Exam 2- AP transmission Flashcards
AP transmission is defined as the movement of an AP from __ to __…
from the membrane of one excitable cell to the membrane of another
neuronal transmission is defined as the movement of an AP from __ to __…
from the terminals of one neuron to the dendrites of another neuron
neuromuscular transmission is defined as the movement of an AP from __ to __…
from the terminals of a neuron to the membrane of a muscle cell
describe the basis of neuromuscular transmission and the structures involved
physical basis of neuromuscular transmission lies in connection of nerve terminal membrane and muscle membrane
- nerve terminal membrane ends in a swelling called the terminal button, this comes very close to the membrane of the muscle, but there’s a space b/w them
- this arrangement is called the neuromuscular junction or the motor end plate (motor end plate is large and complex end-formation by which the axon of a motor neuron establishes synaptic contact with muscle cell
- the space between terminal and muscle membrane is called synaptic cleft
the AP has to jump from terminal membrane to muscle, how does this happen?
transmitted by a mechanism (cannot just jump thru space)
- nerve terminal has a typical sodium pump, Na and K channels, and resting potential of -70 mV
- also has a calcium ATPase pump which uses energy of ATP to pump calcium out of terminal to outside space –> this creates a large calcium concentration (high conc outside and low conc inside terminal membrane)
- also has voltage-gated calcium channels- at rest, these are gated/closed
- inside terminal are synaptic vesicles which contain a chemical substance belonging to a class of neurotransmitters (acetylcholine, Ach)
where are neurotransmitters synthesized?
either in cell body (peptides that can serve as neurotransmitters, synthesized in rough ER, packaged by Golgi, and trafficked and sit in nerve terminal button)of presynaptic neuron OR terminal button itself…it just depends
describe when Ach is synthesized in the terminal button itself
Coenzyme A taken up by mitochondria and comes out as acetyl-CoA
- Acetyl-CoA and choline combined –> acetylcholine, packaged in presynaptic vesicle
- synaptic vesicles also have other transporters- has a proton ATPase and a hydrogen ATPase
- proton ATPase uses ATP tp pump protons into the vesicle –> the protons then tend to diffuse out across vesicle membrane, transport proteins exchange protons for Ach (Ach into vesicle for protons out of vesicle)
once AP travels down axon and depolarizes membrane of nerve terminal button, what happens?
this activates voltage-sensitive calcium channels at terminal button –> calcium enters the cell and calcium current then activates a process that results in migration of vesicles to the terminal button membrane and the opening of vesicles to dump the Ach by exocytosis
- how does calcium do this…? calcium activates snare proteins (a set of complementary proteins- one protein on synaptic vesicle called synaptobrevin and in terminal button membrane is SNAP-25 and syntaxin)
- calcium activates these, changes shape and they twist, which pulls synaptic vesicle to the membrane, the 2 membranes fuse –> synaptic vesicle opens and dumps Ach in synaptic cleft by exocytosis (the Ach diffuses across synaptic cleft and binds to Ach receptor on surface of muscle- the nicotinic receptor)
name of the Ach receptor and why that is its name
nicotinic receptor (the receptor will bind either Ach or the plant substance nicotine)
- discovered pharmacologically, nicotine has same effect as Ach, so the receptors are named for the drug that was first used to document its presence
describe the structure and function of nicotinic receptor
large transmembrane complex- made of 5 subunits: 2 alpha, 1 beta, 1 delta, 1 gamma (form a circle and inside circle is sodium channel)- forms a sodium channel in muscle membrane
- alpha subunits each have a binding site for Ach (Ach binds to each of the alpha subunits), when 2 Ach bind –> the complex changes shape and the Na+ channel is activated –> inward directed Na+ current across the muscle membrane (if enough of the nicotinic receptor Na+ channels activated, depolarization of muscle membrane is enough to take it to threshold- this depolarization to threshold is called an end plate potential) –> threshold reached, voltage changes voltage-gated sodium channels on the membrane
overall: takes 2 Ach binding to nicotinic receptor to activate Na+ channel
what kind of receptor is the Ach nicotinic receptor?
ligand-gated channel (ligand is Ach)
Ach belongs to a class of neurotransmitters, which indirectly…
carries AP across synaptic cleft
AP’s carry very specific pieces on information, when AP transmitted from nerve to muscle, info has to stay intact, it needs a ___
1:1 correspondence of AP’s in nerve and muscle to keep info the same
what happens as soon as Ach diffuses across the synaptic cleft and causes an end-plate potential?
