18.8: Indirect Motor Pathways
The indirect motor or extrapyramidal pathways originate in the brainstem, the lower portion of the brain that connects it to the spinal cord. They consist of several distinct tracts, each with specialized functions. The four main tracts of the indirect motor pathways are the vestibulospinal tract, the reticulospinal tract, the tectospinal tract, and the rubrospinal tract.
The vestibulospinal tract originates in the vestibular nuclei of the brainstem. The vestibular system detects changes in head position and orientation, which is critical for maintaining balance and posture. The vestibulospinal tract receives input from the vestibular system about the position of the head and from the proprioceptors and eyes about the position of muscles, joints, and body in space. By regulating muscle tone, the vestibulospinal tract helps to maintain an upright posture, allowing the body to remain stable in a gravitational field.
The reticulospinal tract also originates in the brainstem, specifically in the reticular formation. This tract has two distinct components: the lateral reticulospinal and medial reticulospinal tracts. The lateral reticulospinal tract is responsible for inhibiting the muscles of the trunk and proximal limbs, while the medial reticulospinal tract excites these same muscles. By balancing the excitatory and inhibitory signals, the reticulospinal tract helps to maintain muscle tone and balance during ongoing movements, such as lifting weights.
The tectospinal tract originates in the superior colliculus, a region of the brainstem involved in processing visual and auditory information. This tract controls sudden head and neck movements in response to visual and auditory stimuli. Additionally, it plays a role in controlling rapid eye movements, which are critical for clear vision during rapid head movements.
Finally, the rubrospinal tract originates in the red nucleus. This tract controls precise and voluntary movements of the distal part of the upper limbs, including the hands and fingers. This tract is essential for tasks that require fine motor control, such as playing a musical instrument or typing on a keyboard.
In summary, the indirect motor pathways are critical for the control of movement throughout the body. By regulating muscle tone, maintaining balance and posture, and allowing us to respond to visual and auditory stimuli, these pathways play a critical role in allowing a person to move efficiently and precisely.
