Upper Motor Neurons: Balance and Posture

Upper Motor Neurons Maintaining Balance and Posture

• Vestibular nuclei are upper motor neurons in the brainstem.
• Projections from vestibular nuclei control axial muscles and proximal limb muscles via medial and lateral vestibulospinal tracts.
• Additional upper motor neurons in vestibular nuclei project to cranial nerve nuclei for eye movement control (III, IV, VI). This maintains fixation while the head moves.
• Direct projections from vestibular nuclei to spinal cord enable quick compensatory responses to postural instability sensed by the inner ear.

Reticular Formation as Upper Motor Neurons

• The reticular formation is a complex network in the brainstem, similar to the intermediate gray matter in the spinal cord.
• It handles various functions, including cardiovascular/respiratory control, sensory motor reflexes, eye movements, sleep-wake cycles, and coordination of movements.
• Descending motor control pathways from the reticular formation to the spinal cord are similar to those of the vestibular nuclei.
• Reticular formation motor centers are influenced by cortical or brainstem centers and initiate adjustments to maintain posture during movement. They predict potential instability and generate corrective movements.

Origin of Pyramidal Tract

• Upper motor neurons in the cerebral cortex (frontal lobe areas) plan and initiate complex voluntary movements.
• These cortical areas receive input from basal ganglia, cerebellum, and sensory regions within the parietal lobe.
• The primary motor cortex (Brodmann's area 4, precentral gyrus) contains upper motor neurons (Betz cells of layer V).

Course of Pyramidal Tract

• Pyramidal tract axons descend to brainstem/spinal motor centers within the corticobulbar and corticospinal tracts.
• They travel through the internal capsule, cerebral peduncles, pons, and form medullary pyramids in the medulla's ventral surface.
• Some axons within the corticobulbar tract innervate cranial nerve nuclei, reticular formation, and red nucleus in the brainstem.
• Most corticospinal tract axons cross over (decussate) in the medulla forming the lateral corticospinal tract.
• Some axons remain uncrossed forming the ventral corticospinal tract and terminate ipsilaterally or contralaterally.
• Ventral corticospinal tract axons primarily innervate muscles controlling axial and proximal limbs.

The Premotor Cortex

• This complex network of interconnected frontal lobe areas lies in front of the primary motor cortex.
• Upper motor neurons within the premotor cortex influence movements through connections with the primary motor cortex, and directly via corticobulbar and corticospinal pathways.

Lateral Premotor Cortex

• Involved in selecting movements based on external cues (visual or verbal commands).
• Damage to this area impacts learning movements in response to external stimuli.

Medial Premotor Cortex

• Involved in selecting movements based on internal cues.
• Damage leads to reduced spontaneous movements, despite ability to execute movements based on external cues.

Summary

• Two sets of upper motor neuron pathways control local circuitry in the brainstem and spinal cord.
• Vestibular nuclei/reticular formation contribute to postural regulation (feedforward/feedback mechanisms).
• The other major pathway originates in the frontal lobe (primarily primary motor cortex and premotor area) for movement execution and selection of movements.
• The motor cortex influences movements directly via corticospinal/corticobulbar tracts, and indirectly via brainstem centers (reticular formation/red nucleus), via reticulospinal/vestibulospinal/rubrospinal tracts..

This quiz explores the role of upper motor neurons in maintaining balance and posture, focusing on vestibular nuclei and the reticular formation. It covers how these structures interact to control muscles and reflexes necessary for stabilization. Test your understanding of the brainstem's influence on body posture and movement coordination.