Ach is broken down by enzyme called Ach esterase, which has 2 isoforms in neuromuscular junction:
1- AchE-R: (read-through isoform), unanchored and soluble in synaptic cleft itself, this isoform is all along protein filaments
2- AchE-S: (synaptic isoform), anchored to muscle membrane in association with nicotinic receptor
describe how much Ach is needed to cause 1 AP and its association with the 2 isoforms
it takes about 200 synaptic vesicles being released from the nerve terminal to release enough Ach to diffuse across the membrane and cause an AP (each synaptic vesicle contains 50,000 molecules of Ach –> so takes 10 MILLION MOLECULES OF ACH RELEASED FROM NERVE TERMINAL TO GENERATE 1 AP ON MUSCLE)
- most of the 10 million go to saturating the AchE-R isoform, so most are broken down before it even gets across the synaptic cleft (makes sure just the right amount of Ach makes it through to interact with Ach(nicotinic) receptors
why is exocytosis such a good mechanism to dump 10 million molecules of Ach?
such a good mechanism to rapidly dump 10 million Ach at a time (if Ach were released by a channel instead, would diffuse out more slowly and all of it would be destroyed by AchE-R, none would survive to make it to muscle membrane)
1 motor neuron activates 1 muscle, ___ molecules of Ach activate 1 AP in muscle, 99.9% of it is there to…
10 million molecules of Ach activate one AP in muscle
99.9% of it there to be broken down so the other little bit can activate Na+ channel in nicotinic receptor (pretty inefficient mechanism)
after Ach is broken down into its components, it’s taken back up by ___ to be recycled to make more ___ and ___
synaptic knob
Coenzyme A & acetyl-SCoA
what is the name of the class of receptors that includes both nicotinic receptor and the other receptor, ___
cholinergic receptors
muscarinic
distribution of nicotinic vs. muscarinic receptors
nicotinic: neuromuscular junction, autonomic post-synaptic dendrite membranes
muscarinic: cardiac muscle, smooth muscle, glands
mechanism of nicotinic vs. muscarinic receptors
nicotinic: Na channel, depolarization
muscarinic: can be inhibitory or excitatory
name the inhibitors of nicotinic receptors
curare, d-tubocurarine
- these plant substances can fit into nicotinic receptors binding sites b/c has enough similar structure to Ach, but not similar enough to activate channel- so just sits on top of binding site and blocks Ach from binding
- (plant substances evolve as defense mechanisms to animals)
what is the name of the class of family of Ach receptors, all activated by Ach
cholinergic receptors
nicotinic receptors are activated by ___ and ___
muscarinic receptors are activated by ___ and ___
Ach & nicotine
Ach & muscarine (plant alkaloid)
nicotinic receptors are always ___ in mechanism
muscarinic can be ___ or ___
excitatory
excitatory or inhibitory
name inhibitors of muscarinic receptors
atropine (muscarine and atropine have similar effects b/c of structural similarities)
why is there more than one type of Ach receptor?
(nicotinic & muscarinic)
- small changes in amino acid composition of the protein (small mutation in nicotinic AA –> new receptor with slightly diff function) –> 5 isoforms of muscarinic
- allows for differentiation of function of the same neurotransmitter
is there any cross-reactivity b/w the 2 types of cholinergic receptors?
no, nicotine does not effect muscarinic receptors and muscarine does not effect nicotinic receptors
do both nicotinic and muscarinic receptors directly form ion channels?
no- nicotinic receptors form sodium channel; but muscarinic receptors do not directly form ion channels
- rather, muscarinic act through diff signal transduction pathways which will activate an ion channel
describe how muscarinic receptors can act in both an excitatory and inhibitory way
1- (inhibitory)- Ach activates muscarinic receptor which activates a channel, ultimately opens a potassium channel and hyperpolarizes it, and inhibits the tissue
2- (excitatory)- can also ultimately open a calcium channel, resulting in depolarization and an AP
names of the nicotinic receptors in neuromuscular junctions and autonomic nervous system
in NMJ- Nm or N1
autonomic- Nn or N2
which of the 5 isoforms of muscarinic receptors are inhibitory and excitatory
M 1-5
M1,3,5 = excitatory
M2,4 = inhibitory
location and pharmacological action of M1 isoform muscarinic receptor
location: CNS, gastric parietal cells
action: CNS excitation, gastric acid secretion
location and pharm action of M2 isoform muscarinic receptor
location: heart
action: cardiac inhibition (bradycardia)
location and pharm action of M3 isoform muscarinic receptor
location:
exocrine glands
smooth muscles (GIT, urinary tract, bronchial muscles)
vascular endothelium
action:
secretion of glands
smooth muscle contraction
vasodilation (via nitric oxide)
location and pharm action of M4&5 isoform muscarinic receptor
location: CNS
action: memory, arousal, attention, and analgesia
there are dozens of NT’s in the body, for a substance to be considered a neurotransmitter, it must follow these criteria:
1- must be found in and synthesized in the pre-synaptic neuron/terminal
2- must be released upon stimulation of the pre-synaptic neuron
3- direct application of the NT to the post-synaptic membrane must have the same effect as stimulation of the pre-synaptic membrane
4- must be a mechanism for the rapid termination of the NT effect
