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.
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