Synaptic transmission Flashcards
What happens when the action potential gets to the end of the axon
Axons connect to other neurons, creating synaptic inputs.
They can connect to cell bodies, dendrites, or other axons, forming a “synapse.”
Synapses features and functions
Axon Terminal: Ends of axons with neurotransmitter vesicles.
Synaptic Cleft: Gap between neurons.
Receptors: Postsynaptic cell has neurotransmitter receptors.
Signal Conversion: Electrical signal → Chemical signal → Electrical signal.
Signal Modification: Allows regulation of signals.
Types: Mostly chemical synapses, some electrical.
Synaptic transmission
- Action potential reaches axon terminal of presynaptic neuron
- Ca2+ enters synaptic knob (presynaptic axon terminal)
- Neurotransmitter is released by exocytosis into synaptic cleft
- Neurotransmitter binds to receptors on postsynaptic neuron’s membrane.
- Binding opens specific channels.
Ligand gated ion channels
- Receptors opening in response to neurotransmitter binding.
- Key in synaptic transmission.
- Mediate local/graded potentials.
- Fast signals between neurons.
Ligand gated vs voltage gated ion channels
Ligand-Gated Ion Channels:
Respond to neurotransmitter binding.
Trigger graded/local potentials at synapses.
Voltage-Gated Ion Channels:
Respond to changes in membrane potential.
Trigger action potentials in axons.
Note: Both are distinct from leak ion channels, which generate and maintain membrane potential
Termination of synaptic transmission
Removal of neurotransmitter by:
1. Enzyme breakdown.
2. Diffusion away and breakdown elsewhere.
3. Reuptake into presynaptic terminal or nearby astrocytes.
Postsynaptic response is a “postsynaptic potential,” decreasing in size as it spreads, requiring action potentials for long-distance signals.
Synapses can be excitatory or inhibitory
Excitatory Synapses: Generate excitatory postsynaptic potentials (EPSPs), depolarizing the membrane closer to action potential threshold.
Inhibitory Synapses: Generate inhibitory postsynaptic potentials (IPSPs), hyperpolarizing the membrane away from action potential threshold.
How does summation of postsynaptic potentials determine action potential threshold?
-temporal summation
-spatial summation
-EPSPs and IPSPs cancel each other out
Temporal summation
Two EPSPs from the same presynaptic neuron occur close in time to depolarise the membrane to threshold
Spatial summation
Two EPSPs from different presynaptic neurons occur close together in time, to depolarise the membrane to threshold
EPSPs and IPSPs cancel each other out
An EPSP and IPSP are triggered at the same time in the postsynaptic neuron, so there’s no net change in membrane potential
Effect of synaptic transmission is dependent on?
- The type of cell it is terminating on
- E.g. another neuron, skeletal muscle, the heart - The type of neurotransmitter the neuron releases
- E.g. an excitatory or inhibitory neurotransmitter - The types of neurotransmitter receptors on the postsynaptic membrane
-Generate fast or slow responses
Axons can also terminate on cells other than neurons
-e.g. heart cell
-Neurons can release chemicals into the bloodstream, acting on distant cells.
-Release neurohormones-> enzymatic breakdown
Comparison of Graded Potentials (GP) and Action Potentials (AP)
Location of Event:
* GP: Cell body and dendrites.
* AP: Axon, beginning at the initial segment.
Distance Travelled:
* GP: Short distance
* AP: Long distance, through entire axon.
Positive Feedback Cycle:
* GP: Absent.
* AP: Present.
Summation:
* GP: Can summate.
* AP: All-or-none; no summation.
Postsynaptic Potentials (EPSP, IPSP, AP)
Functions
Initial Effect of Stimulus
Peak Membrane Potential
Function:
EPSP: Depolarization.
IPSP: Hyperpolarization.
AP: Nerve impulse.
Initial Effect of Stimulus:
EPSP: Opens chemically gated channels.
IPSP: Opens chemically gated K⁺ or Cl⁻ channels.
AP: Opens voltage-gated channels.
Peak Membrane Potential:
EPSP: Depolarizes.
IPSP: Hyperpolarizes.
AP: Peaks at +30 to +50 mV.
