Motor Pathways

Question 0

Lower Motor Neurons: Inputs

Answer 0

• come from three hierarchal sources:
  1. Reflex pathways
  
  • produce stereotyped movements in response to specific stimuli
    2. Pattern generators
  • interneuron groups produce movement combos (grasping)
  • simple, repetitive movements (e.g. walking, breathing).
    3. Descending Upper motor neuron pathways
  • allow higher brain regions to control motor activity
  • minute posture adjustments to goal-directed movements.

Question 1

Lower Motor Neurons: Location

Answer 1

-α-motor neurons: neurons w/ axons directly innervating muscle
-Found in ventral horn of spinal cord gray matter
*and in certain cranial nerve nuclei in the brainstem
-Axons exit spinal cord in the ventral roots
*travel w/ spinal nerves to skeletal muscle.
The axons of brainstem LMNs course with cranial nerves to innervate skeletal muscle of head & neck.

Question 2

Lower Motor Neurons: Convergence on single LMN

Answer 2

• Multiple excitatory & inhibitory inputs converge on a single LMN
• Firing rate of LMN determined by spatial & temporal integration of inputs.
• All motor commands are conveyed to the muscles by LMNs:
  
  • “final common pathway” for motor control

Question 3

Lower Motor Neurons: Somatotopic Organization

Answer 3

• somatotopically organized in the ventral horn of the spinal cord
• LMNs innervating the trunk & proximal muscles of extremities:
  
  • located in medial ventral horn.
• LMNs that innervate the distal muscles of the limbs
  
  • located in the lateral part of the ventral horn.

Question 4

Upper Motor Neurons: Location

Answer 4

• neurons in the cerebral cortex/brainstem
• axons form tracts that descend to the spinal cord
• influence the activity of LMNs

Question 5

Upper Motor Neurons: Function

Answer 5

• Descending pathways control the activity of LMNs
  
  • directly
  • Indirect via interneurons of reflexes & simple pattern generators
• Descending motor pathways:
  
  • medial and lateral descending systems

Question 6

lesion of lower motor neurons

Answer 6

• denervates skeletal muscle.
• produces flaccid paralysis of the affected muscle(s)
  
  • flaccid b/c denervated muscle can’t receive signals to contract
• Flaccidity is characterized by
  
  • reduced muscle tone (hypotonia).
  • reduced or absent myotatic reflex.
  • rapid atrophy of the affected muscle(s)

Question 7

lesion of upper motor neurons

Answer 7

• clinically means lesion of the neurons or axons of the LCST
• produces spastic paralysis of the affected muscles.
• The lesion does not denervate skeletal muscle
  
  • LMNs from damaged UMNs still connected to muscle(s).
• w/o control from UMNs, LMNs have enhanced reflex activity.
• Increased LMN activity produces spasticity in affected muscle(s).
• Spasticity is characterized by
  
  • increased resting muscle tone (hypertonia).
  • increased resistance to passive movement.
  • hyperactive myotatic reflexes (hyperreflexia).
  • return of primitive reflex normally inhibited by supraspinal input
    
    • Ex. Babinski sign
• slow muscle atrophy due to disuse
  
  • can be minimized with physical therapy

Question 8

Lateral Descending Motor System

Answer 8

• 2 crossed tracts: fibers cross before reaching the spinal cord
  
  • descend on the contralateral side
• In the spinal cord, the tracts course in the lateral funiculus
• Axons synapse directly or via an interneuron on LMNs in the lateral portion of the ventral horn
• innervate the distal muscles of the limbs
• Interneurons associated w/ the lateral descending system have short axons
• communicate w/ a small number of LMNs.
  
  • allows lateral descending system to control individual muscles.
• Function: controls fine, skilled movements of distal limb muscles

Question 9

Medial Descending Motor System

Answer 9

• Tracts course in the anterior funiculus of the spinal cord
• Axons synapse directly or via an interneuron on LMNs in the medial portion of the ventral horn
• innervate the axial (trunk & neck) and proximal limb muscles.
• Interneurons have long axons that span several cord segments
  
  • project to both sides
• control movements involving groups of muscles on both sides
  
  • regardless of tracts being crossed, uncrossed or bilateral
• Unilateral lesions do not produce obvious neurological deficits.
• Function: controls axial & proximal musculature
  
  • maintain posture and balance.

Question 10

Lateral Corticospinal Tract (LCST): Origin

Answer 10

• Neurons giving rise to the corticospinal tract are located in the cerebral cortex
  
  • primary motor cortex (>50% of corticospinal fibers arise here)
  • premotor and supplementary motor cortex
  • somatosensory cortex.

