Week 2 - Posture, balance and reflexes

Question 1

What is a motor unit?

Answer 1

A single motor neuron and all of th muscle fibres it innervates

Question 2

Do we control muscle fibres or muscle units?

Answer 2

Muscle units

Question 3

What is the innervation ratio?

Answer 3

The smaller the muscle, the smaller the motor units

Question 4

How are the types of muscles determined?

Answer 4

By the nerves that innevate them

Question 5

Which regions of the brain are involve in planning movement?

Answer 5

Basal ganglia and cerebral cortex

Question 6

What is special about the speed of reflexes?

Answer 6

Some reflex control doesn’t have time to go back to the brain and down the spinal cord, so short circuits it

Question 7

What are the three classes of postural control sensory input?

Answer 7

Somatosensory, vestibular and visual

Question 8

Describe feedback control of posture?

Answer 8

Postural readjustment is often antagonistic to conscious movement. This is feedback and is relativele slow - often already stacking it

Question 9

Describe feed-forward postural control

Answer 9

Feed-forward is anticipatory correction for postural stability - at the same time as making a command for limb movement, an anticipatory movement in a counteracting limb/area will also occur i.e. when moving arm up, will subconsciously tense up legs

Question 10

What is a reflex arc?

Answer 10

Sensory neurons synapse in the spinal cord so that reflexes can occur quickly by activating spinal motor neurons without the relay of passing through the brain first
The brain will receive sensory input while the reflex is carried out

Question 11

If you were about to pull on a handle, would you arm or leg tense up first according to feed forward control?

Answer 11

Leg

Question 12

From where does reflex activation of leg muscles start?

Answer 12

Bottom up i.e. ankles before knees

Question 13

Describe a monosynaptic reflex

Answer 13

Stimulus activates receptor, goes to sensory neuron, goes to spinal cord, goes out to efferent neuron, then target cell receptor, then response.

Question 14

What is different between a monosynaptic reflex and a post synaptic reflex?

Answer 14

Postsynaptic reflex has CNS integration; interneurons affect the reflex
–> in monosynaptic, muscles are only getting witched on. In post, they can be switched off - if you want a reflex relaxation, need a poly-synaptic reflex

Question 15

How do the sensations sensed by Golgi Tendon organs and muscle spindles differ?

Answer 15

GTOs are tension/change of tension sensitive, while muscle spindles are length/change of length sensitive

Question 16

Why are GTOs in tendons and not muscles?

Answer 16

Tension is easy to sense in tendons because they shouldn’t have much stretch. Muscles have a lot more stretch

Question 17

Which muscles contain MS?

Answer 17

Intrafusal muscles

Question 18

Where do you find GTOs?

Answer 18

In the myotendinous junction - here they can montor tension between muscle and tendon

Question 19

What are the two different types of GTOs?

Answer 19

Static - slowly adapting, positional and low intensity - “where is my limb?”
Dynamic - rapidly adapting, intense, fire impulses very quickly (large, fast)

Question 20

What is the purpose of the main reflex of the GTO?

Answer 20

To make sure you drop a mass if it’s too heavy i.e. piano drop reflex

Question 21

How does the piano drop reflex work?

Answer 21

GTOs sense a massive increase in tension and fire into the SC, hooking up with inhibitory interneuron that secretes glycine onto alpha motor neuron that controls the original muscle. This inhibits the muscle, causing immediate relaxation and drop of the piano

Question 22

Describe a muscle spindle

Answer 22

Encapsulated intrafusal fibres within a muscle belly, that respond to stimulation by causing contraction
They are dynamic, and expand and shorten as the muscle expands and shortens

Question 23

Why is there a difference in contraction time between a muscle and it’s muscle spindles?

Answer 23

Muscle spindles have smaller motor neurons than skeletal muscle (gamma vs alpha fibres). This means that when there’s a contraction response, muscle contracts faster than the MS, causing you to lose sensory position from the MS straight away

Question 24

Describe the contraction and sensory regions of a muscle spindle

Answer 24

The sensory region is in the middle and has no actin-myosin fibres - it’s non-contractile. It contains multiple types of sensory neurons, which can convey information to the spinal cord.
Each end of this is hooked up to a contractile end
Contractile areas are hooked up to gamma neurons proximally and distally

Question 25

How is a contraction response initiated in the CNS?

Answer 25

Either by innervation of the skeletal muscle fibre, or by innervation of the gamma neurons of an MS

Question 26

Describe the two different types of sensory neurons coming out of MS

Answer 26

Primary - encircle central portion of intrafusal fibres. Ia fibre, 70-120m/s. Excited by nuclear chain fibres and nuclear bag fibres
Secondary - innervate receptor region on each side of MS. Much slower than primary. Excited only by nuclear chain fibres

Question 27

How do populations of nuclear bag and nuclear chain fibres differ in MS?

Answer 27

Bag fibres - 1-3 per spindle, dynamic and rapidly adapting

Chain fibres - 3-9 per spindle, static and slowly adapting

Question 28

When will you see high frequency of APs from primary sensory neurons of MS? When will you see none? When will you see occasional?

Answer 28

When being stretched - will go crazy
When shortening will see none
When static, let off the off AP

Question 29

Describe the freq of APs in secondary sensory neurons of MS when lengthening vs shortening

Answer 29

When lengthening, AP frequency will increase. When shortening, will decrease

Question 30

What are the three levels of hierarchy in reflex control?

Answer 30

1. Control of a single muscle (myotatic & inverse myotatic stretch reflexes)
2. Coordination about a joint
3. Complex multi-joint reflexes (flexion withdrawal reflex)

Question 31

What is the difference between myotatic and inverse myotatic stretch reflexes?

Answer 31

Myotatic - an increase in tension promotes a greater increase in tension
Inverse myotatic - increase in tension results in a drop in tension i.e. GTOs are activated in a polysynaptic reflex

Question 32

Describe the flexion withdrawal reflex

Answer 32

Easy to understand if imagine stepped on a nail. Flexor muscles are activated and extensory muscles are inhibited - coordination of contraction of multiple joints

Question 33

Describe the crossed extensory reflex

Answer 33

Imagine stepped on a nail. Nociceptors are activated, neurons fire to spinal cord. There’s integration of excitory and inhbiotry interneurons that results in opposite action for each leg. In this case, is that the one experiencing pain is withdrawn, while the other one tenses so it provides stability and enhanced postural support during flexion withdrawal

Question 34

What does coordination around a joint require?

Answer 34

Divergence of sensory input - you need inhibitory interneuron synapses at antagonist muscles and excitatory synapses with synergist muscles

Question 35

What contributes to tone/gain of muscles?

Answer 35

Descending pathways coming down from the brain that descend onto interneurons in the spinal cord.
These tell us how tense our muscles should be and is part of a feed-forward response

Question 36

How do gamma motor neurons regulate how sensitive the stretch reflex is?

Answer 36

By tightening or relaxing fibres within a muscle spindle

Question 37

What does changing how tight nuclear chain fibres or nuclear bag fibres do?

Answer 37

Changes the tone of muscle and how strong reflexes are - if loose, the information isn’t as fast and the CNS doesn’t think it’s that important.