# Synaptic Transmission

Psychology I · Psychophysiology · https://tryals.app/learn/psychology-i/synaptic-transmission-psychophysiology

## The Chemical Bridge

Neurons rarely touch. Across the microscopic **synapse** (or synaptic cleft), the electrical action potential cannot jump directly. It must switch to a chemical message.

When an impulse reaches the axon terminal, **vesicles** release **neurotransmitters** into the cleft. These chemicals drift across and bind to **receptor** sites on the receiving neuron like a key in a lock.

| Signal Type | Effect on Receiving Neuron |
| --- | --- |
| **Excitatory** | Nudges the neuron toward firing |
| **Inhibitory** | Nudges the neuron away from firing |

**Common pitfall:** Picturing the electrical impulse as jumping the gap itself. It does not. The signal converts to chemical across the synapse, then back to electrical.

$$\text{net signal} = \text{excitatory} - \text{inhibitory input}$$

## Resetting and Drugs

Once delivered, the message must be cleared so the synapse can reset. The primary mechanism is **reuptake**, where the sending neuron reabsorbs leftover neurotransmitters for future use.

Drugs exploit this exact machinery to alter behavior and mood:

| Drug Type | Mechanism & Action |
| --- | --- |
| **Agonist** | Mimics or boosts neurotransmitter effect |
| **Antagonist** | Blocks neurotransmitter effect |

For example, many common antidepressants act as reuptake inhibitors. By blocking serotonin reuptake, they leave more of the neurotransmitter active in the synaptic cleft.

## Practice questions

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

### 1. A regular opioid user needs steadily larger doses for the same effect, and feels terrible without any. What has the synapse done?

A. Exhausted its supply of endogenous opioids, which the drug was replacing
B. Downregulated, so normal transmitter levels now underwhelm it
C. Learned to metabolise the drug faster, reducing how much reaches the brain
D. Built a physical barrier preventing the drug from reaching the receptors

**Answer:** B. Downregulated, so normal transmitter levels now underwhelm it

**Why:** The synapse defends its set point by removing receptors, so the same dose lands on a less responsive system, tolerance. Withdrawal is that adaptation exposed: the compensation persists after the drug is gone, leaving normal transmitter levels far short of what the receptor count now expects.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission-psychophysiology/a-regular-opioid-user-needs-steadily-larger-doses-for-the-same

### 2. An agonist and an antagonist can both bind the same receptor with equal affinity. What distinguishes them, and why does that make an antagonist useful as a treatment for overdose?

A. An antagonist destroys the receptor permanently, which is why the drug's effect ends.
B. An antagonist binds more strongly than any agonist, which is what allows it to displace one.
C. Binding is not the same as activating: one triggers, one just occupies.
D. An antagonist causes the neuron to release an opposing neurotransmitter, cancelling the effect.

**Answer:** C. Binding is not the same as activating: one triggers, one just occupies.

**Why:** The receptor binds and activates, and the distinction lives entirely in the second, an antagonist is a key that enters the lock and will not turn, and while it sits there nothing else gets in. Which explains a clinical detail: naloxone wears off faster than many opioids, so a revived patient can relapse. The reversal is a displacement, not a cure.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission-psychophysiology/an-agonist-and-an-antagonist-can-both-bind-the-same-receptor-with

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

A. A molecule can hold a receptor shut simply by sitting in it.
B. Transmission across a chemical synapse is instantaneous, with no delay.
C. Reuptake and enzymatic breakdown both clear transmitter from the cleft.
D. Neurotransmitter is released from vesicles when an action potential reaches the terminal.
E. An agonist and an antagonist have opposite effects on the receptor they share.

**Answer:** C. Reuptake and enzymatic breakdown both clear transmitter from the cleft.; D. Neurotransmitter is released from vesicles when an action potential reaches the terminal.; E. An agonist and an antagonist have opposite effects on the receptor they share.

**Why:** The electrical-to-chemical switch, reuptake's resetting role, and the antagonist's blocking action are true. The impulse does not cross as electricity, and an SSRI enhances serotonin (a functional agonist), so those two are false.

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

### 4. Blocking the reuptake of serotonin will tend to DECREASE serotonin's effect on the receiving neuron.

**Answer:** False

**Why:** False. Reuptake removes serotonin from the synaptic cleft, so blocking it leaves the transmitter there longer and *increases* its effect, which is exactly how SSRIs work. The puzzle the mechanism does not explain is why the receptor change is immediate and the clinical benefit takes weeks.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission-psychophysiology/blocking-the-reuptake-of-serotonin-will-tend-to-decrease-serotonins

### 5. Order the events of chemical synaptic transmission.

**Answer:**

1. An action potential reaches the axon terminal.
2. Vesicles release neurotransmitters into the synaptic cleft.
3. Neurotransmitters diffuse across and bind receptors on the next neuron.
4. The binding excites or inhibits the receiving neuron.
5. Reuptake reabsorbs the leftover neurotransmitter, resetting the synapse.

**Why:** The arriving spike triggers vesicle release; transmitter diffuses, binds receptors, excites or inhibits the next cell, and is then cleared by reuptake. Reuptake comes last because its job is to reset the synapse after the signal.

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

### 6. The action potential does not jump across the synaptic cleft. What crosses instead?

A. Ions flowing directly from one cell into the other
B. Neurotransmitter molecules crossing the gap
C. The electrical signal itself, weakened by the gap
D. Nothing: the cells touch and no gap exists

**Answer:** B. Neurotransmitter molecules crossing the gap

**Why:** The chemical step is the point of the gap. It is where signals can be amplified, inhibited or modulated, and where almost every psychoactive drug does its work.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission-psychophysiology/the-action-potential-does-not-jump-across-the-synaptic-cleft-what

### 7. Sort each drug action as an agonist or an antagonist.

**Answer:**

- Agonist: A drug that mimics a neurotransmitter and activates its receptor, An SSRI that blocks serotonin reuptake, leaving more active in the synapse
- Antagonist: A drug that occupies a receptor without activating it, A drug that blocks a receptor so the transmitter cannot act

**Why:** Agonists enhance a transmitter's effect (mimicking it, or an SSRI leaving more serotonin around); antagonists block it. The SSRI is the trap: it 'blocks' reuptake yet BOOSTS serotonin, making it a functional agonist.

Page: https://tryals.app/practice/psychology-i/synaptic-transmission-psychophysiology/sort-each-drug-action-as-an-agonist-or-an-antagonist