Action potentials Flashcards
What input do skeletal muscles require to produce muscle contractions?
Neurogenic - require synaptic input from neurones to produce muscle contractions
What are neurones?
Designed to respond to stimuli and transmit information over long distances
- Receive information
- Process information
- Propagate signal
- Transmit signal
What are neurones made up of?
Dendrites: points of contact with with other neurones
Axon: permits long distance communication
How does the transmission of information occur in neurones?
Electric and chemical signals
What are excitable cells?
Neurones, muscle cells and endocrine cells
- excitability is a property of cell membrane
What do excitable cells do?
Generate changes in their membrane potential, producing an action potential
Electric current in the neurone is used to rapidly transmit signals through the animal (initiation of contraction in muscles)
- implies rapid and reversible reversal of electrical potential difference across the plasma membrane
What is the significance of the cell membrane in the action potential?
Lipid bilayer: resistant to movement of electrically charged ions
- channels through which ions can pass through the membrane
What is the transmembrane or resting potential?
- Living cells have an electric potential across their membranes
- Inside of the cell more negatively charged, difference in charges = membrane potential (usually between -50 _-100mV) - basis for the resting membrane potential
Differences in intra and extracellular ionic composition
Selective permeability of the plasmic membrane - Na/K pump and potassium channels are largely responsible for the transmembrane potential
Describe the ionic distribution of intra and extracellular fluid?
Maintenance of electro-neutrality and osmotic pressure Extracellular medium - Na+: 140mM - K+: 4mM - Ca2+: 2mM - Cl-: 147mM
How is current flow controlled by ionic channels in membrane? (Resting channels)
- normally open
- not influenced by potential across the membrane
- maintain the resting potential in the absence of signalling
How is current flow controlled by ionic channels in membrane? (Gated channels)
- closed when membrane is at rest
- recognise and select specific ions
- open in response to specific signals
What are the types of gated channels?
Voltage gated Ligand gated (ligand fits to receptor site) Mechanically gated (cytoskeleton stretches channel)
How are ion channels activated?
Changes the resting transmembrane potential results in either:
- local (graded) potentials: changes in transmembrane potential that do not spread far along the membrane from site of initiation
- action potentials: rapid changes in transmembrane potential; means by which electric signals are propagated along axons
Where are the gated channels located along an axon?
Dendrite = ligand-gated channels
Axon/myelin sheath/node of ranvier = voltage-gated Na and K channels
Axon terminal = Voltage gated Ca2+. Na+, K+ channels - mechanically activated
How does depolarisation of a membrane occur?
Increased concentration of Na+ on one side of the membrane
- depolarisation occurs as Na+ moves through Na+ channels
What is hyperpolarisation?
Potassium channels
- if a ligand causes an opening of potassium channels: cell becomes hyperpolarised
- can also be chloride channels
Describe the movement of an action potential
- Momentarily depolarisation of the cell membrane
- Membrane potential reverses from -70mV to +40mV and then repolarises, travels along the axon without a drop in amplitude
- Produced when the membrane potential is depolarised above a critical value (threshold, around 55mV)
- Membrane potential returns to normal rapidly
- Movement of very few sodium ions, these ions moving produce infinitiesimal changes in intracellular ion concentrations
Describe the positive feedback loop of the depolarisation of the axon?
Local depolarisation → increase in Na conductance
↑ ↓
↑ influx of +ve charges
more depolarisation ↓
↑ depolarisation
more influx of +ve charges ↓
↑ more increase in Na conductance
What is initiation?
- Stimulation of sensory receptors, initial depolarisation occurs at nerve endings due to changes in the environment
- synaptic activity, depolarisation to the threshold could be the result of synaptic activity and effect neurotransmitters on the membrane, usually a series of potentials occur in response to a stimulation
- spontaneous activity, same nerve and muscle cells show spontaneous changes in their membrane potential
How does excitablilty vary between regions of axons and among neurons?
Due to set of ion channels that are expressed. Voltage sensitive Na+ channels are concentrated at the initial segment of axon (axon hillock)
- sensory neurons: myelinated axon’s first node of ranvier
- interneurons and motor neurons: axon hillock
What events occur in the depolarising/repolarising phases?
Depolarisation
1. Electric event triggers voltage gated channels to open
2. Na+ v-gated channels open immediately
3. Na+ ions flow into cells
Re-polarisation
4. K+ V-gated channels open more slowly
5. When K+ channels do open, Most Na+ channels have already closed
6. K+ outflow returns membrane potential to -70mV
What is the refractory period?
Period of time in which neuron cannot produce another action potential
- change in state of Na+ channels that is responsible for the refractory period
- limits the rate of the AP generation
What is the absolute refractory period?
For 1-2ms after the spike, neuron unable to produce an AP
What is the relative refractory period?
During the next several ms, threshold falls back to its normal resting value. Becomes progressively easier to produce an AP
Why is an action potential all or none?
If graded potential changes the resting potential to threshold potential, an action potential is always produced
- either a given stimulus produces an action potential, or it does not
- strong stimuli produce no stronger AP than weak ones
- strength of the stimulus is encoded in the frequency of the Ap it generates
What is electrophysiology?
Study of action potentials
- isolating signals from single axons
- recording voltage from a single cell
What is action potential propagation?
Action potential propagates along membrane of axon
- AP conduction: unmyelinated axons
- saltatory conduction: myelinated axons
What is the speed of propagation in an unmyelinated neurone?
1m/sec
What is the speed of propagation in a myelinated neurone?
Action potential regenerated at nodes of ranvier
- propagation = 25-100m/sec
What is acute myelination?
Damages the myelin insulation covering Na+ channel
- poor parts of the axon membrane and produces axonal conduction blockage
What factors determine conduction velocity?
Diameter and myelination
What is the direction of propagation?
Direction of information is always from the dendrites (synaptic knobs)
- due to membrane becoming refractory
- synapses permit conduction in one direction
How is an action potential terminated?
Chemical synapses
Electrical synapses
Neuromuscular junctions