Question 11

Lateral Corticospinal Tract (LCST): Course

Answer 11

• fibers descend ipsilaterally thru the forebrain & brainstem.
• Fibers course in the:
  
  • corona radiata (white matter below the cerebral cortex)
  • posterior limb of the internal capsule
  • crus cerebri of the midbrain (middle 1/3)
  • basilar pons
  • medullary pyramids.
• Corss at the spinomedullary junction (90% of fibers)
  
  • pass dorsolaterally
  • Forms the lateral corticospinal tract.
• Fibers that don’t cross continue down ipsilaterally as anterior corticospinal tract
• LCST descends in the lateral funiculus of the spinal cord

Question 12

Lateral Corticospinal Tract (LCST): Termination

Answer 12

LMNs & interneurons in the lateral part of the ventral horn at all levels of the spinal cord.

Question 13

Lateral Corticospinal Tract (LCST): Function

Answer 13

Mediates fine movements of the distal limbs (skilled voluntary movements.)

Question 14

Anterior Corticospinal Tract (ACST)

Answer 14

• Same origin and initial course as the LCST
  
  • 1˚ motor cortex, premotor cortex and somatosensory cortex
• Course:
• LCST & ACST descend in ipsilateral:
  
  • corona radiata
  • internal capsule
  • crus cerebri
  • basilar pons
  • pyramids.
• tracts are indistinguishable
  
  • referred to collectively as the corticospinal tract
• At spinomedullay junction, 10% of the fibers don’t cross midline.
• The non-crossing fibers continue descending ipsilaterally in the anterior funiculus as the ACST

Question 15

Rubrospinal Tract (RST)

Answer 15

• part of the lateral descending system.
• originates in the red nucleus of the midbrain.
• Course:
  
  • Fibers cross at their site of origin in the midbrain.
  • descends in the lateral funiculus, close to the LCST.
  • It extends only to cervical spinal cord levels.
• Function: facilitates activity of LMNs of the upper limb flexors.
• lesions of RST are not associated w/ clinical motor deficits
  
  • B/c the RST’s functions are also carried out by the LCST.

Question 16

Vestibulospinal Tracts

Answer 16

• Two vestibulospinal tracts arise in the vestibular nuclei of the upper medulla and pons
• descend in the anterior funiculus of the spinal cord.
• Lateral Vestibulospinal Tract
  
  • part of the medial descending system
  • facilitates extensor muscles of the upper & lower limbs.
  • important for maintaining balance–
  • righting reflex: vestibular system detects that one is falling.
• Medial Vestibulospinal Tract
  
  • extends only to the cervical spinal cord.
  • coordinates head movts w/ eye movts when changing gaze

Question 17

Reticulospinal Tracts

Answer 17

• Two reticulospinal tracts originate in the reticular formation of pons & medulla
  
  • descend in the anterior funiculus.
• Pontine reticulospinal tract
  
  • facilitates axial & proximal muscles for posture & adjustments.
  • may contribute to locomotor control.
• Medullary Reticulospinal Tract
  
  • inhibitory influence on axial and proximal limb musculature.
  • may be involved in producing muscle atonia during sleep.

Question 18

Lesion to Corticospinal tract above the pontomedullary junction

Answer 18

• before the decussation of the LCST fibers
• UMN signs will be observed on the contralateral side of the body. Examples:
• Primary motor cortex
  
  • Medial hemisphere- weakness of contralateral lower limb
  • lateral aspect-weak contralateral upper limb, trunk & lower face
• Posterior limb of internal capsule, crus cerebri or basilar pons-
  
  • weakness of entire contralateral body & lower face
• Medullary pyramid- weakness of contralateral body
  
  • face is spared

Question 19

lesion of the LCST in the spinal cord

Answer 19

• UMN clinical signs on ipsilateral side of the body
  
  • muscles innervated by segments below the lesion
• Damage to LCST in cervical cord- affects ipsilateral upper limb, trunk and lower limb
• Damage to LCST in upper lumbar cord- affects ipsilateral lower limb

Question 20

Poliomyelitis

Answer 20

• caused by a neurotrophic virus
• paralytic polio (<1% of infections)
  
  • the virus damages lower motor neurons; rarely symmetrically.
• causes rapid-onset, usually asymmetric weakness, muscle atrophy, loss of muscle tone & myotatic reflexes
• sensory loss is rare.
• Disease can affect CN motor nuclei and/or phrenic nerve nuclei
  
  • weakness of facial musculature & breathing difficulty

Question 21

Amyotrophic Lateral Sclerosis (Lou Gehrig’s Disease)

Answer 21

• although rare, is the most common motor neuron disease
• occurs most frequently in males over age 50.
• Pathology:
  
  • Progressive, bilat degeneration of upper & lower motor neurons
• Typically present with weakness and both UMN & LMN signs
• Sensory and mental status exam are typically normal.
• LMN signs: muscle wasting (atrophy), fasciculations
• UMN signs: increased tone, hyperreflexia, Babinski sign
• Sensory and mental status exam are typically normal
• Eye muscles are typically spared as the disease progresses.

Question 22

Decerebrate rigidity:

Answer 22

• caused by transection of brainstem near pons/midbrain junction
• disrupts almost all of the descending UMN pathways
• spares the LVST since the lateral vestibular nucleus is located in the upper medulla/lower pons.
• characterized by exaggerated extensor posturing of limbs, due to the unopposed action of the LVST.