Neurones & the Action Potential Neurones conduct impulses from one part of the body to another.

an action potential is a short-lasting event in which the electricalmembrane potential of a cell rapidly rises and falls, following a consistent trajectory.

STRUCTURE They have three distinct parts: (1) Cell body, (2) Dendrites, and (3) the Axon The particular type of neuron that stimulates muscle tissue is called a motor neuron. Dendrites receive impulses and conduct them toward the cell body.

Myelinated Axons The axon is a single long, thin extension that sends impulses to another neuron. They vary in length and are surrounded by a many- layered lipid and protein covering called the myelin sheath, produced by the schwann cells.

Resting Potential In a resting neuron (one that is not conducting an impulse), there is a difference in electrical charges on the outside and inside of the plasma membrane. The outside has a positive charge and the inside has a negative charge.

Contribution of Active Transport – Factor 1 There are different numbers of potassium ions (K + ) and sodium ions (Na + ) on either side of the membrane. Even when a nerve cell is not conducting an impulse, for each ATP molecule that’s hydrolysed, it is actively transporting 3 molecules Na + out of the cell and 2 molecules of K + into the cell, at the same time by means of the sodium-potassium pump.

Contribution of facilitated diffusion The sodium-potassium pump creates a concentration and electrical gradient for Na + and K +, which means that K + tends to diffuse (‘leak’) out of the cell and Na + tends to diffuse in. BUT, the membrane is much more permeable to K +, so K + diffuses out along its concentration gradient much more slowly.

RESULTS IN: a net positive charge outside & a net negative charge inside. Such a membrane is POLARISED

Action Potential When the cell membranes are stimulated, there is a change in the permeability of the membrane to sodium ions (Na + ). The membrane becomes more permeable to Na + and K +, therefore sodium ions diffuse into the cell down a concentration gradient. The entry of Na + disturbs the resting potential and causes the inside of the cell to become more positive relative to the outside.

The Action Potential Is a Rapid Change in Membrane Potential 1. Depolarization phase 2. Repolarization phase 3. Hyperpolarization phase Resting potential Threshold potential

DEPOLARISATION As the outside of the cell has become more positive than the inside of the cell, the membrane is now DEPOLARISED. When enough sodium ions enter the cell to depolarise the membrane to a critical level (threshold level) an action potential arises which generates an impulse. In order for the neuron to generate an action potential the membrane potential must reach the threshold of excitation.

12 OUTSIDE INSIDE K + = Potassium; Na + = Sodium; Cl - = Chloride; Pr - = proteins Na + K+K+ K+K+ Force of Diffusion Electrostatic Force Cl - Force of Diffusion Cl - Electrostatic Force Pr - Closed channel open channel open channel no channel 3Na/2K pump Resting Membrane Potential - 65 mV

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Membrane Potentials 1. Resting Potential (just described) 2. Excitatory Post- synaptic potential threshold 4. Inhibitory Post-synaptic potential 3. Action Potential

The Action Potential Is a Rapid Change in Membrane Potential 1. Depolarization phase 2. Repolarization phase 3. Hyperpolarization phase Resting potential Threshold potential

Voltage-gated sodium channels allow the action potential to occur fB8

Voltage-gated channels How voltage-gated channels work At the resting potential, voltage- gated Na + channels are closed. Conformational changes open voltage-gated channels when the membrane is depolarized. Two important types: 1.) Na+ voltage gated channels 2.) K+ voltage gated channels

Resting Potential - Both voltage gated Na+ and K+ channels are closed.

Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.

Na + channels open quickly. K + channels are still closed. P Na+ > P K+

Na + channels self-inactivate, K + channels are open. P K+ >> P Na+

E membrane ≈ E K+ P K+ > P K+ at resting state

Resting Potential - Both Na+ and K+ channels are closed.

Why does the membrane potential increase during stage 3 of the action potential? A. Both the voltage-gated Na+ channels and voltage gated K+ channels are open. B. All of the K+ channels (both leak and voltage gated) are open. C. The voltage gated Na+ channels are open, but the voltage gated K+ channels have not opened yet. D. The voltage gated Na+ channels are open, but the K+ channels (both voltage gated and leak) have not opened yet.

Why does the membrane potential decrease during stage 4 of the action potential? A. The voltage gated K+ channels open. B. The voltage gated Na+ channels open. C. The voltage gated K+ channels close. D. The voltage gated Na+ channels close. E. A and D

Action Potentials Propagate Quickly in Myelinated Axons Action potentials jump down axon. Nodes of Ranvier Schwann cells (glia) wrap around axon, forming myelin sheath Axon Schwann cell membrane wrapped around axon Action potential jumps from node to node

The process of coating axons with myelin is incomplete when humans are born. This is part of the reason why babies are uncoordinated and slow learners. Babies need lots of fat – not only for energy storage but also to myelinate their neurons.

Multiple Sclerosis (MS) Disease results in damage to myelin and impairs electrical signaling. Muscles weaken and coordination decreases.

MIDTERM YOU ARE RESPONSIBLE TILL HERE !