Synapses

Question 1

What is a synapse?

Answer 1

A synapse is a junction (connection point) between two neurons where one neuron sends a signal and the other receives it. Think: one neuron talks, the next listens.

Question 2

What are the roles of neurons at a synapse?

Answer 2

The presynaptic neuron sends the signal; the postsynaptic neuron receives the signal.

Question 3

How many types of synapses are there?

Answer 3

There are three types: chemical synapses (most important), electrical synapses, and conjoint synapses (both).

Question 4

Which type of synapse dominates the human brain?

Answer 4

Chemical synapses dominate the human brain.

Question 5

What is a chemical synapse?

Answer 5

A chemical synapse is where the presynaptic neuron releases a neurotransmitter that affects the postsynaptic neuron.

Question 6

What are the three parts of a chemical synapse?

Answer 6

Presynaptic neuron (releases neurotransmitter), synaptic cleft (tiny gap), postsynaptic neuron (responds).

Question 7

What are the two possible effects of a neurotransmitter?

Answer 7

A neurotransmitter can be excitatory (causes depolarisation, increases firing) or inhibitory (causes hyperpolarisation, decreases firing).

Question 8

What does depolarisation mean?

Answer 8

Depolarisation means the membrane becomes less negative inside.

Question 9

What does hyperpolarisation mean?

Answer 9

Hyperpolarisation means the membrane becomes more negative inside.

Question 10

What happens at an excitatory synapse?

Answer 10

Neurotransmitter opens channels; positive ions (usually Na⁺) enter; the inside becomes less negative.

Question 11

What is an EPSP?

Answer 11

An EPSP (excitatory postsynaptic potential) is a graded depolarisation that brings the neuron closer to threshold.

Question 12

What is the effect of an EPSP on firing?

Answer 12

An EPSP makes an action potential more likely.

Question 13

What happens at an inhibitory synapse?

Answer 13

Neurotransmitter opens channels causing Cl⁻ to enter or K⁺ to leave, making the inside more negative.

Question 14

What is an IPSP?

Answer 14

An IPSP (inhibitory postsynaptic potential) is a graded hyperpolarisation that moves the neuron away from threshold.

Question 15

What is the effect of an IPSP on firing?

Answer 15

An IPSP makes an action potential less likely.

Question 16

Is one EPSP usually enough to cause an action potential?

Answer 16

No. One EPSP is usually not sufficient on its own.

Question 17

What is facilitation?

Answer 17

Facilitation is when an EPSP does not trigger an action potential alone but increases the likelihood of firing with further stimulation.

Question 18

What is a simple analogy for facilitation?

Answer 18

One push on a swing is not enough; several pushes make the swing move.

Question 19

What is summation?

Answer 19

Summation is the adding together of multiple EPSPs to reach threshold.

Question 20

How many types of summation are there?

Answer 20

There are two types: spatial summation and temporal summation.

Question 21

What is spatial summation?

Answer 21

Multiple presynaptic neurons fire at different synapses at the same time, and their EPSPs add together.

Question 22

What is temporal summation?

Answer 22

One presynaptic neuron fires repeatedly before the previous EPSP fades, so EPSPs stack over time.

Question 23

Where are EPSPs and IPSPs summed?

Answer 23

At the axon hillock / initial segment.

Question 24

Where does an action potential start?

Answer 24

An action potential starts at the axon hillock / initial segment, not at the synapse.

Question 25

**What is an electrical synapse?**

Answer 25

An **electrical synapse** is where neurons communicate by **direct electrical current**, with **no neurotransmitter**.

Question 26

**How do electrical synapses work structurally?**

Answer 26

They use **gap junctions**, allowing **ions to flow directly** between neurons.

Question 27

**What are key features of electrical synapses?**

Answer 27

They are **very fast**, **often bidirectional**, have **little modulation**, and are **rare in the adult human cortex**.

Question 28

**Where are electrical synapses commonly found?**

Answer 28

In **some reflexes**, **early brain development**, and **synchronised firing** (e.g. **brainstem**).

Question 29

**What is a conjoint synapse?**

Answer 29

A **conjoint synapse** has **both electrical and chemical properties**, allowing direct current flow **and** neurotransmitter release.

Question 30

**Are conjoint synapses common?**

Answer 30

**No.** They are **less common**, but **conceptually important**.

Question 31

**How close are membranes in an electrical synapse?**

Answer 31

About **3.5 nm apart**, much closer than in chemical synapses.

Question 32

**What is a gap junction?**

Answer 32

A **gap junction** is a **protein channel** linking the **cytoplasm of two neurons**.

Question 33

**What is a connexon?**

Answer 33

A **connexon** is a **half-channel** in one cell membrane that pairs with another connexon.

Question 34

**What is a connexon made of?**

Answer 34

Each connexon is made of **six connexin subunits**.

Question 35

**How many connexons form one gap junction channel?**

Answer 35

**Two connexons** form **one gap junction channel**.

Question 36

**What triggers current flow in an electrical synapse?**

Answer 36

A **voltage change (depolarisation)** in neuron A creates an electrical gradient.

Question 37

**What passes through gap junctions?**

Answer 37

**Ions** (Na⁺, K⁺, Ca²⁺, Cl⁻) and **small molecules** such as **cAMP** and **IP₃**.

Question 38

**What type of postsynaptic response occurs at an electrical synapse?**

Answer 38

A **graded potential**, not automatically an action potential.

Question 39

**If neuron B reaches threshold, where does the action potential start?**

Answer 39

At the **axon initial segment / axon hillock** of neuron B.

Question 40

**Are electrical synapses bidirectional?**

Answer 40

They are **often bidirectional**, though some are **rectifying**.

Question 41

**Why are electrical synapses so fast?**

Answer 41

They skip **Ca²⁺ entry**, **vesicle fusion**, **neurotransmitter diffusion**, and **receptor binding**, so delay is **minimal**.

Question 42

**Are EPSPs and IPSPs action potentials?**

Answer 42

**No.** EPSPs and IPSPs are **graded potentials**, not action potentials.

Question 43

**Do EPSPs and IPSPs travel long distances?**

Answer 43

**No.** They are **local (decremental)** signals.

Question 44

**Do action potentials occur at synapses?**

Answer 44

**No.** Action potentials occur at the **axon hillock**.

Question 45

**Are chemical synapses instantaneous?**

Answer 45

**No.** Chemical synapses depend on **Ca²⁺** and have a **synaptic delay**.

Question 46

**What is the ultra-simple sequence from synapse to firing?**

Answer 46

**Chemical synapse → EPSP/IPSP → facilitation → summation → threshold → action potential**.

Question 47

**What are five must-remember exam facts about synapses?**

Answer 47

1) **Synapse = junction between neurons**. 2) **Chemical synapses** are excitatory or inhibitory. 3) **EPSPs depolarise**, **IPSPs hyperpolarise**. 4) **Spatial and temporal summation** lead to firing. 5) **Electrical synapses use direct current**, no chemicals.