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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.


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

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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