Motor System

Question 1

What is the basic function of the basal ganglia?

Answer 1

-What to do

Question 2

What is the basic function of the cerebellum?

Answer 2

-How to do it

Question 3

Describe the structure of skeletal muscle

Answer 3

-Skeletal muscle is formed of several muscle fasciculi
-Muscle fasiculi = several muscle fibres
muscle fibre = several myofibrils
myofibril = actin and myosin

Question 4

Describe sliding filament theory

Answer 4

1. Myosin head binds to actin
2. Hydrolysis of ATP provides energy for conformational change of myosin head
3. Causes sarcomeric shortening due to the sliding of filaments

Question 5

What route do afferent sensory neurons use?

• where is the central process?
• where is the peripheral process?

Answer 5

• UNIPOLAR sensory afferent neurons bring info to the CNS via the dorsal roots
• central process is in dorsal horn
• peripheral process is in the skin/muscle etc

Question 6

What route do efferent motor neurons use?

• where is the central process?
• where is the peripheral process?

Answer 6

• MULTIpolar motor efferent neurons send motor infomation from the CNS to the muscles
• central process is in ventral horn
• peripheral process is in the skin/muscle etc

Question 7

What is a motor unit?

Answer 7

A motor neurons and all the fibres that it innervates

Question 8

What is a motor neuron pool?

Answer 8

A collection of motor neurons innervating a single skeletal muscle, organised somatotopically

Question 9

What does activation of the alpha motor neuron cause?

Answer 9

Activation of the alpha motor neuron causes contraction of all the fibres in the motor neuron unit
-this are often distributed evening throughout the muscle to provide evenly distributed force

Question 10

What does the number of muscle fibres in a motor unit depend on?

Answer 10

1. Level of control needed (complex = few)
2. Strength needed (higher strength = more fibres)

eye - few muscle fibres - aprox 3
abdominal muscles - over 100

Question 11

Name 3 spinal reflexes

Answer 11

1. Involuntary physiological response to a stimulus, eg withdrawal of hand from flame
2. Unlearned or instinctive response to a stimulus - also called an unconditioned response
3. Muscle spindle feedback - stretch and withdrawal reflex

Question 12

Describe the muscle spindle

Answer 12

1. Muscle spindle is embedded in most muscle
2. Composed of intrafusal fibres which are found parallel to extrafusal fibres
3. Sensory fibres coil around intrafusal fibres
4. Intrafusal muscles are innervated by gamma MNs
5. Thick and thin filaments are at either end
6. Measures changes in muscle LENGTH

a) when the muscle lengthens during contraction/relaxation the muscle spindle is stimulated
b) This opens mechanically gated ion channels in sensory dendrites
c) leads to a receptor potential, which triggers an action potential

Question 13

What is alpha-gamma coactivation?

Answer 13

• When extrafusal fibres have been stimulated to contract by alpha MNs gamma MNs are also stimulated
• The gamma MN stimulates contraction in the 2 ends of the intrafusal fibre, readjusting its length
• This keeps the central region taut and responsive

Question 14

Describe the stretch reflex? - simplest reflex

Answer 14

1. When a muscle lengthens the muscle spindle is activated
2. This increases alpha MN activity
3. Causes muscle fibres to contract, resisting the stretch

Question 15

Describe the withdrawal reflex - protects body from harm, eg flame

Answer 15

1. Heat stimulates temp and danger receptors in the skin
2. Triggers sensory impulse to CNS
3. Synapses with interneurons, then motor neurons
4. Motor impulses to
   a) flexors allow withdrawal
   b) inhibitory impulses to extensors so flexion is not inhibited

Question 16

Describe the walking-running reflex

Answer 16

• Patterns for walking, trotting, galloping are generated in spinal circuitry
• Quadrupeds will walk on treadmills if weight is supported when spinal cord is cute at spinal level
• Will change to the appropriate pattern of limb movement as speed of treadmill is altered
• Stretch of the MUSCLE SPINDLE

Question 17

Describe basic basal ganglia structure/function

Answer 17

The Basal Ganglia

• Upper MNs only
• Group of nuceli deep within the cerebral hemispheres
• General role in motor control

Cerebral cortex - striatum (caudate nucleus and putamen) - globus pallidus, substantia nigra, thalmus

Question 18

Explain route of cortical loops

Answer 18

Sensory input is to cerebral cortex, then passes to basal ganglia for processing, then motor output

Question 19

Explain the route of sub-cortical loops

Answer 19

Sensory input passes to subcortical structures bypassing the cerebral cortex, then gives motor output

Question 20

How is the basal ganglia involved in selection

Answer 20

-Phasic activation occurs in the STRIATUM
-Phasic inhibition occurs in the substantia nigra (SNr)
-
= Basal ganglia can inhibit and excite different inputs to control the output

Question 21

Describe basic cerebellum structure/function

Answer 21

The cerebellum

• Upper MNs only
• 10% of brain weight
• Looped connections with almost all the body’s UMNs
• When damaged;
  1. jerky uncoordinated movements
  2. voluntary movements lose fluidity
  3. intention tremor
  4. dysarthia, disruption of fine control of speech, slurring

Question 22

What are cerebellum reentrant loops?

Answer 22

-Structure of origin to pons to the cerebellum to thalamus and then back to the original structure of origin
=allows the cerebellum to compute motor error and adjusts cortical motor commands accordingly
-HOW you do what you do

*cerebellum is also involved in motor learning as it is in collaboration with basal ganglia and cortical circuits

Question 23

What have functional brain studies shown that the cerebellum is involved in?

Answer 23

• reentrant loops and adjustment of motor commands

- working memory, implicit and explicit learning and memory language, (dyslexia and autism)