Lecture 19- Action Potentials Flashcards by Stephanie Muenzen

Neurons

Basic functional unit of nervous system
Conduct electrical signals, integrate information
Various shapes, sizes- share basic characteristics

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Neuron anatomy- What is the cell body?

Main structure of neuron. Contains organelles.

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Neuron anatomy- What are the 2 types of cytoplasmic extensions?

Dendrites and axons

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Neuron anatomy- What are dendrites?

Protrusions from cell body of neuron. Receive information. Usually many short dendrites.

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Neuron anatomy- What are axons?

Passes signal. Usually one long axon per neuron

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Neuron anatomy- What is the axon hillock?

Base of axon- where signals are generated. Often branched at the end to form synaptic terminals.

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What is a nerve?

Axons of many neurons held together with connective tissue.

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What are the 3 types of neurons?

Afferent neurons, Interneurons, and Efferent neurons

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What are afferent neurons?

Sensory neurons
Sensory receptor -> nervous system

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What are interneurons?

Integration neurons
Only interact with other neurons
Make up 90% of neurons

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What are efferent neurons?

Motor neurons
Nervous system -> effector (muscle, etc)

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What is membrane potential?

Membrane potential is potential energy- potential to do work.

The difference in electrical potential between the interior and the exterior of a biological cell.
Selectively permeable, polarized membrane (difference in charge on either side)
Outside more + charge, inside more - charge.

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What are excitable cells?

Cells that can rapidly change membrane potential
Neurons- today
Muscle cells- later

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What is voltage?

Measurement of membrane potential
Measured by voltmeter

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What is resting potential? What causes it?

Membrane potential of the cell at rest (not excited)
-70mV
Due to:
Na/K pump
Ion channels

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What is the Sodium-Potassium pump?

Contributes to membrane potential + resting potential.
Transmembrane transport protein found throughout neuron.
1 cycle- 3 Na+ out, 2 K+ in
Active transport

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What are the ion channels?

Leaky K+ channels common, always open.
-> K+ is pumped in and diffuses out

Na+ is pumped out and cannot diffuse in
Creates a more positive charge outside cell
Maintains membrane potential (potential E) and resting potential

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How do the sodium-potassium pump and ion channels work to maintain membrane potential?

The sodium-potassium pump actively transports K+ ions into the cell, creating a higher concentration of K+ inside the cell compared to outside. However, these leaky channels are always open, allowing some K+ to diffuse out. The leaky K+ channels prevent the cell from becoming too positive. The Na+ being pumped out allows for a positive charge outside the cell, as the Na+ is attracted to the negative interior of the cell. These processes help to maintain the resting potential of -70mV.

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To generate membrane potential, the outside of the cell must be slightly ________ and the inside of the cell must be slightly __________.

The outside of the cell must be slightly positive and the inside of the cell must be slightly negative.

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The sodium-potassium pump does _______ transport and the potassium channel and sodium channel do __________ transport.

Sodium-potassium pump: Active transport
Potassium + Sodium channels: Passive transport

True or False: Ion channels use active transport to move ions through the plasma membrane.

False; ions pass through ion channels passively via facilitated diffusion.

What is the voltage value of a neuron at resting membrane potential?

-70mV

What is threshold?

Membrane potential required to trigger action potential
-55 mV for most neurons

Action Potential (AP)- How are changes caused in membrane potential?

Electrical signal within neuron
-> Depolarization crosses threshold
-> Induces voltage-gated channels
-> Large change in membrane potential

What are voltage-gated ion channels?

Membrane proteins, allow passage of specific ions When gate is closed: No ions flow across membrane. When gate is open (due to depolarization-> change in membrane potential (voltage)), ions flow through channel. Occurs through facilitated diffusion. Voltage-gated K+, Na+ channels involved in action potentials

AP Chain of Events (from resting state to depolarization)

Neurons start at resting state MP= -70mV Voltage-gated channels closed Stimulus causes Na+ channels to open (NOT voltage gated) -> Na+ enters -> depolarization

Magnitude of Depolarization- small stimulus vs. large stimulus

Small stimulus- Few channels open- weak depolarization, does not reach -55mV. No action potential. Large/strong stimulus- many channels open- strong depolarization. If MP reaches -55mV, action potential.

