Descending Motor Tracts In The Spinal Cord

Reflex activity generated from motor programs in the spinal cord represents the foundation of the motor hierarchy. These programs provide the basic plans by which movement can be achieved in a coordinated fashion. In addition, they facilitate the transfer of information about more complex movements and about volitional movement from brain to appropriate groups of lower motor neurons. Motor input to spinal cord levels is received in two major pathways: (1) the ventromedial pathway, consisting of four major tracts from various brain stem structures primarily concerned with posture and locomotion, and (2) the lateral corticospinal tract, which carries information for volitional movement of proximal and distal muscles under direct cortical control (Fig. 5).

The vestibulospinal tract is composed of two subdivisions, each originating from separate vestibular nuclei in the brain stem and descending in slightly different regions of the cord. One of the main functions of this tract is to keep the body, particularly the head region, balanced during movement and to orient the body and head in the direction of new sensory stimuli. Motion in the fluid of the vestibular labyrinth of the middle ear accompanies movement of the head. Activated hair cells then signal specific vestibular nuclei via cranial nerve VIII. Axons from the lateral vestibular nuclei form the lateral vestibu-lospinal tract, which projects ipsilaterally in the ventro-lateral part of cord throughout its length. This tract terminates in the lateral part of the intermediate zone on interneurons that innervate both alpha and gamma motor neurons of extensor muscles in upper and lower extremities.

The medial vestibulospinal tract originates in the medial vestibular nucleus and descends bilaterally only to midtho-racic levels, traveling in a compact myelinated fiber bundle called the medial longitudinal fasciculus (MLF). It terminates in the intermediate zone and provides inhibitory input to neurons innervating muscles ofthe neck and back. The combined activity of the lateral and medial ves-tibulospinal tracts serves to control balance, posture, and muscle tone; its net effect is to excite antigravity muscles.

The tectospinal tract originates from the superior colliculi, a structure that forms part of the tectum, or roof of the midbrain. The superior colliculus is a coordinating center for visual information, gathering visual input from the retina and the visual cortex, as well as additional sensory information from somatosensory and auditory systems. From these inputs it constructs a spatial map of the immediate environment with respect to the visual field. Cells of the colliculus are retinotopically organized so that a novel sensory stimulus occurring at one point in the visual field leads to stimulation of a corresponding site in the colliculus. In response, collicular neurons become activated and send appropriate signals to motor neurons in the spinal cord, particularly those innervating the head and neck. The head and eyes reflexly move to the appropriate point in space and allow the object of interest to be imaged on the fovea of the retina. The tectospinal

56. Sensory and Motor Pathways Controlling Lower Motor Neurons of the Spinal Cord A Four driving forces on lower motor neurons

1. Afferent Neurons

Ia from distal muscle Ia from its antagonist Ib afferent - distal muscle

3. Medial Tracts medial vestibulo spinal ventral corticospinal tecto spinal pontine reticulo spinal medullary reticulo spinal lateral vestibulo spinal nococeptive from limb

4. Lateral Tracts lateral corticospinal rubro spinal

Y to axial muscles a to axial muscles nococeptive from limb r*

4. Lateral Tracts lateral corticospinal rubro spinal

Motor Neurons

Y to distal muscles _^

a to distal muscles

Y to axial muscles a to axial muscles f

B. Descending medial paths

B. Descending medial paths

cerebral cortex

C. Descending lateral paths tectum pontine reticular formation mudullary vestibular nuclei vestibular nuclei level of spinal cord section in A

C. Descending lateral paths tectum pontine reticular formation mudullary vestibular nuclei vestibular nuclei level of spinal cord section in A

left cerebral cortex basal ganglia red nucleus right cerebellum rubro spinal tract cortical spinal tract left cerebral cortex basal ganglia red nucleus right cerebellum rubro spinal tract cortical spinal tract

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