Unit 7: skeletal muscle Flashcards
the vast majority of skeletal muscles have cells that are innervated by this many motor neurons
ONE
where do the motor neuron cell bodies lie in the spinal cord?
the ventral (anterior) horns
what are two pathways for motor neurons to get “excited”?
- the descending pathways from the brain
- the reflex arcs
what is the sarcoplasmic reticulum (SR)?
where skeletal muscle stores Ca2+
what are the transverse (T) - tubules
these tubules helps propogate the AP across the muscle fibers
what are the two contraction elements in a skeletal muscle fiber?
actin + myosin
these structures shorten the muscle fibers during contraction
what is the mitochondria’s role in the skeletal muscle?
helps provide ATP to the skeletal muscle
also located in presynaptic neuron for ATP use in ion channels
provides “acetate” for ACh formation
differentiate primary vs secondary clefts
primary: one infolding
secondary: two infoldings within one groove
Where are the ACh receptors located on the subneural clefts?
typically on the more superior/surface of the subneural cleft
where do VG Na+ channels lie in the subneural cleft?
they are congregated deeper inside the cleft at the base and along the inferior walls
what is AChE?
an enzyme that breaks down ACh
expressed by the skeletal muscle and parks it in the NMJ
it shuts the stimulus (ACh) down to allow neuronal repolarization
how does AChE break down ACh?
hydrolysis to form acetyl/acetate and choline
choline can be recycled into the neuron
how many ACh-Rs are at a typical NMJ?
5 million
how many ACh-Rs get activated at a typical NMJ during AP?
500,000 or 10% of 5 million
how many ACh molecules typically get released into the NMJ during an AP?
1 million at bare minimum (to elicit muscle contraction)
typically 2 million, but not all the ACh makes it to the skeletal muscle (d/t AChE)
on a N-ACh-R, which subunit binds the ACh molecules?
the alpha subunits bind the 2 ACh molecules
what is an example of a n-ACh-R antagonist?
curare
curare can bind to ___ subunit on the n-ACh-R to shut it down
ONE
(does not need to bind to both alpha subunits)
this sets up exocytosis for ACh release into NMJ
Ca2+
what are the voltage sensors in a skeletal muscle cell and what do they do?
voltage sensors located in the t-tubules & cellular wall recognize an AP that is propogated by the fast Na+ channels
these voltage sensors are AKA dihydropyridine (DHP) receptors
in response to an AP, DHPRs will “tug” on their attachment to the calcium release channels to liberate Ca2+ “popping the cork”
SOME Ca2+ can influx via the dihydropyridine receptors (but not much)
what is a RyR (ryanodine receptor)?
RyR is a Ca2+ release channel gatekeeping Ca2+ in the SR
what is SERCA?
“sarcoplasmic endoplasmic reticulum calcium ATPase”
this structure burns ATP to put Ca2+ BACK into the SR against it’s [ ] gradient
list all the steps to excitation-contraction coupling:
- motor neuron depolarizes from brain/or reflex arcs
- Ca2+ influx to motor neuron – primarily thru voltage sensitive p-type Ca2+ channels
- ACh storage vesicles fuse to presynaptic neural cell wall/membrane
- ACh released into NMJ via exocytosis
- 2 ACh bind to 1 n-ACh-R; primarily sodium INFLUX and secondarily Ca2+ influx
- threshold potential met generating EPP/local depol and (as long as skeletal tissue is healthy) turns into an AP
- AP spreads down muscle fibers in bidirectionally
- AP spreads via VG Na+ channels
- muscle depolarization sensed by DHPRs (in t-tubules and cell wall)
- DHPRs pull on RYR “doors” to release Ca2+ from SR
- Ca2+ influx into sarcoplasm; recycled into SR via SERCA
- Ca2+ removed from sarcoplasm terminates contraction