Neuromuscular and spinal cord control of movements

Question 1

What is the usual contact ratio for synaptic nerve transmission in muscle and how high can it get in the CNS?

Answer 1

• 1:1 in muscle
• Up to 10³:1 in the CNS

Question 2

What is the resting membrane potential across the neuronal membranes?

Answer 2

• • 70 mV

Question 3

1) What are the 2 types of ways that post-synaptic membrane potentials can be altered (i.e 2 types of synaptic transmission)…
2) … and how can these interact with themselves or each other?

Answer 3

1)

1. EPSP (excitatory post-synaptic potential) - makes the membrane potential less negative and therefore brought closer to the threshold for firing
2. IPSP (inhibitory post-synaptic potential) - makes the membrane potential more negative and therefore further away from the threshold for firing of the action potentials

2)

• Summation can occur
• The degree of summation determines how readily a neurone can reach the threshold to reach an action potential

Question 4

Define the neuromuscular junction

Answer 4

The synapse between the motor neurone and the motor end plate (the CSM of the muscle fibre)

Question 5

Describe how activation of muscle occurs by synaptic transmission through the NMJ

Answer 5

• AP propagated through the pre-synaptic neurone
• When the AP reaches the pre-synpatic membrane, there is opening of voltage-gated calcium channels
• Ca²⁺ influx into the pre-synaptic neurone
• This stimulates NT vesicular binding and subsequent exocytosis of ACh into the NMJ
• ACh then binds post-synaptic receptors on the motor end plate
• This propagates the AP throughout the motor fibre by Na⁺ influx
• Actin and myosin mediated muscle contraction - sliding filament theory thing

Question 6

What are mEPPs?

Answer 6

• At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials
• These potentials tend to be graded

Question 7

What are alpha motor neurones, including a general description, where they are found, what they innervate and what their activation causes?

Answer 7

• Lower motor neurones of the brain and spinal cord
• Last neurone from the CNS projecting to the muscle
• Innervate exrafusal muscle fibres of the skeletal muscle
• Activation causes muscular contraction

Question 8

What are extrafusal and intrafusal motor fibres?

Answer 8

• Extrafusal muscle fibres: standard skeletal muscle that causes contraction
• Intrafusal muscle fibres: muscle fibres that contain specialised sensory organs that send information to the CNS

Question 9

What does the motor neurone pool describe?

Answer 9

All alpha neurones innervating a single muscle

Question 10

Where are cell bodies of the alpha neurones located within the spinal cord?

Answer 10

In the ventral horn

Question 11

What is a motor unit?

Answer 11

• A single motor neurone and all of the muscle fibres it innervates (picture a nerve branching out and the branches innervating different muscle fibres)
• It is the smallest unit of muscular contraction

Question 12

1) NO MUSCLE FIBRE IS INNERVATED BY …..
2) Why might this not be the case?

Answer 12

1) …..MORE THAN ONE MOTOR UNIT
2) In certain pathologies there may be sprouting of motor neurone branches such that multiple motor units will innervate the same muscle fibres

Question 13

What are the 3 types of motor units and what are their cellular properties that enable their varied functions?

Answer 13

SLOW (TYPE 1)

• Slowest conduction velocity, due to…..
• Small dendritic tree
• Smallest diameter cell bodies
• Thinnest axons

FAST, FATIGUE RESISTANT (FR, TYPE IIA)

• Both FR and FF have the same properties
• Fast conduction velocity due to…
• Large dendritic tree
• Large cell body diameter
• Thicker axons

FAST, FATIGABLE (FF, TYPE IIB)

• Both FR and FF have the same properties
• Fast conduction velocity due to…
• Large dendritic tree
• Large cell body diameter
• Thicker axons

Question 14

Classify the 3 types of motor units by their 3 property classifications

Answer 14

SLOW (TYPE I)

• Slow twitch
• Low tension generated
• Fatigue resistant

FAST, FATIGUE RESISTANT (FR, TYPE IIA)

• Fast twitch
• Moderate tension generated
• Fatigue resistant

FAST, FATIGUABLE (FF, TYPE IIB)

• Fast twitch
• High tension generated
• Fatiguable

Question 15

What are the 2 neural mechanisms for regulation of muscle force?

Answer 15

1. Recruitment

• ​Size principle - smaller motor units, which are usually slow twitch are recruited first when starting with small contractions including fine control. Larger motor units are recruited during larger contractions, there is gradation from slow to fast, fatigue resistant and fast fatiguable motor units dependent on the amount of muscle force necessary

2. Rate Coding

• ​Increasing neurone firing rate, to increase muscle contraction force by a motor unit
• This is due to summation - i.e. increased neurone firing rate such that there is no relaxation phase in between the stimulation of muscle contraction so as to have a summation effect

Question 16

What are neurotrophic factors and what do they do?

Answer 16

• Factors transported within and between neurones that maintain nerve integrity and function

They…..

• Prevent neuronal death
• Promote neurone growth and peripheral axonal regeneration after injury

Question 17

Describe the changes that occur in motor units after the aging process, the plasticity of motor units. What is the term for this change?

