Ch 2: Behavior & tHE BRAIN

What is a chemical synapse?

A junction where one neuron communicates with another cell by releasing neurotransmitters across the synaptic cleft. Memory cue: neurotransmitters = chemical message 🧪 released across the synaptic cleft gap.

What is an electrical synapse?

A junction where electrical signals pass directly from one cell to another through gap junctions without using neurotransmitters. Memory cue: ⚡ message passes directly through connected channels.

How do chemical and electrical synapses differ?

Chemical synapses communicate by releasing neurotransmitters across the synaptic cleft. Electrical synapses allow electrical signals to pass directly between cells through gap junctions without neurotransmitters.

What is an action potential?

A rapid electrical signal that travels down the axon when the neuron's membrane reaches threshold.

What is a postsynaptic potential?

A change in the electrical charge (membrane potential) of the postsynaptic neuron after neurotransmitters bind to its receptors.

What is a neurotransmitter?

A chemical messenger released by a neuron that crosses the synaptic cleft and binds to receptors on a postsynaptic cell, changing its activity.

What three responses can occur in a postsynaptic neuron after neurotransmitter binding?

1. EPSP: makes the neuron more likely to reach threshold and fire. 2. IPSP: makes it less likely to reach threshold and fire. 3. Neuromodulation: changes intracellular signaling to modify neuronal function more long-term.

What is the presynaptic membrane/terminal?

The sending side of the synapse, located at the end of the axon, that releases neurotransmitters into the synaptic cleft.

What is the postsynaptic membrane/terminal?

The receiving side of the synapse that contains receptors where neurotransmitters bind, causing a postsynaptic response such as an EPSP or IPSP.

What is a postsynaptic receptor?

A protein on the postsynaptic membrane that binds to a specific neurotransmitter and causes a response in the postsynaptic cell.

What is threshold?

The minimum level of depolarization a neuron must reach to trigger an action potential. Threshold is often around −55 mV.

What is resting membrane potential?

The electrical charge difference across a neuron's membrane when it is not firing. The inside of the neuron is more negative than the outside. At rest, it is about −70 mV.

Describe the chemical communication that occurs at a synapse.

An action potential reaches the presynaptic terminal → neurotransmitters stored in vesicles are released into the synaptic cleft → neurotransmitters cross the cleft and bind to postsynaptic receptors → this produces an electrical change such as an EPSP, IPSP, or neuromodulation → the neurotransmitter is cleared from the synaptic cleft.

What happens to neurotransmitter-containing vesicles when an action potential reaches the presynaptic terminal?

The action potential initiates their release. The vesicles merge with the membrane and release neurotransmitters into the synaptic cleft.

What does an EPSP do?

An EPSP is an excitatory postsynaptic potential. It moves the postsynaptic neuron toward threshold and makes it more likely to fire an action potential. Memory cue: EPSP = positive/toward threshold ➕.

What does an IPSP do?

An IPSP is an inhibitory postsynaptic potential. It moves the neuron away from threshold or otherwise makes reaching threshold and firing less likely. Memory cue: IPSP = negative/away from threshold ➖.

What is temporal summation?

Multiple postsynaptic signals from the SAME synapse arrive rapidly one after another and add together.

What is spatial summation?

Postsynaptic signals from DIFFERENT synapses/locations arrive around the same time and add together.

How does a neuron decide whether to fire an action potential?

The postsynaptic neuron combines EPSPs and IPSPs through temporal and spatial summation. If the summed signals reach threshold, around −55 mV, an action potential fires.

What structures should you be able to locate in an image of a synapse?

Synaptic vesicles, presynaptic terminal, neurotransmitters, postsynaptic terminal, postsynaptic receptors, and synaptic cleft.

What is an ion?

A charged particle/atom that can have a positive, negative, or neutral charge.

What is the phospholipid membrane?

The cell membrane made of a phospholipid bilayer that separates the inside of the cell from the outside and controls what can enter and leave. It separates charged ions inside versus outside the neuron, helping create membrane potential.

What is membrane potential?

The difference in electrical charge between the inside and outside of a cell. Memory cue: membrane potential = electrical charge difference across the membrane ⚡.

What is the resting membrane potential of a neuron?

About −70 mV. The inside of the neuron contains more negative charges than positive charges.

What is the electrical gradient/electrical force?

The force that causes ions to move toward an opposite electrical charge and away from a like charge. Memory cue: opposites attract.

If the inside of a neuron is negative, what does the electrical force do to positively charged ions such as Na⁺ and K⁺?

It attracts the positively charged ions into the neuron.

What is the chemical gradient/diffusive force?

The force that causes ions or molecules to move from an area of high concentration toward an area of low concentration.

What two forces influence how ions move across a neuron's membrane?

The chemical/diffusion gradient and the electrical gradient. Together they form the electrochemical gradient.

What is a ligand-gated channel?

An ion channel that opens or closes when a chemical (ligand), such as a neurotransmitter, binds to it. Memory cue: ligand = chemical KEY 🔑 → channel opens 🚪. Important in postsynaptic responses.

What is a voltage-gated channel?

An ion channel that opens or closes in response to changes in membrane voltage. These channels are important in action potentials.

What does the Na⁺/K⁺ pump do?

Uses ATP to pump 3 Na⁺ OUT of the neuron and 2 K⁺ IN, helping maintain the chemical gradients needed for resting membrane potential. Na⁺ is high outside and K⁺ is high inside.

How is the cell membrane selectively permeable?

