Membrane Potential Flashcards
What is membrane potential?
an electrical potential (voltage) difference across their plasma membrane
What is the purpose of a membrane potential?
Membrane Potential provides the basis of signalling in the nervous system as well as in many other types of cells
What is the membrane potential of cardiac and skeletal cells?
-80mv to -95 mV
What is the membrane potential of nerve cells?
– 50 to – 75 mV
How is resting potential set up?
For most cells, open K+ channels dominate the membrane ionic permeability at rest. There is chemical diffusion out of the cell by K+ ions, until electrical gradient becomes large enough for K+ to move back in down electrical gradient. Equilibrium reached
Why does membrane potential differ in cardiac muscle and nerve cells compared to values worked out from the Nerst Equation?
Resting potential close to E(k) but not exactly due to membrane not being perfectly selective for K+
Why does membrane potential differ in smooth muscle and skeletal muscle cells compared to values worked out from the Nerst Equation?
Smooth muscle cells: Lower selectivity for K+ and increased contribution from other channels
Skeletal muscle: Many Cl- and K+ channels open in resting membrane so resting potential close to both E(Cl) and E(k)
How do changes in membrane potential underlie many forms of signalling between and within cells?
- Action potentials in nerve and muscle cells
- Triggering and control of muscle contraction
- Control of secretion of hormones and neurotransmitters
- Transduction of sensory information into electrical activity by receptors
- Postsynaptic actions of fast synaptic transmitters
What is depolarisation?
cell interior becomes less negative e.g. a change from – 70 mV to – 50 mV
What is hyperpolarisation?
Cell interior becomes more negative e.g. a change from – 70 mV to – 90 mV
What are the equilibrium potentials for the common physiological ions?
K+ = - 95 mV
Cl- = - 96 mV
Na+= + 70 mV
Ca2+= + 122 mV
How do nicotinic acetylcholine receptors work?
- Have an intrinsic ion channel
- Opened by binding of acetylcholine
- Channel lets Na+ and K+ through, but not anions
- Moves the membrane potential towards 0 mV - intermediate between the equilibrium potential of sodium and potassium
What is ligand gating?
-The channel opens or closes in response to binding of a chemical ligand e.g. channels at synapses that respond to extracellular transmitters or channels that respond to intracellular messengers
What is voltage gating?
Channel opens or closes in response to changes in membrane potential e.g. channels involved in action potentials
What is mechanical gating?
Channel opens or closes in response to membrane deformation e.g. Channels in mechanoreceptors: carotid sinus stretch receptors, hair cells
Describe how the ears hair cell stretch receptors work
1) Potassium channel closes in cuticular gate
2) Membrane depolarises
3) Calcium channel opens
4) Release of neurotransmitter which goes on to fuse to basement membrane to afferent nerve
5) Neurotransmitter binds to receptor at post synaptic plate and generates an action potential that goes to the CNS for interpretation
What is fast synaptic transmission?
In fast synaptic transmission, the receptor protein is also an ion channel and transmitter binding causes the channel to open
Explain how excitatory synapses work?
Excitatory transmitters open ligand-gated channels that cause membrane depolarization.
They can be permeable to Na+, Ca2+, sometimes cations in general (nAChR).
The resulting change in membrane potential is called an Excitatory post-synaptic potential (EPSP)
How are excitatory post-synaptic potentials different to action potentials?
- Longer time course than AP (around 20ms)
- Graded with amount of transmitter
- Transmitters include Acetylcholine and Glutamate
What are inhibitory synapses?
- Inhibitory transmitters open ligand-gated channels that cause hyperpolarization
- Permeable to K+ or Cl-
- Transmitters include Glycine, GABA
What is direct G-Protein gating?
- localised and quite rapid
1) ion binds to receptor
2) lateral diffusion of G-protein to channel
3) binding of G-protein to channel
4) channel opens
What is gating via an intracellular messenger?
1) ion binds to receptor
2) G-protein binds to enzyme in membrane
3) Signalling cascade activated
4) Intracellular messenger or protein kinase activated
5) Causes channel to open -effects throughout cell -amplification by cascade
How does extracellular K+ concentration influence membrane potential?
extracellular K+ concentration is ~4.5 mM normally. Sometimes altered in clinical situations. Can alter membrane excitability, e.g. in heart
How do electrogenic pumps influence membrane potential?
Na/K- ATPase is indirectly is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients
