Motor System 1: Reflex Control of Movement

Question 1

What are the 2 neuron activation pathways?

Answer 1

• 1st motor neuron: upper motor neuron (UMN) in the precentral gyrus of the brain (1)
• 2nd motor neuron: lower motor neuron (LMN) - a-motor neuron in the spinal cord (2)

Question 2

Outline the concept of neuromuscular coupling and force regulation.

Answer 2

a. Motor neuron forms a neuromuscular junction with a muscle fibre.
b. A single muscle fibre contraction (‘twitch’) produces the same force.
c. Higher firing rate of the a-motor neuron causes muscle fibre twitches to fuse (intracellular Ca2+ accumulation) –> tetany –> larger force production
d. Muscle force is regulated by the activity of a-motor

Question 3

Distinguish between small motor units and large motor units.

Answer 3

Small motor unit: one a-MN controls only a few muscle fibres –> fine motor skills (e.g fingers, eyes)

Large motor unit: one a-MN controls many muscle fibres –> e.g legs

Question 4

Define reflex and provide examples.

Answer 4

Reflex: a relatively predictable, involuntary, and stereotyped response to an eliciting stimulus. Reflexes are defined by their reflex arc.

Examples:
* myotatic (stretch) reflex
* (Golgi) tendon reflex
* Pupillary light reflex
* Withdrawal reflex
* Crossed-extensor reflex

Question 4

Describe key features of spinal reflexes.

Answer 4

• Integration centre: spinal cord
• Reflexes can be mapped to spinal segments
  a. Monosynaptic reflexes occur within one segment
  b. Polysynaptic reflexes can span more than one segment.

Role of the brain: descending tracts modulate spinal reflexes –> sets the excitability of neurons in the integration centre (ranges from very actively react to stimulus - shut down completely)

Question 5

What is the reflex arc (pathway) of the myotatic reflex?

Answer 5

a. Stimulus: muscle stretch –> increase in length
b. Sensor: muscle spindle, ‘intrafusal’ fibres
c. Afferent: type 1a sensory fibres (very fast –> heavily myelinated)
d. Integration: monosynaptic excitatory connection to the a-MN (ventral horn of the spinal cord)
e. Efferent: Aa-fibre/a-MN of extensor muscles increased firing
g. Response: muscle contraction –> decreases in length

Question 6

What is the muscle spindle?

Answer 6

a. Muscle spindle: fusiform (spindle-shaped) structure composed of specialised, small skeletal muscle fibres called intrafusal muscle fibres. They are found within the muscle mass itself.
i. Transmit afferent output: 1a sensory fibres (very fast nerve fibres) connect to the middle of the spindle.
ii. Receive efferent input: y (gamma) fibres connect to the contractile ends of the spindles

b. Regular muscle fibres outside of the spindle are called extrafusal muscle fibres –> the usual contractile ‘working’ fibres (Aa/a-MN input)

Question 7

What is hyperreflexia?

Answer 7

• The monosynaptic reflex is usually inhibited by higher motor centres (brain, brainstem, cerebellum)
• Lesion of the pyramidal tract –> increased gain of the myotatic reflex –> hyperreflexia, clonus
• e.g multiple sclerosis, stroke, ALS

Question 8

Outline the reflex arc for the Golgi tendon reflex.

Answer 8

a. Stimulus: muscle tension –> increase in muscle tension (force/area)
b. Sensor: Golgi tendon organ (GTO) - braided collagen + nerve fibres
c. Afferent: type 1b sensory fibres (not as fast as 1a)
d. Integration: 1b activates inhibitory interneuron, which inhibits a-MN
e. Efferent Aa fibre (reduced firing)
f. Effector: muscle (extrafusal fibres)
g. Response: muscle relaxation

Question 9

Outline the tension control for the golgi tendon reflex.

Answer 9

a. Simple negative feedback mechanism (homeostasis)
b. Regulates maximum muscle tension (measured at the golgi tendon)
c. GTO and muscle fibres are connected in series (muscle spindle was in parallel)
d. Function: protective - excess force/load could rupture the muscle
i. ‘Lengthening reaction’: sudden stop of contraction when tension becomes too large
ii. Fibres that exert maximum effort can recover

Question 10

What is the reflex arc for the flexor/withdrawal reflex?

Answer 10

a. Stimulus: noxious stimuli (e.g pain, heat)
b. Sensors: skin
c. Afferent: slow fibres (type III, IV)
d. Integration centre: spinal cord
e. Efferent: Aa fibre
f. Effector: flexor activation (assisted by antagonist relaxation)
g. Response: retraction (e.g arm retraction)

Question 11

Describe key features of the flexor and crossed extensor reflex.

Answer 11

1. Polysynaptic
2. As triggered leg is flexed –> elevation/retraction, the other leg extends to maintain stability/push the body away from trigger
3. Response 0.2-0.5 seconds after trigger –> suggest that there are many synapses
4. Requires multisegmented polysynaptic activation of both sides of the spinal cord