Page 373 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 358

358 The central nervous system

form a compact bundle which occupies the central third of the cerebral
peduncle. Hence they pass through the ventral pons, where they are broken
up into a number of small bundles between the cells of the pontine nuclei
and the transversely disposed pontocerebellar fibres. Near the lower end of
the pons they again collect to form a single bundle which comes to lie on the
ventral surface of the medulla and forms the elevation known as the
‘pyramid’. As it passes through the brainstem, the pyramidal system gives
off, at regular intervals, contributions to the somatic and branchial arch
efferent nuclei of the cranial nerves. Most of these corticobulbar fibres cross
over in the brainstem, but many of the cranial nerve nuclei are bilaterally
innervated.
Near the lower end of the medulla the great majority of the pyramidal
tract fibres cross over to the opposite side and come to occupy a central
position in the lateral white column of the spinal cord. This is the so-called
‘crossed pyramidal tract’ shown in Fig. 237. Asmall proportion of the fibres
of the medullary pyramid, however, remain uncrossed until they reach the
segmental level at which they finally terminate. This is the direct or
uncrossed pyramidal tract, which runs downwards close to the anteromedian
fissure of the cord, with fibres passing from it at each segment to the oppo-
site side.
In view of the frequent involvement of the pyramidal tract in cere-
brovascular accidents, its blood supply is listed here in some detail:
•◊◊motor cortex — leg area: anterior cerebral artery; face and arm areas:
middle cerebral artery;
•◊◊internal capsule—branches of the middle cerebral artery;
•◊◊cerebral peduncle—posterior cerebral artery;
•◊◊pons—pontine branches of basilar artery;
•◊◊medulla—anterior spinal branches of vertebral artery;
•◊◊spinal cord —segmental branches of anterior and posterior spinal
arteries.

Clinical features

1◊◊It is important to remember that, in the motor cortex, movements are
represented rather than individual muscles; lesions of this pathway result
in paralysis of voluntary movement on the opposite side of the body
although the muscles themselves are not paralysed and may cause involun-
tary movements. This is the essential difference between an ‘upper motor
neuron’ lesion (i.e. a lesion of the central motor pathway) and a ‘lower
motor neuron’ lesion (i.e. a lesion affecting the cranial nerve nuclei, or the
anterior horn cells or their axons). In both types of lesion muscular paraly-
sis results; in the latter, reflex activity is abolished, flaccidity and muscular
atrophy follow, whereas, in pyramidal lesions, there is spasticity, increased
tendon reflexes and an extensor plantar response.
2◊◊Experimental lesions strictly confined to the pyramidal tract are not fol-
lowed by increased muscular tone in the affected part (spasticity), but clini-
cally this is a feature of upper motor neuron lesions; it is attributable to