Spinal Reflexes, UMNs and control of movement

Question 1

Spinal interneurones

• where do they receive input from?
• types of input & their function?

Answer 1

-primary sensory axons
descending axons form the brain
collateral LMNs
other interneurones

-can be excitatory or inhibitory, they integrate incoming info to generate an output
the interneurones themselves can be inhibitory or excitatory

Question 2

Limb movement and the role of inhibitory interneurones

• what do they mediate?
• give an example of this function?

Answer 2

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

-reciprocal inhibition
Myotatic reflex causes the homonymous extensor muscle (quadriceps) to contract, but for the leg to extend the antagonist flexor muscle (hamstring) must simultaneously relax
Note the Ia afferent from the muscle spindle extensor makes an excitatory monosynaptic contact with the α-MN innervating the homonymous muscle. Via a polysynaptic pathway involving an inhibitory interneurone, the Ia fibre also inhibits the α-MN supplying the flexor muscle

Question 3

Initiation of movement by the motor cortex

-how does reciprocal inhibition allow unopposed extension?

Answer 3

-at a joint, voluntary contraction of an extensor will stretch an antagonist flexor, initiating the myotatic reflex. However, descending pathways that activate the aMN controlling the extensor muscles also, via inhibitory interneurones inhibit the Mans supplying the antagonist muscles, allowing, in this case, unopposed extension

Question 4

Role of excitatory interneurones in limb movement

• excitatory interneurones mediate what? (2)
• What is the flexor reflex, explain?
• What is the crossed extensor reflex, explain?
• what does this mean when applied?

Answer 4

-the flexor reflex and the crossed extensor reflex

-noxious stimulus causes limb to flex by:
contraction of flexor muscles via excitatory interneurones
relaxation of extensor muscles via excitatory and inhibitory interneurones

-Noxious stimulus causes limb to extend by:
contraction of extensor muscles via excitatory interneurones relaxation of flexor muscles via excitatory and inhibitory interneurones

-enhances posture support during withdrawal from a painful stimulus

Question 5

A Simple Spinal Central Pattern Generator (CPG) Can Command Rhythmic, Alternating, Activity That Moves a Limb
-what is the essential feature for this?

Answer 5

-excitatory interneurone to display oscillatory (pacemaker) activity
in principle this might command walking at a spinal level

Question 6

what are the 3 levels of motor control hierarchy, what is their function and structures involved?

Answer 6

goes from top (forebrain) to bottom (spinal cord)
High:
strategy
Neocortical association areas + basal ganglia

Middle:
Tactics
motor cortex + cerebellum

Low:
execution
Brain stem + spinal cord

Question 7

Descending spinal tracts and brain control of movement

• where do descending spinal tracts originate from?
• what are the 2 important pathways?
• for each, where are they controlled from, function?

Answer 7

-cerebral cortex
brain stem

-lateral and ventromedial pathways

-Lateral:
cerebral cortex
voluntary control of distal musculature, particularly discrete, skilled, movement

Ventromedial:
brainstem
important for control of posture and locomotion

Question 8

Lateral pathways

• major lateral pathway?
• where are cell bodies located?
• axons form what?
• fibres decussate where to from what? variation?
• where do axons terminate?
• which hemisphere controls the muscles on which side?
• minor lateral pathway?
• where are cell bodies located?
• where do axone decussate
• function?

Answer 8

• corticospinal tract
• the motor cortex and somatosensory areas of the parietal cortex
• they course to the base of the medulla forming the medullary pyramid

-at pyramidal decussation to form lateral corticospinal tract
the rest stay ipsilateral to form ventral corticoapinal tract and decussate more caudally

• the dorsolateral region of the ventral horn and intermediate grey, the location of LMNs and interneurones controlling distal muscles
• left hemisphere controls the right musculature & vice versa
• rubrospinal tract
• in red nucleus (receives input form the motor cortex and cerebellum)
• ventral segmental decussation and descend the spinal cord ventrolateral to the lateral corticospinal tract, terminating in the ventral horn
• Exerts control over limb flexor muscles, exciting LMNS of those muscles

Question 9

Functions of the lateral pathways

• what are lesions of the lateral cord associated with?
• what are lesions of the corticospinal tract associated with?
• relationship of the 2 tracts?

Answer 9

-loss of fractionated movements i.e. shoulders, wrist, and fingers cannot move independently
slowing and impairment of accuracy of voluntary movements
little effect on normal posture

• initially deficits as profound as a lesion on the lateral columns but major recovery can occur
• corticospinal tract dominant but rubrospinal tract can compensate for damage which is why recovery occurs

Question 10

Ventromedial pathways

-Give the 4 component tracts?

Answer 10

vestibulospinal tracts
tectospinal tract
pontine and medullary Reticulospinal tracts

Question 11

Vestibulospinal tracts

• where are cell bodies?
• Pathway of axons? (lateral and medial) function of each?

Answer 11

-vestibular nuclei
receives input from vestibular labyrinths Via CN VIII. cerebellar input also important

-from lateral vestibular nucleus they descend ipsilaterally as the lateral vestibulospinal tract as far as lumbar spinal cord
helps to hold upright and balanced posture by facilitating extensor MNs of antigravity muscles

from medial vestibular nucleus descend as the medial vestibulospinal tract as far as the cervical spinal cord
activate cervical spinal circuits that control neck and back muscles riding head movement

Question 12

Tectospinal tract

• where are cell bodies located?
• where do Axons decussate, whats their pathway?
• function?

Answer 12

-superior colliculus, receives direct input from the retina and also from the visual cortex and afferents conveying somatosensory and auditory info

-in the dorsal segmental decussation
then descend close to the midline as the tectospinal tract to the cervical spinal cord influencing muscles of the neck, upper trunk and shoulders

-e.g. orientation of head and eyes

Question 13

Pontine and Medullary reticulospinal tracts 
-arise from where?
-for each: 
side of descent?
action?
function?
-what are they controlled by?

Answer 13

-from the reticular formation, a diffuse mesh of neurones that are located along the length and at the core of the brainstem

-Pontine
ipsilaterally
enhances antigravity reflexes of the spinal cord
helps to maintain a standing posture by facilitating contraction of extensors in lower limbs

Medullary
descends bilaterally
opposes action of medial tract
releases antigravity muscles from reflex control

-descending signals from the cortex