Motor Systems And Descending Pathways

Question 0

What is a lower motor neurone

Answer 0

Neurone that connects ventral horn grey matter of spinal cord to muscles

Question 1

2 broad categories of muscle in body

Answer 1

Smooth (autonomic inn)
Striated
- skeletal (somatic nervous control- voluntary)
- cardiac (ANS)

Question 2

Categories of LMN

Answer 2

Gamma

Alpha

Question 3

Where do alpha neurones get input from?

Answer 3

Sensory inputs from muscle fibres: give specialised info about proprioception
UMN: initiation and control of voluntary movement
Interneurones: part of circuitry that generates motor programs

Question 4

Proprioception from 1a fibres in muscle spindles

Answer 4

• connected to intrafusal layer, specialised for detection of changes in muscle length.
• 1a axons acts as proprioceptors. These enter spinal cord via dorsal roots and form EXCITATORY synapses with interneurones and alpha motor neurones of ventral root. Very rapid conduction

Question 5

Myotactic reflex

Answer 5

Also called stretch reflex. When muscle is stretched it produces a reflex action that causes it to contract

Stretching increases rate of discharge of 1a axons to synapse with alpha motor neurone
This then sends efferent to muscle to contract
(Gamma loop- activated for resettin)

Question 6

Reciprocal inhibition

Answer 6

Spinal interneurones can inhibit alpha neurones in antagonist muscles
Also used to overcome myotactic reflex in antagonist muscles
Release glycine, glutamate or GABA depending on function

Question 7

Gamma motor neurones

Answer 7

Muscle spindle contains modified skeletal muscle within its capsule - intrafusal fibres
These intrafusal fibres get innervation from gamma neurones

Question 8

Alpha neurones

Answer 8

Innervate extrafusal muscle fibres

Release acetyl choline at their synpases

Question 9

Signal coming from UMN to contract muscle…

Answer 9

• alpha motors will contract extrafusal fibres
• this shortens the muscle, causing the intrafusal layer to be slack—> there would be no 1a AP
• however gamma motors act at same time on intrafusal to shorten it in order to keep the 1a alive

Question 10

Proprioception from 1b fibres

Answer 10

Info comes from golgi tendon organs.
Convey info about muscle tension and force of contraction
1b form synapses w INHIBITORY interneurones which they synapse with alpha motor neurones, inhibiting them

Important for fine motor acts which require steady but not too powerful grip

Question 11

Corticospinal tract

Answer 11

Axons from motor cortex pass through internal capsule and course through cerebral peduncle of midbrain, pons and form tract at base pf medulla
Tract forms bulge called medullary pyramid
At junction between spinal cord 70-90% decussate

Question 12

Corticospinal tracts after decussation

Answer 12

Those that have decussated become LATERAL corticospinal

Those that havent become ANTERIOR corticospinal tract

Question 13

Where do fibres terminate

Answer 13

Dorsolateral region of ventral horns

Question 14

Lesion of primary motor cortex would cause

Answer 14

Loss of motor function on contralateral side of body

Question 15

Damage to lateral spinal cord would cause

Answer 15

Motor deficits on ipsilateral side

Question 16

Lesion in LMN

Answer 16

Weakness 
Atrophy
Decreases tendon reflexes
Decreased tone
Fasciculation

Question 17

Lesion in UMN

Answer 17

Weakness (no paralysis, no/little atrophy)
Increased tendon reflex
Increase tone
Extensor babinski reflex
Loss of descending inhibition