Action potential

Unmyelinated neurons use continuous conduction - the entire axon must depolarize sequentially. Myelinated neurons use saltatory conduction - the myelin allows the depolarization to occur only at the nodes of Ranvier so conduction hops along much quicker.

Describe impulse propagation in un-myelinated and myelinated nerve fibres

The activation of Na+ channels is self-reinforcing once threshold is reached - the opening of a few channels causes further channels to open, causes further depolarization

Describe the positive feedback of sodium ions.

A resting neuron is more negative on the inside of the cell relative to the outside of the cell which is around -70 millivolts. This is because there are more positive sodium ions outside a cell than positive potassium ions inside a cell and the interior of the cell has large proteins that carry a slight negative charge. The neuron is therefore said to be polarised.

Describe the resting neurone membrane potential

This is a temporary reversal of a membrane potential, a brief depolarisation caused by changes in currents. It kicks off a biological chain reaction which sends the electrical signal down the axon

Describe the term "action potential"

These are brief electrical signals in which the polarity of the nerve cell membrane is momentarily reversed .

What are action potentials ?

These are protein complexes that span the lipid bilayer to form a central pathway that allows rapid flow of selected ions across the membrane.

What are ion channels ?

Resting membrane potential (-70mV); stimulus; depolarization (influx of Na+); repolarization (outflow of K+); hyperpolarization (extra K+ out); Resting membrane potential and proper ion distribution restored by Na-K pump

What are the phases of the action potential in order?

The nerve action potential has a short duration (about 2 msec). Nerve action potentials are all-or-nothing. Nerve cells code the intensity of information by the frequency of action potentials. When the intensity of the stimulus is increased, the size of the action potential does not become larger; the frequency or the number of action potentials increases. The inside of a cell at resting membrane potential is always more negatively charged than the outside.

What are the properties of action potentials in neurones?

The delayed closure of the voltage activated potassium ion channels after the membrane repolarizes.

What causes the undershoot (hyperpolarised) phase?

This indicates that the inside of the cell is more negative with respect to the extracellular fluid.

What does a negative membrane potential indicate?

This is when the membrane potential becomes less negative because Na+ ions flow inward through voltage-gated channels

What does depolarisation mean?

This is when the membrane potential becomes more negative, due to the Na+/K+ pump

What does hyperpolarisation mean?

The sodium-potassium pump ensures that the relative concentration of sodium remains high outside of the neuron while potassium remains high inside. It actively pumps 3 Na+ from inside the neuron to the outside and 2 K+ into it. It must use energy (ATP) to do this because they are moving against the concentration gradient (moving from low to high concentration)

What does the Na+/K+ pump do?

Action potentials are only generated when they reach a threshold. If this threshold is not reached, the stimulus is ignored. If threshold is reached, the neuron fires along its entire length without stopping or reversing.

What does the term 'all or none' mean when referring to action potentials?

This is the difference in charge across a membrane. The bigger the difference between the positive and negative areas, the larger the potential.

What is meant by the term membrane potential

Strong and weak stimuli both send the same strength action potentials, however strong stimuli send more frequent action potentials whereas weak stimuli send less frequent stimuli.

What is the difference in electrical signals of a weak and strong stimulus?

This is the period when no stimulus, however strong, can elicit a second action potential.

What is the refractory period?

The direction of the movement of the ion The charge carried by the ion.

Which 2 factors determine the direction of the change in potential.