Skeletal Muscle Innervation


A. Lower Motoneurons

(motoneuron = motor neuron, neuron directly or indirectly controlling an effector organ)

1. Neurons innervating skeletal muscle fibers: alpha (large) motoneurons in the spinal cord and cranial somatic motor nerve nuclei (lower motoneurons)

a. spinal cord: alpha motoneuron pool in ventral (anterior) horn gray matter (sometimes termed “Rexed lamina IX”)

b. cranial nerves: somatic alpha motoneurons in brainstem motor nuclei (medulla, pons, midbrain)

1) CN III, IV, VI: eye movement
2) CN V: mastication
3) CN VII: muscles of facial expression
4) CN IX, X: muscles of the soft palate and larynx
5) CN XI: trapezius and sternocleidomastoid
6) CN XII: tongue muscles

Note: several of the above cranial nerves are mixed nerves and contain sensory and/or autonomic motor fibers

2. Characteristics of skeletal muscle innervation

a. neurons innervating a given muscle grouped together in the ventral horn of the spinal cord and in analogous cranial nuclei (motoneuron pools)

b. alpha motoneuron-muscle fiber synaptic transmission always excitatory and one-to-one (one neuron AP => one muscle fiber AP)

c. organized as motor units (single motoneuron and the several muscle fibers it innervates)

d. muscle force controlled by neuron AP frequency and neuron recruitment; recruitment follows the “size principal” -- low input activates small innervation ratio (muscle fibers/alpha motoneurons) motor units (low force); increasing input recruits progressively larger motor units (higher force)

e. motoneurons completely control and are essential for skeletal muscle contraction (“final common pathway”, all CNS activation of skeletal muscle must be transmitted through alpha motoneurons); following a lesion of its motoneurons, the muscle cannot contract (contrast with smooth and cardiac muscle)

Note: Gamma (small) motoneurons innervating the contractile poles of muscle spindle fibers are interspersed among alpha (large) motoneurons innervating the same muscle

3. Anatomical organization of motoneuron pools

a. within the ventral horn, motoneurons innervating muscles acting at the same joint are located in the same or adjacent spinal segments

b. flexor motoneurons are located lateral in the ventral horn; extensor motoneurons are located medial (note relation to descending motor pathways)

c. motoneurons innervating proximal (upper extremity and axial) muscles are located dorsal in the ventral horn; motoneurons innervating distal muscles are located ventral in the ventral horn

B. Influences on the alpha motoneuron pool

1. Reflexes: simple, stereotyped, involuntary movements (e.g. flexion reflex, myotatic or stretch reflex, inverse myotatic reflex)

2. Input from higher centers (upper motoneurons)

a. brain stem nuclei (tone, posture)

b. pattern generators: more complex movements involving several muscles, often (but not always) voluntarily initiated and maintained without conscious volition (e.g. chewing, swallowing, vomiting, walking)

c. motor cortex (primary motor cortex, M-1); initiation of normal voluntary activity

d. premotor cortex and supplementary motor cortex (movement planning and coordination)

e. basal ganglia (elaboration and modulation of movement)

f. cerebellum (adjust rate and direction of movement, balance, posture/tone)

Note: c and d above are part of the cerebral cortex; d, e, and f above act mainly through their influence on the primary motor cortex