Neuromuscular and Spinal Cord

Question 1

What is the difference between EPSP and IPSP in terms of membrane potential? What do these result in?

Answer 1

EPSP: makes the membrane potential less negative (bringing it closer to the threshold potential)
IPSP: makes the membrane potential more negative (hyperpolarisation- further away from threshold for firing)
Graded effects- activity is a result of 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

Caused by constant low rate release of ACh into the synapse

Question 5

What is the difference between intrafusal and extrafusal skeletal muscle fibres?

Answer 5

Intrafusal: serve as sensory organs (proprioceptors) that detect the amount + rate of change of length of a muscle
Extrafusal: innervated by alpha motor neurones + 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
Motor neurones that innervate the extrafusal fibres of skeletal muscle
Activation causes muscle contraction

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

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

1 motor neurone can innervate several muscle fibres

But every muscle fibre can only be innervated by 1 motor neurone

Question 11

When is a muscle fibre innervated by >1 motor neurone?

Answer 11

Under pathological conditions (e.g. severed nerve): 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 (the smallest functional unit that can generate force.)
Stimulation of 1 motor unit causes contraction of all muscle fibres in that unit

Question 13

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

Answer 13

Muscles that require very fine control (e.g. extrinsic eye muscles) have a 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)

Question 15

Describe the structural and functional differences between slow and fast twitch muscle fibres.

Answer 15

Slow: Smallest diameter cell bodies, Small dendritic trees, Thinnest axons, Slowest conduction velocity
Fast: Larger diameter cell bodies, Large dendritic cells, 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 AP’s travelling down the nerves to the muscle fibres

Question 17

What principle governs recruitment?

Answer 17

Size principle

Smaller units are recruited first (generally slow twitch units)

Question 18

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

Answer 18

Slow –> Fast Fatigue-Resistant –> Fast Fatiguable

Question 19

What are neurotrophic factors?

Answer 19

= a type of growth factor that prevent neuronal death + promote 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 fast

Function of the muscle fibre is determined by the type of nerve that innervates it.

Question 21

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

Answer 21

Type 2B to Type 2A occurs with muscle training

Type 2 to type 1 occurs in severe deconditioning e.g. zero gravity or spinal injury

Question 22

How does muscle composition change with ageing?

Answer 22

Ageing is associated with a loss of type 1 + type 2 fibres with preferential loss of type 2 fibres
Results in a larger proportion of type 1 fibres in aged muscle (slower contraction times)

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

Automatic movements in response to stimuli

Question 25

What is a reflex?

Answer 25

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

Question 26

What are the 2 signals that are generated when the patellar ligament is tapped?

Answer 26

Excitatory signal to quadriceps= contraction | Inhibitory signal to hamstrings (antagonist)= relaxation

Question 27

Why is there a difference in the time taken for signals to reach the quadriceps and hamstrings in the patellar reflex?

Answer 27

Signal going to quadriceps only has 1 synapse (monosynaptic) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are 2 synapses. Thus, signal to quadriceps arrives slightly faster than signal to hamstrings.

Question 28

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

Answer 28

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

Question 29

What names are given to the polysynaptic reflexes?

Answer 29

Flexion withdrawal | Crossed extensor

Question 30

Describe the supraspinal control of reflexes.

Answer 30

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

Question 31

What is the Jendrassik manoeuvre?

Answer 31

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

Question 32

If you decerebrate an animal and test their reflexes, what would you expect to observe?

Answer 32

Revelation of excitatory control from supraspinal areas | Rigidity + spasticity result from brain damage giving over active or tonic stretch reflex

Question 33

What are gamma motor neurones reactive to?

Answer 33

Activity of muscle itself | control contraction of muscle fibers located within the muscle spindle based on afferent input

Question 34

What signs are seen with upper motor neurone lesions?

Answer 34

Hyperreflexia (loss of descending inhibition) Clonus (tonic involuntary behaviour of foot) Babinski’s Sign (great toe will flex, small toes fan out)

Question 35

What is the neuromuscular junction?

Answer 35

A specialised synapse between the motor neurone + motor end plate (muscle fibre cell membrane)

Question 36

What does recruitment allow?

Answer 36

Fine control (e.g. when writing) under which low force levels are required

Question 37

In rate coding, when does summation occur?

Answer 37

When units fire at frequency too fast to allow muscle to relax between arriving AP's

Question 38

What is the reticulospinal tract involved in?

Answer 38

Coordinating automated movements of locomotion + posture e.g. to painful stimuli

Question 39

What is the rubrospinal tract involved in?

Answer 39

Automatic movements of arm in response to posture/ balance changes

Question 40

What is the vestibulospinal tract involved in?

Answer 40

Regulating posture to maintain balance + facilitates mainly alpha motor neurones of the postural, extensor muscles

Question 41

What determines magnitude and timing of a reflex?

Answer 41

Intensity + onset of stimulus

Question 42

How do reflexes differ from voluntary movement?

Answer 42

Once they are released, they can't be stopped

Question 43

Describe what is seen in a flexion withdrawal and crossed extensor reflex

Answer 43

Pain sensation Sensory neurone excited Activation of interneurones in several spinal cord segments Stimulation of flexor muscle to allow withdrawal from pain Stimulation of extensor muscle in other leg to contract to maintain balance

Question 44

What is the Gamma reflex loop?

Answer 44

Feedback loop in nervous system that regulates level of tension in muscles Shortens the spindles in muscle to maintain its sensitivity

Question 45

What is hypo-reflexia?

Answer 45

Below normal or absent reflexes | Mostly associated with lower motor neurone disease