Rising Phase of the Action Potential

If the stimulus is strong enough and the MP reaches -55mV Voltage-gated Na+ channels open -> Rapid depolarization MP reaches +35mV

Falling Phase of the Action Potential

Na+ channels close Membrane impermeable to Na+ Refractory period Voltage-gated K+ channels open at +30mV -> K+ diffuses out -> repolarization during falling phase (Once reaches 30mV, voltage-gated K+ channels open to release positive ions + begin to hyperpolarize)

Undershoot in the Action Potential

Na+ closed, K+ open Hyperpolarization: MP more negative than resting potential K+ channels close-> MP returns to resting potential

AP Summary of Events

1. Resting state 2. Depolarization (caused by stimulus) 3. Rising phase of the action potential 4. Falling phase of the action potential 5. Undershoot Returns to resting state

Intensity of sensation depends on...

- Frequency of stimulus - # of neurons stimulated NOT strength of AP

Why is AP considered "All-or-Nothing"?

Either happens or doesn't. Always the same when it does. Dependent on depolarization reaching -55mV threshold.

Conduction of AP in neurons

Signal propagates as a series of APs along axon Axon hillock to terminal Voltage shift in one region -> triggers Na+ channels further down Refractory period prevents the AP from traveling backward, ensuring a unidirectional signal (the Na+ channels are closed) You can artificially stimulate the axon center, which would result in the AP initiating in two directions.

What is continuous conduction?

Occurs in unmyelinated axons (gray matter) Every spot depolarizes and repolarizes

What is saltatory conduction?

Occurs in myelinated axons (white matter) Requires myelin sheath: Fatty insulation for AP, rich in myelin Made by Oligodendrocytes (CNS), Schwann cells (PNS) Internodes: Regions covered myelin- no depolarization Nodes of Ranvier: No myelin- lots of Na+, K+ channels -> Depolarization only at nodes Signal jumps from node to node 50x faster than unmyelinated More E efficient

Multiple Sclerosis

Degenerative disease Myelin sheaths gradually deteriorate Replaced by scar tissue -> Progressive loss of coordination due to disruption of signal transmission

True or False: Continuous conduction occurs in myelinated axons.

False, continuous conduction occurs in unmyelinated axons.

True or False: Saltatory conduction occurs in myelinated axons.

True

True or False: Voltage-gated ion channels are open at resting membrane potential.

False, voltage-gated ion channels are closed at resting membrane potential

A typical action potential begins at which part of the neuron?

Axon hillock

___________ is the propagation of action potentials along myelinated axons from one node of Ranvier to the next one, increasing the conduction velocity of action potentials.

Saltatory conduction

Are plasma membranes more permeable to sodium or potassium and why?

More permeable to potassium because the potassium ion channels are always slightly open.

Each action potential is followed by a ___________, during which it is impossible to evoke another action potential.

Refractory period

True or False: At resting membrane potential there is more sodium inside of the cell than outside of the cell.

False, at resting membrane potential there is more sodium outside the cell than inside the cell.

What is hyperpolarization and what is its significance?

The change in a cell's membrane potential that makes it more negative. Inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.

True or False: Continuous conduction propagates an action potential faster than saltatory conduction.

False, saltatory conduction propagates an action potential faster than continuous conduction.

True or False: At resting membrane potential there is more potassium inside of the cell than outside of the cell.

True

What occurs immediately after threshold potential is reached?

Sodium channels in the axon hillock open, allowing positive ions to enter the neuron which then completely depolarizes to a membrane potential of +35mV.