Descending Tracts

Question 1

Describe the pathway of a reflex arc

Answer 1

Receptor/transducer –> afferent fibre –> intergration centre –> efferent fibre –> effector
Sensory neurone –> dorsal root –> interneurone –> ventral root –> motor neurone –> muscle

Question 2

Define a lower motor neurone

Answer 2

A somatic efferent that brings about displacement of limbs (movememt) and sets muscle tone
With a cell body in either lamina IX of spinal cord or cranial nerve motor nucleus
Its axon supplies skeletal muscles of the body directly

Question 3

Describe the role of a and Y motor neurones in the spinal cord

Answer 3

Motor unit = a motoneurone and muscle fibres supplied
a - extrafusal fibres of muscle fibres, skeletal muscle contraction
Y - intrafusal fibres of muscle spindles, keeps muscle spindles taut (tone = avoid descending inhibition)

Question 4

List lower motoneurone signs

Answer 4

Flaccid muscle weakness
Hypotonia/atonia
Hyporeflexia/areflexia
Denervation muscle atrophy
Fasciculations
Muscle wastage

Question 5

List some causes of lower motoneurone signs

Answer 5

Trauma to peripheral nerves (toxins, viruses)
Disuse atrophy
Guillain Barré syndrome
Cauda equina syndrome

Question 6

List upper motoneurone signs

Answer 6

Spastic paralysis
Hypertonic
Cog wheel rigidity 
Hyperreflexia
Clonus
Positive babinski sign 
Choreoforms

Question 7

List some causes of upper motoneurone signs

Answer 7

Stroke
MS
Spinal cord injury
Acquired brain injury

Question 8

Define an upper motoneurone

Answer 8

Originate either in motor region of cerebral cortex or in brainstem

Question 9

List the main descending tracts of the motor system

Answer 9

Cortical descending tracts (pre central gyrus):
Corticobulbar, corticospinal
Non-cortical descending tracts (midbrain, brainstem, pons, medulla):
Rubrospinal, reticulospinal, tectospinal, vestibulospinal

Question 10

Describe the corticobulbar tract including function, origin, decussation and termination

Answer 10

Function - voluntary movement
Origin - motor cortex, pre central gyrus
Decussation - brainstem
Termination - contralateral motor cranial nerve nuclei

Question 11

Describe the lateral corticospinal tract including function, origin, decussation and termination

Answer 11

Function - voluntary movement
Origin - motor cortex, pre central gyrus
Decussation - medulla
Termination - contralateral spinal cord

Question 12

Describe the ventral corticospinal tract including function, origin, decussation and termination

Answer 12

Function - voluntary movement
Origin - motor cortex, pre central gyrus
Decussation - spinal cord
Termination - contralateral spinal cord

Question 13

Describe the rubrospinal tract including function, origin, decussation and termination

Answer 13

Function - flexor muscle tone
Origin - red nucleus
Decussation - brain stem
Decussation - neck and upper thoracic spinal cord

Question 14

Describe the reticulospinal tract including function, origin, decussation and termination

Answer 14

Function - automatic movement
Origin - reticular formation
Decussation - partially in brainstem
Termination - spinal cord

Question 15

Describe the tectospinal tract including function, origin, decussation and termination

Answer 15

Function - turns head towards light or sound
Origin - tectum of brain
Decussation - brainstem
Termination - neck, upper thoracic spinal cord

Question 16

Describe the vestibulospinal tract including function, origin, decussation and termination

Answer 16

Function - balance and posture
Origin - vestibular nucleus
Decussation - none
Termination - spinal cord

Question 17

Explain the terms pyramidal and extrapyramidal tracts

Answer 17

Pyramidal - e.g. corticospinal and corticobulbar, pass through the medullary pyramids, fibres that travel from the cerebral cortex to brainstem or spinal cord
Extra pyramidal - e.g. Rubrospinal, reticulospinal, tectospinal, vestibulospinal, part of the motor system causing involuntary movements

Question 18

Understand the causes and presentation of decorticate and decerebrate posturing

Answer 18

Decorticate:
Bent arms, clenched fists, legs straight
Disinhibition of/lesion above red nucleus, damage to cerebral hemispheres/internal capsule/thalamus/midbrain
Decerebrate:
Rigid posture - straight arms and legs, head and neck arched backwards
Brainstem damage below red nucleus, damage to midbrain/cerebellum, excitation of a and Y motoneurones

Question 19

Describe differences between clinical signs subsequent to lesions of the pyramidal system from those of the extrapyramidal system

Answer 19

Pyramidal - upper motor neurone signs
*damage to corticobulbar tract on both sides –> pseudobulbar palsy
Extrapyramidal - decorticate/decrebrate posturing

Question 20

Explain the difference between a bulbar palsy and pseudobulbar palsy

Answer 20

Bulbar palsy - impairment of function of cranial nerves 9-12 due to a lower motor neurone lesion in the medulla or outside the brainstem
Pseudobulbar palsy - impairment of function of cranial nerves 9-12 due to an upper motor neurone lesion of both corticobulbar tracts

Question 21

Define the term ‘spinal reflex’

Answer 21

An involuntary, unlearned, repeatable, automatic reaction to a specific stimulus that does not require the brain to be intact