Voluntary innervation of skeletal muscle

Upper and Lower Motoneurons

Two motoneurons, an upper motoneuron and a lower motoneuron, together form the basic neural circuit involved in the voluntary contraction of skeletal muscle everywhere in the body. ! The lower motoneurons are found in the ventral horn of the spinal cord and in cranial nerve nuclei in the brain stem. Axons of lower motoneurons of spinal nerves exit in a ventral root, then join the spinal nerve to course in one of its branches to reach and synapse directly at a neuromuscular junction in skeletal muscle. Axons of lower motoneurons in the brain stem exit in a cranial nerve.

To initiate a voluntary contraction of skeletal muscle, a lower motoneuron must be innervated by an upper motoneuron (Figure IV-4-3). The cell bodies of upper motoneurons are found in the brain stem and cerebral cortex, and their axons descend into the spinal cord in a tract to reach and synapse on lower motoneurons, or on interneurons, which then synapse on lower motoneurons. At a minimum, therefore, to initiate a voluntary contraction of skeletal muscle, two motoneurons, ; an upper and a lower, must be involved. The upper motoneuron innervates the lower motoneuron, and the lower motoneuron innervates the skeletal muscle.

The cell bodies of upper motoneurons are found in the red nucleus, reticular formation, and lateral vestibular nuclei of the brain stem, but the most important location of upper motoneurons is in the cerebral cortex. Axons of these cortical neurons course in the corticospinal tract.

Clinical Correlate

Lesions to la afferent fibers or lower motoneurons produce areflexia.

Important muscle stretch reflexes to test are:

triceps (Cj-CJ

Midbrain

Upper pons

Upper medulla

Lower medulla

Spinal cord

Midbrain

Upper pons

Upper medulla

Lower medulla

Spinal cord

Figure tV-4-4. Corticospinal Tract -

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