Page 371 - Clinical Anatomy

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356 The central nervous system

dorsal and medial part of the cortex, the trunk and arm in its middle
portion and the face most inferiorly. Since the size of the area of cortical rep-
resentation reflects the density of the peripheral innervation and hence
complexity of the function being performed rather than the area of the
receptive field, there is a good deal of distortion of the body image in the
cortex, the cortical representation of the face and hand being much greater
than that of the limbs and trunk.

Clinical features

1◊◊Lesions of the sensory pathway most commonly occur in the internal
capsule following some form of cerebrovascular accident. If complete,
these result in a total hemianaesthesia of the opposite side of the body. In
partial lesions the area of sensory loss will be determined by the site of the
injury in the internal capsule and, from a knowledge of the sensory (and
motor) loss, it is usually possible to determine with some degree of accu-
racy the site of a lesion in the capsule.
2◊◊Since there is modality segregation below the decussation of the
medial lemniscus, lesions of the sensory pathways at cord level result in
dissociation of sensation, with an area of analgesia contralaterally together
with impairment of tactile sensibility ipsilaterally (for further details, see
pages 366–7).
The auditory, visual and olfactory pathways are dealt with later under
the appropriate cranial nerves.

The motor pathways (Fig. 250)
It is customary to divide the motor pathways of the brain and spinal cord
into pyramidal and extrapyramidal systems. Although the latter is an
imprecise concept, it provides a useful collective term for the many motor
structures not confined to the pyramidal tracts in the medulla.

The pyramidal tract

The pyramidal system is the main ‘voluntary’ motor pathway and derives
its name from the fact that projections to the motor neurons in the spinal
cord are grouped together in the medullary pyramids. The fibres in this
pathway arise from a wide area of the cerebral cortex. About two-thirds
derive from the motor and premotor cortex of the frontal lobes; however,
about one-third arises from the primary somatosensory cortex. In both the
motor and premotor cortex there is an organization comparable to that seen
in the sensory area. Again, the body is inverted so that the ‘leg area’ is situ-
ated in the dorsomedial part of the precentral gyrus encroaching on the
medial surface of the hemisphere, supplied by the anterior cerebral artery.
The ‘face area’ is near the lateral sulcus, while the ‘arm area’ occupies a
central position, both supplied by the middle cerebral artery. Again, the
body image is greatly distorted; the area representing the hand, lips, eyes