Neuroscience 6e Chapter 17 Summary
Two sets of upper motor neuron pathways make distinct contributions to the control of the local circuitry in the brainstem and spinal cord. One set originates from neurons in the frontal lobe and includes projections from the primary motor cortex and the nearby premotor areas. The premotor cortices are responsible for planning, initiating, and controlling complex sequences of voluntary movements, especially movements that are triggered by sensory cues or internal motivations, whereas the primary motor cortex is especially involved with the execution of skilled movements of the limb and facial musculature. The motor cortex influences movements directly by contacting lower motor neurons and local circuit neurons in the spinal cord and brainstem; and indirectly by innervating neurons in brainstem centers (mainly the reticular formation) that in turn project to lower motor neurons and circuits. The other major upper motor neuron pathways originate from brainstem centers—primarily the reticular formation and the vestibular nuclei—and are responsible for postural regulation. The reticular formation is especially important in feedforward control of posture (i.e., movements that occur in anticipation of changes in body stability). In contrast, the neurons in the vestibular nuclei that project to the spinal cord are especially important in feedback postural mechanisms (i.e., in producing movements that are generated in response to sensory signals that indicate an existing postural disturbance). Although the brainstem pathways can independently organize gross motor control, direct projections from the motor cortex to local circuit neurons in the brainstem and spinal cord are essential for the fine, fractionated movements of the face and the distal parts of the limbs that are especially important in activities of daily living and the expression of motor skill.