Lesson 3: Propagation of Action Potentials Flashcards
Membrane Potential
Membrane potential is the result of an unequal ion distribution between the interior and the exterior of a cell.
The solutions of the human body are electrically neutral. When separated by a membrane, minor differences in relative abundance of ions may occur as a result of the
selective movement of ions across the membrane.
When these minor charge differences interact across a thin cell membrane they create a source of electrical potential energy.
This voltage is called the membrane potential.
Resting Potential
Membrane potential is expressed relative to the outside of the cell.
Any membrane with a potential other than zero is considered polarized.
A neuron that is not transmitting a signal has a membrane potential of about
-70mV. This is known as resting potential.
The resting potential is a result of the gradient of ions across the membrane
and the permeability of the membrane to ions.
Establishing Resting
Potential
The resting potential of a neuron is determined by:
- The cytoplasm contains many organic molecules that have net negative charge and
cannot cross the membrane. - This attracts cations that can move across the
membrane. K+ is the most permeable of the cations and will accumulate in cells. This leads the Na+ concentration gradient to be steeper than that of K+.
3.The sodium – potassium pumps in the membrane move ions unequally; for every three Na+ pumped out, two K+ are pumped in.
Changes in Membrane Potential
Signal transmission occurs through changes to the membrane potential.
Since membrane potential is the result of ion concentrations, changes in the membrane potential occur as a result of the movement of ions across the
membrane.
This is controlled by gated ion channels in the membrane which can affect the permeability of the membrane to certain ions.
Gated ion channels open or close in response to stimuli.
An increase in the magnitude of the membrane potential is called a hyperpolarization.
Hyperpolarization is the result of the opening of potassium channels which make the membrane even more permeable to K+ ions, more K+ ions leave the cell, lowering the membrane potential to -90mV.
A reduction in the magnitude of membrane potential is called a depolarization.
Depolarization is the result of the opening of sodium channels which makes the
membrane permeable to Na+, more Na+ ions enter the cell, raising the membrane
potential to +30mV.
If the depolarization shifts the membrane potential enough, this massive change in membrane potential is known as an action potential.
Generating Action Potentials
Generating an action potential occurs in a series of stages.
1.Axon membrane begins at a resting potential. This means that most sodium
and potassium channels are closed.
2.A stimulus depolarizes the membrane, sodium channels open allowing Na+
into the cell.
3.The threshold is crossed, and the membrane potential rapidly increases.
- Sodium channels rapidly close, stopping Na+ inflow and potassium channels
open, causing outflow of K+. - As K+ ions leave the cell, there will be a moment when the membrane potential is more negative than resting potential. This is called the undershoot.
- The potassium channels close and the membrane returns to resting potential.
This is called repolarization. - There is a moment after the action potential passes that the cell cannot initiate another action potential. This is known as the refractory period.
Propagation of Action
Potentials
Neurons send messages as a wave of depolarization down the length of the axon.
The Na+ inflow during the action potential depolarizes the neighbouring region of the axon membrane. The depolarization is large enough to initiate an action potential in this region.
This process is repeated down the length of the axon.
An action potential is an all-or-none event. The magnitude and duration of the action potential remains the constant along the length of the axon.
The Synapse
Neurons do not touch on
another; there are tiny gaps
between them called synapses.
The neuron that carries the
depolarization towards the
synapse is called the presynaptic neuron.
The neuron that receives the
stimulus is called the post
synaptic neuron.When a wave of depolarization reaches the end of a presynaptic axon it triggers the release of
neurotransmitter molecules
The neurotransmitter is released from special vacuoles called synaptic vesicles.
The neurotransmitter diffuses into the gap between the axon and dendrites of the
neighbouring postsynaptic neurons.
The neurotransmitter attaches to special receptor sites and either excites or inhibits the neuron.
Neurotransmitters : Acetylcholine
Responsible for the stimulation of muscles, memory formation and
learning.
Found in the sensory neurons and autonomic nervous system
Botulism and Tetanus are conditions in which bacterial agents block the receptors for acetylcholine. This causes paralysis and sometimes worse.
Neurotransmitters: Norepinephrine
Excitatory neurotransmitter
Responsible for the fight or flight response
Increased by the use of amphetamines
Neurotransmitters: Dopamine
Reduces a neurons ability to fire
Associated with the reward/pleasure center of the brain
Neurotransmitters: GABA
Gamma-aminobutyric acid
Inhibitor of norepinephrine and acetylcholine
Low GABA leads to anxiety, jitters and epilepsy
Diazapam reduces anxiety by binding to GABA receptors
Neurotransmitters: Glutamate
CNS neurotransmitter related to long term memory formation.
Glutamate imbalance can lead to ALS, Lou Gherig’s disease and a large
number of nervous system disorders
Neurotransmitters: Endorphin
Endogenous morphine – structurally similar to opiates and opioids
Involved in pain reduction and pleasure
Neurotransmitters:
Serotonin
Inhibitory neurotransmitter involved in emotion and mood.
Low serotonin can lead to depression, OCD, sleep disorders, carbohydrate binging, migraines, IBS and fibromyalgia
Hallucinogens block the release of serotonin which changes perceptual pathways.
