Neuro 9: Neuromuscular and spinal cord control of movements

Question 1

Contact ratio of presynaptic:postsynaptic membranes in muscle vs CNS

What is bouton

How big is synaptic cleft

Answer 1

muscle 1:1, cns 1000:1

Presynaotic terminal

10-50nm

Question 2

How can post synaptic cell be controlled by presynaptic cells

Answer 2

2 directions:

EPSP (excitatory post synaptic potential). Be made less negative – i.e. be brought closer to threshold for firing

OR

IPSP (inhibitory post synaptic potentials). be brought further away from threshold for firing. Hyperpolarised

Question 3

What determines whether action potential is reached

Answer 3

summation= the process that determines whether or not an action potential will be triggered by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation), and from repeated inputs (temporal summation).

Question 4

Define neuromuscular junction

Answer 4

specialised synapse between the motor neuron and the motor end plate, the muscle fibre cell membrane

Question 5

What is a miniature end plate potential

Answer 5

At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials (mEPP)

NOT activation, just small fluctuations in membrane potential due to small amount of ACh

Question 6

What are alpha motor neurons (aka ventral/anterior horn nerves)

Answer 6

Lower motor neurons of the brainstem and the spinal cord

Innervate extrafusal muscle fibres

Question 7

Differentiate intrafusal and extrafusal muscle fibres

Answer 7

Intrafusal= sensory organs related to reflex

Extrafusal=contractile fibres containing actin and myosin

Question 8

What do alpha motor neurons innerate

Answer 8

Extrafusal muscle fibres of the skeletal muscles

Question 9

Define motor neuron pool

Answer 9

contains all alpha motor neurons innervating a single muscle

i.e. muscle NOT muscle fibre

Question 10

Outline arrangement of alpha motor neurons within the ventral horn

Answer 10

Those innervating flexors more dorsal,

those innervating extensors more ventral

(remember because it matches muscles of leg— extensor muscle is ventral and flexor is dorsal in leg too)

those innervating proximal muscles more medial and those innervating distal muscles more lateral

Question 11

Differentiate motor unit and motor pool What is the motor unit

Answer 11

unit= The neuron + all of the muscle fibres innvervated by that neuron

pool= all the alpha motor neurons innervating a single muscle

Question 12

T/f muscle fibres are only innervarted by branches of the same nerve

Answer 12

T

Question 13

T/f: each nerve only innervates one muscle

T/f: stimulating a motor unit causes contraction of all the muscle fibres in that unit, and you cannot selectively fire certain neurons within a motor unit

Answer 13

F… each nerve innervates lots of muscle fibres, but each muscle fiber is only innervated by that nerve

T

Question 14

How many motor neurons vs skeletal muscle fibes

Answer 14

approximately 420,000 motor neurons and 250 million skeletal muscle fibres

Question 15

How many muscle fibres does the average motor neuron supply

Answer 15

600

Question 16

T/f. Stimulation of one motor unit causes contractin of all the muscle fibres in that unit

Answer 16

T

Question 17

Outline different types of motor unit

Answer 17

SLOW (type 1)- small diameter cell body, small dendritic tree, thinnest axon, slow conduction

FAST, FATIGUE RESISTANT (type 2a) larger diameter cell bodies bodies, larger dendritic tree, thicker axon and faster condiuction velocity

FAST, FATIGUABLE (type 2b)- larger still diameter cell bpdies, larger still dendritic trees, thicker stikk axons and faster still conduction veolicty .

Question 18

T/F each muscle has only one type of motor unit

Answer 18

F- have different types, all mixed up (not in blocks), which determines the property of that muscle!

Question 19

What are the 3 motor unit types classified by

Answer 19

Amount of tension generated
Speed of contraction
Fatiguability of motor unit

Question 20

Q: if muscls contain more than one motor unit type, is the property of the muscle dependent on the proportion of each motor unit?

Answer 20

Yes

Question 21

2 factors regulating force that a single muscle can produce

Answer 21

Recruitment (i.e. how many motor units are used… size principle. Smaller units (type 1) recruited first i.e. slow twitch units. more units recruited as more force required)….. allows FINE CONTROL (i.e. when writing only low force levels needed)

Rate coding
Motor unit can fire at different frequencies, slow units fire at lower frequency. as firing rate increases, so does force produced….

