--- title: "Synaptic Transmission, Summation and Inhibition" description: "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 " canonical: https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B source: https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: The Human Body · Ages 17 to 18 Page: https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B ## Ready when they can - 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. ## Lesson: Signals at the axon initial segment 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. *(drawing: Follow the chemical transmission sequence. Receptors may affect ion channels directly or through further signalling steps.)* 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. **Example.** 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. *(drawing: The model ends at -62 mV, below the -54 mV threshold. This is voltage arithmetic, not a voltage trace over time.)* 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Conduction Speed: Myelination and the Refractory Period](https://lightmysky.com/learn/science/conduction-speed-myelination-and-the-refractory-period-mt_x3M0sv6oD6) ## Opens up - [The Sliding Filament Model of Muscle Contraction](https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8) - [From Photons to Spikes: the Retina](https://lightmysky.com/learn/science/from-photons-to-spikes-the-retina-mt_kviN7WskmP) - [Touch, Temperature and Pain](https://lightmysky.com/learn/science/touch-temperature-and-pain-mt_rnSuJA4-kj) - [Transmitter Systems and Two Kinds of Receptor](https://lightmysky.com/learn/science/transmitter-systems-and-two-kinds-of-receptor-mt_yXaSbrBxua)