The Motor System 23.02.23

Question 1

What is goal-directed and habit motor control?

Answer 1

Goal direct: conscious, explicit, controlled
Habit: unconscious, implicit, automatic

Question 2

What is voluntary and involuntary motor control?

Answer 2

Voluntary: running, walking, talking
Involuntary: eye movement, facial expression, jaw etc…

Question 3

What places in the body evoke motor responses?

Answer 3

Spinal cord
Sensorimotor midbrain
Cortex and limbic system

Question 4

What does activation of muscle fibres create ?

Answer 4

Muscle fibre activated means it contracts and they act in an all or one manner. Depends on LMNs and how they activate different muscle fibres

Question 5

How do we achieve such a wide range of movement and forces?

Answer 5

• Muscles arranged in antagonistic arrangement (tricep and bicep, eye muscles) so combined co-ordination
• recruitment of muscle fibres (fast/slow twitch)

Question 6

How do muscle types vary in individuals?

Answer 6

Varies but changes with time and changing. Appears to be genetically determined

Question 7

How do muscles contract?

Answer 7

Different filaments of muscles interlock and move in and out. Actin (thin) and myosin (thick) they slide against each other producing contraction. Head of myosin filament walking along actin.

Question 8

What neurotransmitter is released for the binding of actin and myosin for muscle contraction?

Answer 8

Acetylcholine triggers biochemical cascade in muscle cells releasing packets of calcium from inside muscle cell.
Causes myosin head to change shape so it can bind with actin filament

Question 9

Why is ATP involved in muscle contraction?

Answer 9

ATP require to break the bond between myosin head and actin filament
Muscle contracted and remains that way until enzyme begins to disrupt actin and myosin

Question 10

What is the motor unit?

Answer 10

Single alpha motor neuron and all the muscle fibres it innervates

Question 11

How many muscle fibres do motor neurons innervates?

Answer 11

Different for each one.
Fewer fibres means greater movement resolution (finger tips and toes)

Question 12

What does an activation of alpha motor neuron cause?

Answer 12

Depolarisation of this neuron and so contraction of all muscle fibres in that unit

Question 13

How do we determine how many muscle fibres are innervated by a single neuron?

Answer 13

1. The level of control the muscle needs (eye needs a lot)
2. Strength of the muscle
   Typically there is a range of motor units size

Question 14

Where do lower alpha motor neurons come from and travel to?

Answer 14

Originate in the gray matter of spinal cord, or in the brainstem.
An alpha motor neuron and muscle fibres it connects represent unit of control of muscle force

Question 15

What is the motor pool?

Answer 15

All the LMN that innervate a single muscle
Contains alpha and gamma motor neurons
Arranged in rod like shape within ventral horn of spinal cord

Question 16

What are cell bodies in the ventral horn activated by?

Answer 16

1. Sensory information from muscle (which is sent to the brain)
2. Descending information from the brain

Question 17

What senses and controls tension of muscles?

Answer 17

Golgi tendon organ sense tension
These are located within the tendon and sends ascending information to the brain via spinal cord about how much force there is in muscle
Critical for proprioception

Question 18

How do the Golgi Tendon organs sense tension?

Answer 18

When a muscle is stretched, tension is exerted on the tendon, the bundles straighten and activate the GTO receptor endings

Question 19

What is the inverse stretch reflex?

Answer 19

• 1b fibres inhibit muscle contraction (via inhibiting alpha motor neurons).
• The synergy between this and interneurons regulates muscle tension and prevents overload

Question 20

What senses stretch and controls stretch of muscles?

Answer 20

Muscle spindles sense the length of muscles and this information forms key part of reflex circuits

Question 21

What is the most simple reflex?

Answer 21

Monosynaptic
Within muscle fibres:
1. Intrafusal muscle (sensory with muscle spindle)
2. Extrafusal muscle (function more to do with being a muscle)

Question 22

What innervates intrafusal muscle fibre to detect stretch?

Answer 22

Gamma motor neurons innervate intrafusal. This is because if intra and extra were both controlled by alpha, when the muscle is slack the system won’t be sensitive enough to these slight changes
So this gamma innervation keeps fibres set at length that optimises muscle stretch detection
- Sensory fibres are coiled around intrafusal fibres

Question 23

Why are reflexes important?

Answer 23

• They can operate without engaging with the brain
• Critical for the avoidance of injury and effective motor control
• They can be simple or complex

Question 24

What is the withdrawal reflex?

Answer 24

• Reciprocal innervation of antagonistic muscles (muscle that opposes the movement of another) explains why the contraction of one muscle induces the relaxation of the other
• This permits the execution of smooth movements

Question 25

Describe the pathway of a monosynaptic stretch reflex (e.g. knee jerk)?

Answer 25

1. Patellar tendon tapped
2. Thigh muscles are stretched
3. Stretch receptors activated
4. Burst of action potentials in the afferent nerve fibres
5. Activate excitatory synapses on the motor neurons that control this muscle
6. Stimulations of motor units
7. Contraction of the muscle
8. Extension of the lower leg

Question 26

How is the sensitivity of the monosynaptic stretch reflex regulated?

Answer 26

Regulated by gamma motoneurons
- These lead to tightening or relaxing of muscle fibres within the muscle spindle

Question 27

Are the ends and centre of a muscle spindle contractile?

Answer 27

• 2 ends of muscle spindle are contractile
• Centre is non-contractile