Neuromuscular and spinal cord control of movements

Question 1

Describe the neuromuscular junction

Answer 1

The axon of a motor neuron divides into a number of branches at the surface of the muscle fibre.
Each branch ends in a bouton which forms a synapse with the muscle fibre, called a neuromuscular junction (nmj). The postsynaptic membrane (endplate), which is thrown into folds, has a high density of nicotinic acetylcholine receptors.
The cleft contains a collagenous basement membrane (basal lamina) to which is bound acetylcholinesterase (AChE). Soluble forms of the same enzyme are also secreted into the cleft.

Question 2

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

Answer 2

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 all the inhibitory and excitatory synapses acting on it at any moment (i.e. If the sum of all EPSPs and IPSPs results in a depolarization of sufficient amplitude to raise the membrane potential above threshold, then the postsynaptic cell will produce an action potential)

Question 3

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

Answer 3

SNARE proteins

Triggered by calcium influx

Question 4

What does the acetylcholine do?

Answer 4

Binding of ACh to nicotinic acetylcholine receptors (ligand-gated ion channels) causes opening of the channel, allowing Na+ influx and K+ efflux => depolarisation => action potential which is propagated over the muscle fibre membrane.

Question 5

If you record the post-synaptic membrane potential at any one time, you will see some small changes in membrane potential. Explain why.

Answer 5

At rest, spontaneous secretion of ACh from a single vesicle causes a miniature endplate potentials (mepp) of 0.4 mV.
Firing of the motor nerve releases ACh from hundreds of vesicles causing an endplate potential => a large depolarisation that triggers muscle fibre action potentials and contraction.

Question 6

Define the term ‘motor unit’.

Answer 6

A motor unit consists of a motor neuron plus all the muscle fibers it innervates, which ranges from six to a few thousand. In mammals, each muscle fiber gets input from just one motor neuron.

Question 7

Describe the progression from twitch => tetanus, in terms of muscle fibre contraction

Answer 7

An action potential in a motor neuron causes a twitch, a single contraction, in all the fibres it supplies.
At high firing rates individual twitches summate to produce tetanus, a prolonged maximal contraction. Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

Question 8

State the different types of motor unit

Answer 8

Slow twitch (S) motor units
Fast twitch motor units - fatigue resistant (FR) and fast fatiguable (FF) fast twitch motor units

Question 9

Characterise/describe slow twitch motor units

Answer 9

• Most numerous
• smallest diameter cell bodies
• small dendritic trees
• thinnest axons
• slowest conduction velocity
• Drive type 1
  muscle fibres that are adapted for aerobic metabolism and capable of sustaining low forces for very long periods (which form the bulk of the postural muscles of the trunk and legs).

Question 10

Characterise/describe fast twitch, fatigue resistant motor units

Answer 10

• larger diameter cell bodies
• larger dendritic trees
• thicker axons
• faster conduction velocity
• With repetitive stimuli, fatigue resistant units can sustain moderate force for 5 minutes or so before a steady decline sets in that takes many minutes.

Question 11

Characterise/describe fast twitch, fast fatiguable motor units

Answer 11

• larger diameter cell bodies
• larger dendritic trees
• thicker axons
• faster conduction velocity
• Fast fatigue (FF) motor units can achieve the greatest force of the three types, but with repetitive stimuli the force falls precipitously after about 30 seconds.

Question 12

What type of muscle fibres are innervated by FF and FR units respectively?

Answer 12

FF motor units innervate type 2b muscle fibres (anaerobic)

FR motor units innervate type 2a muscle fibres (aerobic/anaerobic)

• generally muscles involved in executing fast movements.

Question 13

Define the term ‘motor pool’

Answer 13

Motor neurons that innervate the same muscle form a common motor pool
Motor pools are *topographically localized in motor nuclei of the brainstem and spinal cord. Spinal motor nuclei extend over several spinal segments. Axons of motor neurons leave the ventral horn of the spinal cord to run in the spinal nerve of the same spinal segment.

*most medial part of the ventral horn contains lower motor neurone pools that innervate axial muscles/proximal limb muscles whereas more lateral parts contain lower motor neuron pools that innervate distal limb muscles.

Question 14

The force of contraction of a muscle is determined by the motor pool in two ways. What are they?

Answer 14

The rate at which individual motor neurons fire

The number of active motor neurons in the pool that are firing.

Question 15

State the name of the process that describes the increase in number of active motor units to achieve larger contraction force. What important principle governs this process and how?

Answer 15

Recruitment

The order of recruitment is determined by the SIZE PRINCIPLE (the size of the motor neurons and how synapses onto them are organised) so that the earliest units to be recruited are S, followed by FR and finally FF; allows for fine control (e.g. when writing)

Question 16

State the name of the process that describes the increase in firing rate to achieve a larger contraction force.

Answer 16

Rate coding

Question 17

What would result if type 1 muscle fibres are denervated and the axon of an FF unit sprouts to establish new connections with the denervated fibres?

Answer 17

The denervated fibres would acquire the characteristics of type 2b muscle fibres => SO, muscle fibre properties are determined by the motor neurons which innervate them

Question 18

How do neurotrophic factors contribute to the plasticity of the motor unit?

