Skeletal Muscle 1: Neuromuscular junction Flashcards
What innervates the muscle and what is its neurotransmitter?
Alpha motor neurons
ACh
What is a motor unit?
A motor nerve and all the muscle fibres it innervates
Do motor neurons different fibres types? What are the fibre types?
No. They only innervate one type of fibre type at a time.
Type 1 (Slow) Type 2 (Fast)
How does the contractile function of skeletal muscle relate to the excitation?
Increasing excitatory input progressively recruits more muscle fibres. Therefore the contractile function can be graded.
What is the size principle?
The size principle ensures efficiency by;
Ensuring small, oxidative, motor units are recruited first. (Fewer large glycolytic motor units therefore last to be recruited).
Describe the presynaptic events at the neuromuscular junction;
The AP reaches the motor end plate and voltage gated Ca channels open due to the depolarisation of the terminal.
Ca influxes and triggers the movement of ACh filled vesicles to the active zone of the membrane and stimulates exocytosis.
ACh diffuses across the synpatic cleft binding to ACh receptors before being broken down by ACh esterase.
Are motor units assynchronous?
All muscle fibres within a motor unit contract in syncrony
What does the motor end plate have structurally to enhance the efficiency of the synaptic cleft?
Junctional folds exist to increase the SA / number of ACh receptors at the motor end plate
How many ACh molecules must bind to the nicotinic ACh receptor?.
2
Are the neuromuscular junctions myelinated?
No
What are the two distinct domains of the motor end plate?
The crests of the junctional fold. Contains many AChRs and AChR clustering proteins i.e Rapsyn, Utrophin
In the depths of the junctional fold there are high concentrations of VGSCs (Voltage gated sodium channels)
What sort of transduction in the neuromuscular junction?
Chemo-mechanical.
Describe the post synaptic events of the NMJ;
2 ACh binding at transmitter gated channels
Channels open
Na+ inflow
K+ outflow
Depolarisation of the motor end plate (EPP)
Opening of voltage gated Na channels, Na+ inflow and depolarisation. = Propogation of Muscle AP.
Why are they called alpha toxins?
Bind to the Alpha unit of the nicotinic AChR. (two alpha untis for activity)
What recycles ACh?
ACh Esterase.
- Bound to collagen.
- Breaks ACh into water and choline. Choline recycled.
- AChE is target of nerve agents.
