Lecture 6 - Neuromuscular Junctions

Question 1

Describe the cellular characteristics of a single axon terminal

Answer 1

Characteristics of a single axon terminal include
- Many mitochondria
- Synaptic vesicles with acetylcholine (These release about 300,000 neurotransmitters via exocytosis)
- Dense bars: (Anchored to the presynaptic membrane and associated with synaptic vesicles to which they are tethered by short filaments)
The axon also has a Synaptic gutter and a synaptic cleft

Question 2

Describe the synaptic gutter and the synaptic cleft

Answer 2

Synaptic gutter (trough): This is a groove or furrow in the surface of a sarcolemma in which the axon terminal makes contact with the sarcolemma. Subneural clefts are smaller clefts or troughs in the bottom of the synaptic trough. 
- Synaptic cleft: 20-30 nm wide, this is a very narrow but real gap between the axolemma of the axon terminal and the sarcolemma of the innervated muscle fiber.

Question 3

Describe how the sarcolemma in the skeletal muscle interacts with the axon muscle.

Answer 3

• Sarcolemma has acetylcholine-gated ion channels:
• 275,000 mw
• 2 αproteins, 1 βprotein, 1 γ protein, 1 δ protein
• Tubular channel remains closed until two acetylcholine molecules attach to its α subunits.
• Acetylcholinesterase
• Acts to generate end-plate potential

Question 4

Where are vesicles for the neurotransmitters formed in the neuron?
How are they transported?
What first happens when the action potential arrives to the terminus of the axon?
Review Figure 7-2 (Slide 11)

Answer 4

• 40 nm vesicles are formed in the Golgi apparatus and are carried by axonal transport to the axon terminus where they are filled with Ach (Refer to Figure 7-2)
• Ach is synthesized in the cytosol of the nerve axon terminal.
• When the action potential arrives at the terminus of the axon, voltage-gated calcium channels open and calcium ions enter the axon terminus.
• (ECF Ca2+ conc. = 1-2 mM; intracellular Ca2+ =

Question 5

What do dense bars do?

Answer 5

Not really sure. Nobody is. It’s thought that they guide vesicles to the site of exocytosis.

Question 6

After the Ach binds to the ligand-gated channels on the sarcolemma, how does end-plate potential occur.

Answer 6

• The vesicles fuse to the neuronal membrane and empty their contents into the synaptic cleft.
• Two Ach molecules bind to each ligand-gated channel on the sarcolemma.
• Both sodium and potassium ions pass through the channels, but sodium ions are more permeable.
• The principal effect is for large numbers of sodium ions to pass through the muscle fiber membrane (sarcolemma), creating the end-plate potential (50-75 mV), which initiates an action potential on the sarcolemma.

Question 7

After the action potential is in the sarcolemma, what causes the sliding filament mechanism?

(Optional: Review 7-3)

Answer 7

• The action potential on the sarcolemma continues down the T-tubules and activates voltage-gated dihydropyridine channels.
• Dihydropyridine channels activate ryanodine receptors (ryanodine-sensitive calcium ion release channels) on the sarcoplasmic reticulum membranes, allowing calcium ions to move quickly through the ryanodine receptors into the cytosol at the A-I boundaries.
• The ryanodine receptor is also activated by the calcium released into the cytosol, thus allowing more calcium ion to be released. (An example of positive feedback)
• Calcium ions bind to troponin in the sarcomeres, resulting in the “sliding filament” mechanism.
• High cytosolic calcium ion concentrations (after calcium release from the sarcoplasmic reticulum) promotes ryanodine channel closure.

Question 8

List the mechanisms by which acetylcholine is removed from the synaptic cleft and describe the role of acetylcholinesterase

• What is meant by excitation-contraction (electro-mechanical) coupling?

Answer 8

• Within a few seconds after each action potential clathrin coated pits appear in the axon nerve terminal. The pits separate from the plasmalemma and are then refilled as new synaptic vesicles.
• Acetylcholine is removed from the synaptic cleft: Degradation into choline and acetate by acetylcholinesterase, followed by Reuptake of choline by axon end terminal, and lastly diffusion of Ach away from site.
• Electro-mechanical coupling is the overarching term that refers to what goes in and what comes out.

Question 9

Once again, what are the first 11 steps to skeletal muscle contraction

Answer 9

Fuck, again? Fine.
(1. Action Potential in Alpha Motor Neuron), (2. Calcium-Ion Influx into Axon Terminal), (3. Exocytosis of Synaptic Vessicles), (4. Ach released into the Synaptic Cleft), (5. Diffusion of Ach across Cleft), (6. Binding of Ach on Ach Receptors on Sarcolemma), (7. Opening of Ligand-Gated Sodium Channels), (8. Sodium-ion influx), (9. End-plate depolarization), (10. Opening of voltage-Gated Sodium Channels), (11. Sarcolemma Action Potential)

Question 10

What are the Next 12 steps of a skeletal muscle contraction (beginning with the sarcolemma action potential)

Answer 10

(1. Sarcolemma action potential), (2. Depolarization of T-Tubules), (3. Conformational Change in DHP receptors), (4. Conformational change in Ryanodine Receptors), (5. Opening of ryanodine receptor calcium channels), (6. Release of Calcium from Sarcoplasmic Reticulum), (7. Calcium Concentration in the cytosol goes from 0.1 umol/Liter to 10umol/Liter), (8. Binding of Calcium to Troponin C), (9. Conformational Change in Troponin), (10. Tropomyosin is pulled away from active sites on actin), (11. Exposure of Actin Sites on Actin), (12. Binding of Myosin Heads to Actin Active Sites)

Question 11

What are 3 drugs that mimic the effects of acetylcholine?

Answer 11

• Methacholine, carbachol, and nicotine:
• They have the same effect on muscle fibers as acetylcholine
• But they are not broken down by acetylcholinesterase; they cause spasm.

Question 12

What are two drugs that inactivate acetylcholinesterase?

Answer 12

Neostigmine, physostigmine, and diisopropyl fluorophosphates

• These Also cause muscle spasms

Question 13

What is curare?

Answer 13

Prevents passage of impulses from nerve ending into muscle

Question 14

What is the underlying cause of myasthenia and what are it’s effects?

Answer 14

• Autoimmune disease
• Antibodies attack acetylcholine receptors.
• End plate potentials are too weak to initiate opening of the voltage-gated sodium channels.
• Neostigmine can be used to inactivate acetylcholinesterase
• By inactivating acetylcholinesterase, more receptors can bind to what’s there, and bind there for longer, which might allow enough ligand-gated channels to get a high enough voltage to get an action potential.