Page 351 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 336

336 The central nervous system

pathway commences at the pyramidal cells of the motor cortex, decussates
in the medulla, then descends in the pyramidal tract on the contralateral
side of the cord. At each spinal segment, fibres enter the anterior horn and
connect up with the motor cells there—the tract therefore becomes progres-
sively smaller as it descends.
2◊◊The direct pyramidal (anterior cerebrospinal or uncrossed motor) tract is a
small tract descending without medullary decussation. At each segment,
however, fibres pass from it to the ventral horn (anterior) motor cells of the
opposite side.

Ascending tracts (Fig. 237)
1◊◊The posterior and anterior spinocerebellar tracts ascend on the same side of
the cord and enter the cerebellum through the inferior and superior cerebel-
lar peduncles respectively.
2◊◊The lateral and anterior spinothalamic tracts. Pain and temperature fibres
enter the posterior roots, ascend a few segments, relay in the substantia
gelatinosa, then cross to the opposite side to ascend in these tracts to the
thalamus, where they are relayed to the sensory cortex.
3◊◊The posterior columns comprise a medial and lateral tract, termed respec-
tively the fasciculus gracilis (of Goll) and fasciculus cuneatus (of Burdach). They
convey 1st order sensory fibres subserving fine touch and proprioception
(position sense), mostly uncrossed, to the gracile and cuneate nuclei in
the medulla where, after synapse, the 2nd order fibres decussate, pass to the
thalamus and, after further synapse, 3rd order fibres are relayed to the
sensory cortex. Some fibres pass from the medulla to the cerebellum along
the inferior cerebellar peduncle.

Blood supply
The anterior and posterior spinal arteries descend in the pia from the intracra-
nial part of the vertebral artery. They are reinforced serially by branches
from the ascending cervical, the cervical part of the vertebral, the intercostal
and the lumbar arteries.

Clinical features

1◊◊Complete transection of the cord is followed by total loss of sensa-
tion in the regions supplied by the cord segments below the level of
injury together with flaccid muscle paralysis. As the cord distal to the
section recovers from a period of spinal shock, the paralysis becomes
spastic, with exaggerated reflexes. Voluntary sphincter control is lost
but reflex emptying of bladder and rectum subsequently return, provided
that the cord centres situated in the sacral zone of the cord are not
destroyed.
2◊◊Destruction of the centre of the cord, as occurs in syringomyelia and in
some intramedullary tumours, first involves the decussating spinothalamic
fibres so that initially there is bilateral loss of pain and temperature sense