Structural Organization of skeletal muscle

Question 1

Striated muscles are….

Answer 1

Cardiac and skeletal

Question 2

Non-striated muscle is…

Answer 2

Smooth muscle

Question 3

Voluntary muscle is?

Answer 3

Skeletal muscle

Question 4

Involuntary muscle is?

Answer 4

Smooth and cardiac

Question 5

What is the primary role of skeletal muscle?

Answer 5

To produce the force and movement necessary for life

Question 6

What % of the body is skeletal muscle?

Answer 6

35-55%

Question 7

Why is skeletal muscle voluntary?

Answer 7

Because its contraction is controlled by input from the nervous system via alpha motor neurons. It is an elastic and excitable cell.

Question 8

How many muscles fibres synapse per motor neuron?

Answer 8

Alpha motor neurons branch and synapse onto a number of skeletal muscle fibres.

Question 9

What is a ‘motor unit’?

Answer 9

A single alpha motor neuron and the muscle fibres it innervates

Question 10

How is contraction modulated by recruitment?

Answer 10

The more motor units, and therefore muscle fibres that are recruited, the large the force and strength of contraction of the muscle.

Question 11

What follows its electrochemical gradient INTO the excitable cell?

Answer 11

Ca2+ (1.5mM) —–> Ca2+ (0.1mM)

Na+ (145mM) ——-> Na+ (10mM)

Question 12

What would follow its electrochemical gradient OUT of the cell?

Answer 12

K+ (145mM) ——–> K+ (4mM)

Question 13

What is the size principle of recruitment?

Answer 13

“under load, motor units are recruited smaller –> larger. So smaller oxidative units that are more sensitive to change are recruited first, THEN large glycotic units.

Question 14

What is the purpose of schwann cells?

Answer 14

To enable fast propagation of AP

Question 15

What are the presynaptic events?

Answer 15

1) AP in the presynaptic cell reaches the nerve terminal
2) Depolarisation opens voltage gated Ca2+ channels, calcium follows its electrochemical gradient into the presynaptic cell
3) Ca2+ influx triggers exocytosis of vesicles containing neurotransmitter acetyl choline.
4) ACh diffuses across the synaptic cleft
5) ACh is broken down by acetylcholinesterase in the synaptic cleft

Question 16

What happens to the pre-synaptic membrane?

Answer 16

It is recycled, so there is no net loss!

Question 17

What is within the presynaptic terminal boutons?

Answer 17

Mitochondria, neurotransmitters within vesicles, dense bar/active zone.

Question 18

What is the dense bar for?

Answer 18

Site of neurotransmitter release

Question 19

What are the steps of vesicle cycling?

Answer 19

1) Neurotransmitters are actively transported into synaptic vesicles
2) Synaptic vesicles gather at the active zone
3) synaptic vesicles dock at the active zone
4) vesicles are primed
5) converted into a state of competence for Ca2+ triggered pore opening
6-8) vesicles endocytose and recycle

Question 20

What generates End Plate Current (EPC)

Answer 20

The near synchronous bindings of more then 200,000 ACH receptor ion channels. 17,000 Na+ and less K+ leave, resulting in depolarisation

Question 21

What causes ACh to debind from ACh receptor ion channels?

Answer 21

Aceytlcholinesterase activity

Question 22

How is the falling time course of EPC determined

Answer 22

By the random closure of individual channels over a period of time

Question 23

Are AChR specific or non-specific?

Answer 23

Non-specific

Question 24

How big is the synaptic cleft ACh has to travel?

Answer 24

15nm

Question 25

How does acetylcholinesterase work

Answer 25

Hydrolyses ACH into choline and acetate

Question 26

Where does choline and acetate go post-hydrolysis?

Answer 26

Choline: diffuses back into the presynaptic terminal and is reabsorbed (to make more ACh)
Acetate: Diffuses into surrounding medium

Question 27

List the post synaptic events at the NMJ

Answer 27

1) 2 ACh molecules bind to ACh receptor ion channels
2) Channels open
3) Na+ inflow, K+ outflow
4) depolaristion of motor end plate occurs (EPP)
5) opening of Na+ voltage gated channels
6) Na+ inflow
7) depolarisation of muscle fibre
8) propagation of AP

Question 28

What comes first, EPP or EPC?

Answer 28

EPC. The movement of Na+ and K+ through ACh receptors causes the EPC, which in turn causes EPP

Question 29

Where would you find AChE and what does it do?

Answer 29

This enzyme is found anchored to the collagen fibrils of the basement membrane, where is hydrolyses ACh

Question 30

What can target AChE and what would happen if this occurred?

Answer 30

Insecticides and military nerve gases (sarin)

If this occurred ACh would not breakdown, would remain bound to AChR and the muscle would remain activated, in a state of rigor.

Question 31

What do presynaptic abnormalities affect?

Answer 31

Vesicle cycling and calcium channel opening

Question 32

What do Botulinum and tetanus do?

Answer 32

Prevent vesicle docking, muscle remains relaxed

Question 33

What does a-latrotoxin do, and what animal is it found in

Answer 33

triggers exocytosis of vesicles, found in black widow spiders.

Question 34

What is Lambert-Eaten Syndrome?

Answer 34

a presynaptic auto-immune disease that decrease the number of voltage gated calcium channels, so in one nerve stimulus there is less NT release

Question 35

What is Myasthenia Gravis (MG) ?

Answer 35

a common NMJ disorder of the postsynaptic. An autoimmune disorder, where there is less ACh receptors, so it takes a lot more NT to get the same effect. Antibodies bind to the AChRs, inhibiting AP initiation. There is a weakness and fatigue in facial muscles, causing droopyness.

Question 36

What are the four characteristics of skeletal muscle

Answer 36

1) Excitable
2) contract
3) Extensible
4) elastic

Question 37

What are the Na+/K+ ATPase pumps for

Answer 37

to restore the membrane potential after an AP

Question 38

What is the synaptic cleft?

Answer 38

Where the NT travels

Question 39

Alpha toxins are?

Answer 39

Blockers of nicotonic AchR
Snake venom or curare
cause paralysis as AP cant be propagated