Guyton Physiology (13th ed) Ch 7: Excitation of Skeletal Muscle

Neuromuscular junction

A junction between a nerve ending and the midpoint of a muscle fiber. Only one junction per fiber is present.

Motor end plate

Complex of branching nerve terminals that invaginate into the surface of the muscle fiber but lie outside the muscle fiber plasma membrane.

Synaptic gutter (Synaptic trough)

The invaginated muscle fiber plasma membrane where the motor end plate lies.

Synaptic Cleft or Space

The space between the nerve terminal and muscle fiber membrane. (20-30nm)

Subneural cleft

Numerous smaller folds of the muscle membrane that increases the surface area at which the neurotransmitter can act.

Acetylcholine (Ach)

An excitatory neurotransmitter synthesized in the cytoplasm of the nerve terminal, but is absorbed rapidly into small synaptic vesicles.

300,000 vesicles/end plate

How many synaptic vesicles are normally found in the terminals of a single end plate?

Acetylcholinesterase (AChE)

An enzyme that destroys Ach a few milliseconds after it has been released from the vesicles into the synaptic space.

125 vesicles

How many vesicles are released into the synaptic space when a nerve impulse reaches the junction?

Voltage-gated calcium channels

What membrane channels are found on each side of a dense bars in the inside surface of the neural membrane?

Synapsin

Ca-calmodulin dependent protein kinase, when activated, is responsible for phosphorylation of _____________ which anchors Ach vesicles to the cytoskeleton.

Exocytosis

Vesicles dock at release sites, fuse with the neural membrane and empty Ach into the synaptic space via _______________.

Ach-gated ion channels

Ach receptors found on the post-synaptic muscle fiber membrane. Located near the mouth of the subneural clefts immediately below the dense bar areas.

2 Alpha, 1 Beta, 1 Delta, 1 Gamma

Five subunits of fetal acetylcholine receptor complex

2 Alpha, 1 Beta, 1 Delta, 1 Epsilon

Five subunits of adult acetylcholine receptor complex

Alpha

Ach receptors remain constricted until 2 Ach molecules attach to the _______ subunits, causing a conformational change that opens the channel.

0.65nm

Diameter of the Ach-gated channel. Large enough to allow passage of impt ions (Na+, K+, Ca2+)

15,000-30,000

How many Na+ ions can the Ach-gated channel transmit in a millisecond?

True

Chloride and other negative anions cannot pass through the Ach channels because of the strong negative charges at the mouth. (True or False)

End plate potential (EPP)

A local positive potential change inside the muscle fiber membrane after influx of large numbers of Na+ ions -> action potential -> muscle contraction.

False

AchE is released into the synaptic space from vesicles, similar to Ach. (True or False)

Curare

A drug that blocks the gating action of Ach on the Ach channels by competing for the receptor sites.

Botulinum toxin

A bacterial poison that decreases the quantity of Ach released by nerve terminals.

Fatigue

_____________ of the neuromuscular junction occurs upon stimulation of the nerve fiber for >100x/sec for several minutes.

Golgi apparatus

What cell organelle in the spinal cord motor neuron is responsible for synthesis of the Ach vesicles?

Cytosol

Ach is synthesized in the ___________ of the nerve fiber terminal.

10,000

How many molecules of Ach are concentrated in each vesicle?

Acetate ion, choline

Degradation products of acetylcholine after action of acetylcholinesterase.

Clathrin

Contractile protein attached to the nerve fiber membrane at the areas of release of the original vesicles, forming coated pits.

20 secs

How long does it take for clathrin to contract and break away from the pre-synaptic membrane, forming new vesicles?

True

The action of Ach-like drugs persists for several minutes to hours because they are not destroyed by AchE. (True or False)

Methacholine, carbachol, nicotine

Drugs that cause localized areas of depolarization at the post-synaptic terminal where Ach receptors are located, causing muscle spasm. (MCN)

B

All of these drugs combine with AchE inactivating it for several hours, except:

A. Neostigmine

B. Nicotine

C. Phytostigmine

Diisopropyl fluorophosphate

A powerful and lethal nerve gas poison that inactivates AchE for weeks

Curariform drugs

Group of drugs that can prevent passage of impulses from the nerve ending into the muscle.

A

Which of the following is not true of Myasthenia gravis?

A. Autoimmune neuromuscular disorder causing muscle weakness due to Abs that destroy Ach

B. End-plate potentials too weak to initiate opening of voltage-gated Na+ channels

C. Can be ameliorated by anti-AchE drugs

D. NOTA

C

The following is true, except:

A. Skeletal muscle RMP= similar to large nerves

B. Duration of action = 5x longer than nerves

C. Velocity of conduction = 13x faster than nerves

Transverse tubules (T tubules)

Structure responsible for transmission of action potentials deep within the vicinity of the separate myofibrils.

Excitation-Contraction Coupling

Maximum muscle contraction resulting from transmission of action potentials along T tubules that penetrate all the way through the large muscle fibers -> release of Ca+ ions in the immediate vicinity of myofibrils -> contraction

True

The T tubules communicate with the ECF and are internal extensions of the cell membrane. (True or False)

Terminal cisternae (Terminal cisterns)

Large chambers of the sarcoplasmic reticulum that abut the T tubules.

False

A special feature of the sarcoplasmic reticulum is the excess sodium ions within its tubules that are released when an action potential occurs. (True or False)

Dihydropyridine receptors

T tubule receptors that sense a change in volage and are linked to calcium release channels in the tubular cisternae.

Ryanodine receptor channels

Calcium release channels in the sarcoplasmic reticulum cisterns that are activated during an action potential.

False

A continuously active calcium pump in the walls of the sarcoplasmic reticulum pumps Ca+ ions towards the myofibrils, initiating more contractions. (True or False)

Calsequestrin

A protein found inside the sarcoplasmic reticulum that binds up to 40x more Ca+

500x

Full excitation of T tubule and sarcoplasmic reticulum causes a _____x increase in Ca+ concentration, which is 10x the level required for maximum muscle contraction