Peripheral nerves and ganglia

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Peripheral Nerves and Ganglia


The peripheral nerve lesions are common in clinical practice and can be caused by a wide variety of diseases like trauma, neoplasms, infection, metabolic diseases (diabetes) and chemical toxins such as lead.


Therefore, it is of paramount importance for a physician to know the basic structure of peripheral nerves. Further, he also needs to know the structure and function of nerve fibres, and the process of their myelination, so that he could understand the mode of conduction of nerve impulses, and appreciate the process of nerve degeneration and regeneration.



Nerve Fibres


An axon of a nerve cell is termed nerve fibre. The bundles of nerve fibres found in the central nervous system (CNS) are referred to as nerve tracts while the bundles of nerve fibres found in the peripheral nervous system are called peripheral nerves. Two types of nerve fibres are present in the nervous system, viz. myelinated and non-myelinated.



Myelinated and Non-myelinated Nerve Fibres


In the peripheral nervous system, all axons (nerve fibres) are enveloped by the specialised Schwann cells which provide both structural and metabolic support to them.


In general, small diameter axons, for example those of the autonomic nervous system (ANS) and small pain fibres, are simply enveloped by the cytoplasm of Schwann cells; these nerve fibres are said to be non-myelinated. The large diameter fibres are wrapped by a variable number of concentric layers of Schwann cell plasma membrane forming the so-called myelin sheath, and such nerve fibres are said to be myelinated. Within the CNS, the myelination is similar to that in the peripheral nervous system except that the myelin sheath is formed by cells called oligodendrocytes.



Myelination (formation of myelin)


The myelination is the process by which nerve fibres acquire myelin sheaths which enhance the conduction of nerve impulses.


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 (Fig. 3.1)

The myelination begins near the origin of the axon and ends just before its terminal branches.



The axon invaginates the side of a Schwann cell, as a result the plasma membrane of 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 (Fig. 3.1A).


The Schwann cell now rotates around the axon so that mesaxon becomes wrapped repeatedly around the axon forming spirals around it. As the process continues, the cytoplasm is extruded from the spirals into the Schwann cell body. On maturity, the inner layers of plasma membrane fuse with each other so that axon becomes surrounded by several layers of modified membrane which together constitute the myelin sheath. 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. It insulates the axon from extracellular environment thus preventing ion fluxes across the plasma membrane of the nerve fibre/axon.


The thickness of myelin sheath depends on the number of spirals of Schwann cell membrane. In electron micrographs of cross-sections of myelinated nerve fibres, the myelin is seen to be laminated consisting of major and minor dense lines. The darker major dense line (about 2.5 nm thick) consists of two inner protein layers of the plasma membrane that are fused together. The lighter minor dense line (about 10 nm thick) is formed by the approximation of the outer surfaces of adjacent plasma membranes and is made up of lipid.


Each Schwann cell extends for a short distance along the nerve fibre and at its termination its role is supplemented by an another Schwann cell with which it interdigitates closely.


In the CNS, oligodendrocytes responsible for the process of myelination, follows the similar pattern as of Schwann cell in the PNS; a single oligodendrocyte, however, forms the myelin sheath around several axons.


A myelinated nerve fibre, therefore, consists of an axon, a myelin sheath and a neurilemmal/Schwann sheath. The myelin sheath is segmented, the segments being separated at regular intervals by nodes of Ranvier. The areas between the nodes are called internodes.



Functions of the myelin sheath



The non-myelinated fibres are also surrounded by Schwann cells (Fig. 3.2). Several axons become longitudinally invagi-nated into the cytoplasm of a Schwann cell so that each fibre is embedded in a groove in the Schwann cell cytoplasm. The Schwann cell plasma membrane fuses along the opening of the groove, thus effectively sealing the nerve fibre within an extracellular compartment. As many as 15 or more axons may share a single Schwann cell.





Each axon is surrounded by a single layer of plasma membrane of schwann cell, hence it is unmyelinated. There are no nodes of Ranvier. Consequently the action potential travels along the whole length of axolemma without the accelerating factor of node-to-node (saltatory) conduction. This accounts for slow rate of conduction of nerve impulse in the unmyelinated fibres.



Conduction of Action Potential along an Axon


Like all the cells, the resting (unstimulated) neuron maintains an ionic gradient across its plasma membrane, thereby creating an electrical potential called resting membrane potential. Thus, in resting neuron its plasma membrane remains polarized. The excitability (a fundamental property of neurons) 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 action potential then spreads along the plasma membrane. This is followed by the process of repolarization in which membrane rapidly re-establishes its resting potential.


The speed of conduction of the action potential, along an axon depends on the myelination of the axon (Fig. 3.3). The action potentials are conducted more rapidly in myelinated than in non-myelinated axons. In non-myelinated fibres, the action potential passes continuously along the axolemma, progressively exciting neighbouring areas of membrane. In myelinated fibres, the myelin sheath serves as an insulator. Consequently a myelinated nerve fibre can be stimulated only at the nodes of Ranvier, where the axon is naked and the ions can pass freely through the plasma membrane between the extracellular fluid and the axo-plasm. Therefore, in these fibres the action potential jumps from one node to the next. The action potential at one node sets up a current in the surrounding tissue fluid, which quickly produces depolarization at the next node. The action potential conduction in a myelinated fibre is like a grasshopper jumping, whereas action potential conduction in a non-myelinated fibre is like a grasshopper walking. The action potential will naturally move more rapidly by jumping.



This leaping of the action potential from one node of Ranvier to another in the myelinated nerve fibres is called saltatory conduction (L. saltare = to leap).


In addition to myelination, the diameter of axons affects the speed of conduction of action potential. The conduction of action potential is faster along large diameter axons than small diameter axons because large diameter axons provide less resistance to action potential propagation. In the large motor fibres (alpha fibres), the rate of conduction may be as high as 70-120 meters per second.


The smaller sensory fibres have slower conduction rate (Table 3.1).






Classification of Peripheral Nerve Fibres


According to the axonal diameter (including myelin sheath if present) and speed of conduction, the peripheral nerve fibres are classified into three main groups: A, B and C.








The type B and C fibres are primarily found in the ANS, which supplies internal organs such as stomach, intestine. The responses necessary to maintain internal homeostasis such as digestion need not be as rapid as to external environment.


The group A fibres are further classified into somatic sensory (I, II, III) and motor (a, (3, 7) subgroups. Table 3.1 shows the types of nerve fibres and their maximum diameters and conduction rates.

Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Peripheral nerves and ganglia
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