Lecture 20: Synaptic Transmission Flashcards
what happens when the AP gets to the end of the axon?
axons terminate on other neurons, forming synaptic inputs and terminate on the cell body, thus forming a synapse.
what is a synapse?
the end of an axon is an axon terminal and is full of neurotransmitters. the small gap between that and the postsynaptic cell is called the synaptic cleft. the electrical signal of the AP is converted into a chemical signal and then back into an electrical signal.
what are the key steps of synaptic transmission?
- AP 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 that are an integral part of chemically gated channels on subsynaptic membrane of postsynaptic neuron
- binding of neurotransmitter to receptor opens that specific channel.
2 main roles of ligand gated ion channels in synaptic transmission?
- essential in the process of synaptic transmission as it assists the receptor to bind to a specific neurotransmitter.
- mediates graded potential and creates a fast signal to travel from one neuron to the next.
compare ligand gated vs voltage gated ion channels.
ligand gated is responsible for triggering graded potentials at a synapse, whereas voltage gated ion channels are responsible for triggering action potentials in an axon.
how is synaptic transmission terminated?
terminated by the removal of the neurotransmitter by:
1. breakdown by enzymes
2. diffusion away from the synapse and broken down elsewhere
3. reuptake into the presynaptic terminal or nearby astrocytes
compare excitatory (EPSPs) and inhibitory (IPSPs) synapses. (graded potential summations)
excitatory synapses generate excitatory postsynaptic potentials by depolarising the membrane (closer to initiation AP threshold). whereas, inhibitory synapses generate inhibitory postsynaptic potentials by hyperpolarising the membrane (further from AP initiation threshold).
explain how graded potentials are short-distance signals.
graded potentials decrease in size as they spread. the membrane depolarises and spreads to create currents that depolarise adjacent membrane area, spreading the wave of depolarisation. however, due the current can be lost through the leaking plasma membrane and voltage declines.
what are the different summation postsynaptic potentials types that determine whether the cell will reach the action potential threshold.
temporal summation; two ESPSs 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; triggered at the same time in postsynaptic neuron, so there’s no net change in membrane potential.
what are the different summation postsynaptic potentials types that determine whether the cell will reach the action potential threshold.
temporal summation: two ESPSs 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; triggered at the same time in postsynaptic neuron, so there’s no net change in membrane potential.
explain the neuron input and output.
input from other neurons trigger EPSPs and IPSPs and then there is an AP initiation and propagation therefore EPSPs, IPSPs are triggered in post synaptic neuron.
compare graded potentials and action potentials.
graded potential:
- occurs in cell body and dendrites
- short distance
- various sizes, decays w distance
- chemical ion channel
- absent positive feedback cycle
- voltage independent; occurs when no stimulus is present
- stimulus responses can summate to increase amplitude of graded potential (temporal or spatial)
action potential:
- occurs at axon
- long distance
- always the same size, does not decay with distance
- voltage ion channel
- present positive feedback cycle
- voltage regulated, occurs when Na channels inactivate and K channels open
- summation does not occur
what is synaptic transmission dependent on?
dependent on:
1. the type of cell it is terminating on.
2. the type of neurotransmitter the neuron releases.
3. the types of neurotransmitter receptors on the postsynaptic membrane.
what are the two main types of receptors involved in neurotransmission? + explain function
- ligand-gated ion channels; fast and direct neurotransmission. the channels open to allow ions to flow into or out of cell, thus there is a direct voltage movement across the membrane to generate graded potentials.
- g protein coupled receptors; slower and indirect neurotransmission. this receptor causes a cascade of effects within the cell which doesn’t directly cause graded potentials.
what are some examples of neurotransmitters that cause an excitatory response by acting on a ligand gated ion channel (EPSP).
glutamate, acetylcholine.
