The Reticulospinal and Vestibulospinal Tracts Are Medial Brainstem Motor Pathways Important for Keeping the Body Upright Against the Pull of Gravity
A major responsibility of the medial descending brainstem motor pathways is to maintain the body subconsciously in an upright position against the pull of gravity. The reticulospinal and vestibulospinal tracts play a major role in this involuntary control of the axial and proximal extensor musculature that prevents the animal from falling to the ground.
The reticulospinal tract is particularly important in controlling the magnitude of the steady-state contraction level, or muscle tone, of these antigravity muscles. The vestibulospinal tract plays an essential role in activating the antigravity muscles in response to destabilization of the body with respect to gravity. Keep in mind that subconscious control of the postural musculature is an integral part of the ability to execute skilled voluntary movement of the distal musculature successfully, because voluntary movement requires a stable “platform" on which it can proceed.The reticulospinal tract originates from cell bodies in the reticular formation of the brainstem (see Figure 10-3). This is a netlike complex of many small clusters of cell bodies (nuclei) and loosely organized axonal projections, located near the midline. Oncc thought to be a diffuse and fairly nonspecific system, the reticular formation is now known to contain a number of functionally specific nuclei. In addition to being the origin of a medial descending brainstem motor pathway to the spinal cord, ascending projections of the reticular formation play an important role in modulating consciousness, arousal, and attention. The reticular formation receives a vast array of sensory information and also plays an important role in pain perception, respiration, and circulatory function.
Axons of the reticulospinal tract synapse within medial regions of the spinal cord gray matter that primarily control the axial and proximal extensor musculature (see Figure 10-3).
Collectively, the tract projects to virtually all rostrocaudal levels of the cord. Portions of the reticulospinal tract that originate from cells in reticular nuclei of the pons tend to have an excitatory effect on lower motor neurons to the antigravity muscles. Portions of the tract coming from the reticular nuclei of the medulla tend to have an inhibitory effect on the lower motor neurons to antigravity muscles. These opposing portions of the reticulospinal tract interact to regulate antigravity muscle tone. Influences from other regions of the brainstem, the cerebellum, and the spinal cord endow the pontine reticular nuclei with a high level of spontaneous activity. The effects of such spontaneous excitatory activity on antigravity muscle tone can be tempered by activation of the inhibitory medullary reticular nuclei. Descending projections from the cerebral cortex to the brainstem represent a significant forebrain source of relative control over the two portions of the reticulospinal tract. This cortico-reticulospinal route emphasizes the point that some of the descending brainstem motor pathways provide an indirect way for more rostral levels of the motor system hierarchy to influence spinal lower motor neurons (see Figure 10-3).The descending cortical projections to the origins of the reticulospinal tract endow that tract with two important motor functions, in addition to its critical role in the subconscious modulation of antigravity muscle tone. The first function is related to skilled voluntary movement requiring a stable postural background, as previously noted. Just before the execution of such a voluntary movement, the reticulospinal tract subconsciously activates the appropriate axial and proximal musculature that will compensate for the postural destabilization that will be produced by the intended voluntary movement (usually of the distal musculature). The reticulospinal tract also plays a role in the voluntary execution of crude (nonskilled), stereotypical movements of the proximal limb musculature, such as those involved in simple pointing or locomotion.
As noted in Chapter 8, γ motor neurons are usually activated along with α motor neurons so that muscle spindles maintain their sensitivity to stretch even when the muscle is shortened during contraction. This α-γ co-activation is a principle common to the excitation of lower motor neurons by upper motor neurons. Under certain circumstances, however, it appears that this process can be dissociated, such that the γ motor neuron-mediated sensitivity of the muscle spindle, and thus the sensitivity of the stretch reflex, can be adjusted apart from extrafusal muscle contraction. Although the reticulospinal tract participates in α-γ co-activation of lower motor neurons, it appears to be strongly associated with the ability to regulate γ motor neuron activity independently. It is likely that this ability of the reticulospinal tract to modulate independently the sensitivity of the stretch reflex underlies its significant role in adjusting antigravity muscle tone.
