64
CHAPTER 7
and soon cross to reach the opposite lateral funiculus. Afferent impulses ascending in the
spino-tectal tract are concerned with head and eye movements.
(5)
Posterior-lateral tract.
It is formed by fibres of the posterior roots which are divided
into ascending and descending branches. The fibres ascend one or two segments in the
posterior tract and give up collaterals which terminate around cells of the posterior grey
column, from where the new axons travel to the opposite lateral spino-thalamic tract and
convey nociceptive and thermal sensibility.
(b)
Descending tracts
(1) Lateral cortico-spinal tract.
This is the most important upper motor neuron tract
originating from the giant pyramidal cells in the motor cortex, descending through the
capsula interna and traversing the pedunculus cerebri and pons into the oblongata. In
the lower part of the oblongata two-thirds of the fibres decussate and descend in the
lateral funiculus of the spinal cord, while the rest of the fibres descend into the same side
of the cord forming the anterior cortico-spinal tract. The majority of the cortico-spinal
fibres terminate in the cord opposite to the side of the cortex from which they spring and
synapse either directly with motor ganglion cells of the anterior grey column or they
synapse on intermmcial neurones in the neighbouring part of the base of the anterior
grey column. From these neurones they are brought into contact with the motor cells of
the anterior grey column.
(2)
Rubro-spinal tract.
This tract originates from cells of the red nucleus of the tegmentum
and descends on the opposite side of the cord anteriorly to the lateral cortico-spinal
tract, forming synapses in the anterior grey column. Thus this tract brings the cells of
anterior grey column into contact with the corpus striatum.
(j)
Olivo-spinal tract.
The fibres of this tract, which originate in the olivary nucleus of
the oblongata, synapse with ganglion cells of the anterior grey column in the cervical
region only.
(4) Descending autonomic tracts.
These are crossed and uncrossed fibres originating from
the hypothalamic and other stations of the brain stem regulating functions of the auto–
nomic system. They descend to the spinal cord mainly in an area between the posterior
border of the spino-thalamic tract and the anterior border of the pyramidal tract and
synapse to the lateral funiculus.
(5)
Lateral retro-spinal tract.
This is another important descending upper motor neuron
tract which originates from the reticular formation of the brain stem and descends
mainly in the contralateral side of the spinal cord and synapses with the internuncial
and motor cells of the anterior grey column. Its significance in connection with the
reflex behaviour in spinal cord lesions will be discussed in the section on Neurophysiology.
