Neurophysiology: Membrane Potentials, Action Potentials, Synapses, and Conduction

What is the resting membrane potential of a neuron?

−70 mV

What maintains the resting membrane potential?

Na⁺/K⁺ pump and K⁺ leak channels

What is the function of the Na⁺/K⁺ pump?

Pumps 3 Na⁺ out, 2 K⁺ in to maintain −70 mV

Which ion has more leak channels in the membrane?

K⁺

What is depolarization?

Membrane potential becomes less negative

What causes depolarization?

Na⁺ ions entering the cell

What is repolarization?

Membrane potential returns toward resting

What causes repolarization?

K⁺ ions leaving the cell

What is hyperpolarization?

Membrane potential becomes more negative than resting

Which ions are responsible for hyperpolarization?

K⁺ ions leaving the cell

What is the threshold potential for an AP?

−55 mV

What is the value of the peak of an action potential?

+30 mV

What is the refractory period?

Time when a neuron cannot fire another AP

What is the absolute refractory period?

No AP can occur, Na⁺ channels inactivated

What is the relative refractory period?

AP can occur if stimulus is stronger than usual

Where do graded potentials occur?

Dendrites and soma

Where do action potentials occur?

Axon

Can graded potentials summate?

Yes

Can action potentials summate?

No

Are graded potentials all-or-none?

No

Are action potentials all-or-none?

Yes

Do graded potentials lose strength over distance?

Yes

Do action potentials lose strength over distance?

No

What type of stimulus triggers a graded potential?

Neurotransmitters or sensory stimuli

What type of stimulus triggers an action potential?

Threshold is reached at axon hillock

EPSP makes the membrane more...?

Positive (closer to threshold)

IPSP makes the membrane more...?

Negative (further from threshold)

Which potentials are localized?

Graded potentials

Which potentials propagate along the axon?

Action potentials

Can EPSPs and IPSPs combine?

Yes, they can summate

What is the first step of an action potential?

Resting potential (−70 mV)

What is the second step of an action potential?

Graded potentials sum at axon hillock

What is the third step of an action potential?

Threshold reached (−55 mV)

What is the fourth step of an action potential?

Depolarization (Na⁺ enters, +30 mV)

What is the fifth step of an action potential?

Na⁺ channels inactivate, K⁺ channels open

What is the sixth step of an action potential?

Repolarization (K⁺ leaves)

What is the seventh step of an action potential?

Hyperpolarization (membrane dips below −70 mV)

What is the eighth step of an action potential?

Return to resting potential via Na⁺/K⁺ pump

During depolarization, which channel opens?

Voltage-gated Na⁺ channels

During repolarization, which channel opens?

Voltage-gated K⁺ channels

What happens to Na⁺ channels after depolarization?

They inactivate

What is the purpose of the refractory period?

Prevents backward propagation of AP

What happens if threshold is not reached?

No AP occurs

Is the action potential amplitude variable?

No, always the same (+30 mV)

What is continuous conduction?

AP travels along entire unmyelinated axon

What is saltatory conduction?

AP jumps between nodes of Ranvier on myelinated axons

Why is saltatory conduction faster?

Current jumps over myelin, fewer APs needed

Which uses more energy, continuous or saltatory conduction?

Continuous conduction

What happens at the axon hillock?

Graded potentials summed to see if threshold is reached

Which phase of AP causes K⁺ to leave the cell?

Repolarization

What is the function of the axon?

Transmit action potentials from soma to axon terminal

What is the axon hillock?

Trigger zone where graded potentials are summed

What are nodes of Ranvier?

Gaps in myelin where AP regenerates

What is myelin made of?

Schwann cells (PNS) or oligodendrocytes (CNS)

Why is myelin important?

Speeds up conduction and saves energy

What type of axons use continuous conduction?

Unmyelinated axons

What type of axons use saltatory conduction?

Myelinated axons

How does AP travel along unmyelinated axons?

Step-by-step along the entire membrane

How does AP travel along myelinated axons?

Jumps node-to-node

What limits the frequency of APs?

Refractory periods

Can AP go backward?

No, due to refractory period

Why is conduction velocity faster in larger diameter axons?

Less resistance to current

What is the effect of temperature on conduction speed?

Higher temperatures increase speed

What is a synapse?

Junction between neurons where chemical signals pass

Name the two main types of synapses.

Electrical and chemical

Which type of synapse is more common?

Chemical synapse

What happens when an AP reaches the axon terminal?

Voltage-gated Ca²⁺ channels open

Why does Ca²⁺ enter the axon terminal?

Triggers vesicles to release neurotransmitters

What is exocytosis?

Release of neurotransmitters into synaptic cleft

Where do neurotransmitters bind?

Postsynaptic receptors

What is the effect of an excitatory neurotransmitter?

EPSP → depolarizes postsynaptic neuron

What is the effect of an inhibitory neurotransmitter?

IPSP → hyperpolarizes postsynaptic neuron

How is neurotransmitter removed from the synapse?

Reuptake, enzymes, or diffusion

Give one example of an excitatory neurotransmitter.

Glutamate

Give one example of an inhibitory neurotransmitter.

GABA

What is summation at the synapse?

EPSPs and IPSPs combine to determine AP generation

What is temporal summation?

Multiple EPSPs from same presynaptic neuron in quick succession

What is spatial summation?

EPSPs from multiple presynaptic neurons combine

What determines if a postsynaptic neuron fires?

Whether threshold at axon hillock is reached

Which ion causes vesicle release at the synapse?

Ca²⁺

What happens if a neurotransmitter binds to an ionotropic receptor?

Directly opens ion channel

What happens if a neurotransmitter binds to a metabotropic receptor?

Activates second messenger system

Can inhibitory and excitatory inputs occur at the same time?

Yes, the net effect determines action potential

What are the main parts of a neuron?

Dendrites, soma, axon hillock, axon, axon terminal

What is the function of dendrites?

Receive signals from other neurons

What is the function of the soma?

Contains nucleus and integrates signals

What is the function of the axon terminal?

Releases neurotransmitters

What is the function of the myelin sheath?

Insulates axon, speeds conduction

What is the function of the nodes of Ranvier?

Regenerate action potential

Where does summation of graded potentials occur?

Axon hillock

Where does integration of signals occur?

Soma

Where is the resting potential maintained?

Throughout the entire neuron

What is the primary role of an axon?

Conduct action potential to synapse

Which ions are most important for action potential?

Sodium (Na⁺) and Potassium (K⁺)

During depolarization, which channels open?

Voltage-gated Na⁺ channels

During repolarization, which channels open?

Voltage-gated K⁺ channels

Which channels are leaky and maintain resting potential?

K⁺ leak channels

How does the Na⁺/K⁺ pump restore resting potential?

Pumps 3 Na⁺ out and 2 K⁺ in

What direction does Na⁺ move during depolarization?

Into the cell

What direction does K⁺ move during repolarization?

Out of the cell