Neuronal Excitability & Action potentials Flashcards
What are electric signals?
signals produced as a result of transient, rapid fluctuations in their membrane potentials
What is the resting potential of a neurone at rest?
-70mV
what are the ions that affect membrane potential?
K+, Na+ and A- (larger anions)
What are the events that occur in the axon membrane that allow it to reach resting potential?
. Sodium potassium exchange pump requires ATP as it pumps the ions against their concentration gradient
2. 3 sodium ions actively transported out and 2 potassium ions in.
3. There are more sodium ions in the tissue fluid outside the axon than in the cytoplasm so am electrochemical gradient is formed. K+ can then diffuse in via leak channels and Na+ diffuses out however the membrane is more permeable to k+ hence the potential difference is established
4. The membrane is polarised as resting potential of -70mV is reached (-ve on the inside)
5. Membrane is impermeable to sodium ions as voltage activated sodium ion channels are closed due to a change in the tertiary structure of the protein
what two factors is the membrane potential derived from?
-electrical force = mutual attraction and repulsion between particles and the ions
-diffusion
Give three ways the movement of ions is controlled across the axon membrane
- phospholipid bilayer prevents sodium and potassium ions simply diffusing across it
- channel proteins allow the sodium and potassium ions to pass through the phospholipid bilayer by facilitated diffusion
- some carrier proteins can actively transport potassium ions in and sodium ions out of the axon as it functions as a sodium potassium pump
what is the goldman equation?
models the interactions that generate the resting potential based on ionic charge and concentrations and considers the relative membrane permeability to each ion
what to factors determine the magnitude of influence of an ion over the membrane potential of a cell?
-the ion’s driving force: Em-Eion
-ions permeability so how easily it can cross the membrane
what is the membrane potential as calculated by the goldmann’s equation and why is this different to the membrane potential found by electrodes?
-69mV compared to -70mV and this is due to Na+/K+ ATPase ion pump which generates -1mV
How does the Na+/K+ ATPase ion pump work?
- ATP binds to sodium potassium pump
- this attracts 3 na+ ions to phosphrylate ADP
- triggers conformation change
- transporter protein closes on the inside and opens on the outside as reduced affinity for Na+ hence ADP is released
- dephosphrylation and 2 K+ ions bind
- change to the tertiary structure of the transporter protein
- 2K+ move inside
when is a membrane considered polarised?
the Em is < or > 0mV
when is a membrane depolarized?
when the Em becomes more postive than at resting potential
when is a membrane repolarised
when the membrane returns to resting potential after depolarisation
when is a membrane hyperpolarised?
when the membrane becomes more polarised so the inside becomes more negative than at resting potential
Give the events of action potential
- Membrane depolarises when sodium ions diffuse in rapidly through open channels as the channels are highly permeable to sodium ions
- Membrane potential changes to +40mV
- local circuit created with the neighbouring resting potential further along the axon
- Na+ diffuse through the axoplasm along the circuit into the region ahead
- The membrane potential in the region ahead is reduced and made less negative
- At threshold of -55mV, voltage gated sodium ion channels open causing depolarisation of the next region
What are the phases involved in an action potential occuring?
1.Resting potential
2. Threshold
3. rising phase: depolarisation
4. Falling phase: repolarisation
5. Recovery phase: hyperpolarisation
Give the steps of repolarisation
- At +40mV, voltage activated potassium ion channels open
- The potassium ions diffuse out rapidly
- The membrane becomes hyper polarised at -80mV as its now lower than the resting potential
- The refractory period takes place which is the time taken to reset polarisation
What happens during the refractory period?
Once an action potential has been created in any region of an axon, the inward movement of sodium ions is prevented as the sodium voltage-gated channels are closed so it’s impossible for a further action potential to be generated as the sodium potassium exchange pump resets the resting potential
What happens after the action potential?
-resting potential is restored
-Na+ and K+ distribution is altered
-action potentials can still occur: concentration gradients of ions exist
How is the action potential an all or nothing response?
- low intensity of mV below threshold causes the signal to die off
- if the intensity is reached, mP reaches threshold so signal is amplified and reaches a peak value
How do we detect differences in stimuli strength?
a larger stimulus activates more nerve cells and so a higher threshold of neurones so generates an increased frequency of impulses
what are the different ion channels and what do they transport?
-leak channels do K+
-ion pumps / transporters do Na+/K+
-voltage gated channels do na+ and K+ in action potential propagation and ca2+ to release NT in preSN knob
-ligand gated channels in post synaptic neuronal potential EPSP Na+ and K+, IPSP in Cl-/K+ channels
what is a soma?
the cell body of a neurone
what are the differences between axons and dendrites
-axons may be myelinated while dendrites are not
axons do not contain ribosomes while dendrites contain many ribosomes
-usually one axon per cell but many dendrites branch from one cell
-axons carry info out of the cell, dendrites carry info into the cell
what are the similarities between axons and dendrites
-both extensions from the soma of neurones
-both carry electrical signals
How does an action potential pass along a myelinated axon?
Through saltotory conduction
- sensory and motor neurones are myelinated by being tightly surrounded by Schwann cells
- The myelin insulates sections of the axon against ion movement
- This greatly extends local circuits so depolarisation only occurs at the nodes of ranvier so propagation of nerve impulses through action potentials is much faster than in unmyelinated neurones
What are the factors that affect the speed of conductance of an action potential?
myelinated neurones the action potential travels at 100ms-1 compared to unmyelinated at 0.5ms-1 as a result of saltatory conduction
- thickness of of the unmyelinated neurone as a thicker neurone reduces diffusion resistance for sodium ions along local circuits hence speeds up the action potential
What are the two types of summation?
- Spatial summation = multiple Presynaptic neurones release enough neurotransmitter to exceed threshold of post synaptic neurone so a new action potential is triggered
- temporal summation = where a single presynaptic neurone releases neurotransmitter at a higher frequency so the concentration of neurotransmitter exceeds the threshold value of the postsynaptic membrane triggering a new action potential
