Lecture 5.2: Action Potentials Flashcards
Resting Membrane Potential
All cells have an electrical potential (voltage) difference across their plasma membrane (membrane potential)
Membrane potential of cells under resting condition is defined as the resting membrane potential
Changes in the membrane potential provide the basis of signalling in the nervous system and in many other types of cells
Factors Influencing Resting Potential
The ionic permeability of the cell membrane
The ionic concentrations on either side of the cell membrane
K+ is the main ion affecting the resting membrane potential
• K+concentration gradient
• Selective permeability of membrane to K+
Depolarisation
A decrease in the absolute value of the membrane potential
Cell interior becomes less negative
E.g. -70mv to +40mv
Opening Na+ or Ca2+ channels will cause depolarisation
Hyperpolarisation
An increase in the absolute value of the membrane potential.
Cell interior becomes more negative
E.g. -70mv to -90mv
Opening K+ or Cl- channels will cause hyperpolarisation
Equilibrium potentials for physiological ions
EK= - 95 mV
ENa = + 70 mV
ECl = - 96 mV
ECa = + 122 mV
How do neurones “communicate”?
Neurons (nerve cells) communicate through electrical signals
Incoming signals are received across intercellular gaps called synapses (via neurotransmitters) on extensions of the cell membrane called dendrites
Where do synaptic connections occur?
• nerve cell – nerve cell
• nerve cell – muscle cell
• nerve cell – gland cell
• sensory cell – nerve cell
Synaptic Transmission
At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane
Sympathetic Nervous System
Prepares the body for fight and flight response
Excitatory (this is a massive oversimplification)
Parasympathetic Nervous System
Restores the body to a calm and composed state and prevents it from overworking
Inhibitory (this is a massive oversimplification)
Excitatory Synapses
• Excitatory transmitters open ligand-gated channels permeable to Na+ ,Ca2+, sometimes cations in general (nAChR)
• Cause membrane depolarisation called an Excitatory postsynaptic potential (EPSP)
- Longer time course than action potential
- Graded with amount of transmitter
- Transmitters include: Acetylcholine, Glutamate
Inhibitory Synapses
• Inhibitory transmitters open ligand-gated channels permeable to K+ or Cl-
• Cause membrane hyperpolarization
• The resulting change in membrane potential is called an Inhibitory
postsynaptic potential (IPSP)
Transmitters include: Glycine, g-aminobutyric acid (GABA)
Summation of inputs: where are they received?
Excitatory and inhibitory inputs are received at dendrites
Summation of inputs: where are the summed?
Inputs are summed in the perikaryon at the axon hillock
Summation of inputs: when is an action potential generated?
If the summed input above a threshold of ≈ -55mV, an action potential is started in the axon
Initiation of an action potential
If the axon hillock membrane potential exceeds the threshold of -55mV
Voltage-gated sodium channels open as the cell depolarises
Positive feedback
“All or nothing”
Phases of an action potential 1: Depolarisation + AP peak
Sodium channels are fast to open
Rapid influx of Na+ ions cause the upstroke of the AP
Membrane potential goes towards ENa
Na channels also inactivate rapidly stopping Na+ influx
Phases of an action potential 2: Repolarisation
Na channels inactivate and close
Depolarisation causes K+ channels to open
Membrane potential goes to EK
Phases of an action potential 3: Hyperpolarisation + Threshold Reestablished
Efflux of K+ leads to an ‘undershoot’ and a short period of hyperpolarisation
At this time the membrane is refractory – unable to conduct another AP
Resting membrane potential is restored
Na+-K+-ATPase restores the resting ion balance
Absolute Refractory Period
Most VSGCs (voltage-gated sodium channels) are in the inactivated state
Relative Refractory Period
VGSCs are resetting to the closed conformation
Hyperpolarisation of the membrane makes membrane less excitable
At the synapse
As an AP reaches the synapse the change in membrane potential triggers Ca2+ entry
Increased Ca2+ causes release of neurotransmitters stored in vesicles into the
synapse (via exocytosis)
Transmits a chemical message to down-stream cells
Neurotransmitters are the broken down in the synapse (e.g. acetylcholine broken down by acetylcholinesterase, taken up by presynaptic neurone again via endocytosis)
