LECTURE OUTLINE
Somatic Motor Pathways
Motor Pathways
CNS issues motor commands in response to information provided by
sensory systems, sent by the somatic nervous system (SNS) and the
autonomic nervous system (ANS)
Conscious and subconscious motor commands control skeletal muscles by
traveling over 3 integrated motor pathways
The corticospinal pathway – voluntary control of motor activity
Corticobulbar tracts
Corticospinal tracts
The medial and lateral pathways – modify or direct skeletal muscle
contractions by stimulating, facilitating, or inhibiting lower motor neurons
Motor Pathways
•Contain a sequence of TWO neurons from the cerebral cortex or brain stem
to the muscles
•Upper motor neuron : has cell body in the cerebral cortex or brain stem,
axon decussates before terminating on the lower motor neuron
•Lower motor neuron: has cell body in the ventral horn of the spinal cord,
axon runs in the ipsilateral ventral root of the spinal nerve and supply the
muscle.
COMPARISON BETWEEN UMN AND LMN
Descending Spinal Tracts
Originate from the cerebral cortex & brain stem
Concerned with:
Control of movements
Muscle tone
Spinal reflexes & equilibrium
Modulation of sensory transmission to higher centers
Spinal autonomic functions
Direct (Pyramidal) System
Regulates fast and fine (skilled) movements
Originate in the pyramidal neurons in the precentral gyri,
Impulses are sent through the corticospinal tracts and synapse in the
anterior horn
Stimulation of anterior horn neurons activates skeletal muscles
Part of the direct pathway, called corticobulbar tracts, innervates cranial
nerve nuclei
Indirect (Extrapyramidal) System
Complex and multisynaptic pathways
The system includes:
•Rubrospinal tracts: control flexor muscles
•Vestibulospinal tracts: maintain balance and posture
•Tectospinal tracts: mediate head neck, and eye movement
•Reticulospinal tracts concerned with motor function
Descending Spinal Tracts
Corticospinal Tracts
Concerned with voluntary, discrete skilled movements, especially those of
distal parts of the limbs (fractionated movements)
Innervate the contralateral side of the spinal cord
Provide rapid direct method for controlling skeletal muscle
Rubrospinal Tract
Controls the tone of limb flexor muscles, being excitatory to motor neurons
of these muscles
Origin: Red nucleus
Axons course ventro-medially, cross in ventral tegmental decussation,
descend in spinal cord ventral to the lateral corticospinal tract
Cortico-rubro-spinal pathway (Extrapyramidal)
Tectospinal Tract
Mediates reflex movements of the head and neck in response to visual
stimuli
Origin: Superior colliculus
Axons course ventro-medially around the periaqueductal gray matter, cross
in dorsal tegmental decussation, descend in spinal cord near the ventral
median fissure, terminate mainly in cervical segments
Cortico-tecto-spinal pathway (Extrapyramidal)
Vestibulospinal Tracts
Lateral Vestibulospinal Tracts
Origin: lateral vestibular (Deiter’s) nucleus
Axons descend ipsilaterally in the ventral funiculus
Terminate on ventral horn cells throughout the length of spinal cord
Has excitatory influences upon extensor motor neurons, control extensor
muscle tone in the antigravity maintenance of posture
Vestibulospinal Tracts
Medial vestibulospinal tract
Origin: medial vestibular nucleus
Axons descend bilaterally in the ventral funiculus, with the medial
longitudinal fasciculus
Most of the fibers end in the cervical region, some reaching upper thoracic
segments
Involved in movements of the head required for maintaining equilibrium
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Reticulospinal Tracts
Influence voluntary movement, reflex activity and muscle tone by
controlling the activity of both alpha and gamma motor neurons
Mediate pressor and depressor effect on the circulatory system
Are involved in control of breathing
Origin: pontine & medullary reticular formation
Medial (pontine) reticulospinal tract descends ipsilaterally
Lateral (medullary) reticulospinal tract descends bilaterally
Both tracts located in the ventral funiculus
Descending Autonomic Fibers
The higher centers associated with the control of autonomic activity are
situated mainly in the hypothalmaus
The fibers run in the reticulospinal tracts
Terminate on the autonomic neurons in the lateral horn of thoracic & upper
lumbar (sympathetic) and sacral segments (parasympathetic) levels of the
spinal cord
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