10) Action Potentail Flashcards
Excitability and excitable tissue
The property of cells to respond rapidly to the action of stimulants
Nerve
Muscle
Glands
Action potential definition and function
The rapid change in electrical potential that occurs between the inside and outside of then cell
Is called nerve impulse
The function is to carry information along nerve fibres
Cause contraction of muscle fibres
Stimulate secretion by glands
Characteristics of action potential
Ap is generated when the membrane potentials rises above the threshold
The sign of the membrane changes during ap
All or nothing
Experiment with Cole and Curtis
When the membrane is excited it drops 40 fold
The resistance of the cytoplasm and extracellular fluid doesn’t change
Permeability of membrane increases
Hodgkin and Huxley
Investigation of the dependence of then membrane permeability to sodium and potassium ion on voltage and time
Development of mathematical model of the Ap
Changes of conductivity during excitation
At the start the conductivity for sodium ions rises rapidly
A skirt growth for the conductivity or potassium ions takes place
Changes of ion flux
The action potential is generated by sodium and potassium in opposite directions
Ap is generated due to the different development of these currents over time
Initiation of action potential
The sodium ions enter the cell driven by the electrochemical gradient
The value of the membrane potential decreases becomes less negative
Threshold potential
If the membrane potential reaches a specific critical value then voltage gated sodium channels open
The threshold is the membrane potential which opens the voltage gated channels
Depolarisation phase
This is a process of positive feedback
This causes the voltage gated sodium ions channel to open. This causes more sodium ions to enter the cell and the inside of the cell becomes more positive and increases the membrane potential
Operation of the sodium channels
M gate is the activation gate and it opens when the potential exceeds the threshold
h gate closes 1-3 seconds after the m is opened
Peak of action potential
Is close to but lower than the equilibrium potential for the sodium ions
Repolarisation
The sodium channel closes and then potassium channel opens. The membrane potential reverts back to the resting potential
Hyper polarisation
The membrane potential reaches the rating potential by the potassium channel is slow to close
This causes more potassium to enter the cell and cause undershoot
Refractory period
Absolute
This is when a new action potential can’t be gee rated whatever the strength of the stimulus
Relative
This is when it can be generated but the threshold is higher than usual
Phases of action potential
Initiation of the action potential
Depolarisation
Peak
Repolarisation
Hyperpolarisation
All or nothing
When the membrane depolarisation by the stimulus and it exceeds the threshold
Function of the sodium potassium ion pump
The sodium potassium ion pump has no effect on the depolarisation and repolarisation phases
Cells can generate tens of thousands of action potentials with the pump
Shape and amplitude of the action potential
The action potential generated is almost identical
The impulses propagating down the nerve fibre have identical shape and peak values
Coding of stimulus strength and duration
A strong stimulus imitates the next impulse earlier during the refractory period than a weak stimulus
The burst lengths carries information about the duration of the stimulus
