Neuromuscular and Spinal Cord

Question 1

What is the difference between EPSP (excitatory post-synaptic potential) and IPSP (inhibitory post-synaptic potential) in terms of membrane potential?

Answer 1

EPSP – makes the membrane potential less negative (bringing it closerto the threshold potential)
IPSP – makes the membrane potential more negative (hyperpolarisation)
You get graded effects – whether the neurone fires or not is dependent on the summation of inputs

Question 2

Which proteins are involved in the release of acetylcholine at synapses?

Answer 2

SNARE proteins

Question 3

What trigger acetylcholine release?

Answer 3

Calcium influx

Question 4

If you record the post-synaptic membrane potential at any one time, you will see some small changes in membrane potential. What are these caused by?

Answer 4

Miniature end plate potentials

They are caused by the constant dumping of acetylcholine into the synapse

Question 5

What is the difference between intrafusal and extrafusal muscle fibres?

Answer 5

Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors- muscle spindles) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement

Question 6

What are alpha motor neurones? State some other names given to alpha motor neurones.

Answer 6

Anterior horn cells, ventral horn cells or lower motor neurones
They are motor neurones that innervate the extrafusal fibres of skeletal muscle

Question 7

What is the name given to the sensory receptors in muscle that feedback to the CNS and allow an excitatory reflex to be generated?

Answer 7

Spindles (stretch receptors)

Question 8

What is a motor neurone pool?

Answer 8

Collection of lower motor neurones that innervate a single muscle

Question 9

Describe the arrangement of alpha motor neurones within the ventral horn.

Answer 9

Dorsal – flexors
Ventral – extensors
Medial – proximal
Lateral - distal

Question 10

What is an important rule to remember regarding the connections between alpha motor neurones and muscle fibres?

Answer 10

One motor neurone can innervate several muscle fibres

But every muscle fibre can only be innervated by one motor neurone

Question 11

Under what conditions can this rule be broken?

Answer 11

Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated

Question 12

Define motor unit.

Answer 12

A single motor neurone together with all the muscle fibres that it innervates - it is the smallest functional unit that can generate force.

Question 13

Describe and explain the difference in innervation ratio across different muscles in the body using examples.

Innervation ratio= how many muscle fibres a single neuron innervate in a motor unit.

Answer 13

Muscles that require very fine control (e.g. extrinsic eye muscles) havea low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)

Question 14

What are the 3 types of motor unit?

Answer 14

Slow (Type 1)
Fast fatigue-resistant (Type 2A)
Fast fatiguable (Type 2B)

slow muscle fibres contract slowly with little force but can stay contracted for long time

Fast fibres are more easily fatigued (cannot contract for as long) and contract more powerfully

Note these are all subtypes skeletal muscle motor units

Question 15

Describe the structural and functional differences of the neurons innervating the slow and fast twitch muscle fibres.

• cell body size
• size of dendritic trees
• axon thickness
• conduction velocity

Answer 15

Slow motor units (type 1) are innervated by neurons that are: 
 Smallest diameter cell bodies 
 Small dendritic trees
 Thinnest axons 
 Slowest conduction velocity 
Fast motor units (types 2a and 2b) neurons that are: 
 Larger diameter cell bodies 
 Large dendritic trees
 Thicker axons 
 Faster conduction velocity

Question 16

What are 2 methods by which the brain regulates the force that a single muscle can produce?

Answer 16

Recruitment – recruiting more motor units for the muscle contraction
Rate Coding – increasing the frequency of action potentials travelling down the nerves to the muscle fibres

Question 17

What principle governs recruitment?

Answer 17

Size principle

Smaller units are recruited first, which are generally slow fibres

Question 18

Describe the order of recruitment of motor units with increasing force generation.

Answer 18

Slow –> Fast Fatigue-Resistant (2a)–> Fast Fatiguable (2b)

Slow muscle fibres tend to be in smaller motor units. Fast muscle fibres tend to be in larger motor units

Question 19

What are neurotrophic factors?

Answer 19

Factors produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injury.

Question 20

What happens to a slow fibre when a fast nerve is transplanted onto it and what does this show?