The cell membrane allows some substances to cross more easily than others, controlling what enters and leaves the cell.

How do the cell membrane, ion channels, and ion pumps control ion movement?

The cell membrane is a selectively permeable barrier that prevents ions from freely crossing. Ion channels allow specific ions to cross according to their electrochemical gradients. Ion pumps use ATP to move ions against their gradients and maintain concentration differences.

How do diffusion and electrical forces combine to control ion movement?

Diffusion moves ions from high → low concentration. Electrical force attracts ions toward opposite charges and repels them from like charges. Together they form the electrochemical gradient, which determines ion movement when channels are open. The forces can agree or oppose each other.

What four steps can you use to determine the direction an ion will move?

1. Chemical force: high → low concentration. 2. Electrical force: opposites attract and like charges repel. 3. Combine both forces to determine the electrochemical gradient. 4. The ion can move only if an appropriate channel/permeability exists.

Na⁺ is high outside a neuron and the inside is negative. If a Na⁺ channel opens, which direction does Na⁺ move?

IN. The chemical force pushes Na⁺ from its high concentration outside toward its lower concentration inside, and the negative interior electrically attracts positively charged Na⁺. Both forces favor movement IN.

A neuron has high K⁺ inside and low K⁺ outside. If a K⁺ channel opens, which direction does the chemical force push K⁺?

OUT. K⁺ moves from its high concentration inside toward its lower concentration outside.

If the inside of a neuron is negative, which direction does the electrical force pull K⁺?

IN. K⁺ is positively charged, so it is electrically attracted to the negative interior of the neuron.

For K⁺ with high concentration inside and a negative interior, how do the chemical and electrical forces interact?

They oppose each other. The chemical/diffusion force pushes K⁺ OUT, while the electrical force pulls K⁺ IN.

Describe the resting membrane potential and its ionic basis.

At rest, the membrane is about −70 mV. Na⁺ concentration is high outside and K⁺ concentration is high inside. The Na⁺/K⁺ pump and the membrane's selective permeability/ion channels maintain these distributions.

What is depolarization?

The membrane becomes less negative/more positive, moving toward threshold. a rapid influx of sodium (Na⁺) ions rushing into the cell through voltage-gated sodium channels

What is hyperpolarization?

The membrane becomes more negative, moving farther from threshold.

How is Na⁺ entry related to an EPSP?

Na⁺ entry generally causes depolarization, producing an EPSP and making an action potential more likely. Memory cue: Na⁺ IN = EPSP ➕.

How is Cl⁻ entry related to an IPSP?

Cl⁻ entry generally causes inhibition, producing an IPSP and making the neuron less likely to reach threshold/fire. Memory cue: Cl⁻ IN = IPSP ➖.

What does subthreshold mean?

A change in membrane potential that does not reach threshold, so no action potential occurs.

Where and how do EPSPs and IPSPs summate?

EPSPs and IPSPs summate at/near the axon hillock/initial segment through temporal and spatial summation. If the combined membrane potential reaches threshold, an action potential is generated. Memory cue: EPSPs + IPSPs → SUM → threshold? → AP ⚡.

What happens during the rising phase of an action potential?

Voltage-gated Na⁺ channels open → Na⁺ enters → rapid depolarization. Memory cue: Na⁺ IN.

What happens during the falling phase of an action potential?

Na⁺ channels inactivate and voltage-gated K⁺ channels open → K⁺ leaves → repolarization. Memory cue: K⁺ OUT.

What happens during the undershoot of an action potential?

K⁺ channels remain open briefly, causing K⁺ to continue leaving and the membrane to become more negative than resting potential before returning to rest. This is hyperpolarization.

Describe the ionic basis of an action potential from threshold back to resting potential.

Threshold → voltage-gated Na⁺ channels open → Na⁺ enters → depolarization → Na⁺ channels inactivate and K⁺ channels open → K⁺ exits → repolarization → brief hyperpolarization → resting potential.

What is action-potential propagation?

The action potential is regenerated along the axon as depolarization triggers voltage-gated channels farther down the membrane.

What is a Node of Ranvier?

A gap between sections of myelin where the action potential is regenerated.

How does myelin affect action-potential propagation?

Myelin insulates the axon and increases the speed of action-potential propagation.

How does action-potential propagation differ in unmyelinated and myelinated axons?

Unmyelinated: the action potential is regenerated continuously along the axon → slower. Myelinated: current travels rapidly beneath myelin and the AP is regenerated at Nodes of Ranvier (saltatory conduction) → faster.

What is an autoimmune disorder?

A disorder in which the immune system mistakenly attacks the body's own cells or tissues.

What is multiple sclerosis (MS)?

An autoimmune disorder in which the immune system damages myelin in the CNS, disrupting action-potential conduction.

What could occur if Cl⁻ channels are not located on the cell membrane?

Cl⁻ cannot enter normally → reduced inhibitory effects/IPSPs → the neuron may become more excitable and more likely to fire. Memory cue: Less Cl⁻ entry = less inhibition.

What is the effect of tetrodotoxin (TTX) on Na⁺ channels and action potentials?

TTX blocks voltage-gated Na⁺ channels, preventing Na⁺ entry and therefore preventing action potentials from being generated or propagated. Memory cue: TTX blocks Na⁺ → NO depolarization → NO AP.

What occurs in multiple sclerosis to affect action-potential propagation?

MS damages/destroys CNS myelin (demyelination), causing action potentials to travel more slowly or fail to propagate properly. Memory cue: MS → myelin damage → signal slows/fails.