Control of Movement One

Question 1

What are the three causes of movement under normal physiological conditions?

Answer 1

• Voluntary
• Reflexes (somatic not autonomic, i.e stretch reflex)
• Rhythmic motor pattern i.e breathing, chewing, locomotion (walking)

Question 2

Where ins the CNS do motor neurons extend from?

Answer 2

Brainstem (cranial nerves -9/12)
Spinal Cord (spinal nerves 30)

Question 3

What major CNS landmarks are involved in motor skills

?

Answer 3

Motor regions of cerebral cortex
Thalamus
Basal ganglia
Cerebellum
brainstem
Spinal cord

Question 4

What is another name for motor neurons?

Answer 4

Lower Motor Neurons (LMNs)

Question 5

Where are the nuclie located for the LMNs?

Answer 5

Two locations

• Brain stem (III, IV, V, VI, VII, IX, X, XI and XII cranial nerves)
• Spinal cord (Ventral Horn)

Question 6

What are the types of motor neurons?

Answer 6

Alpha motor neurons

Gaba Motor neurons

Question 7

Whats the function of Alpha motor neurons ?

Answer 7

• Innervate Extrafusal muscle fibers

- Directly responsible for the generation of force by muscles

Question 8

What is the function of Gaba motor neurons ?

Answer 8

• Innervate intrafusal muscle fibres

- Responsible for controlling the excitability of stretch receptors in muscle spindles

Question 9

What is the function of forebrain input in motor control?

Answer 9

Voluntary movements and muscle tone

Question 10

What is the role of the spinal cord and brainstem in motor control?

Answer 10

Reflex movements and rhythmic motor patterns

Question 11

What is crucial for the regulation of motor control?

Answer 11

Sensory feedback i.e propioreceptors

Question 12

What are Alpha motor neurons considered to be?

Answer 12

The final common pathway

Question 13

What is the final common pathway?

Answer 13

Alpha motor neurons

Question 14

Why are A-LMNs considered to be the final common pathway?

Answer 14

The final decision to move is made by alpha motor neurons. Excitatory and inhibitory synpatic inputs all converge here.

Question 15

What are the sources of synaptic inputs for alpha motor neurons?

Answer 15

Four:

1) Descending tracts i.e corticospinal and reticulo spinal
2) Spinal interneurons 1a, 1b
3) Propriospinal neurons (regulate function between limbs)
4) Afferent fibres (muscle receptors) i.e 1a afferents from the muscle spindles

Question 16

What is the motor unit?

Answer 16

The functional and anatomical elements of the motor system

Question 17

What are the components of an individual muscle unit?

Answer 17

1) Cell body of a-MN
2) Axon with all its branches
3) All neuromuscular junctions (end plates) formed by the motor neuron
4) All muscle fibers (extrafusal) innervated by this motor neuron

Question 18

How many neuromuscular junctions does a mature muscle fibre have?

Answer 18

Only one!

Question 19

How many neuromuscular junctions does a immature muscle fibre have?

Answer 19

Can have multiple but these become pruned with growth

Question 20

How many muscle fibers can a single A-MN innervate?

Answer 20

5-2000 muscle fibers (innervation ratio)

Question 21

How are A-MNs classified?

Answer 21

based on their anatomical (size), Physiological and Biochemical properties.

Question 22

What are two major classes of A-MNs?

Answer 22

S-type (type 1) = Slow Twitch

FF-type (type 2B) = Fast Twitch, fatigable

Question 23

What are some properties of the s-type AMNs?

Answer 23

Number of units in a muscle =  Many
Size of AMN cell body = Small
Fatigue = Little or none
Metabolism = Aerobic
Mitochondria = high density
Twitch time = Long (>50ms)
Power = Weak

Colour = Red as capillary and myoglobin rich

Question 24

Are FF and S type fibres recruited simultaneously?

Answer 24

No. S type always first before ff type

Question 25

Whats the size principle?

Answer 25

The fact that the S type is always recruited before the FF type regardless of task

Question 26

What are FF types useful for then?

Answer 26

They are useful for large powerful contractions (high levels of muscle force)

Question 27

What are some physiological consequences of the size principle?

Answer 27

• Some s type units fire almost always (except during REM)
• S-type units are best for carrying sustained light loads (non-fatigable)
• weak contractions can be graded with greater precision than strong contractions
• It is necessary to exercise with heavy loads to prevent f type atrophy

Question 28

When are S-type fibres not recruited?

Answer 28

During REM sleep

Question 29

What controls the force of muscle contraction?

Answer 29

Frequency coding - High Hz firing = Increased force

Recruitment - Increased MN = increased recruitment

Question 30

Whats the twitch and contraction theory?

Answer 30

An AP from a A-MN causes a twitch (single contraction of all muscle fibres it innervates)

• Multiple AP’s (frequency) leads to sustained contraction through summation of twitches (no time for Ca to exit)

Question 31

What is neuromuscular matchmaking?

Answer 31

properties of muscle fibres are determined by Alpha MNs, as demonstrated by the CROSS-INNERVATION EXPERIMENT conducted by J.Eccles