# Synaptic Transmission

Psychology I · Behavioral Biology · https://tryals.app/learn/psychology-i/synaptic-transmission

## Crossing the Synaptic Cleft

Neurons never touch. They communicate across the **synaptic cleft**, a microscopic gap between cells. When an action potential arrives, voltage-gated **calcium (Ca2+) channels** open, driving Ca2+ influx. This causes **synaptic vesicles** to fuse with the membrane and release neurotransmitters.

The released molecules cross the cleft and bind postsynaptic **receptors**. Binding triggers an **EPSP** (excitatory postsynaptic potential, depolarizing via Na+ influx) or an **IPSP** (inhibitory postsynaptic potential, hyperpolarizing via Cl- influx or K+ efflux).

| Receptor Family | Mechanism | Speed & Duration |
| --- | --- | --- |
| **Ionotropic** | Ligand-gated ion channels | Fast, brief |
| **Metabotropic** | G-protein-coupled | Slow, long-lasting |

## Summation and Clearance

A single EPSP rarely reaches threshold. **Summation** solves this through two distinct mechanisms: **temporal summation** adds signals from one synapse firing rapidly, while **spatial summation** combines inputs from multiple synapses simultaneously at the axon hillock.

After signaling, neurotransmitters must be cleared immediately via **reuptake** (pumping molecules back into the presynaptic terminal) or **enzymatic degradation** (destroying them in the cleft, like acetylcholinesterase).

> **Common pitfall:** Assuming every synaptic signal is excitatory. Roughly half of all synaptic traffic consists of inhibitory IPSPs. Cognition depends just as heavily on well-timed inhibition as on excitation.

## Practice questions

6 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. An inhibitory synapse sits on the axon hillock. An excitatory synapse producing a potential of exactly the same size sits far out on a distal dendrite. Why does the inhibitory one have the greater influence on whether the neuron fires?

A. Because the axon hillock disregards any input that does not arrive at it directly.
B. Because inhibitory potentials are intrinsically larger than excitatory ones wherever they occur.
C. Because postsynaptic potentials shrink as they spread.
D. Because IPSPs propagate along the membrane faster than EPSPs do, so they arrive first and pre-empt them.

**Answer:** C. Because postsynaptic potentials shrink as they spread.

**Why:** Postsynaptic potentials are passive and leak away as they spread, so two equal potentials are no longer equal by the time they reach the axon hillock where the decision is taken. It is why inhibitory synapses cluster on the soma, anatomy is doing arithmetic, and position counts alongside amplitude.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/an-inhibitory-synapse-sits-on-the-axon-hillock-an-excitatory-synapse

### 2. Put the events of chemical synaptic transmission in causal order.

**Answer:**

1. An action potential arrives at the presynaptic terminal button.
2. Voltage-gated Ca2+ channels open and Ca2+ flows into the terminal.
3. Synaptic vesicles fuse with the membrane and release neurotransmitter into the cleft.
4. Neurotransmitter diffuses across the cleft and binds postsynaptic receptors.
5. Reuptake or enzymatic degradation clears the neurotransmitter from the cleft.

**Why:** The arriving spike opens Ca2+ channels; Ca2+ influx is the trigger that makes vesicles fuse and release transmitter, which then diffuses, binds receptors, and is finally cleared. Forgetting that calcium is the trigger is the most common slip here.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/put-the-events-of-chemical-synaptic-transmission-in-causal-order

### 3. Sort each scenario by the kind of summation it illustrates.

**Answer:**

- Temporal summation: One synapse fires three times in rapid succession, its EPSPs piling up, A single input fires again before its first EPSP has decayed
- Spatial summation: EPSPs from fifty different synapses arrive at the same instant, Excitation from many locations minus inhibition from others is tallied at once

**Why:** Temporal summation stacks repeated EPSPs from one synapse over time; spatial summation adds inputs from many synapses arriving together (and subtracts IPSPs). The distinction is whether the inputs are separated in time or in location.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/sort-each-scenario-by-the-kind-of-summation-it-illustrates

### 4. Ionotropic receptors act in milliseconds and metabotropic ones over seconds. Why does the second messenger cost so much time?

A. Because metabotropic receptors are located further from the synapse
B. Because metabotropic runs through a G-protein cascade first
C. Because metabotropic transmitters diffuse more slowly across the cleft
D. Because ionotropic receptors are more numerous and so respond sooner

**Answer:** B. Because metabotropic runs through a G-protein cascade first

**Why:** Steps cost time, and buy amplification and duration. Fast signalling is ionotropic; modulation that must outlast the signal is metabotropic.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/ionotropic-receptors-act-in-milliseconds-and-metabotropic-ones-over

### 5. Select every statement that is TRUE about synaptic transmission.

A. Neurotransmitter crosses the cleft by traveling down the axon.
B. Calcium entry into the terminal triggers vesicle release.
C. Roughly as much synaptic traffic is inhibitory as excitatory.
D. An IPSP makes the postsynaptic neuron more likely to fire.
E. Ionotropic receptors act faster than metabotropic receptors.

**Answer:** B. Calcium entry into the terminal triggers vesicle release.; C. Roughly as much synaptic traffic is inhibitory as excitatory.; E. Ionotropic receptors act faster than metabotropic receptors.

**Why:** Inhibition is roughly as common as excitation, ionotropic beats metabotropic on speed, and Ca2+ triggers release - all true. An IPSP makes firing LESS likely, and transmitter diffuses across the cleft (it is not conducted down the axon), so those are false.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/select-every-statement-that-is-true-about-synaptic-transmission

### 6. Electrical synapses exist and are faster, yet almost all human synapses are chemical, slower, and requiring a whole apparatus of vesicles, transmitters and receptors. What does that expense buy?

A. Computation, since a chemical synapse can be modulated
B. Protection from electrical interference between adjacent neurons
C. Nothing, chemical synapses are an evolutionary accident later systems were built around
D. Speed over long distances, which electrical synapses cannot achieve

**Answer:** A. Computation, since a chemical synapse can be modulated

**Why:** A gap junction passes the signal faithfully and can do nothing else. Converting to chemistry buys a place to intervene: the same input can be amplified, inverted to inhibition, or strengthened by learning. Every drug in this unit works in that gap, the expense is what makes the synapse programmable.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission/electrical-synapses-exist-and-are-faster-yet-almost-all-human