Lecture 10: Muscle physiology 1

Question 1

Define motor unit

Answer 1

A single alpha motor neuron and all the muscle fibres it innervates (not necessarily physically connected) is called a motor unit. These fibres are all same type: type 1 slow or type 2 fast.

Question 2

What is the size principle in muscle response

Answer 2

Creates a graded response in voluntary skeletal movements which is more efficient. Smaller, oxidative motor units are recruited first before larger glycolytic motor units.

Question 3

Describe the structure of the neuromuscular junction

Answer 3

Long axon of a motor neuron splits into lots of presynaptic terminal boutons which is where the presynaptic membrane interfaces with the post synpatic membrane of muscle fibre across the synaptic cleft.

Question 4

Where are the Ach receptors and voltage gated Na+ channels on the post synpatic membrane

Answer 4

Ach receptors are on the crests. VGNa+ channels are in the junctional folds down on the bottom

Question 5

What are the 5 pre synaptic events in the NMT

Answer 5

1. An action potential in the presynaptic cell reaches the nerve terminal
2. Depolarisation of terminal opens v gated Ca2+ channels resulting in Ca2+ influx
3. Increased Ca2+ triggers exocytosis of vesicles and release of AcH
4. ACh diffuses across the synaptic cleft and binds to Ach receptors
5. AcH is broken down by acetylcholinesterase

Question 6

What does vesicle cycling mean

Answer 6

This is how there is always a good supply of ACh ready to be released.
It is the process where endosomes either from an early endosome or recycled from an old vesicle that has been acidified uptake the neurotransmitter and wait at the active zone to be primed for Ca2+ influx triggered fusion pore opening. After which they endocytose.

Question 7

What are the 4 post synaptic events

Answer 7

1. Binding of 2 ACh to each post synaptic t-gated channel allowing influx of Na+ and exit of K+
2. Depolarisation of the motor end plate (EPP)
3. Opening of voltage gated Na+ channels
4. Influx of Na+ causing propogated AP in muscle fibre

Question 8

What is End plate current and end plate potential

Answer 8

EPC : net depolarising current caused by synchronous opening of ACh ion channels. The time course is defined by the mean open time of the AChr channels

EPP: is the depolarisation (voltage change) of the end plate before the AP due to the influx of Na+ and efflux of K+ which was triggered by the opening of the ACh channels. It has a longer time course/ relatively slow change in voltage

Question 9

What toxins can be used to isolate End plate potential

Answer 9

alpha toxins and curare

Question 10

What is the result of conditions that interfere with the transmission of electrical impulse across the neuromuscular junction?

Answer 10

It can result in in//complete block of transmission - muscle weakness. This is made worse by exercise and better by rest

Question 11

What is Myasthenia gravis caused by and its symptoms

Answer 11

Autoimmune disorder of NMJ, where the antibodies against ACh receptor are made and these bind to the receptors. This leads to inhibition of AP initiation.
Symptoms is face muscles and respiratory muscles affected. Can be treated with immunosuppresive drugs and anti acetylcholine esterase drugs .