Structural Organization of skeletal muscle Flashcards
Striated muscles are….
Cardiac and skeletal
Non-striated muscle is…
Smooth muscle
Voluntary muscle is?
Skeletal muscle
Involuntary muscle is?
Smooth and cardiac
What is the primary role of skeletal muscle?
To produce the force and movement necessary for life
What % of the body is skeletal muscle?
35-55%
Why is skeletal muscle voluntary?
Because its contraction is controlled by input from the nervous system via alpha motor neurons. It is an elastic and excitable cell.
How many muscles fibres synapse per motor neuron?
Alpha motor neurons branch and synapse onto a number of skeletal muscle fibres.
What is a ‘motor unit’?
A single alpha motor neuron and the muscle fibres it innervates
How is contraction modulated by recruitment?
The more motor units, and therefore muscle fibres that are recruited, the large the force and strength of contraction of the muscle.
What follows its electrochemical gradient INTO the excitable cell?
Ca2+ (1.5mM) —–> Ca2+ (0.1mM)
Na+ (145mM) ——-> Na+ (10mM)
What would follow its electrochemical gradient OUT of the cell?
K+ (145mM) ——–> K+ (4mM)
What is the size principle of recruitment?
“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.
What is the purpose of schwann cells?
To enable fast propagation of AP
What are the presynaptic events?
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
What happens to the pre-synaptic membrane?
It is recycled, so there is no net loss!
What is within the presynaptic terminal boutons?
Mitochondria, neurotransmitters within vesicles, dense bar/active zone.
What is the dense bar for?
Site of neurotransmitter release
What are the steps of vesicle cycling?
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
What generates End Plate Current (EPC)
The near synchronous bindings of more then 200,000 ACH receptor ion channels. 17,000 Na+ and less K+ leave, resulting in depolarisation
What causes ACh to debind from ACh receptor ion channels?
Aceytlcholinesterase activity
How is the falling time course of EPC determined
By the random closure of individual channels over a period of time
Are AChR specific or non-specific?
Non-specific
How big is the synaptic cleft ACh has to travel?
15nm
How does acetylcholinesterase work
Hydrolyses ACH into choline and acetate
Where does choline and acetate go post-hydrolysis?
Choline: diffuses back into the presynaptic terminal and is reabsorbed (to make more ACh)
Acetate: Diffuses into surrounding medium
List the post synaptic events at the NMJ
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
What comes first, EPP or EPC?
EPC. The movement of Na+ and K+ through ACh receptors causes the EPC, which in turn causes EPP
Where would you find AChE and what does it do?
This enzyme is found anchored to the collagen fibrils of the basement membrane, where is hydrolyses ACh
What can target AChE and what would happen if this occurred?
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.
What do presynaptic abnormalities affect?
Vesicle cycling and calcium channel opening
What do Botulinum and tetanus do?
Prevent vesicle docking, muscle remains relaxed
What does a-latrotoxin do, and what animal is it found in
triggers exocytosis of vesicles, found in black widow spiders.
What is Lambert-Eaten Syndrome?
a presynaptic auto-immune disease that decrease the number of voltage gated calcium channels, so in one nerve stimulus there is less NT release
What is Myasthenia Gravis (MG) ?
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.
What are the four characteristics of skeletal muscle
1) Excitable
2) contract
3) Extensible
4) elastic
What are the Na+/K+ ATPase pumps for
to restore the membrane potential after an AP
What is the synaptic cleft?
Where the NT travels
Alpha toxins are?
Blockers of nicotonic AchR
Snake venom or curare
cause paralysis as AP cant be propagated
