Spinal Cord and Descending Tracts

Question 1

overview of motor systems

Answer 1

• the spinal cord a-motor neuron is the final common pathway of motor control
  
  • receives input from local circuit neurons within the spinal cord (interneurons) and directly from the brainstem and motor cortex
• the cerebellum and basal ganglia modulate motor output
  
  • basal nuclei assists initiation and termination of movement
  • cerebellum assists posture and movement coordination by detecting error signals
• thalamus is the gateway for cerebellar and basal nuclei input to the cerebral cortex

Question 2

the spinal cord

Answer 2

• the ‘neural highway’
  
  • information is sent to and from the brain and body via the spinal cord
• there are 31 pairs of spinal nerves
  
  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 saccral
  • 1 coccygeal
• each spinal nerve consists of a ventral (motor) and dorsal (sensory) root

Question 3

sensory and motor tracts

Answer 3

• corticospinal and rubrospinal tracts form the lateral descending system
• reticulospinal and vestibulospinal tracts form the medial descending system

Question 4

somatotopic arrangement

Answer 4

• motor neurons exhibit ‘somatotopy’
  
  • somatotopy is the point for reference of a part of the body in the central nervous system
  • proximal and distal muscles are represented in proximal and distal spinal cord

Question 5

corticospinal tract

Answer 5

• largest descending tract
  
  • also known as the pyramidal tract
  • response for voluntary control of movement
  • contains direct and indirect connections to motor neurons
• indirect connections are known as corticobulbar tracts
• direct pathways contain lateral and anterior/ventral aspects
  
  • 90% of corticospinal pathways are lateral (crossed)
  • pyramidal decussation is why the brain controls the contralateral side of the body

Question 6

lateral corticospinal tract

Answer 6

• projects from motor cortex (precentral gyrus) to spinal cord
• upper motor neuron descends through the cerebral peduncles in the midbrain
• decussates at the base of medullary pyramids
• descends through lateral corticospinal tract
• a-motor neuron (or lower motor neuron) innervates skeletal muscles in distal parts of limbs such as
  
  • muscles of the hamd (e.g. flexor carpi ulnaris)
  • muscles of the lower leg (e.g. tibialis anterior)

Question 7

anterior/ventral corticospinal tract

Answer 7

• projects from motor cortex (precentral gyrus) to spinal cord
• upper motor neuron descends through the cerebral peduncles in the midbrain
• does not decussate in medulla
• a-motor neuron (or lower motor neuron) innervate skeletal muscles in trunk and proximal part of limbs
  
  • muscles of the upper arm (e.g. brachioradialis)
  • muscles of the upper leg (e.g. rectus femoris)

Question 8

differences between species and dexterity

Answer 8

• the extent of direct connections to the spinal cord differs between species
• in ‘higher’ animals (e.g. chimps, humans), there are more direct connections to spinal motor neurons
  
  • corticospinal tract is evolutionary
  • correlates with manual dexterity
• reliance upon direct connections means humans are less able to adapt to spinal cord injury versus other species
  
  • cats can learn to walk after spinal resection

Question 9

stimulating the corticospinal tract

Answer 9

• the motor cortex can be stimulated using magnetic or electrical stimulation
• transcranial magnetic stimulation has many applications
  
  • assess integrity of corticospinal tract
  • first demonstrated by Anthony Barker and colleagues in Sheffield

Question 10

the babinski sign

Answer 10

• the babinski sign is diagnostic of a lesion to the corticospinal tract
  
  • characterised by abnormal plantar responses
    
    • normal plantar response consists of a brisk flexion of all toes when the sole of the foot is stroked by a blunt instrument - descending input from cortex inhibits the spinal reflex
    • abnormal plantar response consists of extension of great toe while the other toes fan out - descending input from cortex does not inhibit the spinal reflex

Question 11

corticobulbar tract

Answer 11

• corticobulbar tract forms connections from the motor cortex to brainstem
  
  • the corticobulbar tract is a relay to the brainstem
  • terminates mainly on interneurons
  • importance in volitional control of breathing
• upper motor neuron projects from cerebral cortex to brainstem (pons and medulla) via cerebral peduncles and innervates interneurons or cranial nerves
• cranial nerves are lower motor neurons and include:
  
  • CN VII from pons (facial, facial express ion)
  • CN XII from medulla (hypoglossal, tongue)
• interneurons innervate the reticular formation
  
  • project to various sites involved in somatic, autonomic, sensory/neuromodulatory functions

Question 12

rubrospinal tract

Answer 12

• the rubrospinal tract originates in the red nucleus
• the red nucleus is a spherical collection of cell bodies in the midbrain (rostral to pons)
  
  • highly vascularised
  • large input from cerebellum and primary motor cortex
• primarily involved in activation of flexor motor neurons
• axons originating from the red nucleus cross the midline of the ventral midbrain (ventral tegmental decussation)
• fibres terminate on interneurons that project to the ventral horn
• primary function is voluntary control of muscle
  
  • stimulation of red nucleus produces contralateral flexion and inhibition of extension
  • also involved in compensation

Question 13

rubrospinal tract: compensation

Answer 13

• the rubrospinal tract can compensate for damage to the corticospinal tract
• Lawrence and Kuypers (1968) surgically severed corticospinal tract in monkeys
  
  • movements were significantly altered immediately after lesion
  • improvements were rapid and reached near identical function after a few weeks
  • only remaining deficit was the inability to use individual fingers
  • corticospinal and rubrospinal tracts form the lateral descending system and are primarily involved in voluntary control of human movement

Question 14

reticulospinal tract

Answer 14

• originates in the brainstem reticular formation
• reticular formation is a complex network of brainstem nuclei and poorly anatomically defined
• reticular formation coordinates many body functions necessary for survival
• reticulospinal tracts include:
  
  • lateral tracts (medullary)
  • medial tracts (pontine)
• projects to somatic (e.g. respiratory muscle) and autonomic (e.g. heart and kidneys) neurons

Question 15

reticulospinal tract: medial vs lateral

Answer 15

• medial (or pontine) reticulospinal tract arises from the pons
  
  • projects ipsilaterally to entire spinal cord
  • facilitate extensor spinal reflexes
• lateral (or medullary) reticulospinal tract arises from the medulla
  
  • projects bilaterally to entire spinal cord
  • suppresses extensor activity
• unlike other descending tracts, there is no somatotopic arrangement

Question 16

vestibulospinal tract

Answer 16

• originates in the vestibular nuclei in the pons-medulla junction
  
  • vestibular nuclei receives input from CN VIII (vestibulocochlear nerve)
  • CN VIII arises from the inner ear (otolith organs)
• lateral and medial tracts
• vestibulospinal information contracts and relaxes muscles to maintain balance, posture and muscle tone when the head moves
  
  • descending tracts innervate neck muscles
  • ascending tracts innervate oculomotor nuclei to control eye movements

Question 17

vestibulospinal tract: medial versus lateral

Answer 17

• medial vestibulospinal tract
  
  • arises from ipsilateral and contralateral medial vestibular nuceli at the pons-medulla junction
  • descends in the ventral funiculus of the cervical spinal cord and terminates in the ipsilateral ventral horn
• lateral vestibulospinal tract
  
  • arises from neurons of the lateral vestibular nucleus at the pons-medulla junction
  • descends the entire length of the spinal cord
  • receives inhibitory inputs from the cerebellum
• both tracts produce excitation of extensors and inhibition of flexors
• main functions are to control the muscles that maintain upright posture and balance