Level 3 Unit 2 Part 26: Motor control of the torso and neck

Motor control of the torso and neck also starts with a motor plan being developed in association motor cortices, which is then sent to the superior part of the primary motor cortex. The axons of these upper motor neurons descend in the corticospinal tract through the deep cerebrum and brainstem.

The corticospinal tract in the cerebrum and brainstem contains upper motor neuron axons for the limbs as well as the torso and neck. At the junction of the brainstem and the spinal cord, most of the corticospinal tract axons decussate (the pyramidal decussation) and form the lateral corticospinal tract in the spinal cord. Most of these decussating axons are involved in motor control of the limbs. A minority of corticospinal tract axons do not decussate at the pyramidal decussation, but instead descend in the ipsilateral anterior column of the spinal cord, where they are called the anterior corticospinal tract. Most of these axons that do not decussate are involved in motor control of the torso and neck.

Several other tracts containing upper motor neurons project to the spinal cord from several brainstem nuclei that receive input from motor cortex and other areas; most of these also appear to be involved in motor control of the torso and neck. Motor control of the limbs is mostly from the contralateral brain, but motor control of the torso and neck appears to be more bilateral.

A few muscles of the shoulders and neck are controlled by the accessory nerves instead of spinal nerves. These are the sternocleidomastoid muscles of the neck, and the trapezius muscles of the shoulders. The sternocleidomastoid muscle is examined by assessing the strength of head rotation, and the trapezius muscle is examined by assessing the strength of shoulder elevation (shrugging).

Stance (standing) and gait (walking) are motor activities involving muscles of both the torso and the limbs, and which also have both voluntary and involuntary aspects. Multiple areas of the spinal cord, brainstem, and cerebrum play a role in aspects of stance and gait.

Next:

Level 3 Unit 2 Part 27: Motor control of the face

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