Lecture 13 - NMJ & Skeletal Muscle

Question 0

Once the signal reaches the neuromuscular junction, how does the signal continue to bring about muscle contraction

Answer 0

1. Pre-synaptic cell releases ACh which acts n the nAChRs on the motor end plate
2. Receptors open, allow the passage of Na into and K out of the cell
3. Phosphodiesterase cleaves the ACh so that the response is short lived
4. The depolarisation of the membrane flows along the membrane and down the T tubules.
5. The depolarisation reaches the DHP, which connects to the RyR channels on the SR, opening them
6. Once open, Ca flows out of the SR into the cytosol.
7. Binds to troponin etc.

Question 1

Outline the differences between the regular synapse and the neuromuscular junction

Answer 1

The NMJ is dedicated and highly specialised to connect nerves with muscle fibres

• convoluted to allow more space for receptors
• wall to wall receptors
• phosphodiesterase

Question 2

What is a motor unit?

Answer 2

These are all the individual muscle fibres that are innervated by the same lower motor neuron

• many units within one muscle
• every fibre in a unit will have the same twitch characteristic

Question 3

Describe the differential recruitment of motor units

Answer 3

The motor units that are recruited first produce small amounts of force. This ensures that fine, dexterous movement can be produced.

The larger motor units, that produce more force, are recruited last

Question 4

The motor units that are recruited first are …

Answer 4

Fatigue resistant

Question 5

Describe the different types of muscle fibres, in terms of twitch speed

Answer 5

Fast twitch I - high force, recruited last, fatigue

Fast twitch II - intermediate force, recruited intermediately, fatigue resistant

Slow twitch - recruited first, low force, fatigue resistant

Question 6

What accounts for the different characteristics of muscle fibres: eg. Fast and slow twitch?

Answer 6

Different protein structure etc.

Question 7

Describe summation of action potentials and muscle contraction

Answer 7

Action potentials cannot summate, however, muscle contraction can.

Many action potentials in quick succession will cause summation of the force produced by the muscle, eventually leading to tetanus

Question 8

Describe the sequence of events in muscle contraction

Answer 8

1. Action potential
2. Calcium influx into fibre
3. Muscle contraction