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George E. Carvell, PhD, PT   University of Pittsburgh
 
SPINAL CORD: VENTRAL HORN Medial Motor Nuclei
The Ventral Horn contains Medial and Lateral Motor Nuclei and surrounding Interneurons that provide sensory-motor integration and the generation and regulation of signals that activate skeletal muscle in trunk and limbs. Each motor nucleus contains a group of local interneurons and motoneurons that collectively are sometimes called Segmental Motor Centers (SMCs). SMCs have dedicated their neuronal lives to helping you move in a smooth, controlled fashion. SMCs are influenced by a balance of Peripheral, Spinal and Descending Pathway Inputs and they influence the Brainstem and Cerebellum. SMCs send information about on-going motor output to the Cerebellum by way of the Ventral Spinocerebellar Tract and to Brainstem Centers via Ascending Spinoreticular Tracts.
    The medial motor nucleus is located at all levels of the spinal ventral gray matter. It contains alpha and gamma motoneurons that innervate, respectively, the extrafusal and intrafusal muscle of the ipsilateral axial skeleton (neck, back, thoracic cage, & abdominal muscles. Extrafusal muscle (EFM) is the skeletal muscle that is found in the motor units that move the joints of the spine and the ribs. Intrafusal muscle (IFM) is the specialized
muscle found within the muscle spindle capsule. Contraction of the IFM alters the spindle sensitivity to change proprioceptive feedback. For most of our actions, both EFM & IFM are co-activated. SMC Interneurons that modulate the discharge of alpha and gamma motoneurons are located in the medial motor nucleus and in the gray matter between the medial and lateral motor nuclei in the ventral horn. The medial motor nucleus has major connections with the long propriospinal neurons and with commissural interneurons (located close to the midline in the gray matter) that provide distributed, bilateral connectivity over many segments.