, thus effectively sealing the nerve fibre within an extracellular compartment.

Conduction of action potential along an axon

• Resting membrane potential: the resting neuron maintains an ionic gradient across its plasma membrane thereby creating an electrical potential called the RMP
• Excitability involves a change in membrane permeability in response to appropriate stimuli so that the ionic gradient across the plasma membrane is reversed and the plasma membrane becomes depolarized.
• A wave of depolarization known as an action potential and spreads along the plasma membrane.
• This is followed by the process of repolarisation in which the membrane rapidly re-establishes its resting potential.
• Saltatory conduction: leaping of the action potential from one node of ranvier to another in the myelinated nerve fibres is called saltatory conduction.

Classification of peripheral nerve fibres

• Type A fibres: are large-diameter, myelinated axons and conduct action potential at a grate speed (15 to 120 m/sec)
• Type B fibres are medium-diameter, myelinated axons and conduct action potential at a slow speed. (3 to 15 m/sec)
• Type C fibres: are small-diameter non-myelinated axons that conduct action potentials at a very slow speed (2 m/sec or less)

Peripheral nerves

consisting of 12 pairs of cranial nerves from III to XII and four of them (III, VII, IX, and X) have both somatic and autonomous fibres

1. Optic nerve
2. Olfactory nerve
3. Oculomotor nerve
4. Trochlear nerve
5. Trigeminal nerve
6. Abducent nerve
7. Fascial nerve
8. Vestibulocochlear nerve
9. Glossopharyngeal nerve
10. Vagus nerve
11. Accessory nerve
12. Hypoglossal nerve

Spinal nerves

There are 31 pairs of spinal nerves that arise from spinal cord.

Formation of the plexus: except for thoracic nerves from T3 to T11,the anterior primary rami of all the spinal nerves join together and/or branch to form a network of nerves known as nerves plexus.

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Cervical plexus:

the cervical plexus is located below the sternocleidomastoid muscle. Its sensory fibres carry exteroceptive information from scalp, neck, and chest, and proprioceptive information from muscles, tendons, and joints, its motor fibres innervate cervical muscles and diaphragm. Sympathetic sudomotor and vasomotor fibres pass through this plexus to blood vessels and glands.

Brachial plexus:

The brachial plexus is formed by the C5, C6, C7,C8,and T1 ventral and dorsal roots.C4 contributes only very slightly to the formation of this plexus. Injuries to roots, trunks and cords of the brachial plexus may produce characteristic defects which are described here.

• Erb’s paralysis: site of injury
• Klumpke’s paralysis
• Injury to the nerve to serratus anterior (nerve of bell)
• Injury to lateral cord
• Injury to medial cord

Lumbosacral plexus:

Made up of L1 through S5 spinal nerves, innervates muscles of the lower limb, divided into two portions:

1. Lumbar portion: L1 through L4, supplies mostly muscles of the thigh.
2. Sacral portion L5 through S5 supplies mostly muscles of leg and foot.

Radial

Radial nerves is a continuation of the posterior cord of the brachial plexus and receive fibres from C5 to T1roots.it supplies the three heads of the triceps and winds around the shaft of the humerus in the spiral groove. At the level of the spiral groove, it gives a posterior antebrachial cutaneous branch. below the insertion of deltoid muscles, it lies relatively superficially on the lateral aspect of the arm. at the level of lateral epicondyle, it lies lateral to the biceps. Radial nerves enter the forearm between the brachialis and brachioradialis .in the proximal forearm, the radial nerve divides into posterior interosseous and superficial radial nerves.

median

It’s a mixed nerve derived from C5-T1 roots via medial and lateral cords of the brachial plexus. Forearm flexors and thenar muscles provide sensory innervation to the lateral aspect of palm and dorsal surface of terminal phalanges along with the palmar surface of the thumb, index, middle and half of the ring finger.

Ulnar

Receives fibres from C7, C8, and T1 roots through the medial cord of the brachial plexus. It descends medially in the upper arm close to median nerves and brachial artery.in the upper, ulnar nerves. Has no branch. at the elbow, it lies in the condylar groove, behind the medial epicondyle and then enters the cubital tunnel.

Femoral

It is formed by the dorsal portion of the anterior rami of L2-, L4 roots.it passes through the lateral border of the psoas and travels between the psoas and iliacus muscles.it emerges from the pelvic under the inguinal ligament and lies lateral to femoral vessels in the femoral triangle. About 4-5 cm below inguinal ligament femoral

Physiology of peripheral nervous system

Autonomic nervous system

The autonomic nervous system (ANS) controls all the body’s ‘autonomic’ functions, those outside voluntary control. ANS neurons span both the CNS and PNS and control homeostasis.

Divisions of the Autonomic nervous system

1) Sympathetic nervous system: preganglionic neurons found in the lateral horn of spinal cord from upper thoracic to the mid-lumbar cord (T1-T3). postganglionic cell bodies found in vertebral and prevertebral ganglia. Uses noradrenalin as postganglionic transmitter. it is generally said that sympathetic stimulation mobilises the body energy for flight or fight.

Functions of the sympathetic nervous system

• Dilation 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
• Bronchodilation 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 bladders and bowel
• Motor for ejaculation.

2) Parasympathetic nervous system: preganglionic neurons have cell bodies in the brainstem and sacrum. Postganglionic cell bodies are bound adjacent to or within the walls of the organ they supply. Uses acetylcholine (Ach)as a postganglionic transmitter.

Somatic nerves system:

The somatic nervous system is made up of nerves that are connected to skin, muscles and sensory organs. the SNS or voluntary nervous system is concerned with reactions to external stimulation. This system is under conscious control and is responsible for skeletal muscle contraction by way of the 31 pairs of spinal nerves. this system enables our voluntary control of muscles, as well as our reception of sights, sounds, sensations, tastes and smells.

Reference

1. Textbook of physiology volume -14 th edition Prof. A K Jain MD department of physiology maulana Azad medical college new Delhi
2. Textbook of clinical neuroanatomy-Prof.Vishram Singh, MS department of anatomy Santhosh world medical academy, Ghaziabad.
3. Clinical neurophysiology UK Misra MD, DM, MNAMS Professor and head department of neurology Sanjay Gandhi Postgraduate Institute of Medical Sciences.