'Γhe vestibulospinal tract originates from cell bodies in the vestibular nuclear complex, which lies primarily in the medulla, just ventral to the fourth ventricle. This complex consists of several subnuclei that receive their principal synaptic input from eighth cranial nerve fibers carrying sensory input from the vestibular apparatus of the inner ear (see Chapter 11). The vestibular apparatus provides sensory information about the position of the head with respect to gravity and about acceleration of the head through space, thus indicating body position and disturbances of balance. The vestibular nuclear complex also receives significant input from the cerebellum, but not from forebrain levels of the motor system hierarchy.
As in the reticulospinal tract, axons of the vestibulospinal tract synapse within medial regions of the spinal cord gray matter that primarily control the axial and proximal extensor musculature (see Figure 10-3). Also like the reticulospinal tract, vestibulospinal tract axons collectively project to virtually all rostrocaudal levels of the spinal cord.
A disturbance of balance detected by the vestibular apparatus results in excitation of the antigravity musculature in an attempt to counteract the disturbance. Although the vestibulospinal tract principally functions to produce compensatory adjustments to postural disturbances, it seems to make some contribution to antigravity muscle tone as well.Some aspects of the functions of these two descending brainstem motor pathways can be better understood by considering the clinical state called decerebrate rigidity. This condition occasionally results from severe forebrain disease. It also results from surgical transection of the brain at the rostral midbrain level, as discovered by the British neurophysiologist Charles Sherrington. As noted earlier, the portion of the reticulospinal tract originating in the pons, which excites lower motor neurons to antigravity muscles, has a high degree of spontaneous activity. Excitation of the portion of the reticulospinal tract originating in the medulla inhibits the lower motor neurons to antigravity muscles. When the forebrain is disconnected from the brainstem, descending projections from cerebral cortex cannot excite these medullary reticular neurons projecting to the spinal cord, and thus a significant source of inhibition to the lower motor neurons of antigravity muscles is removed. The excitation of the lower motor neurons produced by the spontaneous activity of the pontine reticulospinal neurons has now lost a significant source of opposition, and therefore much greater muscle tone exists in the antigravity muscles. The animal now assumes a hobbyhorselike posture, often so rigid that the animal stands in a fixed position. Subsequently cutting a portion of the vestibulospinal tract reduces some of this rigidity, so the tract apparently plays some normal role in regulating antigravity muscle tone, in addition to its principal role in responding to postural destabilization with respect to gravity.
As noted, the reticulospinal and vestibulospinal tracts make important contributions to the control of the axial and proximal musculature to keep the body upright.
However, the to-and-fro rhythmicity of walking and running is organized by circuits of spinal interneurons that control the lower motor neurons in a repetitive, oscillating manner. Although these spinal neural networks are capable of producing this simple oscillatory behavior without control by more rostral parts of the motor system hierarchy, the reticulospinal tract plays an important role in initiating this locomotor behavior and in controlling its speed.The tectospinal tract is a medial brainstem motor pathway that is principally involved in reflex orientation of the head toward environmental stimuli. The cells of origin of the tectospinal tract are located in the superior colliculus of the midbrain (often called “rostral colliculus” in quadrupeds; see Figure 10-3). As in the other two medial brainstem motor pathways to the spinal cord, the tectospinal tract axons synapse within medial regions of the spinal cord gray matter that primarily control the axial and proximal musculature. However, these axons only project as far as the upper cervical regions of the cord. This is consistent because the tectospinal tract principally controls the musculature that moves the head. The superior colliculus processes visual, auditory, and somatosensory information about the relative position of stimuli in the environment with respect to the organism. The superior colliculus can also control rapid reflex movements (saccades) of the eyes to the stimulus. The tectospinal tract is involved in producing a movement of the head toward the stimulus that corresponds with the rapid eye movement so that the animals gaze is fixated directly on the stimulus.