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George E. Carvell, PhD, PT   University of Pittsburgh
 
SPINAL CORD: VENTRAL HORN Lateral Motor Nuclei
The motor nuclei contain alpha and gamma motoneurons that innervate, respectively, the extrafusal and intrafusal muscle of the ipsilateral trunk or limb. Extrafusal muscle (EFM) is the skeletal muscle that is found in the motor units that move the joints. Intrafusal muscle (IFM) is the specialized muscle found within the muscle spindle capsule.
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.
Contraction of the IFM alters the spindle sensitivity and thus alters proprioceptive feedback. For most of our actions, both EFM & IFM are co-activated. SMC Interneurons that modulate the discharge of these alpha and gamma motoneurons are located in the lateral motor nucleus and in the gray matter between the medial and lateral motor nuclei in the ventral horn. The lateral motor nucleus has major interconnections with short propriospinal neurons and to a lesser extent with some long propriospinal neurons.
1 = Commissural Neurons
2 & 3 = Medial Motor Nucleus Motoneurons
4-7 = Lateral Motor Nucleus Motoneurons