Lecture 19- Action Potentials

Question 1

Neurons

Answer 1

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

Question 2

Neuron anatomy- What is the cell body?

Answer 2

Main structure of neuron. Contains organelles.

Question 3

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

Answer 3

Dendrites and axons

Question 4

Neuron anatomy- What are dendrites?

Answer 4

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

Question 5

Neuron anatomy- What are axons?

Answer 5

Passes signal. Usually one long axon per neuron

Question 6

Neuron anatomy- What is the axon hillock?

Answer 6

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

Question 7

What is a nerve?

Answer 7

Axons of many neurons held together with connective tissue.

Question 8

What are the 3 types of neurons?

Answer 8

Afferent neurons, Interneurons, and Efferent neurons

Question 9

What are afferent neurons?

Answer 9

Sensory neurons
Sensory receptor -> nervous system

Question 10

What are interneurons?

Answer 10

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

Question 11

What are efferent neurons?

Answer 11

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

Question 12

What is membrane potential?

Answer 12

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.

Question 13

What are excitable cells?

Answer 13

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

Question 14

What is voltage?

Answer 14

Measurement of membrane potential
Measured by voltmeter

Question 15

What is resting potential? What causes it?

Answer 15

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

Question 16

What is the Sodium-Potassium pump?

Answer 16

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

Question 17

What are the ion channels?

Answer 17

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

Question 18

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

Answer 18

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.

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

-70mV

Question 23

What is threshold?

Answer 23

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

Question 24

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

Answer 24

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

Question 25

What are voltage-gated ion channels?

Answer 25

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

Question 26

AP Chain of Events (from resting state to depolarization)

Answer 26

Neurons start at resting state
MP= -70mV
Voltage-gated channels closed

Stimulus causes Na+ channels to open (NOT voltage gated)
-> Na+ enters
-> depolarization

Question 27

Magnitude of Depolarization- small stimulus vs. large stimulus

Answer 27

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.

Question 28

Rising Phase of the Action Potential

Answer 28

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

Question 29

Falling Phase of the Action Potential

Answer 29

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)

Question 30

Undershoot in the Action Potential

Answer 30

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

Question 31

AP Summary of Events

Answer 31

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

Question 32

Intensity of sensation depends on…

Answer 32

• Frequency of stimulus
• # of neurons stimulatedNOT strength of AP

Question 33

Why is AP considered “All-or-Nothing”?

Answer 33

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

Question 34

Conduction of AP in neurons

Answer 34

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.

Question 35

What is continuous conduction?

Answer 35

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

Question 36

What is saltatory conduction?

Answer 36

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

Question 37

Multiple Sclerosis

Answer 37

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

Question 38

True or False: Continuous conduction occurs in myelinated axons.

Answer 38

False, continuous conduction occurs in unmyelinated axons.

Question 39

True or False: Saltatory conduction occurs in myelinated axons.

Answer 39

True

Question 40

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

Answer 40

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

Question 41

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

Answer 41

Axon hillock

Question 42

___________ 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.

Answer 42

Saltatory conduction

Question 43

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

Answer 43

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

Question 44

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

Answer 44

Refractory period

Question 45

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

Answer 45

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

Question 46

What is hyperpolarization and what is its significance?

Answer 46

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.

Question 47

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

Answer 47

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

Question 48

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

Answer 48

True

Question 48

What occurs immediately after threshold potential is reached?

Answer 48

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