Electrical Signaling in Neurons

Size and Scale Order

1. Cells

2. Organelles

3. Viruses

4. Macromolecules

5. Molecules

6. Ions

First Step

Dendrites receive signals

Second Step

Axon transmits the signal to the next neuron.

Third Step

The message alternates between an electrical signal within the neuron and a chemical signal released at the synapse

Fourth Step

The presynaptic cell transmits the signal to the postsynaptic cell

Fifth Step

The electrical signal travels through the first neuron.

Sixth Step

At the end of the presynaptic neuron, a neurotransmitter is released (from presynaptic to postsynaptic).

Seventh Step

Neurotransmitter activates the next cell resulting in an electrical signal in the post synaptic neuron

What is a neuron?

A cell that receives and sends information in the nervous system.

What is a neural circuit?

A group of connected neurons that work together to produce a response.

What is an action potential?

A brief electrical signal that travels down a neuron's axon.

What direction does an action potential travel?

From the axon hillock down the axon toward the axon terminals.

What is the axon?

The long part of a neuron that carries action potentials toward the axon terminals.

What are axon terminals?

The ends of the axon that release neurotransmitters.

What are synaptic terminals?

Another name for axon terminals.

What is a synapse?

The junction where a neuron communicates with another cell.

What is a chemical synapse?

A synapse where a neuron uses neurotransmitters to communicate with another cell.

What is the synaptic cleft?

The tiny gap between the sending cell and the receiving cell.

What are neurotransmitters?

Chemical messengers that carry signals across a chemical synapse.

What happens when an action potential reaches the axon terminal?

Voltage-gated calcium channels open, allowing calcium ions to enter the terminal.

What does calcium do at the axon terminal?

It triggers synaptic vesicles to release neurotransmitters.

What are synaptic vesicles?

Tiny sacs that store neurotransmitters inside the axon terminal.

How are neurotransmitters released?

Vesicles fuse with the cell membrane and release neurotransmitters into the synaptic cleft.

How do neurotransmitters reach the receiving cell?

They cross the synaptic cleft and bind to receptors on the receiving cell.

What do neurotransmitters do when they bind to receptors?

They open or close ion channels or trigger other changes in the receiving cell.

What happens to neurotransmitters after they send their message?

They may be broken down, taken back up into cells, or diffuse away.

What is an excitatory neurotransmitter effect?

It makes the receiving neuron more likely to fire an action potential.

What is an inhibitory neurotransmitter effect?

It makes the receiving neuron less likely to fire an action potential.

What is a resting membrane potential?

The electrical difference across a neuron's membrane when it is not firing an action potential.

What is the typical resting membrane potential of a neuron?

About -70 mV.

What does mV stand for?

Millivolts, a unit used to measure electrical voltage.

Why is the inside of a resting neuron negative?

There are more positive charges outside relative to inside, partly because of ion distribution and negatively charged substances inside the cell.

Where is sodium (Na+) more concentrated?

Outside the neuron.

Where is potassium (K+) more concentrated?

Inside the neuron.

What is an ion?

An atom or molecule with an electrical charge.

What is diffusion?

The movement of particles from an area of high concentration to an area of low concentration.

Why does sodium tend to move into a resting neuron?

There is more sodium outside than inside, and the negative interior attracts positively charged sodium.

Why does potassium tend to move out of a resting neuron?

There is more potassium inside than outside, so it tends to diffuse outward.

What is a leak channel?

A channel that allows certain ions to cross the membrane without needing to open in response to a signal.

What do potassium leak channels allow?

They allow potassium ions to move out of the neuron.

What does the sodium-potassium pump do?

It uses ATP to move 3 sodium ions out and 2 potassium ions into the neuron.

Does the sodium-potassium pump use energy?

Yes, it uses ATP.

Why is the sodium-potassium pump important?

It maintains the different sodium and potassium concentrations needed for the resting membrane potential and neuronal signaling.

Can the sodium-potassium pump keep up with all ion movement by itself?

No. Leak channels also allow ions to move, and the pump maintains the concentration differences over time.

What is a concentration gradient?

A difference in the concentration of a substance between two areas.

What is an electrical gradient?

A difference in electrical charge that attracts or repels charged particles.

What is an electrochemical gradient?

The combined effect of a concentration gradient and an electrical gradient on an ion.

What happens when sodium channels open during an action potential?

Sodium enters the neuron, making the inside less negative.

What is depolarization?

When the membrane potential becomes less negative.

What happens when potassium channels open during an action potential?

Potassium leaves the neuron, helping the inside become more negative again.

What is repolarization?

When the membrane potential moves back toward its resting negative value after depolarization.

What is threshold?

The membrane voltage that must be reached to trigger an action potential.

What happens if a neuron reaches threshold?

It fires an action potential.

What happens if a neuron does not reach threshold?

It does not fire an action potential.

What is an all-or-none response?

An action potential either occurs fully once threshold is reached or does not occur.

What is a sensory neuron?

A neuron that detects information from the environment or body and carries it toward the nervous system.

What is a motor neuron?

A neuron that sends signals to muscles or glands.

What is an excitatory signal?

A signal that makes a receiving neuron more likely to reach threshold.

What is an inhibitory signal?

A signal that makes a receiving neuron less likely to reach threshold.

What is summation in a neuron?

The combining of incoming excitatory and inhibitory signals to determine whether threshold is reached.

What does summation determine?

Whether the neuron fires an action potential.

What is an EPSP?

An excitatory postsynaptic potential; a small change that makes the receiving neuron more likely to fire.

What is an IPSP?

An inhibitory postsynaptic potential; a change that makes the receiving neuron less likely to fire.

What is the postsynaptic cell?

The receiving cell at a synapse.

What is the presynaptic cell?

The sending cell that releases neurotransmitters.

How does a neural message travel between neurons?

It travels electrically along the neuron and chemically across a chemical synapse.

What is a neural network?

A larger group of interconnected neurons that process information together.

What is parallel processing?

When information travels along multiple pathways at the same time.

What is feedback in a neural circuit?

When activity in a circuit affects later activity in that same circuit.

What is rhythmic activity?

A repeating pattern of activity and inactivity.

What are examples of rhythmic activity in humans?

Walking, breathing, and sleep-wake cycles.

What is the balance between excitation and inhibition?

The right amount of activation and suppression that allows a neural circuit to function properly.

What can happen if a neural circuit has too much excitation?

It may become excessively active.

What can happen if a neural circuit has too much inhibition?

It may become insufficiently active.

Why are neural circuits efficient?

Even a small number of connected neurons can produce complex behaviors.

What is the Tritonia sea slug example used to explain?

How a small neural circuit can produce a repeating movement that helps an animal escape danger.

What triggers the Tritonia sea slug's escape response?

Sensory neurons detect chemicals associated with a predator.

What happens first in the Tritonia swim circuit?

Sensory neurons fire action potentials and excite motor neurons.

What do the motor neurons do in the Tritonia swim circuit?

They activate the back muscles, causing the sea slug to arch.

What does the inhibitory neuron do in the Tritonia swim circuit?

It inhibits the motor neurons, allowing the back muscles to relax.

Why does the Tritonia swim movement repeat?

The neurons activate and inhibit one another in a repeating cycle.

When does the Tritonia sea slug stop its escape swimming?

When the sensory neurons stop firing.

How can experience change the Tritonia swim circuit?

Injury can make sensory neurons more excitable, while repeated harmless touches can make the response weaker.

What does it mean when a neuron is hyperexcitable?

It fires more easily or stays active longer than usual.