Answer 18

They are a type of growth factor (produced within nerve) that prevent neuronal death promote growth of neurons after injury

Question 19

What are common motor unit/muscle fibre conversions, and under what circumstances?

Answer 19

• Type 2b to type 2a is most common following training.
• Type 1 to 2 possible in cases of severe deconditioning (e.g. microgravity during spaceflight) or spinal cord injury
• Ageing associated with loss of type 1 and 2 fibres (sarcopenia) but also preferential loss of type 2 fibres => larger proportion of type 1 fibres in aged muscle

Question 20

Axons in which tract are responsible for causing voluntary movements?

Answer 20

Pyramidal/Corticospinal tract

Question 21

What are the roles of the extrapyramidal tracts?

Answer 21

(Chiefly found in the reticular formation of the pons and medulla) Involved in the control of involuntary movements e.g.:

• rubrospinal tract = automatic movements of arm in response to posture/balance changes
• reticulospinal tract = coordinate automated movements of locomotion and posture
• vestibulospinal tract = regulates posture to maintain balance

Question 22

What is a reflex?

Answer 22

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 23

What are the components of a reflex arc?

Answer 23

Sensory neuron, a motor neuron and usually interneurons interposed between the two, which may be excitatory or inhibitory;

polysynaptic reflex arc = two or more interneurons
disynaptic reflex arc = one interneuron
monosynaptic reflex arc = no interneuron

Question 24

What type of reflex is the muscle spindle reflex?

Answer 24

aka stretch reflex;

they are monosynaptic reflexes which cause a muscle to contract when it is stretched

Question 25

Describe the sensory component of the stretch reflex

Answer 25

• Modulation of motor unit output is made by sensory input from the muscle spindles which measure the length and rate of change of length of the muscle
• Each muscle spindle is a fluid- filled capsule of connective tissue containing about seven modified muscle fibres called intrafusal fibers
• Intrafusal fibers have contractile ends but their central regions are non-contractile
• Muscle spindles lie in parallel with the standard (contractile) extrafusal fibres so any force acting on the whole muscle acts in the same way on the spindle

Question 26

Name the types of motor neurones which innervate the intrafusal and extrafusal fibres

Answer 26

The neurons which drive the extrafusal fibers to produce muscle contraction are referred to as alpha motor neurons.

Gamma motor neurons innervate the muscle spindles - all intrafusal fibres have their contractile ends innervated by these gamma motor neurons => this maintains the sensitivity of the muscle spindle to changes in length over a wide range of lengths (gamma reflex loop)

Question 27

Distinguish between lower and upper motor neurone

Answer 27

Lower motor neurones send their axons out of the brainstem and spinal cord to innervate skeletal muscles of the head and body, respectively (whether voluntary or reflexive).

Interneurones are the major source of synaptic input to all lower motor neurones; lie primarily in the intermediate zone of spinal cord (topographically arranged in the same way as lower motor neurones)

Upper motor neurones, whose cell bodies lie in the brainstem or cerebral cortex, have axons which descend in white matter tracts to synapse with interneurones or (more rarely) with lower motor neurones directly.

Question 28

Give an example of how the stretch reflex can be elicited

Answer 28

Tapping the patellar ligament between its insertion into the tibia and the kneecap
=> resultant stretch causes the contraction of the quadriceps femoris (monosynaptic component of reflex arc)
=> Also an inhibitory signal, via an inhibitory interneuron, going to the hamstrings (antagonist)

Note that signal to the quadriceps (being monosynaptic) arrives slightly faster than the signal to the hamstrings.

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 larger

Question 32

Describe the reflex circuitry that mediates the withdrawal of a limb from a painful stimulus

Answer 32

Flexion reflex involves slowly conducting afferent axons and several synaptic links (polysynaptic reflex arc). Stimulation of nociceptive sensory fibres leads to withdrawal of the limb from pain source by excitation of ipsilateral flexors and reciprocal inhibition of ipsilateral extensors.
This is accompanied by an opposite reaction in the contralateral limb (mediated by excitatory/inhibitory interneurones). This crossed extension reflex provides postural support during withdrawal of the affected limb from the painful stimulus.

Question 33

Describe the supraspinal control of reflexes.

Answer 33

Higher centres of the CNS exert inhibitory and excitatory regulation upon lower motor neurons and interneurons involved in the stretch reflex (inhibitory control dominates in normal conditions)

Question 34

What is the Jendrassik manoeuvre?

Answer 34

Tap someone’s patellar tendon with a tendon hammer whilst they are clenching their teeth (or pulling hands apart). 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

Decerebration reveals the control from supraspinal areas => Hyperreflexia and Increased muscle tone (rigidity)

Question 36

What signs are seen with upper motor neurone lesions?

Answer 36

Hyperreflexia
Clonus
Babinski’s Sign

Question 37

What is Babinski’s sign?

Answer 37

Sharply stroking the sole of the foot elicits extension of the big toed fanning of the other toes. A similar response occurs in human infants before maturation of the corticospinal pathway.