3. The Action Potential

Importance of graded potentials decay

- Depolarization is confined to a small region of membrane - Graded potentials only function as signals over short distances

Characteristics of a action potential SELF PROPOGATING? (6)

- Large fluctuations in membrane potential (from -70 to +30mV) - Very rapid (1-4ms) - Fequent (100s per second) - All cells produce graded potentials, but only excitable membranes conduct a actual action potential - Will only occur if depolarization reaches (-55mV) - Action potentials feature propogration, whereby it travels along the membrane of the axon for very long distances. In addition, it can be self-activated by the body without stimulis, which is the mechanism which the nervous system used to communicate from cell to cell.

What is the threshold potential?

-55mV. Membrane potential at which channels open to initiate an action potential.

Action Potential Mechanism

1. Depolarization(a): voltage gated sodium channels open first from a neurotransmitter binding to a ligand-gated ion channel, causing sodium to rush into the cell and reach it's critical threshold potential(-55mV) causing more sodium channels to open(positive feedback loop). This overshoots the equilibrium potential, making the inside of the cell more and more positive, until it reaches it's peak membrane potential value of +30mV. 2. Shortly after depolarization(b): VGsodium channels are quickly closed by an external factor: inactivation gate "ball and chain", for limiting sodium flux by blocking the channel after it opens, initiating repolarization, breaking the positive feedback loop. Meanwhile, the potassium channel is open, causing potassium ions to slowly exit the cell and repolarize. 3. Repolarization(a): Sodium channels eventually closes because of repolarization, causing the ball and chain out of the socket of the activation gate, returning the sodium channel back to it's closed state. 4. Hyperpolarization(b): While sodium channels are closed, potassium channel are still open but slowly closing because of being slower. This causes an overshoot influx of potassium ions to leave the cell and create a increasingly negative membrane potential, below the resting potential of -70, around -75mV. 5. Resting membrane potential restored: In a negative feedback loop, the potassium channels reduce permeability and induce their own closing, returning to the resting potential.

Steps of ions motion in a membrane to produce a graded potential

1. From the extraceullular fluid, positive sodium ions flow rapidly across membrane in open sodium channels 2. Local current loops: In the intracellular fluid, positive charges(mainly potassium ions) flow away from the source of depolarization to more negative regions because of the postivie and negative attraction. 3. Adjcacent depolarization: Simultaneously, outside the cell, positive charge will move from positive region of the resting membrane toward less positive regions created by depolarization. 3. Depolarization propogation: Decreased difference in charge nearby leads to depolarization and signal moving along the membrane.

What does the speed of AP depend on?

1. Myelination 2. Fibre Diameter

Importance of the Absolute Refractory period

1. Prevents another stimulis to produce an action potential, limitng the number of action ptoentials produced by membrane in a period of time 2. Seperates action potentials 3. Determines the direction of action potential propogation

Describe action potential generation in a neuronal pathway

1. Receptor potential: Initial depolarization by a graded potential that contacts the Afferent Neurons generated by sensory receptors at peripheral nerve ends. 2. Synaptic potential: Graded potential that has reached the interneurons and efferent neurons, which is generated by the synaptic input to the neuron that may be depolarizing or hyperpolarizing

Action potential curve description

1. Resting Potential: -70mV 2. Depolarizing reach to threshold-50mV threshold 3. Depolarization to +30/40mV 4. Repolarization to -70mV 5. HYperpolarization to below -70mV/-75mV. 6. Returning to resting potential at -70mV.

Names of membrane potentials

1. Resting Potential: -70mV 2. Threshold potential: -55mV 3. Overshoot peak: +40 4. Reploarizing: -70mV 5. Hyperpolarizing: -90mV

Benefits of saltatory condction

1. Speed: Propogation via saltatory conduction is faster than propagation in non-myelinated fibres of the same axon diameter. 2. Metabolically more efficient: ions cross the membrane at the nodes of ranvier, so membrane pumps need to restore fewer ions. 3. Saves room: Because axons can be thinner, there can be more sapce in the nervous system.

What are the two types of membrane potentials with major functional significance

1. graded potentials: short distance signals 2. action potentials: long distance signals

What is depolarizing

A membrane's electrical potential becoming more positive than -70.

What is the Absolute refractory period?

After the first action potential, a second stimulis will not cause result in a action potential, no matter how strong the stimulis is.

Reason for the Relative Refractory period to occur?

Becase the sodium channel has returned to its closed state of not being plugged by the ball and chain/inactivation gate mechanism, it can initiate a action potential if a strong enough stimulis causes hyperpolarization to the extent it reaches the critical threshold value.

How does a neuron transmit information

Changing it's membrane's permeability to ions

What happens to graded potentials as they move over distance

DECREMENTAL: The decay of graded potentials occurs, as there is a leakage of charge across plasma membrance reduces current at sites further laong the membrane.

Types of graded potential page 150 figure 6.16

Excitatory synapse and inhibitory.

In what form is a initial depolarization started as?

Graded potential

Why do graded potentials lose their charge in short distances?

Local current is decremental, because the membrane is permeable to ions through open leak channel, so a change in membrane potential decreases as the distance increases.

What is name of a action potential travelling across a myelnated fibre?

Saltaotry conduction

Difference in conduction velocities from fibre diameter with real life example

Small diamter unmyelinated fibres = 0.5m/s - onset of a dull, throbbing ache Large dimater myelinated fibres = 100m/s - dropping a heavy object on toe and geting a immediate,sharp pain

What are the feedback loops of sodium and potassium channels?

Sodium channels: Positive feedback loop in the beginning of action potential mech. As more and more sodium channels open from depolarization, causing more sodium to flow into the cell. Potassium channels: Negative feedback loop at the end of action potential mech. as the potassium channel starts to close, more and more close together because the membrane is repolarizing and becoming more negative/returning to the resting potential.

What is the Relative Refractory Period?

The period of stages from initially reaching the resting potential from repolarization, hyperpolarization, then returning to the resting potential, where a action potential can occur.

What is the mangitude of a graded potential dependent on?

The proportionate size of a stimulis.

Reason for the absolute refractory yperiod

The stages of action potential generation from reaching the threshold potential, to repolarizing and reaching the resting potential (not including hyperpolarization), because the Inactivation gate that has closed the channel with the ball and chain must be removed by repolarizing to occur first and for the sodium channels to all close before the channels can open again to a second stimulis.

What is a graded potential

Varying changes(graded) in membrane potential that occur in a relatively small region of the plasma membrane.

What property of excitable membranes allow the generation of action potentials?

Voltage-gated ion channels

What is a excitatory graded stimulis

When a graded potential is depolarizing, therefore more likely to cause a action potential.

What is a inhibitory graded potential

When a graded potential is hyperpoarizing, therefore less likely to cause a action potential.

What is the process of summation?

When aditional stimuli occurs before a graded potential has died away, adding to the depolarization and increasing it's magintude from the initial first stimulis