P3 - The H Reflex

Question 1

What is a Muscle Action Potential (MAP)?

Answer 1

caused by action potentials in an efferent nerve passing across the nerve/muscle synapse (neuromuscular junction)

• -> individual muscle fibres
• -> contraction of skeletal muscle

Question 2

What is an EMG and what does it record?

Answer 2

Electromyogram (EMG)

• -> When a whole muscle is active, the currents associated with each muscle fibre action potential combine
• -> Compound Muscle Action Potential (CMAP)
• -> this signal is recorded by the EMG

Question 3

Why is the EMG usually of an irregular shape?

Answer 3

EMG usually of irregular shape because the individual muscle fibre action potentials, which sum to make the EMG, fire asynchronously.

Question 4

What is a motor unit?

Answer 4

A motor unit is defined as one motor neuron, its axon and all of the muscle fibres that it innervates.

Question 5

What is a Motor Unit Action Potential (MUAP)?

Answer 5

When a motor neuron fires, the impulse - the action potential- is transmitted along the axon to the neuromuscular junction. After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibres of that particular motor unit. The sum of all this electrical activity is known as a motor unit action potential (MUAP). This electrophysiological activity from multiple motor units is the signal typically evaluated during an EMG.

Question 6

Is the tendon jerk reflex di-synaptic or monosynaptic?

Answer 6

monosynaptic:

1) receptor
2) sensory neuron
3) motor neuron
4) muscle

Question 7

Describe the recruitment of motor neurons in voluntary activation

Answer 7

Orderly recruitment:

• of motor units from smallest to largest
• Hennemann Size Principle
• If you apply V=IR then it is easy to see that if a small motor unit and a large motor unit receive the same synaptic current, then the small neuron produces a large Excitatory Post Synaptic Potential (EPSP) and will therefore be activated before the larger motor neuron.

Question 8

Do small or big motor neurons have a higher resistance?

Answer 8

The small motor neuron has a smaller surface area and has fewer parallel ion channels, and therefore has a higher resistance (R) than the large motor neuron which has a large surface area and more parallel ion channels thus lower resistance.

Question 9

Describe the recruitments of axons in an artificial/direct nerve stimulation

Answer 9

‘Reverse recruitment’

• large diameter axons will depolarise first
• Sensory axons have a slightly larger diameter than motor efferents, and thus have a lower internal resistance. The larger the area, the greater the number of ions there will be (per unit length of neuron) to conduct current. It is easier to get current in and this makes the larger axons easier to depolarise as they have a lower excitation threshold.

Question 10

What is orthodromic transmission?

Answer 10

Orthodromic transmission:

the signals/action potentials propagate in the correct direction

Question 11

What is antidromic transmission?

Answer 11

Antidromic transmission:

the signals/action potentials transmitted in the wrong direction

Question 12

Where do signals propagate in direct nerve stimulation?

Answer 12

In both directions: orthodromically and antidromically

Question 13

Is the soleus muscle important for power or posture?

Answer 13

both, but mostly posture (EMGs show neural activity even when just standing still)

Question 14

Stretch reflex: is the EMG amplitude higher with or without the Jendrassik manoeuvre? why?

Answer 14

• The amplitude of the EMG is larger whilst performing the Jendrassik manoeuvre
• More motor neurons activated (the effort used in that task spreads the motor command to spinal motor neurons throughout the spinal cord, thus motor neurons that did not reach the threshold for discharge in the relaxed state are easier to depolarise)

Question 15

Are there differences in latency between the Jendrassik manoeuvre and the relaxed state?

Answer 15

• The latency of the reflex arc is slightly reduced for the Jendrassik manoeuvre compared to the relaxed state.
• The motor overflow that results from the Jendrassik manoeuvre brings the motor neurons in the lumbosacral spinal cord segments closer to the threshold for firing. They therefore fire more easily and thus may depolarise slightly earlier than in the relaxed state. The latency of the reflex response is around 1ms shorter.

Question 16

What is latency?

Answer 16

latency: time between tendon tap and beginning of muscle contraction

Question 17

What are the M and H waves? which comes first?

Answer 17

Direct stimulation of tibial nerve:

• M-wave first –> motor axons directly stimulated (action potentials travel shorter distance, directly from stimulated area to soleus muscle)
• H-wave second –> sensory axons directly stimulated (action potentials travel to spinal cord, synapse with motor neuron and then back along leg to soleus muscle)