Neuroscience Graded Potentials and Action Potentials

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Questions and Answers

What is the threshold potential that must be reached to initiate an action potential?

-90 mV

-55 mV (correct)

-70 mV

+30 mV

During depolarization, which ion channels open allowing sodium ions to enter the cell?

Mechanically gated K+ channels

Voltage-gated K+ channels

Ligand-gated Na+ channels

Voltage-gated Na+ channels (correct)

What characterizes the absolute refractory period?

K+ channels are still open

A strong stimulus can initiate an action potential

A neuron can fire another action potential

Inactivated Na+ channels need to return to resting state (correct)

What happens to the membrane potential during repolarization?

It decreases back to -70 mV Signup and view all the answers

How much sodium actually enters the cell during the depolarization phase?

20,000 Na+ Signup and view all the answers

Which phase of the action potential involves the outflow of potassium ions?

Repolarization Signup and view all the answers

What occurs during the relative refractory period?

K+ channels are still open and Na+ channels are closed Signup and view all the answers

What is the typical resting membrane potential of a neuron?

-70 mV Signup and view all the answers

What is the role of refractory periods in action potential propagation?

They separate action potentials and determine their direction. Signup and view all the answers

Which type of conduction involves step-by-step depolarization along the axolemma?

Continuous conduction Signup and view all the answers

In saltatory conduction, where does depolarization primarily occur?

At nodes of Ranvier. Signup and view all the answers

How does the flow of sodium ions affect action potential propagation?

It triggers the opening of voltage-gated Na+ channels in adjacent areas. Signup and view all the answers

What is a nerve impulse also referred to as?

Action potential. Signup and view all the answers

Which statement is true regarding the propagation speed of a nerve impulse?

It is independent of the strength of the stimulus. Signup and view all the answers

What distinguishes saltatory conduction from continuous conduction?

Saltatory conduction allows impulses to jump between nodes of Ranvier. Signup and view all the answers

What happens to the action potential as Na+ flows into the cell during depolarization?

It influences adjacent areas, causing their voltage-gated Na+ channels to open. Signup and view all the answers

Which type of nerve fibers are the largest and conduct impulses at the fastest rate?

A fibers Signup and view all the answers

What characterizes the conduction speed of C fibers compared to A fibers?

C fibers conduct slower than A fibers. Signup and view all the answers

What is the primary mechanism by which stimulus intensity is encoded?

The frequency of impulses generated Signup and view all the answers

Which effect do local anesthetics such as lidocaine have on action potentials?

They block the opening of voltage-gated Na+ channels. Signup and view all the answers

What type of synapse directly allows electrical activity to pass between two neurons?

Electrical synapse Signup and view all the answers

How do neurotransmitters at a chemical synapse influence the postsynaptic neuron?

They bind to receptors on the postsynaptic neuron. Signup and view all the answers

Which type of fiber is primarily involved in transmitting visceral sensory information?

B fibers Signup and view all the answers

What impact does a firm pressure stimulus have compared to a light touch?

It activates a greater number of sensory neurons. Signup and view all the answers

What initiates the release of neurotransmitters at a chemical synapse?

Inward flow of calcium ions Signup and view all the answers

Which statement correctly describes an excitatory postsynaptic potential (EPSP)?

It is associated with opening ligand-gated Na+ channels. Signup and view all the answers

What is the key difference between EPSP and IPSP?

EPSP leads to Na+ influx while IPSP often involves Cl- or K+ influx. Signup and view all the answers

Which of the following describes the synaptic delay in chemical synapses?

Approximately 0.5 msec Signup and view all the answers

What occurs during the binding of neurotransmitter to ligand-gated receptors?

The membrane potential of the postsynaptic cell becomes more positive. Signup and view all the answers

What is the role of the postsynaptic cell in response to inhibitory postsynaptic potential (IPSP)?

To become hyperpolarized and less likely to reach threshold. Signup and view all the answers

What characterizes the information transfer in chemical synapses?

Information transfer is one-way. Signup and view all the answers

What results from the influx of Cl- or K+ ions during synaptic transmission?

IPSP formation Signup and view all the answers

What defines the magnitude of graded potentials?

It decreases with the distance from the original event. Signup and view all the answers

What does summation of graded potentials refer to?

The accumulation of graded potentials from several stimuli occurring quickly. Signup and view all the answers

Where do graded potentials primarily occur within a neuron?

In the dendrites and cell body. Signup and view all the answers

Which is true about action potentials?

They occur in a sequence of depolarization and repolarization. Signup and view all the answers

What happens to the membrane potential during depolarization?

