intro to brain anatomy 2

What initiates an action potential in a presynaptic neuron?

A graded potential that reaches the threshold level triggers an action potential in the presynaptic neuron.

What happens when an action potential reaches the axon terminal?

It invades the synaptic terminal and triggers the release of chemical neurotransmitters.

How does the action potential lead to neurotransmitter release?

It opens voltage-gated Ca²⁺ channels, and calcium entry causes vesicles to release neurotransmitters into the synaptic cleft.

what happens after the neurotransmitter is released into the synaptic cleft?

It binds to receptors on the postsynaptic membrane, causing ion channels to open.

What does neurotransmitter binding cause in the postsynaptic neuron?

It generates a graded potential — either excitatory or inhibitory.

What is an excitatory postsynaptic potential (EPSP)?

A graded potential that makes the membrane less negative, increasing the chance of firing an action potential.

What is an inhibitory postsynaptic potential (IPSP)?

A graded potential that makes the membrane more negative, decreasing the chance of firing an action potential

Where are neurotransmitters stored before release?

They are synthesised and stored in vesicles within the presynaptic neuron.

What event triggers neurotransmitter release?

An action potential travels down the axon and invades the presynaptic terminal.

What does the action potential do at the synaptic terminal?

It depolarises the terminal membrane, opening voltage-gated calcium channels (VGCCs).

What happens when VGCCs open?

Ca²⁺ ions enter the presynaptic terminal from outside the cell.

What does Ca²⁺ do once inside the terminal?

It triggers vesicle fusion with the presynaptic membrane.

How is the neurotransmitter released into the synaptic cleft?

Through exocytosis — the vesicle merges with the membrane and releases the transmitter.

What happens after the transmitter is released?

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

What does receptor binding cause in the postsynaptic neuron?

It opens ion channels, causing ion flow and a postsynaptic response (either excitatory or inhibitory).

How is the neurotransmitter signal stopped?

The transmitter is broken down by enzymes or taken back up into the presynaptic neuron (reuptake).

What happens to the vesicle after neurotransmitter release?

It is retrieved from the terminal membrane via endocytosis and recycled for future use.

What are the two main types of neurotransmitter receptors?

Ligand-gated ion channels (ionotropic receptors) and G protein-coupled receptors (GPCRs or metabotropic receptors).

Where are neurotransmitter receptors located?

They are found on the cell surface of the postsynaptic membrane, where they mediate the effects of neurotransmitters.

What are ligand-gated ion channels (ionotropic receptors)?

They are membrane pores that open when a neurotransmitter (ligand) binds, allowing ions to flow across the membrane.

what is the effect of ion flow through ionotropic receptors?

It causes either depolarization (excitatory) or hyperpolarization (inhibitory), depending on the ion type.

How fast are ligand-gated ion channels?

They act on a very fast, millisecond timescale — ideal for rapid synaptic transmission

What is a classic example of an ionotropic receptor?

The nicotinic acetylcholine receptor (nAChR) — a fast, ligand-gated ion channel.

What are G protein-coupled receptors (GPCRs)?

They are receptors that activate G-proteins when a neurotransmitter binds, triggering slower, longer-lasting responses.

How do GPCRs affect the postsynaptic neuron?

G-proteins can signal to ion channels (changing membrane potential) and also activate enzymes like adenylyl cyclase, which triggers second messenger pathways.

How fast are GPCR-mediated responses?

They are relatively slow, acting over seconds, but often have longer-lasting effects than ionotropic receptors.

Can a single neurotransmitter act on both receptor types?

Yes — one neurotransmitter (e.g. acetylcholine) can bind to both ionotropic and metabotropic receptors, but their distribution and function differ.

What is a graded potential?

A graded potential is a small, localized change in membrane potential caused by neurotransmitters binding to receptors on the postsynaptic membrane.

Where do graded potentials typically occur in a neuron?

In the dendrites and soma (cell body) of the neuron.

What causes a graded potential to occur?

The binding of neurotransmitters to ligand-gated ion channels on the postsynaptic membrane.

How do excitatory neurotransmitters affect membrane potential?

They cause depolarization, making the membrane potential less negative and the neuron more likely to fire an action potential.

How do inhibitory neurotransmitters affect membrane potential?

They cause hyperpolarization, making the membrane potential more negative and the neuron less likely to fire.

What ions are typically involved in depolarization during a graded potential?

Sodium ions (Na⁺) entering the cell.

What ions are typically involved in hyperpolarization during a graded potential?

Chloride ions (Cl⁻) entering or potassium ions (K⁺) leaving the cell.

What does it mean that graded potentials are “graded”?

Their size and duration vary depending on the strength of the stimulus (e.g., the amount of neurotransmitter released).

How is the strength of a graded potential determined?

By the amount of neurotransmitter released and the number of receptors activated.

Do graded potentials follow the all-or-none rule?

No, graded potentials are proportional to the strength of the stimulus.

Do graded potentials travel long distances?

No, they travel only short distances because they decay rapidly

Why do graded potentials decay as they travel?

Due to leakage of ions and resistance within the neuron.

What is summation in the context of graded potentials?

The process of adding multiple graded potentials together to determine if the neuron reaches threshold for an action potential.

What happens if the graded potentials do not reach threshold?

: No action potential occurs; the graded potential simply fades out.

Are graded potentials excitatory, inhibitory, or both?

They can be both, depending on the neurotransmitter and the type of ion channel activated.

What type of receptors do neurotransmitters bind to initiate graded potentials?

Ligand-gated ion channels on the postsynaptic membrane.

: What is the major excitatory neurotransmitter in the brain?

Glutamate.

What is the major inhibitory neurotransmitter in the brain?

GABA (Gamma-Aminobutyric Acid).

What effect does glutamate have on the postsynaptic membrane?

It causes depolarization, making the neuron more likely to fire an action potential.

What effect does GABA have on the postsynaptic membrane?

It causes hyperpolarization, making the neuron less likely to fire an action potential.

Why is glutamate considered excitatory?

Because it brings the membrane potential closer to threshold, increasing the chance of an action potential.

Why is GABA considered inhibitory?

Because it makes the membrane potential more negative, moving it away from threshold and preventing action potentials.

How does GABA cause hyperpolarization?

By opening Cl⁻ (chloride) channels (letting Cl⁻ in) or K⁺ (potassium) channels (letting K⁺ out).

What does it mean when we say GABA "stops action potentials"?

it prevents the neuron from reaching threshold by hyperpolarizing the membrane, so no action potential is triggered.