Answer 17

• Senile Sarcopenia
• Loss of both type I and type II motor units
• However preferential loss of type II (fast) motor units
• This results in higher type I : type II motor unit proportions and therefore slower contraction
• N.B Type I = slow twitch and type II = FR (IIa) and FF (IIb)

Question 18

Apart from ageing, what are the other circumstances or ways in which plasticity of motor units can occur?

Answer 18

• No way of changing fast to slow or vice versa
• With exercise, shift from type IIb (fatiguable) to type IIa (fatigue resistant)
• Severe spinal cord injury or other deconditioning - from type I to type II
• Microgravity during spaceflight - type I to type II

Question 19

What type of movements are pryramidal tracts (including the corticospinal tract etc) and extrapyramidal tracts involved in?

Answer 19

• Pyramidal tracts - voluntary movements
• Extra-pyramidal tracts - involuntary movements

Question 20

List 4 extrapyramidal tracts and what functions they are involved in - except for one of them

Answer 20

1. Rubrospinal tract - automatic movements of the arm in response to posture / balance changes
2. Reticulospinal tract - coordinated movements of locomotion / posture resulting fom painful stimuli
3. Vestibulospinal tract - regulates posture to maintain balance and facilitates α-motor neurone of the postural extensor muscles - allows us to maintain head / neck position
4. Olivospinal tract

Question 21

What are reflexes?

Answer 21

• Nerve impulses passing inward from a receptor to a nerve centre and then outwards as an effector (muscle / gland), without reaching the level of consciousness
• An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli

Question 22

What are the components of a reflex arc?

Answer 22

1. Sensory receptors
2. Sensory neurones
3. Integrating centre (i.e. neurones that relay within the CNS - spinal cord, from the sensory to the motor neurones e.g. interneurones in the spinal cord)
4. Motor neurones
5. Inhibitory interneurones
6. Effectors - muscles or glands

Question 23

If there is suspected damage to the CNS or PNS, which may be affecting the reflex arc, how might it be affecting the reflex arc and how might you test this? Also what would indicate the function of the motor arm of the reflex arc is preserved?

Answer 23

• If the sensory arm - the afferent part of the reflex arc is damaged, there is less output from the motor arm - the efferent part of the reflex arc
• Reflex testing to test if there is sensory or motor loss
• If you can voluntarily contract a muscle than the motor arm is preserved, if however you strike the tendon to stimulate the reflex arc and there is absence of the reflex, then the sensory arm must be damaged

Question 24

What is the difference between monosynaptic and polysynaptic reflex arc pathways?

IGNORE THIS FLASHCARD I THINK ITS WRONG - COME BACK AND EDIT

Answer 24

• Monosynaptic means there is only one synapse within the chemical pathway in the reflex arc - this happens in reflexes with only one sensory neurone and one motor neurone (no interneurones). So the only synapse is between the sensory and motor neurone
• Polysynaptic reflex pathways refers to reflex arc pathways where there are more than one synapes, whem there are interneurones interfacing between the sensory and motor neurones. So there’s a synapse between the sensory neurone and the interneurone, and a synapse between the interneurone and the motor neurone, and between interneurones as well?

Question 25

In the reflex arc, how is there integration of both agonist and antagonist pairs of muscles to coordinate a movement in a reflex and give an example? Outline the whole reflex arc in order to carry this out, starting with the stimulus stimulating the sensory receptor

Answer 25

• There is both stimulation of agonist muscular contraction as well as inhibition of antagonist muscular contraction e.g. in the patellar reflex there is stimulation of quadriceps and inhibition of hamstrings muscles

1. Stimulatory signal stimulates the sensory receptor
2. Sensory receptor → sensory (afferent) neurone
3. Afferent neurone travels to the integrating centre in the spinal cord
4. From here the afferent neurone synapses with both the afferent motor neurone fibre and also an inhibitory interneurone
5. The motor afferent fibre goes on to stimulate contraction of the agonist (the muscle producing the desired contraction)
6. Meanwhile the inhibitory interneurone inhibits the antagonist in the muscle pair

Question 26

Give an example of a monosynaptic reflex arc, as well as the antagonist muscle pair integration in the reflex arc to complete the reflex

Answer 26

• Striking the patellar tendon
• This stimulate the afferent signal to the spinal cord…
• Which then stimulates the motor arm of the pathway to stimulate muscular contraction of the quadriceps muscle agonist
• The afferent signal also synapses with the interneurone and thus with the afferent (motor) pathway to the hamstrings - the antagonist muscle which is inhibited
• So the stimulation of the quadriceps (agonist) and inhibition of the hamstrings (antagonist) results in the leg kicking up

Question 27

What is the Hoffman reflex and how is it used clinically in testing which part of the reflex arc is damaged?

