Skeletal muscle 3

Question 1

What is the motor unit consist of

Answer 1

A motor unit has one motor neuron and all the muscle fibres it innervates

Question 2

Draw/explain how a muscle can have many motor units of different fibre type

Answer 2

Lecture slide

Question 3

Is skeletal muscle voluntary or involuntary

Answer 3

Voluntary muscle as contraction is controlled by input from the nervous system via alpha motor neurons.

Question 4

whats a motor unit in terms of alpha motor neuron

Answer 4

A single AMN and the muscle fibre it innervates = motor unit

Question 5

one rule with motor units

Answer 5

The motor nerves innervate cells of the SAME fibre type, type 1 (slow) and type 2 (fast)

Question 6

Recruitment of motor units

Answer 6

Operation of the size principle ensures that efficency is met in that SMALL, OXIDATIVE motor units are recruited first.
The larger the unit, the last it will be selected, eg glycolytic motor units.

Question 7

Draw the structure of the neuromuscular junction

Answer 7

Lecture slide

Question 8

Presynaptic events:
Describe the overall process (5 major steps)

Answer 8

1. An action potential (AP) in the presynaptic cells reaches the nerve terminal
2. Depolarisation of the terminal opens the voltage gated Ca2+ channels, causing Ca2+ influx
3. Increased conc of Ca2+ triggers exocytosis of vesicles and quantal release of Ach neurotransmitter
4. Ach diffuses across the synaptic cleft and binds to the Ach receptors
5. Ach is broken down by acetylcholinesterase in the synaptic cleft

Question 9

Draw vesicle cycling

Answer 9

Early endosome, Neurotransmitter uptake, docking, priming, fusion, endocytosis, vesicle acidification.

Firstly, the neurotransmitter molecules are actively transported into synaptic vesicles and these cluster in front of the active zone. The vesicles then dock at the active zone where they are primed to convert them into a state of competence for Ca2+ triggered fusion-pore opening. After fusion-pore opening the vesicles endocytose and are recycled.

Question 10

Post synaptic events:
End plate current (EPC)

Answer 10

At the motor end plate on the muscle fibre, EPC is generated by the near synchronus opening of more than 200,000 indivual Achr ion channels, each permitting around 17,000 Na+ ions to enter the muscle EP (also allowing a smaller number of K+ ions to leave causing a net inward depolarising current)

Indiviaul AchR ion channels open and contribute to the discrete increments of EPC

Channels close as Ach unbinds and is removed from the synapse by Ach esterase activity

The falling time course of EPC is deteremines by random closure of indivual channels that are open for a rnage of time

Question 11

what is total EPC

Answer 11

Total EPC: sum of all indivual channel currents

Question 12

End plate potental:
role and where does it travel to

Answer 12

EPP are produced by EPC and are always suprathreshold and therefore trigger AP in muscle fibre.

EPP is the depolarization caused by EPC acting on the muscle membrane. EPC is the kinetic interaction of acetylcholine (ACh) with the postjunctional nicotinic acetylcholine receptor (nAChR). EPP is generated when ACh interacts with the nAChR, causing the receptor ion channel to open and depolarize the region around the end plate.

Travels passively along the post synaptic cell (muscle fibres) for only a short distance before dissapting

Question 13

EPP vs EPC time courses

Answer 13

EPC has time course defined by mean open time of achr channels

EPP has longer time course defined by both Achr Channel open time AND passive electrical charging/discharging time of muscle membrane

Question 14

Overall sequence of events in neuromuscular transmission

Answer 14

1. AP in presynaptic cell reaches nerve terminal
2. Depolarisation of nerve terminal opens voltage agtes Ca2+ channels
3. INcreased conc oc Ca2+ triggers exocytosis and vesicles and quantal release of Ach

4. Binding of 2 ach to each post synaptic transmitter gated channel
5. channel opens
6. influx of Na and Ca, efflux of K
7. Depolarisation of motor end plate (EPP)
8. Opening of voltage gated Na channels
9. Na influx
10. Propagted AP in muscle fibre

Question 15

EPP vs EPC

Answer 15

EPP is a transient depolarization of the muscle membrane, while EPC is an influx of positive ions

Question 16

Epimysium
Perimysium
Endomysium

Answer 16

Epimysium - surround entire muscle
Perimysium- surrounds muscles bundles or fascicles (bundle of fibres)
Endomysium - surrounds each muscle fibre or muscle cell

Question 17

Sarcolemma vs transverse tubular membrane system

Answer 17

Sarcolemma: is the plasma membrane of the skeletal muscle fibre (cell). It is an exictbale membrane, similar to that of the neural cell membrane

transverse tubular membrane system: deep invaginations of sarecolemma into the muscle cell whihc oncduct the propgrated AP causing localised contraction of filaments.

Question 18

Abnormal neuromusclar transmission

Answer 18

Myasthernia gravis
AID where antibodies against the Ach recceptor are made
Binding of the Ab to receptors in endplate causes reduction in functional receptors = less AP

affects: eyelids, facial expression, swallowing

treat: anticholingerics

Question 19

Force contraction is modulated by

Answer 19

number and characteristics of motor units recruited

Question 20

Structure of NMJ

Answer 20

At the NMJ, the motor axon loses sheth and brancesm each branch makes synaptic contact with the muscle fibre.

The crests are the motor end plates and have lots of ACHRS

The depths of crests ahve lots of voltage gated Na channels

Question 21

abnormal pre synaptic and abnormal post synaptic

Answer 21

Pre:
Diabetes

Post:
alpha toxins and anti-ach esterases