Spinal Reflexes, Upper Motor Neurons And Control Of Movement

Question 1

Where do spinal interneurons receive input from?

Answer 1

Primary sensory axons e.g. Ia and Ib fibres, descending axons from the brain, collaterals of LMNs and other interneurons

Question 2

What do inhibitory interneurons mediate?

Answer 2

The inverse myotatic response and reciprocal inhibition between extensor and flexor muscles

Question 3

Describe the myotatic reflex

Answer 3

The myotatic reflex causes the homonymous extensor muscle e.g. quadriceps to contract, but for the leg to extend the antagonist flexor muscle (hamstring) must simultaneously relax

Question 4

How are Ia afferent fibres involved in reciprocal inhibition?

Answer 4

The Ia afferent from the muscle spindle extensor makes an excitatory monosynaptic contact with the alpha-MN innervating the homonymous muscle. Via a polysynaptic pathway involving an inhibitory interneuron, the Ia fibre also inhibits the alpha-MN supplying the flexor muscle

Question 5

How is reciprocal inhibition important in the initiation of movement by the motor cortex?

Answer 5

At a joint, voluntary contraction of an extensor will stretch an antagonist flexor, initiating the myotatic reflex. However, descending pathways that activate the alpha-MN controlling the extensor muscles also, via inhibitory interneurons, inhibit the alpha-MNs supplying the antagonist muscles allowing, in this case, unopposed extension (the reverse also applies)

Question 6

What do excitatory interneurons mediate?

Answer 6

The flexor reflex and the crossed extensor reflex

Question 7

Describe the flexor reflex

Answer 7

Noxious stimulus causes limb to flex by:

• Contraction of flexor muscles via excitatory interneurons
• Relaxation of extensor muscles via excitatory and inhibitory interneurons

Question 8

Describe the crossed extensor reflex

Answer 8

Noxious stimulus causes limb to extend by:

• contraction of extensor muscles via excitatory interneurons
• relaxation of flexor muscles via excitatory and inhibitory interneurons

Question 9

What structures are involved in high level strategy functions?

Answer 9

Neocortex also association areas

Basal ganglia

Question 10

What structures are involved in middle level tactic functions?

Answer 10

Motor cortex

Cerebellum

Question 11

What structures are involved in low level execution functions?

Answer 11

Brain stem

Spinal cord

Question 12

Where do descending spinal tracts originate from?

Answer 12

The cerebral cortex and brain stem

Question 13

What are lateral pathways (descending tract) involved in?

Answer 13

Important for voluntary control of distal musculature, particularly discrete, skilled movements e.g. hands and fingers in a fractionated manner

Question 14

Where are lateral pathways under control from?

Answer 14

Cerebral cortex

Question 15

Where are ventromedial pathways under control from?

Answer 15

The brainstem

Question 16

What are ventromedial pathways involved in?

Answer 16

Important for the control of posture and locomotion

Question 17

Name the major lateral pathway

Answer 17

Corticospinal tract/pyramidal tract

Question 18

Where are the cell bodies of the corticospinal tract located?

Answer 18

The motor cortex and the somatosensory areas of the parietal cortex

Question 19

Axons of the corticospinal tract course to the base of the medulla forming a tract. Name this tract

Answer 19

Medullary pyramid

Question 20

Where do most fibres decussate in the corticospinal tract?

Answer 20

The pyramidal decussation to form the lateral corticospinal tract (75-90%). The remainder stay ipsilateral to form the ventral corticospinal tract and decussate more caudally

Question 21

What does the corticospinal tract control?

Answer 21

Distal muscles, particularly flexors

Question 23

Where do axons of the rubrospinal tract decussate?

Answer 23

The ventral segmental decussation and descend the spinal cord ventrolateral to the lateral corticospinal tract, terminating in the ventral horn

Question 24

What does the rubrospinal tract control?

Answer 24

Exerts control over limb flexor muscle, exciting LMNs of those muscles

Question 25

What are lesions of the lateral columns associated with?

Answer 25

Loss of fractionated movements i.e. shoulders, elbow, wrist and fingers cannot be moved independently. Slowing and impairment of accuracy of voluntary movements. Little effect on normal posture e.g. standing and sitting

Question 26

What does a lesion of the corticospinal tract alone result in?

Answer 26

Deficits as profound as by a lesion of the lateral columns but, over time, major recovery can occur (although weakness of distal flexors and inability to move fingers independently persists). Such recovery is reversed if the rubrospinal tract is also lesioned

Question 27

In higher mammals the direct corticospinal tract has largely replaced the functions of the indirect rubrospinal tract so why is the rubrospinal tract still so important?

Answer 27

The rubrospinal tract remains capable of compensating significantly for damage to the corticospinal tract

Question 28

Where are the cell bodies of the vestibulospinal tracts found?

Answer 28

The vestibular nuclei that receive input via CN VIII, from the vestibular labyrinths. Cerebellar input is also important

Question 29

What do axons from the lateral vestibulospinal tract do?

Answer 29

Help with balanced posture by facilitating extensor MNs of antigravity muscles e.g. of the leg

Question 30

What do axons from the medial vestibulospinal tract do?

Answer 30

These activate cervical spinal circuits that control neck and back muscles guiding head movements

Question 31

Where are cell bodes from the tectospinal tract found?

Answer 31

The superior colliculus

Question 32

What is the tectospinal tract involved in (i.e. function)?

Answer 32

Influencing muscles of the neck, upper trunk and shoulders in response to visual stimulus

Question 33

Where do the reticulospinal tracts arise from?

Answer 33

Reticular formation, a diffuse mesh of neurons that are located along the length and at the core of the brainstem

Question 34

What does the pontine reticulospinal tract do?

Answer 34

Enhances antigravity reflexes of the spinal cord. Helps to maintain a standing posture by facilitating contraction of the extensors of the lower limbs

Question 35

What does the medullary (lateral) reticulospinal tract do?

Answer 35

Opposes the action of the medial/pontine tract. Releases antigravity muscles from reflex control

Question 36

Where are the cell bodies of the rubrospinal tract located?

Answer 36

Red nucleus, which receives input from the motor cortex and the cerebellum