Answer 27

• Simultaneous stimulation of the sensory and motor neurones with the same duration and amplitude to stimulate 2 contractions of a muscle, e.g. the quadriceps muscle. Then measurement of the muscle contraction using an EMG in order to determine if its the sensory or motor component that is damaged
• Direct motor response causing M wave - AP causes direct stimulation of the motor neurone to stimulate contraction, this is the first contraction which is seen on the EMG as the M wave
• Hoffman reflex (i.e. normal reflex arc) causing H-wave - AP stimulation of sensory neurone which then synapses to the motor neurone along the normal reflex arc, this then causes the second contraction which is seen on the EMG as the H-wave
• You can see these different waves from the 2 contractions on the EMG, the first is the M-wave and then the H-wave, you can see if one of the waves is defective through this. If the motor / afferent arm is defective, then the M-wave will be distorted and if the the efferent / sensory arm is defective, then the H-wave will be distorted

Question 28

How does the stimulus correlate to the size of the reflex generated?

Answer 28

• By duration and amplitude
• i.e the longer and more intense the stimulus, the greater the size of the reflex

Question 29

In upper motor neurone lesions that may occur with injury or with decerebration, how will this affect reflex responses - a form of supraspinal control of reflexes

Answer 29

• The cerebral cortex has inhibitory effects on reflexes
• So there is upregulation of the reflex control of various muscles
• This leads to rigidity and spasticity resulting in over-active or tonic stretch reflexes

Question 30

How does stimulus strength relate to response amplitude in the H and M-waves - not really a relationship but more a particular thing?

Answer 30

Sensory nerves are more amenable to electrical stimuli because they are larger so you can get a response from a sensory nerve (H-wave) at lower stimulus intensity than the M wave

Question 31

What is the Jendrassik manouevre - note this manouevre is a good example of supraspinal control of reflexes

Answer 31

• Clenching your jaws or fists can increase the patellar reflex response
• Because you are reducing the supraspinal control of the reflexes by occupying the supraspinal structures with the actions of the manouevre - almost like distracting it

Question 32

mGive 5 mechanisms by which higher centres can exert supraspinal control of reflexes

Answer 32

1. Activating α-motor neurones
2. Activating terminals of afferent fibres
3. Activating gamma-neurones
4. Activating inhibitory interneurones
5. Activating propriospinal (postural) neurones

Question 33

Give 4 higher centres and pathways which have reflex involvement and detail their functions in this capacity

Answer 33

1. Corticospinal tract - finer limb movement and control and body movement control
2. Rubrospinal tract via the red nucleus - automatic movements of the arm in response to posture change / movement
3. Vestibulospinal tract via the vestibular nuclei - automatic alteration of posture to maintain balance
4. Tectospinal tract via the tectum - automatic head movements in response to visual information

Question 34

1) What are gamma-motor neurones - what do they project to?
2) Conversely, what do α-motor neurones project to?

Answer 34

1)

• Project to the muscle spindles of the intrafusal muscle fibres
• Intrafusal muscle fibres cannot produce movement - they are too weak
• Function in the gamma reflex loop etc

2)

• α-motor neurones project to the extrafusal muscle fibres
• These muscle fibres generate movement

Question 35

Describe the gamma reflex loop

Answer 35

• When the muscle spindles are loose (i.e. not taut) - this is not ideal to generate a good afferent signal to the extrafusal muscle fibres via the alpha motor neurones
• So gamma-motor neurones generate action potentials which project to the intrafusal muscle fibres and the spindles in order to make it taut
• The muscle spindle initiates afferent impulses upon stretch to the dorsal horn and eventually out through the ventral horn in the α-motor neurones in a reflex movement
• The degree of afferent impulses initiated through the muscle spindle is proportional to the length of the muscle spindle. Therefore gamma-motor neurone activity in the gamma reflex loop increases sensitivity to stimuli and increases the reflex ultimately generated by α-motor neurones

Question 36

1) Strokes lead to an example of what type of motor lesion and what problems does this cause?
2) What are the clinical signs of this defect as a result of stroke?

Answer 36

1)

• Example of upper motor neurone lesions
• Resulting in a loss of descending inhibition of reflexes
• This results in hyperreflexia

2)

• Clonus - muscular spasm involving repeated, often rhytmic contractions
• Babinski’s sign - if you stroke the bottom of the foot, the toes will span out reflexively - positive in people with stroke or small infants (<18 months) due to damage to or underdeveloped corticospinal tracts respectively

Question 37

Describe the arrangement of α-motor neurones in the ventral horn of the spinal cord

Answer 37

The flexors are usually more dorsal and the extensors are usually more ventral

Question 38

What kind of defect in terms of reflexes will lower motor neurone lesions result in?

Answer 38

Hyporeflexia

Question 39

What happens in polysynaptic reflex arcs and give an example?

Answer 39

• This is when there is both the reflex in the desired limb for example say when you step on something sharp on one leg to withdraw this leg
• …but then you need other compensatory levels to the reflex to correct the change to the posture or balance e.g. if you move one leg in the reflex response, you might fall, so you need the reflex to also communicate with the opposing leg to adjust to balance yourself
• So there is ascension or descending of the reflex to different levels in the spinal cord in order to have an effect in other areas
• The example given here is the flexor withdrawal (removing leg from stimulus) and crossed extensor (adjusting opposite leg to maintain balance)