The lateral vestibulospinal tract originates in the lateral vestibular nucleus, also known as Deiter’s nucleus. This tract descends ipsilaterally through the ventral funiculus of the spinal cord and ends on interneurons associated with the medial motor neuron groups. The lateral vestibulospinal tract excites motor neurons that supply extensor muscles of the proximal part of the limb that are important for postural control. In addition, this pathway inhibits flexor motor neurons because it also excites the reciprocal group Ia interneurons that receive group Ia input from extensor muscles, which in turn inhibit flexor motor neurons. The excitatory input to the lateral vestibular nucleus is from both the semicircular canals and the otolith organs, whereas the inhibitory input is from the Purkinje cells of the anterior vermis region of the cerebellar cortex. An important function of the lateral vestibulospinal tract is to assist in postural adjustments after angular and linear accelerations of the head.
The medial vestibulospinal tract originates from the medial vestibular nucleus. This tract descends in the ventral funiculus of the spinal cord to the cervical and midthoracic levels, and it ends on the medial group of interneurons. Sensory input to the medial vestibular nucleus from the labyrinth is chiefly from the semicircular canals. This pathway thus mediates adjustments in head position in response to angular acceleration of the head.
The Tectospinal Tract
The tectospinal tract originates in the deep layers of the superior colliculus. The axons cross to the contralateral side, just below the periaqueductal gray matter. They then descend in the ventral funiculus of the spinal cord to terminate on the medial group of interneurons in the upper cervical spinal cord. The tectospinal tract regulates head movement in response to visual, auditory, and somatic stimuli.
In addition to the lateral and medial systems, less specifically organized systems descend from the brainstem to the spinal cord. These include several pathways in which monoamines serve as synaptic transmitters.
The locus coeruleus and the nucleus subcoeruleus are nuclei located in the rostral pons, and they are composed of norepinephrine-containing neurons. These nuclei project widely throughout the CNS, and their projection to the spinal cord travels in the lateral funiculus. Their terminals are on interneurons and motor neurons. The dominant effect of the pathway is inhibitory.
The raphe nuclei of the medulla also project widely throughout the CNS and give rise to several raphe-spinal pathways. With regard to motor function, the ventral horn projection may enhance motor activity.
In general, the monoaminergic pathways act to alter the responsiveness of spinal cord circuits, including the reflex arcs. In this way, they induce widespread changes in excitability rather than discrete movements or specific changes in behavior.
Motor Deficits Caused by Lesions of Descending Motor Pathways