VPB 121: Veterinary Physiology - II (2+1)

NERVE FIBRES

• Nerve fibres are axons of neurons which can be either insulated or naked. Insulation is provided by sheaths which may be either myelin sheath of lipoid layer or thin tubular covering of neurilemma or schwann sheath.
• The nerve fibres with myelin sheath are known as myelinated and those without myelin sheath are known as unmyelinated fibres. Myelinated fibres are seen in both CNS and PNS whereas fibres with schwann cell, sheathing core characteristically seen in fibres lie outside CNS.

Structure of Myelinated nerve fiber

• Axon is composed of delicate neurofibrils embedded in an axoplasm along with mitochondria and endo plasmic reticulum. Axon has a cell membrane known as Axolemma separating the axon from the surrounding structures.
• Axolemma is surrounded by thick myelin sheath made-up of nucleated membranous Schwann cell sheath. This structure poses the axon to be highly refractive and provides white colour to the axon.
• Myelin sheath consists of lipid-Sphingomyelin. The myelin sheath is interrupted at regular intervals by constriction which appears to be a node and is known as Nodes of Ranvier. At each node, there is a constriction of neurilemmal sheath towards the axon. These sheath cells are absent in the myelinated fibers of CNS.

Myelinogenesis

• It is the process of myelin formation. It begins with crowding and excessive growth of Schwann cells completely encasing the whole length of the axon.
• The cell membrane of the Schwann cells surrounding the axons rotate in a manner to wrap around the axons and forms closely packed helically arranged layers of double membrane. Each membrane is composed of bilipid layer sand witched between the protein layer. Thickness of myelin sheath is determined by the number of membrane layers wrapped round the axon.
• Myelination of the sensory tract precedes the motor tract. Myelination is influenced by Vitamin B 12, Folic acid and Thyroxin hormone.

Stages of Myelination

• First stage begins with enclosure of axon with myelin forming cells. Mesoaxon is formed by the union of enveloping processes from outer surface.
• Elongation of mesoaxon in a spiral manner which coils around the axon occurs. Loss of cytoplasm of turns of mesoaxon tightens the spiral turns around the axon.

Conduction of impulse in myelinated fiber

• Nerve impulse travel at rapid rate in myelinated fibers. Electrical membrane has effective contact with the interstitial fluid only at the nodes of Ranvier.
• In myelinated fibers the local circuit flow is from one node to the other, known as Saltatory conduction.

• Due to this the conduction velocity in these fibers is greater as the conduction is restricted to patches of the membrane which are then depolarized. The conduction speed is 20 times greater than the unmyelinated fibers.

Unmyelinated nerve fiber

• Lacks myelin sheaths and are found in grey matter of spinal cord, some parts of brain and in ANS.
• Conduction velocity is about 1 meter and is dependant upon the diameter of the fiber.