Other Motor Pathways

Question 1

describe the function of the rubrospinal tract

Answer 1

• cells in the red nucleus receive excitatory connections from the ipsilateral motor cortex and from contralateral cerebellar nuclei
• most rubrospinal fibers end in cervical segments at synapses on lower motor neurons innervating arm muscles
• each red nucleus therefore primariliy facilitates voluntary flexion of the muscles of the contralateral upper limb, while minimally influencing muscles of the lower limb

Question 2

describe the route of the rubrospinal tract

Answer 2

• start: the red nucleus receives cortical projections through the corticorubral tract and hence facilitates voluntary movement
• route: axons of red nuclear efferents decussate in the midbrain
  
  • rubrospinal axons descend through the lateral brainstem and spinal cord, often forming excitatory synapses on alpha and gamma motor neurons
• end: alpha and gamma motor neurons in the ant. horn of the cervical spinal cord

Question 3

Answer 3

Question 4

describe the function of the medullary (lateral) reticulospinal tract

Answer 4

• descend to all levels of the spinal cord and facilitates flexion/contraction of the limbs

Question 5

describe the pathway of the medullary (lateral) reticulospinal tract

Answer 5

• start: medullary reticular nuclei receive bilateral cortico-reticular input and somatosensory projections through the spinoreticular tract (collaterals of ALS)
• route: many axons from the medullary reticular nucleus descend through the ipsilateral medulla and spinal cord
  
  • some axons cross in the medulla and descend contralaterally in the cord
  • both ipsilateral and contralateral tracts end mainly on interneurons that excite alpha and gamma motor neurons
• end: interneurons that excit alpha and gamma motor neurons

Question 6

Answer 6

Question 7

describe the function of the pontine (medial) reticulospinal tract

Answer 7

• the excitatory actions of the pontine (medial) reticulospinal tract predominantly affect the gamma motor neurons (not alpha) of axial and limb extensor muscles

Question 8

describe the pathway of the pontine (medial) reticulospinal tract

Answer 8

• start: the pontine reticular nuclei receive bilateral cortical input rhough corticoreticular fibers
  
  • they also receive somatosensory projections through the spinoreticular tract (collaterals of ALS)
• route: axons from the pontine reticular nuclei descend ipsilaterally through the medulla and ventral spinal cord
• end: ipsilateral ventral horn of the spinal cord
  
  • synapses involve interneurons that excite mainly gamma motor neurons

Question 9

describe the function of the spinoreticular tract

Answer 9

• this tract relays sensory info. (pain, temp, pressure) from one side of the body to the opposite to reticular nuclei
• its influence on the pontine reticular nuclei facilitates contraction of the extensor muscles of the ipsilateral limb and trunk

Question 10

describe the pathway of the spinoreticular tract (sensory pathway)

Answer 10

• start: pain and temp. receptors
• route: this sensory pathway arises from collateral branches of the anterolateral system (ALS)
• end: reticular nuclei in pons and medulla

Question 11

Answer 11

Question 12

describe the function of the lateral vestibulospinal tract

Answer 12

• the lateral vestibulospinal tract predominantly excites the alpha motor neurons of the axial and limb extensor (anti-gravity muscles)
  
  • anti-gravity muscles = soleus muscles, the extensors of the leg, the gluteus maximus, the quadriceps femoris and the muscles of the back
• this tract regulates balance-related muscular responses

Question 13

describe the pathway for the lateral vestibulospinal tract

Answer 13

• start: the lateral vestibular nucleus receives input from vestibular organs (inner ear) and the cerebellum
• route: axons of lateral vestibular nuclei cells descend in the ipsilateral medulla and spinal cord
• end: the axons of the lateral vestibular nucleus synapse in the ant. horn of the spinal cord on excitatory interneurons that, in turn, excite alpha motor neurons (NOT gamma)

Question 14

Answer 14

Question 15

describe the pathway of the medial vestibulospinal tract

Answer 15

• start: the medial vestibular nucleus receives input from vestibular organs in the inner ear and cerebellum
• route: axons of medial vestibular nuclear efferents descend mainly ipsilaterally into the spinal cord and run as the caudal extension of the MLF
• end: the tract ends with synapses on alpha motor neurons at the cervical and upper thoracic levels

Question 16

describe the function of the medial vestibulospinal tract

Answer 16

• the medial vestibulospinal tract synapses on motor neurons in the cervical and upper thoracic levels
• causes muscular contraction of neck in coordination of eye movement

Question 17

Answer 17

Question 18

describe the gamma loop

Answer 18

• gamma motor neurons (intrafusal muscle) and alpha motor neurons (extrafusal) are simultaneously active leading to coordinated contraction of the muscle fibers
• if extrafusal fibers of a muscle were allowed to contract in isolation under the influence of alpha motor neurons, muscle spindles would lose tension and slacken
  
  • in this hypothetical condition, muscle spindles would be unable to measure subsequent muscular stretch

Question 19

describe decorticate posturing

Answer 19

• a lesion rostral to the red nucleus impairs corticobulbar and corticospinal fibers and produces decorticate posturing
  
  • corticospinal: interrupted, mainly flexion impaired
  • corticobulbar: interrupted, paralysis of motor CNs
  • rubrospinal tract: intact and liberated from cortical control, flexion of arms
  • medullary reticulospinal tract: intact and liberated from cortical control, flexion of extremities
  • pontine reticulospinal tract: intact and liberated from cortical control, extension of extremities
  • vestibulospinal tract: intact, extension of extremities (particularly the lateral vestibulospinal tract)

Question 20

explain what is seen

Answer 20

Question 21

describe decerebrate posturing

Answer 21

lesion below red nucleus (lower midbrain/pons)

• corticospinal interrupted: mainly flexion imparied
• corticobulbar interrupted: paralysis of motor CNs
• rubrospinal interrupted: no flexion of arms
• reticulospinal: extension of extremities
• vestibulospinal: extension of extremities