Page 374 - Clinical Anatomy

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The brain 359

concomitant involvement of the extrapyramidal system, hence demon-
strating the over simplification of the pyramidal and extrapyramidal
concept.
3◊◊The pyramidal tract is most frequently involved in cerebrovascular acci-
dents where it passes through the internal capsule. Indeed, the artery sup-
plying this area — the largest of the perforating branches of the middle
cerebral artery—has been termed the artery of cerebral haemorrhage.
4◊◊Alist of the more important related signs is given here for involvement
of the pyramidal tract at each level.
•◊◊Cortex— isolated lesions may occur here, resulting in loss of voluntary
movement in, say, only one contralateral limb, but often the sensory cortex
is also involved. Aphasia in dominant hemisphere lesions, (usually left),
involving Broca and Wernicke’s areas and the cortex between them, is not
uncommon.
•◊◊Internal capsule—usually all parts of the tract are involved, giving a com-
plete contralateral hemiplegia with associated sensory loss. The lesion may
extend back to involve the visual radiation, giving a contralateral homony-
mous field defect (hemianopia).
•◊◊Cerebral peduncle and midbrain — the fibres from the 3rd nerve are often
concomitantly involved so that there are the associated signs of a 3rd nerve
palsy.
•◊◊Pons— here the 4th nerve is often involved, alone or together with VII.
There may then be a hemiplegia affecting the arm and leg of the opposite
side and an abducens and a facial palsy of the lower motor neuron type on
the same side as the lesion.
•◊◊Medulla — because of the proximity of the pyramids to one another,
medullary lesions often affect both sides of the body. Paralysis of the tongue
on the side of the lesion is due to involvement of the 12th nerve or its
nucleus. The respiratory, vasomotor and swallowing centres may also be
affected.
•◊◊Spinal cord—the paralysis following lesions of the spinal cord is ipsilat-
eral and accurately depends on the level at which the pyramidal tract is
involved. Lower motor neurone lesion signs can be detected at the level of
the spinal trauma (direct injury) and upper motor neurone lesion signs
below. The proximity of the pyramidal tracts to the ascending sensory path-
ways accounts for the concomitant sensory changes which are usually
found.

The extrapyramidal system
The extrapyramidal motor system should, by definition, include all those
motor projections which do not pass physically through the medullary
pyramids. It was once thought to control movement in parallel with and,
to a large extent, independently of the pyramidal motor system and the
pyramidal/extrapyramidal division was used clinically to distinguish
between two motor syndromes: one characterized by spasticity and para-
lysis whereas the other involved involuntary movements, or immobility
without paralysis. It is now clear that many ‘extrapyramidal’ structures,