Lecture 19- Action Potentials Flashcards
Neurons
Basic functional unit of nervous system
Conduct electrical signals, integrate information
Various shapes, sizes- share basic characteristics
Neuron anatomy- What is the cell body?
Main structure of neuron. Contains organelles.
Neuron anatomy- What are the 2 types of cytoplasmic extensions?
Dendrites and axons
Neuron anatomy- What are dendrites?
Protrusions from cell body of neuron. Receive information. Usually many short dendrites.
Neuron anatomy- What are axons?
Passes signal. Usually one long axon per neuron
Neuron anatomy- What is the axon hillock?
Base of axon- where signals are generated. Often branched at the end to form synaptic terminals.
What is a nerve?
Axons of many neurons held together with connective tissue.
What are the 3 types of neurons?
Afferent neurons, Interneurons, and Efferent neurons
What are afferent neurons?
Sensory neurons
Sensory receptor -> nervous system
What are interneurons?
Integration neurons
Only interact with other neurons
Make up 90% of neurons
What are efferent neurons?
Motor neurons
Nervous system -> effector (muscle, etc)
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.
What are excitable cells?
Cells that can rapidly change membrane potential
Neurons- today
Muscle cells- later
What is voltage?
Measurement of membrane potential
Measured by voltmeter
What is resting potential? What causes it?
Membrane potential of the cell at rest (not excited)
-70mV
Due to:
Na/K pump
Ion channels
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
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
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.
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.
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
- Resting state
- Depolarization (caused by stimulus)
- Rising phase of the action potential
- Falling phase of the action potential
- Undershoot
Returns to resting state
Intensity of sensation depends on…
- Frequency of stimulus
- # of neurons stimulatedNOT 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.
