The Polyneuropathies Functional Anatomy

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Entry to and exit from the central nervous system is achieved by paired spinal nerve roots (30 in all).

These dorsal and ventral roots lie in the spinal subarachnoid space and come together at the intervertebral foramen to form the spinal nerve.

The dorsal root contains sensory fibres, arising from specialised sensory receptors in the periphery.

Dorsal root

Dorsal root ganglion

Pial membrane

Posterior ' horn

Dorsal root

Dorsal root ganglion

Posterior ' horn

Spinal nerve

Ventral root

Spinal nerve

SSW-. Anterior horn

Ventral root

The dorsal root ganglia are collections of sensory cell bodies with axons extending peripherally as well as a central process which passes into the spinal cord in the region of the posterior horn of grey matter and makes appropriate central connections.

Sensation can be divided into:

- Pain and temperature

- Simple touch

- Discriminatory sensation - proprioception, vibration.

These different forms of sensation are carried from the periphery by axons with specific characteristics. The central connections and pathways vary also (see page 196).

The anterior horns of the spinal cord contain cell bodies whose axons pass to the periphery to innervate skeletal muscle - the alpha motor neurons. Smaller cell bodies also project into the anterior root and innervate the intrafusal muscle fibres of muscle spindles — the gamma motor neurons.

Each alpha motor neuron through its peripheral ramifications will innervate a number of muscle fibres. The number of fibres innervated from a single cell varies from less than 20 in the eye muscles to more than 1000 in the large limb muscles (innervation ratio). The alpha motor neuron with its complement of muscle fibres is termed the motor unit.


Peripheral nerves are composed of many axons bound together by connective tissue. A 'mixed' nerve contains motor, sensory and autonomic axons.

The blood supply to these bundles is by means of small nutrient vessels within the epineurium - the vasa nervorum.

Cross section of nerve



-- Epineurium

Bundles of axons

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