Answer 20

It becomes faster
This shows that the function of the muscle fibre is INFLUENCED by the type of nerve that innervates it.
The action potentials can’t be the only thing being delivered to the muscles by the nerves, NEUROTROPHIC factors of the nerves may influence the muscle characteristics

Question 21

How easy is it to switch from one motor unit type to another?

Answer 21

Type 2B to Type 2A can happen with training
There is usually no way of changing from type 2 to type 1 or vice versa except in the case of severe deconditioning e.g. zero gravity or spinal injury. Without gravity, theres no weight and hence astronauts dont have to use muscles to do anything. This may cause deconditioning/muscle wasting

Question 22

How does muscle composition change with ageing?

Answer 22

Ageing is associated with a loss of type 1 and type 2 fibres with preferential loss of type 2 fibres
This means that a large proportion of muscle fibres in aged muscle are type 1
This loss of muscle is called sarcopenia

Question 23

What tract is responsible for voluntary movements?

Answer 23

Pyramidal/Corticospinal tract

Question 24

What is the role of extrapyramidal tracts?

Answer 24

It is responsible for automatic movements in response to stimuli (these are movements that your body makes without you being aware of it) and also in regulating pyramidal tracts

Question 25

What is a reflex?

Answer 25

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

Question 26

What are the components of a reflex arc?

Answer 26

Afferent signal
Relay neurone
Motor neurone

Question 27

What are the two signals that are generated when the patellarligament is tapped?

Answer 27

There is an excitatory signal going to the quadriceps

There is also an inhibitory signal going to the hamstrings (antagonist)

Question 28

Why is there a difference in the time taken for these signals to reach the relevant muscles?

Answer 28

The signal going to the quadriceps only has one synapse (monosynaptic reflex) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are two synapses.
This means that the signal to the quadriceps arrives slightly faster than the signal to the hamstrings.
NB he said the monosynaptic reflex strictly only involves the quadricep contraction, the hamstring inhibition is an additional thing that is not part of the reflex hence you call the reflex monosynaptic

Question 29

How did Hoffman plan on standardising the reflex test?

Answer 29

He considered bypassing the stretch that is caused by the tendon hammer on the patellar ligament and directly stimulating the nerve, which has sensory and motor fibres.
This would mean that the stimulus is identical every time (in duration and amplitude) and the magnitude of the reflex elicited will not be due to variations in input (how hard you tap the patellar tendon/where you tap it)

Question 30

What are the two twitches that are seen when you stimulate the nerve behind the knee?

Answer 30

M wave – twitch due to the direct conduction of the impulse down the motor neurone to the muscle fibre
H wave – due to the action potential passing down the sensory neurone back to the spinal cord and then coming out via a motor neurone to stimulate the muscle

Question 31

Why do sensory nerves show a response at lower stimulus intensity than motor nerves?

Answer 31

They are more susceptible to electrical stimuli because they’re larger

Question 32

Example of a polysynaptic reflexes?

Answer 32

Flexion withdrawal/Crossed extensor reflex

Question 33

Describe the supraspinal control of reflexes.

Answer 33

There is a large descending control over reflexes that only becomes noticeable when these descending controls are removed.

E.g they may exert inhibitory action on reflexes. If u remove the cerebral cortex and then stimulate the reflex, the reflex would be much greater

Question 34

What is the Jendrassik manoeuvre?

Answer 34

Tap someone’s patellar tendon with a tendon hammer whilst they are clenching their teeth. The response elicited is 2-3 times greater.

Question 35

If you decerebrate an animal (but keep them alive) and test their reflexes, what would you expect to observe?

Answer 35

Hyperreflexia
Increased muscle tone

this is due to loss of descending inhibitory signals

Question 36

What is the gamma reflex loop?

Answer 36

It shortens the spindles in muscle to maintain its sensitivity
There is also facilitation from higher centres, which increase the sensitivity of the motor neurone to afferent input

Question 37

What signs are seen with upper motor neurone lesions?

Answer 37

Hyperreflexia (loss of descending inhibitory control)
Clonus - involuntary rhythmic muscular contractions (loss of descending inhibitory control)
Babinski’s Sign

Question 38

Hyporeflexia is often associated with upper or lower motor neuron lesions

Answer 38

lower motor neuron lesions (because neurons involved in the reflex itself are mostly lower motor neurons)