Block 5 - Spinal Chord and Motor Control

Question 1

What is an alpha motor neuron and where is it found?

Answer 1

Neuronal cell.
Supplies all skeletal muscles, control every movement in the body
Found in the ventral horn of the spinal chord (or brainstem for eye muscles)

Question 2

What is a motor unit?

Answer 2

Everything that connects to one motor neuron.
Motor neuron plus the muscle fibres it is attached to.

Question 3

What are the three components of the spinal chord?

Answer 3

Roots: dorsal=sensory, ventral=motor, origin of peripheral nerves
White matter: ascending and descending neuronal pathways
Grey matter: neuronal cell bodies (including alpha motor neurons)

Question 4

How are muscle fibres attached to the axon of a motor neuron?

Answer 4

NMJs - neuromuscular junctions

Question 5

On average how many muscle fibres do small and large MUs (motor units) attach to?

Answer 5

Smaller MUs: One alpha motor neuron supplies 100 muscle fibres

Larger MUs: One alpha motor neuron supplies 2000 muscle fibres

Question 6

List the 3 types of MU within a muscle

Answer 6

S (slow): slow contracting, resistant to fatigue, small force and aerobic

FR ( fast fatigue resistance): faster contracting, fatigue resistant, low force, mainly aerobic

FF (fast fatigable): fast contracting, fast fatigue, hugh force, anaerobic

Question 7

How does the brain increase force in the muscles? Give 3 ways.

Answer 7

1) Rate coding - increasing the firing rate of motor neurons
2) Recruitment - increasing the amount of motor units firing
3) Use a recruitment order - increasing the size/ type of motor units firing

Question 8

How do recruitment orders work?

Answer 8

Use small MUs first - low muscle tension
ADD increasingly larger motor units - high muscle tension

Question 9

What is msucle tetanus?

Answer 9

Sustained muscle contraction caused when action potentials fire above a certain rate - smoothes out the contractions

Question 10

Which areas of the brain are involved in controlling movement?

Answer 10

a-motor neurons are controlled by the primary motor cortex.

Other areas involved in controlling movement:
- cerebellum - error correction and learning movements
- basal ganglia - movement selection

Question 11

What gives us awareness of where our body is in space?

Answer 11

1) Vision - eyes in head
2) Vestibular system - inner ears in head
3) Proprioception - muscles, joints and skin
4) Efference Copy - no feedback

Question 12

What are the two streams of vision?

Answer 12

Dorsal - where
Ventral - what

Question 13

How does the vestibular system work?

Answer 13

Rotations and translations of the head (and hence the body) are sensed using sense organs in the inner ears.

Rotations - semicirular canals
Translations - otoliths

Question 14

How does proprioception work?

Answer 14

Muscle spindles - muscle length and stretch sensors
Golhi tendon organs - muscle tension (or force) sensors.

Other - eg for control of posture
Cutaneous receptors (eg pressure receptors in the feet)
Joint receptors (eg in the ankle)

Question 15

What is an efference copy?

Answer 15

The brain makes an internal copy of the outflowing (efferent) motor signal

The reason we can’t tickle ourselves

Question 16

Muscle fibre function in proprioception

Answer 16

Sensory fibres wrap around muscle fibres (called intrafusal fibres)
Arranged in parallel with muscle fibres

Two types:
II afferents - measure muscle length (detect join angle)
Ia afferents - detect change in lenth (detect join movement in velocity and acceleration)

Question 17

Golgi tendon organ’s role in proprioception

Answer 17

Sensory fibres embedded in tendon of msucle
Arranged in series with muscle

Detect muscle tension and forces applied to joints

Question 18

How can you measure sensory imputs by looking at postural control?

Answer 18

SOT (sensory organisation test)

Remove vision by closing eyes
Remove proprioceptors by causing platfrom to sway with body

Allows contributions of vision, vestibular and proprioceptive feedback to be separated out

CDP (computerised dynamic posturography)
Measures body sway using a force platform

Question 19

Adantages and disadvantages of efference copies

Answer 19

Advantages:
Good for controlling fast movements (don’t need to wait for sesnory feedback to arrive)
Good for movements which are not usually affected by external forces (eg eye movement)

Disadvantages:
Doesn’t allow for unexpected changes (usually external applied forces)
No absolute reference (needs updating using sensory feedback)
Less accurate for slower movements

Question 20

What causes the stretch reflex?

Answer 20

Muscle spindle is stretched causing afferent neuron to fire

Monosynaptic connection in the spinal chord leads to contaction of agonist muscle

Inhibition of antagonist muscle through inhibitory interneuron

Question 21

Why is the stretch reflex important?

Answer 21

• Transfer motor control to the spinal chord
• Improved speed response
• Control of posture