Functions of Myelin Sheath

Myelin sheath is responsible for faster conduction of impulse through nerve fibers. In these nerve fibers, impulses jump from one node to another. Transmission of impulses from one node to another is by means of saltatory conduction.

Myelin sheath also has a high insulating capacity. Because of this, myelin sheath restricts nerve impulse within single nerve fiber, and prevents stimulation of neighboring nerve fibers.

Neurilemma

Surrounding myelin sheath, there is a thin membrane called neurilemmal sheath. This is also called neurilemma or sheath of Schwann. It contains Schwann cells. Cytoplasm is thin and modified to form the thin sheath of neurilemma enclosing the myelin sheath. At node of Ranvier (where myelin sheath is absent), the neurilemma invaginates and runs up to axolemma in form of a finger-like process. Neurilemma is necessary for formation of myelin sheath (myelinogenesis). Neurilemma is absent in central nervous system.

In non-myelinated nerve fiber, the neurilemma continuously surrounds axolemma.

Nervous impulses conduction

Fig.11. Mode of conduction through nerve fibers.

A. Non-myelinated nerve fiber — Continuous conduction.

2. Myelinated nerve fiber — Saltatory conduction: impulse jumps from node to node. AP = action potential.

CONDUCTION THROUGH MYELINATED NERVE FIBER—SALTATORY CONDUCTION

Conduction of impulse through a myelinated nerve fiber is about 150 times faster than through a non-myelinated fiber. This is because, myelin sheath forms an effective insulator and flow of current through this sheath is negligible. But action potential jumps from one node to another node of Ranvier. So, velocity of conduction is faster. This type of jumping of action potential from one node to another is called saltatory conduction.

Mechanism of Saltatory Conduction

The myelin sheath is not permeable to ions. So, entry of sodium from extracellular fluid into nerve fiber occurs only in the node of Ranvier, where myelin sheath is absent. This causes depolarization in the node, and not in the internode. Thus, depolarization occurs at successive nodes. So, action potential jumps from one node to another. Hence, this is called saltatory conduction (saltare = jumping).

NERVOUS FIBERS CLASSIFICATION

1. Depending on structure:

a) Myelinated Nerve Fibers

Myelinated nerve fibers are covered by myelin sheath.

b) Non-myelinated Nerve Fibers

Nerve fibers of this type do not have myelin sheath.

2. Depending upon distribution:

а)Somatic Nerve Fibers - supply the skeletal muscles of the body.

b) Visceral or Autonomic Nerve Fibers - supply the various internal organs of the body.

3. Depending on source of origin:

a) Cranial Nerves - arising from the brain.

b) Spinal Nerves - arising from the spinal cord.

4. Depending on function:

а) Motor or Efferent Nerve Fibers - carry motor impulses from central nervous system to different parts of the body.

b) Sensory Nerve or Afferent Fibers - carry sensory impulses from different parts of the body to the central nervous system.

5. Depending on chemical neurotransmitter:

а) noradrenergic - secrete noradrenaline;

b) cholinergic - secrete acetylcholine.

6. Depending on diameter and conduction

Fibers type	Fibers diameter (mcm)	Transduction velocity (m/sec)	Main function
Aα (Type I )	13-22	70-120	skeletal muscles efferent fibers; receptors (muscular spindles) afferent fibers
Aβ (Type II )	8-13	40-70	afferents from pressure and touching receptors
Aγ	4-8	15-40	receptors (muscular spindles) efferent fibers; part of afferents from pressure and touching receptors
Aδ (Type III)	1-4	5-15	afferents from skin temperature and pain receptors, partially pressure
B	1-3	3-14	autonomic nervous system preganglionar efferents
C (Type IV)	0,5-1,5	0,5-2	autonomic nervous system postganglionar efferents; pain and warmth skin receptors afferents

Velocity of impulse through a nerve fiber is directly proportional to thickness of fibers. Except С type of fibers, all nerve fibers are myelinated, B type is partially myelinated.