F4. neurotransmission Flashcards
what is membrane excitability?
“Excitability” refers to the ability to dynamically alter the electrical potential (voltage) across the plasma membrane
what is potential difference in membranes?
Any difference in the concentration of charged molecules across the membrane, will generate a potential difference which may cause excitement
what is the resting membrane potential for cells?
The “resting” membrane potential for cells is negative (-50 to -90 mV).
Describe ion channels
-Membrane-spanning proteins that open a selective pore, allowing ion entry or exit
-Change cell excitability (mainly Na+ and K+)
-Influx of “signalling” ions – e.g. Ca2+ ONE NOTE- action potential propagation
-concentration differences in ions and potential difference provide driving force for ion movement across membranes
Describe voltage-gated ion channels
-Membrane depolarization (inside the cell becomes more positive) causes structural re-arrangements in the ion channel protein
-Voltage gated potassium channel opens in response to depolarization ONE NOTE
Describe action potential generation 1
Depolarization opens Na+ channels (fast)
Na+ floods into the cell down its electrochemical gradient
This depolarizes the membrane even more
-40mV is threshold for action potential generation
ONE NOTE
Describe action potential generation 2- depolarisation peak
K+ channels open (more slowly than Na+ channels)
K+ floods out of the cell down its electrochemical gradient
Na+ channels inactivate preventing further depolarization
This repolarizes the membrane
ONE NOTE
Describe action potential generation 3
Need to reestablish resting potential as it becomes more negative than resting potential
K+ channels inactivate.
Ion pumps and transporters use energy from ATP or counter-transport to re-establish resting membrane conditions
ONE NOTE
Describe saltatory conductance
-Mechanism for increasing speed and reliability of conduction
-Glial cells/oligodendrocytes coat axons in insulating myelin sheaths
-Ion channels are clustered at unmyelinated “Nodes of Ranvier”
-Action potential “jumps” between nodes
-back movement doesn’t occur as resting potential is always reestablished
ONE NOTE
Describe chemical transmission
-Once the action potential reaches a synapse (neuron to neuron junction in the ganglion) or terminals to other cell types (e.g. smooth muscle, endocrine cells), chemical transmitter is released.
-presynaptic cells has presynaptic vesicles packed with neruotransmitter
-postsynaptic cell has neurotransmitter receptors one ligand-ion gated channels
ONE NOTE
Describe the synthesis and storage of transmitter
-Synthetic enzymes generate NT from inactive precursors
-Vesicular transporters use active transport to concentrate NT within the vesicle for storage
Describe vesicle release
An incoming action potential depolarizes the terminal
Voltage-gated calcium channels open
Calcium triggers vesicle fusion and transmitter release
ONE NOTE
Describe the activation of postsynaptic receptors
-Nicotinic receptors for the transmitter on the postsynaptic cell are activated.
Ligand-gated ion channels open, influx of Na+, triggering AP generation
ONE NOTE
Describe Nicotinic acetylcholine receptor
nAChR
Open pore lets Na+ and K+ (and some Ca2+) ions through
Describe clearance of transmitter
-The synaptic “signal” must be turned off
Enzymatic degradation (acetylcholinestrase) or active uptake of transmitter eliminates it from the extracellular space (noradrenaline transporter)
ONE NOTE
Describe autonomic innervation of tissues
-The nerve terminals of autonomic projections run through almost all tissues
-Form boutons (varicosities) that lie close to target cells
-Sites for vesicular transmitter release
-Activate receptors on target cells
Describe release at autonomic terminals
-Boutons release onto smooth muscle, endocrine cells etc.
-Activate G protein coupled receptors (GPCRs)
-Activation or inhibition of second messenger pathways (metabortropic signalling)
ONE NOTE
Describe sympathetic terminals
-Noradrenaline acts on alpha and beta adrenoceptors
-a and B receptors have variable effects depending on cell type (aR cause smooth muscle contraction and BR cause smooth muscle relaxation)
ONE NOTE
Describe parasympathetic terminals
-Acetylcholine acts on muscarinic (M) receptors
-M1, M2 and M3 receptors have variable effects depending on cell type
-M1,M3 causes smooth muscle contraction and M2 causes cardiac muscle relaxation
Describe sodium concentration difference
higher concentration outside the cell
Describe potassium concentration difference
higher concentration inside the cell
Describe chloride concentration difference
higher concentration outside the cell
Describe calcium concentration difference
higher concentration outside the cell