It becomes more positive. Signup and view all the answers

Which channels open first during the generation of an action potential?

Voltage-gated Na+ channels. Signup and view all the answers

What is a characteristic feature of graded potentials?

They are decremental and localized. Signup and view all the answers

What principle governs the generation of an action potential?

The all-or-none principle. Signup and view all the answers

Study Notes

Graded Potentials

Graded potentials are localized changes in membrane potential.
The magnitude of the potential change varies depending on the strength of the stimulus.
Graded potentials are decremental; the potential change decreases with distance from the stimulus.
Graded potentials can summate, meaning that multiple stimuli can combine to create a larger potential change.
Graded potentials occur most often in the dendrites and cell body of a neuron.

Action Potential

An action potential (AP) is a rapid sequence of events that reverses the membrane potential and then restores it to resting state.
During an AP, voltage-gated Na+ and K+ channels open in a specific sequence.
The all-or-none principle states that if a stimulus reaches threshold, an AP will occur with the same magnitude, regardless of the stimulus strength.

Depolarization Phase

A stimulus causes a graded potential to reach threshold (-55 mV).
Voltage-gated Na+ channels open, allowing Na+ to flow into the cell.
The influx of Na+ rapidly depolarizes the membrane, reaching a peak of +30 mV.

Repolarization Phase

Voltage-gated K+ channels open slowly, and K+ flows out of the cell.
The outflow of K+ repolarizes the membrane, returning it to the resting potential (-70 mV).

After-Hyperpolarization

If enough K+ leaves the cell, it will reach a membrane potential of -90 mV.
K+ channels close, and the Na+/K+ pump restores the membrane potential to -70 mV.

Refractory Period

The refractory period is a time during which a neuron cannot generate another action potential.
The absolute refractory period occurs when Na+ channels are inactivated and cannot be reopened, even with a strong stimulus.
The relative refractory period occurs when K+ channels are still open, but Na+ channels have closed. A suprathreshold stimulus can still initiate an AP during this period.

Propagation of Action Potentials

The AP spreads along the axon membrane.
The influx of Na+ at one point depolarizes the adjacent region, triggering the opening of voltage-gated Na+ channels and propagating the AP.
The traveling AP is called a nerve impulse.

Continuous vs. Saltatory Conduction

Continuous conduction occurs in unmyelinated fibers.
Saltatory conduction occurs in myelinated fibers.
Saltatory conduction is faster than continuous conduction because the AP only occurs at the nodes of Ranvier, where voltage-gated ion channels are concentrated.

Speed of Impulse Propagation

The speed of impulse propagation is not dependent on stimulus strength.
Larger, myelinated fibers conduct impulses faster due to their size and saltatory conduction.

Types of Nerve Fibers

Type A fibers are the largest and fastest, myelinated, and transmit somatic sensory and motor signals to skeletal muscle.
Type B fibers are medium-sized, myelinated, and transmit visceral sensory and autonomic preganglionic signals.
Type C fibers are the smallest and slowest, unmyelinated, and transmit sensory and autonomic motor signals.

Encoding of Stimulus Intensity

The nervous system differentiates between stimuli of different intensities through:
- Frequency of impulses: Stronger stimuli generate higher frequency impulses.
- Number of sensory neurons activated: Stronger stimuli activate a larger number of neurons.

Clinical Effects of Action Potential Inhibition

Local anesthetics, such as procaine and lidocaine, block voltage-gated Na+ channels, preventing action potential generation.
This blocks pain signals from reaching the brain.

Synapses

A synapse is a specialized junction between two neurons.
Synapses can be electrical or chemical.
Electrical synapses allow direct current flow between neurons through gap junctions.
Chemical synapses involve the release of neurotransmitters from the presynaptic neuron, which bind to receptors on the postsynaptic neuron.

Chemical Synapses

When an AP reaches the axon terminal, voltage-gated Ca2+ channels open.
Ca2+ influx triggers the release of neurotransmitters.
Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron.
The amount of neurotransmitter released determines the magnitude of the postsynaptic potential change.

Excitatory & Inhibitory Potentials

Neurotransmitters can be excitatory or inhibitory.
An excitatory postsynaptic potential (EPSP) is a depolarizing graded potential that increases the likelihood of the postsynaptic neuron reaching threshold.
An inhibitory postsynaptic potential (IPSP) is a hyperpolarizing graded potential that decreases the likelihood of the postsynaptic neuron reaching threshold.

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Description

This quiz covers key concepts related to graded potentials and action potentials in neuroscience. It includes discussions on their definitions, mechanisms, and phases, emphasizing their role in neuron function. Understanding these processes is essential for studying the nervous system.