Question 22

When does summation occur

Answer 22

Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

This relates to RATE CODING

Question 23

How does recruitment work

Answer 23

Slow units used first, then fast resistant, and then fast fatigue,

In derecruitment, fast fatigue is lost first, then fast resistant.

It’s cumulative though so slow stays on throughout

Question 24

How does rate coding regulate the force produced

Answer 24

Look at the red graph…. as you increase the amount of force being produced, as well as the number of units being recruited (first slow, then FR then FF), the rate of firing also increases (shown by the increase in Hz on the Y axis)

Question 25

What are neurotrophic factors.

Answer 25

This is a type of growth factor, prevent neuronal death and promote growth of neurons after injury (but this doesn’t happen in CNS- not the nerve but the ENVIRONMENT)

Question 26

What is the impact of swapping nerves between a fast twitch and slow muscle???

Answer 26

Fast becomes slow and slow becomes fast!

The motor neurone has some effect on the properties of the muscle fibres which it innervates

BASICALLY, WHETHER AN EXTRAFUSAL MUSCLE FIBRE IS SLOW, FAST RESISTANT OR FAST FATIGUABLE DEPENDS ON THE NERVE THAT INNERVATES THEM

Question 27

What changes can occur to the types of msucle units (i.e. slow, IIA and IIB)

Answer 27

switch between type of fibres

IIB–> IIA in TRAINING

type I to II in cases of severe deconditioning or spinal cord injury

in microgravity in space, slow –> fast musce fibres

Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle.. slower contraction

Question 28

Look at the location of corticospinal tract in relation to other tracts on spinal cord

What is rubrospinal tract

Answer 28

Rubrospinal small tract originating from red nucleus in brainstem… invlved in voluntary movement control mostly of upper limb…. travels adjacent to corticospinal tract

Question 29

Give an example of pyramidal and extra pyramidal tract

Answer 29

Pyramidal- lateral and anterior corticospinal tract

Extrapyramidal tracts- e.g. vestibulospinal tract

Question 30

Define reflex

Answer 30

An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness

Question 31

If you stretch a muscle, it contracts again (i.e. in patellar reflex). Why

T/f Reflexes always result in the same magnitude of motor response regardless of the stimulant

Answer 31

It has an afferent.

Stretching the muscle (with a cut sensory neuron) will not achieve contraction.

F:whose magnitude and timing are determined respectively by the intensity and onset of the stimulus

Question 32

If you stimulate an

afferent EXTENSOR, what are the effects on the efferent extensor and flexor

Answer 32

For AFFERENT extensor stimuation:

extensor efferent: this will cause excitation (so the extensor muscle contracts)

flexor efferent: this will excite an inhibitory neuron so there inhibition of the flexor motor nerve and thus muscle (causing relaxation)

Question 33

Outline the monosynaptic stretch reflex

Answer 33

. Patellar reflex… stretching of the extensor excited senroy neuon, which directly synapses onto motor neuron which contracts that muscle (MONOSYNAPTIC)

In addition, there is a disynaptic part here because the sensry afferent also synaspses onto a inhibitory interneuron which inhibits the flexor

Question 34

What is the Hoffman Reflex

Answer 34

Basically doing the patellar reflex without the variations of how hard you hit/did you hit in the right place etc. etc.

To negate the need for stretching the muscle, Hoffman instead gives an electrical signal to the nerve (although this stimulates both the motor and sensory nerve)

Question 35

Why do you see 2 twtiches in the Hoffman’s reflex

Answer 35

You see a DIRECT MOTOR RESPONSE (because the motor nerve is stimulated as well as sensory) and an action potential passes immediately down the motor nerve to the muscle, and causes a small M wave.

AFTER, you get the H wave. This wave is caused by stimulation of the afferent sensory nerve, which then goes to the spinal cord, and stimulates the motor nerve.

