PHYS: Posture, balance, and reflexes

Question 1

What are the three classes of sensory input contributing to postural control?

Answer 1

• Somatosensory (including proprioceptive)
• Vestibular
• Visual

Question 2

What is a vestibule?

Answer 2

A small space or cavity at the beginning of a canal.

Question 3

What does the vestibular system control?

Answer 3

Sense of balance and spatial orientation.

Question 4

What is a monosynaptic reflex?

Answer 4

Reflex involving transmission of information from a sensory neuron to a motor neuron across a single synapse in the spinal cord.

Question 5

How do polysynaptic reflexes differ from monosynaptic reflexes?

Answer 5

Polysynaptic reflexes involve two to several synapses in the spinal cord, involving one or more interneurons.

Question 6

• Where do interneurons exist?

- What is their function?

Answer 6

• In the spinal cord.

- Relays signals between afferent and efferent neurons.

Question 7

Name the two sensory receptor organs of note from this lecture.

Answer 7

• Muscle spindles.

- Golgi tendon organs.

Question 8

Describe a feed-forward strategy

Answer 8

Postural responses are triggered centrally before voluntary movements

Question 9

What is a spinal reflex?

Answer 9

Entire neural circuit bypasses higher centres

Question 10

To what are Golgi Tendon organs sensitive?

Answer 10

Tension/change of tension

~16 micrometre diameter

Question 11

What do muscle spindles monitor?

Answer 11

• Change in muscle length

- Rate of change in muscle length

Question 12

Where are Golgi tendon organs located?

Answer 12

Within myotendinous junctions

Question 13

Where are Muscle Spindles located?

Answer 13

Encapsulated within the muscle belly

Question 14

What is the mechanism of action of Golgi tendon organs?

Answer 14

They monitor tendon tension (both static and dynamic) and transmit via large, fast, lb type fibres to synapse with inhibitory interneurones.

These inhibit alpha motor neurone firing, preventing damage

Question 15

What is another phrase for muscle spindles?

Answer 15

Intrafusal fibres.

Extrafusal fibres = regular muscle fibres

Question 16

What are the two types of sensory endings of muscle spindles?

Where are they located?

Answer 16

1. Primary ending (Ia) - encircles central portion of each intramural fibre, input from nuclear bag & chain, senses velocity and length.
2. Secondary ending (II) - innervate the receptor region on sides, input only from nuclear chain, senses length.

Question 17

What are the two types of intrafusal fibres?

Hint: one relates to static, one relates to dynamic information

Answer 17

1. Nuclear bag fibres (1-3 per spindle); dynamic - provide a rapidly adapting response
2. Nuclear chain fibres (3-9 per spindle); static - provide a slowly adapting response

Question 18

What are the three levels of hierarchy in reflex circuitry?

Answer 18

1. Control of individual muscles
2. Control of muscles around a joint
3. Coordination of muscles at several joints

Question 19

What is the monosynaptic stretch reflex?

Hint: most simple reflex

Answer 19

A 2 neuron circuit: single synaptic connection between a Ia afferent fibre from a muscle spindle and an alpha motor neuron

e.g. patellar tendon tap

Question 20

What is the inverse myotactic reflex?

Hint: requires an inhibitory interneuron

Answer 20

Stimulation of Ib afferent neutron from a Golgi tendon organ causes reflex inhibition of the motoneurons via Ib inhibitory interneuron

Question 21

Explain coordination about a joint

Hint: involves synergists and antagonists

Answer 21

Requires divergence of sensory input:

• Inhibitory interneuron synapse at antagonist muscle
• Excitatory synapses also with synergist muscles

Question 22

Explain an example of complex multi-joint reflexes

Hint: e.g. stepping on a nail

Answer 22

1. Flexion withdrawal reflex
   - Reflex withdrawal from noxious stimuli
   - Flexor muscles activated, extensor muscles inhibited
2. Crossed extensor reflex
   - Enhance postural supporting during flexion withdrawal on contralateral limb
   - Flexor muscles inhibited, extensor muscle activated

Question 23

Explain modulation of reflex strength

Answer 23

Descending input from higher motor centres can modify sensitivity of of reflex response

Occurs via gamma motor neurons to muscle spindles - tightening or relaxing fibres within spindle

Question 24

What is a motor unit?

Answer 24

A single motor neurone and the multiple muscle fibres that it innervates

Question 25

What is the innervation ratio of motor units?

Answer 25

The ratio between the alpha motor neurone and the number of muscle fibres it innervates.

Associated with the degree of dexterity needed in the movement

Question 26

Explain the difference in motor unit ratios between big strong muscles (e.g. gastrocnemius) and fine control muscles (e.g. rectus lateralise controlling the eye)

Answer 26

Gastroc = 579 MUs, and 1,934 Myofibres/MU –> Generates a lot of force, but not good dexterity

Rectus lat. = 4,150 MUs, and 5 Myofibres/MU –> Not a lot of force, but perfect dexterity

Question 27

What are the 3 skeletal muscle fibre types?

Answer 27

Type I - slow contracting, fatigue resistant (e.g. postural muscles)
Type IIa - fast contracting, fatigue resistance
Type IIb - fast contracting, fast fatigue

Question 28

What is the metabolic type and mitochondrial density of the 3 skeletal muscle fibre types?

Answer 28

Slow = oxidative, high mitochondrial density

Fast, fatigue resistant = intermediate, medium mitochondrial density

Fast, fatiguable = glycolytic, low mitochondria density