Nervous conduction and transmission Flashcards
What is the resting membrane potential?
A potential difference across the membrane of negative 20-90mV (approx -70mV). The membrane is ‘polarised’
At resting membrane potential, which side of the membrane is more negative?
Inside (ICF) is more negative than the outside (ECF)
How is the membrane polarised despite there being equal numbers of positive and negative charges on each side of the membrane?
Charges are unevenly distributed - all negative just inside the membrane, all positive just outside the membrane.
Significance of different concentrations of ions in ICF and ECF
e.g. much greater Na+ conc in ECF, greater K+ in ICF. Indicates presence of Na+/K+ pump.
During resting membrane potential, which channels are open/closed?
Some K+ channels are open. Na+ channels are closed.
How does the resting membrane potential arise?
The Na+/K+ pump moves 3 Na+ out and 2 K+ in. K+ is able to diffuse out of cell via open K+ channels down the concentration gradient. This creates a more positive cell exterior than interior.
How can the membrane potential be altered?
By applying an electrical current to the cell (a stimulus)
In which direction does a hyperpolarising current move the membrane potential?
Further away from 0 (more negative)
In which direction does a depolarising current move the membrane potential?
Closer to 0 (more positive)
Why are depolarisation and hyperpolarisation considered to be ‘graded’ responses?
The amplitude depends on the size of the stimulus (until threshold is reached)
When will an action potential occur?
When the depolarisation reaches the threshold (approx -55mV) this causes voltage-gated Na+ channels to open. Influx of Na+ causes the MP to reach about +35mV.
Why is an action potential considered to be an ‘all or nothing’ event?
If the threshold is reached, the amplitude of the AP is independent of the stimulus intensity.
What happens once the MP reaches +35mV in an action potential?
Voltage-gated Na+ channels shut, voltage-gated K+ channels open. K+ diffuse out of the cell which leads to repolarisation (MP becomes more negative).
Why does hyperpolarisation occur?
Due to a small K+ overshoot.
Stages of an action potential
A stimulus causes depolarisation that reaches the threshold (-55mV) resulting in Na+ channels opening and Na+ diffusing in creating further depolarisation. At +35mV, Na+ channels close and K+ channels open causing K+ diffuse out resulting in repolarisation. A K+ overshoot causes hyperpolarisation. Resting potential is then restored.
How do local anaesthetics work?
Stop nerve conduction by binding to an internal site on Na+ channels which blocks them.
What is the refractory period?
Period of in excitability after an AP has been initiated. Due to inactivation of voltage-gated Na+ channels
Advantage of having a refractory period
AP are unidirectional - previous Na+ channels are inactivated
How are APs propagated?
As waves of depolarisation. The current flows in ICF and ECF from positive to negative regions which alters the membrane potential in adjacent regions.
Effect of axon diameter on propagation speed
Speed of AP propagation increases with axon diameter
Why do squid giant axons relate to propagation in human axons?
Squids’ giant axons are up to 1mm in diameter with a conduction velocity of 35m/s. The axons are too large and conduction too slow for vertebrates so a way of increasing conduction speed while decreasing diameter evolved (myelination)
Maximum speed of conduction in a myelinated neuron
120 m/s
How are axons myelinated?
Axons are gradually wrapped in concentric layers of Schwann/glial plasma membrane (neurilemma).
Function of myelination
Insulates the axon and improves conduction speed.
