SPINAL CORD AND CRANIAL NERVE NUCLEI

SPINAL CORD AND CRANIAL NERVE NUCLEI

Motor Neurons

Activities of skeletal muscles are executed by the impulses discharged from alpha motor neurons situated in ventral (anterior) gray horn of spinal cord and nuclei of many of the cranial nerves present in brainstem.

Alpha motor neurons in the spinal cord, which innervate the extrafusal fibers of skeletal muscles are responsible for the contraction of muscles in upper limbs, trunk and lower part of the body. The gamma motor neurons, which innervate the intrafusal fibers o muscle, are responsible for the maintenance of muscle tone.

Motor neurons of the cranial nerve nuclei situated in brainstem send their signals to the muscles of neck and upper part of trunk via cranial nerves.

Final Common Pathway

Activities of a particular skeletal muscle depend upon the excitation of alpha motor neuron (also known as lower motor neuron) in the spinal cord or cranial nerve nuclei. This is the only pathway, through which the signals from other parts of nervous system reach the muscles. Hence, the alpha motor neurons are called ‘final common pathway’ of motor system.

Functions of Motor Neurons

Motor neurons responsible for the contraction of skeletal muscles are arranged topographically in the

ventral gray horn of spinal cord. Neurons situated in the medial part of ventral gray horn innervate the

muscles near midline of the body called axial muscles and muscles in the proximal portions of limbs called proximal muscles. These two types of muscles are involved in the adjustment of posture and gross

movement. Motor neurons in lateral part of ventral gray horn innervate the muscles in distal portions of the limbs called distal muscles. Distal muscles are involved in the well coordinated skilled voluntary movements. Motor neurons in cranial nerve nuclei of brainstem innervate the extrinsic muscles of eyeball and muscles of face, tongue, neck and upper part of trunk. These muscles are concerned with ocular movements and movements of facial expressions, chewing, swallowing and movements of head and shoulder. Motor neurons are situated in the nuclei of cranial nerves III, IV, V, VI, VII, IX, X, XI and XII.

CEREBRAL CORTEX

Cortical areas concerned with origin of motor signals are the primary motor area, premotor area and

supplementary motor area in frontal lobe and sensory area in the parietal lobe. Cortical areas send their output signals to spinal cord via corticospinal tracts and to brainstem via corticobulbar tracts. About 30% of the fibers forming corticospinal and corticobulbar tracts take their origin from primary and supplementary motor cortex, 30% from premotor area and remaining 40% from parietal lobe particularly from somatosensory area.

CEREBELLUM

Cerebellum plays an important role in planning, programming and integrating the skilled voluntary

movements. It is also concerned with the maintenance of muscle tone, posture and equilibrium. Cerebellum vestibular apparatus, cerebral cortex, brainstem and basal ganglia. It interprets these impulses and sends signals to motor cortex, reticular formation and nuclei of brainstem.

BASAL GANGLIA

Basal ganglia play an important role in the coordination of skilled movements, regulation of automatic

associated movements and control of muscle tone by sending output signals to motor cortex, reticular

formation and spinal cord.