Resting Membrane and Action Potentials Flashcards
What is a neuron at rest?
Unstimulated/inactive
Can change when excited by input
How does a negative charge build-up inside the cell?
The membrane bilayer is selectively permeable
Ions cannot cross easily which results in a polarized membrane (-70 mV)
Ions are unequally distributed across the membrane
How do we record membrane potential?
Use intracellular and extracellular electrodes to detect the difference between the inside and outside of cells
What is the ionic distribution outside of the cell?
Higher [Na+] outside of the cell
- the membrane extremely resists passage
- driven inside cell by electrostatic forces and concentration gradient
Higher [Cl-] concentration outside of the cell
- membrane slightly resists passage
- more at equilibrium
What is the ionic distribution inside of the cell?
Higher [K+] inside of the cell
- driven inside by electrostatic pressure and outside by a concentration gradient
- membrane is highly permeable to K+ because they are more leaky channels
Negatively charged proteins inside
-cannot cross the membrane
What are the factors that maintain resting potential?
Concentration gradient
Electrostatic pressure
Passive forces
Active forces
What is a concentration gradient?
Tend to equally distribute
Move from high to low concentration
What is electrostatic pressure?
Like repels like
Opposites attract
What are passive forces?
Random motion due to selective permeability to ions
What are active forces?
Sodium-potassium pumps; maintain the stability of resting membrane potential
Re-distribution of ions
How do sodium-potassium pumps provide long-term maintenance of resting membrane potential?
Active transporter
Maintains equilibrium potential by continuously transferring 3Na+ out and 2K+ in
How do postsynaptic potentials create excitatory signals?
Excitatory neurotransmitters bind and cause rapid change to the postsynaptic cell
Causes depolarization of the cell (cell becomes less negative)
An excitatory postsynaptic potential increases the likelihood of an action potential
What is an EPSP?
Excitatory postsynaptic potential
Graded response
Bigger stimulation = bigger PSP
larger positive ion influx = larger EPSP
How do postsynaptic potentials create inhibitory signals?
Inhibitory neurotransmitters bind and cause rapid change to the postsynaptic cell Causes hyperpolarization (cell becomes more negative) An inhibitory postsynaptic potential decreases the likelihood of an action potential
What is an IPSP?
Inhibitory postsynaptic potential
Large negative ion influx = larger IPSP
What is a graded response?
The amplitude of the signal proportional to stimulus intensity
Passive decay over space and time
What is a rapid transmission?
Instantaneous rate of transmission
How does the integration of PSPs help the cell depolarization reach threshold?
One EPSP will not generate an action potential
We need to add or combine incoming signals within a given neuron
What is the threshold of excitation?
Synapses closer to the axon hillock have a larger effect on firing due to the incremental transmission of far away PSPs
The integration of inputs must result in about -65 mV at hillock for an action potential
What is spatial summation?
Occurs at axon initial segment
Integration of PSPs across postsynaptic locations
Local EPSPs and IPSPs occurring simultaneously combine to amplify or cancel out
What is temporal summation?
Integration across time on the same synapse
Multiple rapid inputs from the same presynaptic site
What is an action potential?
Reversal of membrane potential by stimulation
Why doesn’t an action potential degrade?
Does not degrade along the length of the axon due to the voltage-gated channels
What type of response is an action potential?
All or nothing is the membrane depolarizes to the threshold voltage
How are action potentials affected by higher or longer stimulation?
The firing frequency increases
What happens to the action potential when it just meets the threshold?
There are fewer action potentials produced
What happens to the action potential when it goes way about the threshold?
There are many action potentials produced
What channels generate and propagate action potentials?
Voltage-gated sodium channels
Where are voltage-gated ion channels found?
Along the axon
When do voltage-gated ion channels open?
in response to membrane potential depolarization
When do voltage-gated ion channels close?
Close to continued depolarization
What happens when sodium channels open?
Sodium rushes into the cell based on the electrostatic and concentration gradient
The membrane potential reaches +50 mV
When do sodium channels close?
When depolarization reaches a threshold
What happens when potassium channels open?
Potassium rushes out of the cell based on the electrostatic and concentration gradient
Membrane repolarizes and becomes slightly hyperpolarized
These channels close gradually
How is membrane potential reestablished?
By random motion of ions and the sodium-potassium pump
What are the differences between PSPs and APs?
EPSPs/IPSPs
- decremental over space and time
- fast
- passive
APs
- non-decremental
- conducted more slowly than PSPs
- passive and active
What is the absolute refractory period?
Voltage-gated sodium channels are inactivated for about 1 msec post-AP initiation
Cannot start a new AP in the same location on the axon
What is the purpose of the absolute refractory period?
Prevents backwards movement of the AP
What is the relative refractory period?
Happens during the after hyperpolarization
Requires larger than threshold stimulus to initiate new AP
More intense stimulation can overcome hyperpolarization to increase the firing rate
How are action potentials conducted across the axon?
A row of voltage-gated sodium channels creates a domino effect that causes a wave of depolarization
Spreading can occur in two directions
-anterograde
-retrograde
What is orthodromic conduction?
Anterograde movement
-hillock to boutons
What is antidromic conduction?
Retrograde movement
-boutons to hillock
What is the velocity of axonal conduction of APs?
Large diameter = faster because there is less friction
Myelinated = faster
Large are myelinated = really fast
What is saltatory conduction?
AP jumps from node to node
Requires less energy
Sodium channels are only located on nodes
What is conduction in myelinated axons like?
Faster action potentials
Passive (rapid and decremental) from node to node
-diminished before the next node but is enough to open the next sodium channel
Are all axons myelinated?
Long, peripheral nerves
Cells spanning locations within the CNS
What does myelin degeneration do?
Impairs AP conduction
What does MS attack?
Immune system attacks myelin
- schwann cells
- oligodendrocytes
What are the steps of chemical transmission?
- AP reaches the end of the axon terminal
- Calcium enters the cell
- Presynaptic density release neurotransmitter into the cleft via exocytosis
- Neurotransmitter binds receptors on post synaptic density
- Receptors influence post synaptic neurons
- Neurotransmitter degradation/recycling
What are the 5 types of synaptic contacts?
Axodendritic Axosomatic Dendrodendrtic Dendroaxonic Axoaxonic
What is an axodendritic synaptic contact?
Axon to dendrite
Most common
What is an axosomatic synaptic contact?
Axon to soma
What is a dendrodendritic synaptic contact?
Dendrite to dendrite
Reciprocal
-can be transmitted in either direction
What is a dendroaxonic synaptic contact?
Dendrite to axon
What is an axoaxonic synaptic contact?
Axon to axon
Mediate presynaptic inhibition or facilitation
-selectively influence synapse rather than entire neuron
What is a directed synapse?
Site of neurotransmitter release close to postsynaptic contact
Most common
What is an undirected synapse?
The site of release and contact is far
Has varicosities
Common for monoamines
The neuroendocrine system releases neurohormones into the bloodstream
