Descending Motor Pathways (continued)
Vestibulospinal tracts. The two vestibulospinal tracts originate in 2 of the 4 vestibular nuclei (Figure 2.11). The lateral vestibulospinal tract originates in the lateral vestibular nucleus. It courses through the brainstem and through the anterior funiculus of the spinal cord on the ipsilateral side, before exiting ipsilaterally at all levels of the spinal cord. The medial vestibulospinal tract originates in the medial vestibular nucleus, splits immediately and courses bilaterally through the brainstem via the medial longitudinal fasciculus (MLF) and through the anterior funiculus of the spinal cord, before exiting at or above the T6 vertebra.
Function. The vestibulospinal tracts mediate postural adjustments and head movements. They also help the body to maintain balance. Small movements of the body are detected by the vestibular sensory neurons, and motor commands to counteract these movements are sent through the vestibulospinal tracts to appropriate muscle groups throughout the body. The lateral vestibulospinal tract excites antigravity muscles in order to exert control over postural changes necessary to compensate for tilts and movements of the body. The medial vestibulospinal tract innervates neck muscles in order to stabilize head position as one moves around the world. It is also important for the coordination of head and eye movements.
Figure 2.11 |
Reticulospinal tracts. The two reticulospinal tracts originate in the brainstem reticular formation, a large, diffusely organized collection of neurons in the pons and medulla (Figure 2.12).
The pontine reticulospinal tract originates in the pontine reticular formation, courses ipsilaterally through the medial longitudinal fasciculus and through the anterior funiculus of the spinal cord, and exits ipsilaterally at all spinal levels. The medullary reticulospinal tract originates in the medullary reticular formation, courses mainly ipsilaterally (although some fibers cross the midline) through the anterior funiculus of the spinal cord, and exits at all spinal levels.
Function. The reticulospinal tracts are a major alternative to the corticospinal tract, by which cortical neurons can control motor function by their inputs onto reticular neurons. These tracts regulate the sensitivity of flexor responses to ensure that only noxious stimuli elicit the responses. Damage to the reticulospinal tract can thus cause harmless stimuli, such as gentle touches, to elicit a flexor reflex. The reticular formation also contains circuitry for many complex actions, such as orienting, stretching, and maintaining a complex posture. Commands that initiate locomotor circuits in the spinal cord are also thought to be transmitted through the medullary reticulospinal tract. Thus, the reticulospinal tracts are involved in many aspects of motor control, including the integration of sensory input to guide motor output.
Figure 2.12 |
Tectospinal tract. The tectospinal tract (Figure 2.13) originates in the deep layers of the superior colliculus and crosses the midline immediately. It then courses through the pons and medulla, just anterior to the medial longitudinal fasciculus. It courses through the anterior funiculus of the spinal cord, where the majority of the fibers terminate in the upper cervical levels.
Function. Little is known about the function of the tectospinal tract, but because of the nature of the visual response properties of neurons in the superior colliculus (the optic tectum), it is presumably involved in the reflexive turning of the head to orient to visual stimuli.
Figure 2.13 |
