The Polyneuropathies Functional Anatomy

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The function of the peripheral nervous system is to carry impulses to and from the central nervous system. These impulses regulate motor, sensory and autonomic activities.

The peripheral nervous system is comprised of structures that lie outside the pial membrane of the brainstem and spinal cord and can be divided into cranial, spinal and autonomic components.

Cell body

STRUCTURE OF THE NERVE CELL AND AXON

Node of Ranvier

Axon

Node of Ranvier

Myelin sheath

Axon

Schwann cell

Each axon represents an elongation of the nerve cell - this lying within the central nervous system, e.g. anterior horn Dendrites cell, or in an outlying ganglion, e.g. dorsal root ganglion. The cell body maintains the viability of the axon, being the centre of all cellular metabolic activity.

Many axons are surrounded by an insulation of myelin, which is enveloped by the Schwann cell membrane. Myelin is a protein-lipid complex. The membrane of the Schwann cell 'spirals' around the axon resulting in the formation of a multilayered myelin sheath.

Myelin sheath

Schwann cell

Nucleus-^

Developing myelin sheath

Schwann cell

Axon

Nucleus-^

Developing myelin sheath

Schwann cell

Axon

Final stage of myelination

All axons have a cellular sheath - Schwann cell - but not all axons are myelinated.

Schwann cells with associated myelin are 250-1000 /mi in length and separated from each other by the node of Ranvier. The axon is bare at this node and, during conduction, impulses jump from one node to the next - saltatory conduction. The rate of conduction in myelinated nerves is markedly increased in comparison with unmyelinated fibres. Myelin thus facilitates fast conduction. In unmyelinated fibres conduction depends upon the diameter of the nerve fibre, this determining the rate of longitudinal current flow.

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