On the graph you see that H wave gets a response amplitude at a lower stimulation… this is because the sensory nerves are larger so more amenable to electrical stimuli

Question 36

Differentiate flexion withdrawal and cross extensor

Answer 36

As well as monosynaptic (i.e. excitation) and disynaptic (i.e. inhibition) reflexes, we have polysynaptic reflexes too. NOTE: THIS IS NOT REFERRING TO HOW MANY DIFFERENT MUSCLES INVOLVED, BUT TO HOW MANY SYNAPSES INVOLVED

Flexion withdrawal= lifting your foot off a pin. The afferent sensory neuron will stimulate the flexor muscle, but also synapse onto 2 different interneurons. The first interneuron will synapse onto inhibitory neurons of the extensor muscle. The second interneuron will leave the grey matter and travel up or down the spinal cord, and then synapse onto motor neurons at different spinal cord levels which innervate the flexor muscle to contract. This is POLYSYNAPTIC

Cross extensor= other leg contracts to remain sturdy when other leg is lifted. In addition the above 2 interneurons that the sensory afferent synapsed onto, it can also synapse onto a 3rd. It can synapse onto an interneuron which then crosses the spinal cord and synapses onto an excitatory motor neuron of the extensor muscle on the CONTRALATERAL side in order to remain upright. The sensory neuron can also synapse onto another interneuron which crosses the spinal cord and travels up or down on either side to excite motor neurons at different spinal cord levels which innervate the extensor on the contralateral leg to ensure you remain upright.

Question 37

What is Jendrassik manoeuvre, what does it show and what is the response

Answer 37

Clenching teeth/making fist enhances petallar tendon reflex—> demonstrating SUPRASPINAL CONTROL OF REFLEXES

Often a larger reflex response will be observed when the patient is occupied with the maneuver, as the maneuver may prevent the patient from consciously inhibiting or influencing his or her response to the hammer

Question 38

Whats meant by Supraspinal control of reflexes

Answer 38

Higher brain centres influence excitatory or inhibitory control over reflexes. In normal conditions, inhibitory control from the brain dominates over reflex.

If you remove cerebral hemisphere, you remove inhibitory control from the cerebral hemisphere and the reflex response is a lot larger than normal

Question 39

What are gamma motor neurons.

Explain the gamma reflex loop

Answer 39

Motor neurons innvervating intrafusal fibres. They change the sensitivity of the muscle fibers, as well as ensuring the muscle spindles (= sensory receptors located within muscles that allow communication to the spinal cord and brain about proprioception and velocity) are kept taught.

if the knee is extended and the muscle goes slack,
The spindle is shortened to maintain its sensitivity
THIS IS THE GAMMA REFLEX LOOP

Question 40

A cause of hyperreflexia

Answer 40

STROKE… it can remove the descending inhibition from cortical areas so bigger reflex!

Question 41

What conditions are indicated by hyperreflexia and hyporeflexia

Answer 41

Hyperreflexia –> upper motor neuron damage (loss of inhibition)
Hyporeflexia –> lower motor neuron damage (because there are less functioning motor neurons emerging from the anterior horn so reflex won’t be as strong)

Question 42

What can a different babinski sign indicate

Answer 42

Upper motor neuron disease (look at this sign)

Question 43

Differentiate the time difference between stimulus and the efferent effect in monosynaptic and disynaptic

Answer 43

I.e. if you stimulate the afferents in extensor, this will cause contraction of efferent. Monosynaptic pathway, afferent synapses directly onto efferent nerve

But if you stimulate afferent in flexor, you’d want the extensor to be inhibited. This involves stimulation of inhibitory neuron (synpase), which then synapses onto the efferent neuron. As there are 2 synapses here to deliver inhibition, the effect of the efferent wil be about double as long as the fist time around

Question 44

What can spasticity and rigidity be caused by

Answer 44

Brain damage leading to loss of inhibitory influence of CNS on the reflexes. This causes bigger reflexes (due to loss of inhibitory influence), and also a higher level of tone after the muscle has been stretched (because the brain upregulates its attempt to control the reflex, which doesn’t actually control the reflex, just causes tone when you don’t want it!)

Question 45

How do higher brain centres influence stretch reflex

Answer 45

Activating alpha motor neurons
Activating inhibitory interneurons
Activating propriospinal neurons
Activating gamma motor neurons
Activating terminals of afferent fibres

Question 46

Which brain areas and pathways are involved in stretch reflex regulation

Answer 46

Cortex – corticospinal (fine control of limb movements, body adjustments)
Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
Tectum – tectospinal (head movements in response to visual information).

Question 47

Give an exampe of a monosynaptic and polysynaptic reflex

Answer 47

Mono=stretch reflex

Poly= flexion withdrawal with orssed extensor reflexes