Motor Control

Question 1

Voluntary movement requires what?

Answer 1

Immensely complex neural interactions

Question 2

What does planning movement require?

Answer 2

Prefrontal Cortex:
- Neurons involved in decisions to move; planning the desired movement outcome

Premotor Cortex:
- Neurons involved in organising movement sequences to achieve the outcome

Question 3

What does initiating movement require?

Answer 3

Primary motor cortex:
- Made of cell bodies of upper motor neurons
- Neurons involved in directing voluntary movement

Question 4

What does the basal nuclei do in terms of modifying movement?

Answer 4

Basal nuclei:
- Influences posture and automatic movements
- Regulates muscle tone
- Refines movements - selects which to allow and which to inhibit
- How? By altering sensitivity of neurons projecting into the corticospinal or other pathways

Question 5

What does the cerebellum do in terms of modifying movement?

Answer 5

Cerebellum:
- Stores and facilitates learning, planning and execution of motor programmes
- Monitors and compares sensory input (eg. proprioceptive and balance) to compare actual movement to planned movement
- Organises timing of muscle contractions and monitors ongoing activity (eg. for posture and balance)

Question 6

Summarise movement control

Answer 6

Prefrontal cortex:
- decision to move
- signals sent to premotor cortex
Premotor cortex:
- select movement sequences
- signals sent to primary motor cortex, basal nuclei and cerebellum
Primary motor cortex:
- initiate movements
- signals sent down corticospinal pathway
Basal nuclei and Cerebellum:
- Modify movements
- signals sent back and forth to each other as well as to frontal cortex and down descending pathways

Question 7

What is the corticospinal pathway from the brain to the muscle (somatic motor control)?

Answer 7

1. upper motor neurons in primary motor cortex fire action potentials that propagate along axons extending down the spinal cord
2. to activate lower motor neurons in spinal cord, to fire action potentials that propagate along axons within peripheral nerves
3. to skeletal muscle

Question 8

What are motor units?

Answer 8

A motor unit is made of a single lower motor neuron plus all the skeletal muscle fibres it innervates.
- Small motor unit:
- a motor neuron plus the few muscle fibres it activates
- can produce more precise movement (eg. of hand, lip, tongue)
- Large motor units:
- A motor neuron plus the many muscle fibres it activates
- Can produce more forceful movements (eg. of limbs)

Question 9

How does the somatotopic map relate to motor units?

Answer 9

• Areas of the body controlled by many small motor units have more representation in the cortex
• Areas of the body controlled by few large motor units have less representation on the cortex
  (The primary motor cortex is made up of upper motor neuron cell bodies. Signals from the left cortex control the right side of the body and vice versa).

Question 10

Voluntary vs Reflex movements

Answer 10

Voluntary:
- wide range of movements of varying speed, duration and complexity
- typically involves complex patterns of sensory and motor processing
- enables us to interact with the environment
- indicated voluntarily by neurons in the brain
- variable latency (~100+ ms)

Reflex:
- rapid reproducible, automatic motor response to external stimulus
- employs a simple neural circuit involving neurons within peripheral nerves and spinal cord
- protective; prevents injury
- does not require involvement of neurons in the brain
- consistent latency (~40 ms)

Question 11

Describe voluntary movement response to touch time

Answer 11

Voluntary response to touch:
- ~100+ ms latency (latency: time delay between ball touching the foot and the kick)
- Highly variable
- Path: from foot to brain, then to quadriceps muscle
- Focus and training can reduce response time, as it reduces the number of neural connections involved

Question 12

Describe the stretch reflex - response time

Answer 12

Reflex response to tendon tap:
- ~ 30-40 ms latency
- Very consistent, reproducible response to sudden stretch
- Protects muscle from tearing
- Path: from stretched muscle spindles, to spinal cord, to stretched muscle
- Cannot be changed with training

Question 13

Describe muscle spindles

Answer 13

• Stretch receptors - detect muscle stretch
• Proprioceptors - stretch provides information about limb position in relation to torso
  Structure:
• Dendritic endings of a sensory neuron wrap around specialised infrastructural muscle fibres
• dendritic endings have mechanically-gated Na+ channels that open when the membrane gets stretched

Question 14

How does the stretch reflex happen?

Answer 14

1. Tendon tap causes sudden, fast stretch of quadriceps muscle
2. Muscle spindles (proprioceptors/stretch receptors within the muscle) get stretched
3. Mechanically-gated Na+ channels open in dendritic endings of the sensory neuron of the muscle spindle
4. Na+ entry causes depolarisation that leads to an action potential (AP), which propagates along sensory axon to spinal cord
5. synaptic transmission from sensory axon terminal depolarisation in motor neuron cell body
6. AP fires and propagates along motor axon to NMJ of quadricepts
7. stimulating quadriceps to contract, causing foot to kick out

Question 15

Describe the withdrawal reflex

Answer 15

Reflex response to painful stimuli:
- Nociceptors acitvated
- Sensory neuron depolarises and AP fires and propagates along spinal cord
- Sensory neuron stimulates interneurons:
- leading to excitation of motor neurons that stimulate flexors
- and inhibition of motor neurons that stimulate extensors
- enables withdrawal of the affected limb