Axonal and Synaptic neurotransmission 17.02.23 Flashcards
What is an axonal transmission vs a synaptic transmission?
Axonal: transmission of information from location A to location B
Synaptic: Integration/ processing of information and transmission between neurons
At rest, what charge does a neuron have?
Has a negative electric charge
What substances cross the neuron’s semi-permeable membrane?
- Potassium and Chloride which are electrically charged, cross readily
- Some cross with difficulty (Na+)
- Some not at all (large organic proteins)
Are A- (anions, protein) inside or outside the cell?
They are restricted to inside of the cell
Are Na+ ions inside or outside the cell?
Mostly outside neuron
Are K+ ions inside or outside the cell?
Mostly inside neuron
Are Cl- ions inside or outside the cell?
Mostly outside neuron
How does the sodium potassium pump distribute ions?
- Active transport Na+ ions out of neuron and K+ in
- 3 Na+ for every 2 K+
- Requires energy supplied by ATP
What is the final resting potential of a neuron?
70 mV
- Na+ high conc outside so Na+ movement into the neuron is restricted
- K+ and Cl- can move back and forth across membrane so reach steady state
When will an action potential be formed?
- Membrane potential is stable until receptors open ion channels and Na+ channels open
- Action potential created when the membrane potential is depolarised beyond the threshold of excitation
- Na channels then close and cell becomes repolarised
What is an EPSP?
Excitatory post synaptic potential
- Postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential.
- Typically, one EPSP is not enough to reach the AP threshold
- Therefore, and AP is generated via the combined effects of multiple synapses
What is an IPSP?
Inhibitory Post synaptic potential
- Decreases the probability of an action potential being elicited
What begins to depolarise the cell membrane?
- EPSPs begin to depolarise the cell membrane
- The threshold is -60mV, when this is reached Na+ channels open and polarity reverses to +30 inside
When do voltage gated Na+ channels close?
- Membrane potential reverses with the inside going positive, this is when the Na+ channels close and K+ channels open
- K+ rushes out
- Restoring membrane potential
Why is an action potential self perpetuating?
- voltage changes are caused by the opening or closing of ion channels
- but the channels are opened by voltage changes
- Thus, voltage changes control the ion channels which control the voltage changes
- It is a cycle
What speeds up axonal conduction?
Myelination
What is saltatory conduction?
- Decremental conduction between nodes (action potential jumps from each node of Ranvier) but it is re-boosted each time
- Very fast along axon
- this happens in most CNS neurons
What happens when the action potential reaches the terminal buttons?
- Voltage activated calcium channels open
-This leads to an influx of calcium, which changes the state of certain membrane proteins in the presynaptic membrane, and results with exocytosis (release) of the neurotransmitter in the synaptic cleft. - Then a normal synapse occurs and receptors on the post-synaptic membrane accept the neurotransmitter and the action potential continues
Why doesn’t the neurotransmitter remain active in the synapse and continually make new action potentials?
- Enzyme degradation (acetylcholinesterase is the enzyme that breaks down the neurotransmitter acetylcholine)
- Reuptake (actively transported back to presynaptic axon)
- Diffuses away from receptor site
What does acetyl choline do?
Key neurotransmitter at the neuromuscular junction - it activates muscles
What are some fast neurotransmitters?
- acetylcholine
- glutamate
- GABA
(short lasting effects)
What are some slow neuromodulators?
- Dopamine
- Noradrenaline
- Serotonin
(slower timescale)
What drugs affect the transmitter at the neuromuscular junction?
- Cigarettes (nicotine)
- Poison arrows (curare)
- Spider toxins (black widow)
What is noradrenaline affected by?
- Antidepressant drugs (blocks re-uptake and blocks break down)
- Stimulants (amphetamine - increases release and blocks re-uptake)
How do hallucinogenic drugs work?
Stimulate serotonin and can activate numerous serotonin receptor subtypes
What does the resting membrane potential usually sit at?
Sits between -50 and -75 mV and the inside more negative
Where are the higher concentrations of K+, Na+ and Cl-?
- K+ ions normally found at higher concentrations inside the cell
- Na+ and Cl- are at higher concentrations outside the cell
What causes action potentials to only flow in one direction?
Refractory periods
What happens to axonal transmission with multiple sclerosis?
- Degeneration of myelin and development of scar tissue
- This disrupts, and eventually blocks, neurotransmission along myelinated axons
What is temporal summation?
input signals arrive from the same presynaptic cells at different times
What is spatial summation?
input signals arrive from different locations in the postsynaptic neuron
