Synaptic Transmission, Summation and Inhibition
At a chemical synapse, calcium entry triggers transmitter release and receptor binding changes the next cell's response. Overlapping inputs can combine, while inhibitory synapses reduce the chance of firing.
What a learner can do afterwards
- Orders the steps from an arriving impulse to a response in the postsynaptic membrane.
- Distinguishes spatial from temporal summation with an example of each.
- Explains how an inhibitory synapse acts on the postsynaptic membrane.
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At a chemical synapse, a spike opens voltage-gated calcium channels in the sending, or presynaptic, terminal. Calcium entry triggers vesicles to fuse with its membrane and release neurotransmitter by exocytosis. Transmitter diffuses across the cleft and binds receptors on the receiving, or postsynaptic, cell. Binding affects ion flow and the cell's response. Electrical synapses instead pass current directly between cells.
Postsynaptic potentials are small voltage changes that fade with time. Temporal summation combines repeated inputs at one synapse when a later response begins before the earlier one fades. Spatial summation combines overlapping responses from different synapses; arrivals need not be exactly simultaneous. Refractory periods limit presynaptic spike frequency, while response duration also controls overlap.
Excitatory postsynaptic potentials (EPSPs) depolarise toward threshold. Inhibitory postsynaptic potentials (IPSPs) reduce the chance of firing. Inputs spread through dendrites and the cell body. Their combined effect at the axon initial segment, beside the hillock, determines whether a new spike begins. One strong EPSP may be enough; weaker inputs may need to overlap.
In a simplified model, add signed voltage changes at the initial segment to its starting voltage. From -68 mV, two overlapping EPSPs of +5 mV each and an IPSP of -4 mV give -68 + 5 + 5 - 4 = -62 mV. With threshold at -54 mV, no spike starts. Real inputs do not always add exactly; timing and location also matter.
Inhibitory receptors can affect potassium or chloride channels. Potassium leaving removes positive charge and can make the inside more negative. Chloride conductance can inhibit by hyperpolarising the membrane or by reducing an excitatory input's effect without a large voltage change. The latter is shunting inhibition. The response depends on receptors and ion gradients; inhibition need not make the membrane more negative.
Transmitter removal limits continued receptor stimulation. Depending on the transmitter, molecules diffuse away, are taken up by neurones or glia, or are broken down by enzymes. Acetylcholinesterase breaks down acetylcholine. Blocking this enzyme leaves acetylcholine available longer. GABA is mainly cleared by uptake. Transmitter clearance and an axon's refractory recovery are different processes.
Calcium triggers chemical transmitter release. Overlapping inputs combine. The initial segment fires if threshold is reached. Inhibition reduces firing, and clearance limits receptor stimulation.
Lesson, questions and flashcards written for this dataset. Factual background includes OpenStax chapters on communication between neurones.
Readings and revised flashcards written for this dataset, with OpenStax chapters consulted for facts. Video references: Khan Academy. Video reuse terms have not been verified.
More to read
From a spike to a chemical signal
At a chemical synapse, the sending terminal releases transmitter for a receiving cell. An arriving action potential depolarises the terminal and opens voltage-gated calcium channels. Calcium entry activates the release machinery. Vesicle membranes fuse with the terminal membrane, allowing transmitter to leave by exocytosis. The vesicles do not burst apart.
Transmitter molecules diffuse across the cleft and bind suitable receptors on the receiving cell. Those receptors affect ion channels directly or through further signalling steps. Without the required binding, the usual receptor-mediated response cannot occur. This sequence describes chemical synapses. Electrical synapses use direct connections through which current passes between cells.
When small responses overlap
A postsynaptic voltage response fades with time. If another response starts before it has faded, the two can overlap. Repeated inputs at one synapse can produce temporal summation. Spatial summation combines responses produced at different synapses. Their inputs need not arrive at precisely the same instant.
Excitatory postsynaptic potentials, or EPSPs, depolarise toward threshold. Inhibitory postsynaptic potentials, or IPSPs, reduce the chance of firing. Their combined effects influence the axon initial segment beside the hillock. A new action potential usually starts there if threshold is reached.
The refractory period limits how rapidly a presynaptic axon can deliver spikes. Summation also depends on how long the receiving cell's responses last. These are related timing factors, not the same process.
Inhibition and transmitter removal
Inhibition reduces the chance that excitation will trigger a spike. Potassium flowing out removes positive charge and can make the membrane more negative. Chloride conductance can also inhibit. Its effect depends on ion gradients: it may hyperpolarise the membrane or reduce an excitatory input's voltage effect without a large voltage change. The latter effect is shunting inhibition.
Transmitter removal limits how long released molecules can stimulate receptors. The route depends on the transmitter. Molecules may diffuse away, enter neurones or glia through uptake, or be broken down by enzymes. Acetylcholinesterase breaks down acetylcholine. Blocking that enzyme can prolong acetylcholine's availability. GABA is mainly cleared by uptake. Transmitter clearance is distinct from recovery of an axon membrane after a spike.
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