Lecture 6 Flashcards
Describe the cellular characteristics of a single axon terminal (synaptic vesicles, mitochondria, dense bars)
Characteristics of a single axon terminal:
• Many mitochondria
• Synaptic vesicles with acetylcholine:
- Release neurotransmitters via exocytosis:
- 300,000
• Dense bars:
- Anchored to the presynaptic membrane and associated with synaptic vesicles to which they are tethered by short filaments
Describe the characteristics of a motor end plate (synaptic gutter, subneural clefts, synaptic cleft)
• 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.
Describe the structure and subunits of an Ach-gated channel
Sarcolemma of the skeletal muscle:
• Has acetylcholine-gated ion channels:
- 275,000 mw
- 2 α proteins, 1 β protein, 1 γ protein, 1 δ protein
- Tubular channel remains closed until two acetylcholine molecules attach to its α subunits.
• Acetylcholinesterase
End plate potential
Identify the subunits Ach attaches
2 α proteins
Where are the vesicles for neurotransmitters formed in the neuron, how are they transported
• 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
- Ach is synthesized in the cytosol of the nerve axon terminal. - Calcium ions are thought to draw synaptic vesicles closer to neurolemma next to the voltage gated calcium channels.
Compare concentrations of Ca2+ ion outside axon and inside axoplasm
- (ECF Ca2+ conc. = 1-2 mM; intra Ca2+ =
How does Ca2+ enter the axon during the transmission of an action potential?
- When the action potential arrives at the terminus of the axon, voltage-gated calcium channels open and calcium ions enter the axon terminus
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.
State the # of Ach molecules that attach each ligand-gated channel
Two Ach molecules bind to each ligand-gated channel on the sarcolemma.
Define end-plate potential
Large numbers of sodium ions pass through the muscle fiber membrane (sarcolemma), creating the end-plate potential (50-75 mV), which initiates an action potential on the sarcolemma.
List mechanisms Ach is removed from the synaptic cleft and describe role of acetylcholinesterase
Acetylcholine is removed from the synaptic cleft:
• Degradation into choline and acetate by acetylcholinesterase
• Reuptake of choline by axon end terminal
• Diffusion of Ach away from site.
List examples of drugs that mimic Ach but are not broken down by acetylcholinesterase and describe their effect on muscle contraction
Methacholine, carbachol, and nicotine:
• Have the same effect on muscle fibers as acetylcholine
• But are not broken down by acetylcholinesterase;
• They cause spasm.
List drugs that inactive acetylcholinesterase and describe their effect on muscle contraction
Neostigmine, physostigmine, and diisopropyl fluorophosphates:
• Inactivate acetylcholinesterase
• Cause muscle spasms.
Describe the effect of curare on skeletal muscle contraction
• Prevents passage of impulses from nerve ending into muscle
Describe the underlying cause of myasthenia gravis and its effects
- Autoimmune disease
- Antibodies attack acetylcholine receptors.
- End plate potentials are too weak to initiate opening of the voltage-gated sodium channels.
Explain how neostigmine can help to alleviate the effects of myasthenia gravis
Neostigmine can be used to inactivate acetylcholinesterase.
