Part 5 – Synapses, Neurotransmitters, and Signal Integration

What is a synapse?

A junction that mediates information transfer from one neuron to another neuron or to an effector cell.

What are the two main types of synapses?

Electrical synapses and chemical synapses.

What are electrical synapses?

Connections where neurons are joined by gap junctions that allow direct ion flow between cells.

Where are electrical synapses found?

In some brain regions, embryonic nervous tissue, and cardiac or smooth muscle.

What are chemical synapses?

Junctions where neurotransmitters transmit signals across a synaptic cleft.

What are the two main parts of a chemical synapse?

The axon terminal of the presynaptic neuron and the receptor region on the postsynaptic membrane.

What separates the presynaptic and postsynaptic membranes?

A fluid-filled synaptic cleft (about 30–50 nm wide).

What happens at the presynaptic terminal when an action potential arrives?

Voltage-gated Ca²⁺ channels open, and calcium enters the terminal.

What does calcium entry into the axon terminal cause?

It triggers synaptic vesicles to release neurotransmitter into the synaptic cleft by exocytosis.

What happens to the neurotransmitter once released?

It diffuses across the cleft and binds to receptors on the postsynaptic membrane.

What happens after neurotransmitter binds to its receptor?

Ion channels open, causing graded potentials on the postsynaptic neuron.

How is neurotransmitter action terminated?

By reuptake into the presynaptic terminal, enzymatic degradation, or diffusion away from the cleft.

What determines whether a neurotransmitter causes excitation or inhibition?

The type of receptor it binds to on the postsynaptic membrane.

What is a postsynaptic potential?

A graded potential caused by neurotransmitter binding.

What is an excitatory postsynaptic potential (EPSP)?

A local depolarization that brings the postsynaptic membrane potential closer to threshold.

What ions are involved in EPSPs?

Sodium (Na⁺) enters and potassium (K⁺) leaves simultaneously, causing a net depolarization.

What is an inhibitory postsynaptic potential (IPSP)?

A local hyperpolarization that drives the postsynaptic membrane potential away from threshold.

What ions are involved in IPSPs?

Usually potassium (K⁺) exiting or chloride (Cl⁻) entering the cell.

Can EPSPs and IPSPs occur simultaneously?

Yes — their combined effects determine whether threshold is reached at the axon hillock.

What is summation?

The process of adding together multiple EPSPs and IPSPs to reach threshold.

What is temporal summation?

When a single presynaptic neuron fires rapidly in succession, causing EPSPs to build up over time.

What is spatial summation?

When multiple presynaptic neurons fire simultaneously at different synapses on the same postsynaptic cell.

What happens if the sum of graded potentials reaches threshold?

An action potential is generated at the axon hillock.

What happens if the sum of graded potentials is below threshold?

No action potential occurs.

What is synaptic delay?

The time required for neurotransmitter release, diffusion, and binding — about 0.3–5 milliseconds.

Why is transmission across a chemical synapse slower than an electrical one?

Because of the time needed for neurotransmitter processes in the synaptic cleft.

What are neurotransmitters?

Chemicals used by neurons to transmit signals across synapses.

How many neurotransmitters have been identified?

Over 50 different types.

What are the main chemical classes of neurotransmitters?

Acetylcholine, biogenic amines, amino acids, peptides, and gases.

Where is acetylcholine (ACh) released?

At neuromuscular junctions and by some autonomic neurons.

How is acetylcholine degraded?

By the enzyme acetylcholinesterase (AChE).

What are biogenic amines?

Neurotransmitters derived from amino acids, such as dopamine, norepinephrine, epinephrine, serotonin, and histamine.

What functions are biogenic amines associated with?

Mood, emotional behavior, and biological clock regulation.

What are amino acid neurotransmitters?

GABA, glycine, aspartate, and glutamate.

What is the function of GABA?

It is the main inhibitory neurotransmitter in the brain.

What is the function of glutamate?

It is the main excitatory neurotransmitter in the CNS.

What are peptide neurotransmitters (neuropeptides)?

Chains of amino acids that act as neuromodulators, such as endorphins, substance P, and somatostatin.

What is the function of endorphins?

They act as natural painkillers and produce euphoria.

What is the function of substance P?

It mediates pain transmission in the PNS and CNS.

What are novel messengers (gas neurotransmitters)?

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S).

What is the function of nitric oxide (NO)?

It acts as a signaling molecule involved in learning, memory, and vasodilation.

What are the two main types of neurotransmitter receptors?

Channel-linked receptors and G-protein–coupled receptors.

What are channel-linked receptors?

Ionotropic receptors that mediate direct, fast synaptic transmission.

What are G-protein–coupled receptors?

Metabotropic receptors that produce slow, long-lasting effects via intracellular second messengers.

What is neural integration?

The summation and processing of incoming signals by neurons to produce coordinated output.

What is an example of neural integration?

A postsynaptic neuron integrating both excitatory and inhibitory inputs before deciding whether to fire.

What is synaptic plasticity?

The ability of synapses to change in strength, number, or efficiency — important for learning and memory.