Biology 211 Anatomy & Physiology I Dr. Thompson Electrophysiology

Recall: A neuron carries an electrical signal produced by the movement of ions across its plasma membrane The mechanism by which it does this should be familiar – it is very similar to how the plasma membrane of a muscle cell (its sarcolemma) generates and carries electrical signals as we have previously discussed …

When resting, the plasma membrane of a neuron is polarized. Sodium ions are concentrated on its outer surface. Potassium ions and large negative ions (proteins, phosphate, sulfate, etc) are concentrated on its inner surface. Sodium channels and potassium channels are closed so very few ions are passing across the membrane.

An action potential begins when the plasma membrane begins to depolarize Sodium gates (or "gated channels") open on one section of the membrane. (later, we’ll discuss what causes this to happen) Large amounts of sodium ions flow into the cell carrying their positive charges, making the inner surface of the plasma membrane more positive..

A few milliseconds later, potassium gates open as the sodium gates close. Potassium ions, with their positive charges, flow out of the cell, again making the outer surface of the plasma membrane more positive. The plasma membrane has begun to repolarize.

The potassium gates then also close. The cell quickly pumps sodium ions back to the outside of the membrane and potassium ions back to the inside of the membrane. The plasma membrane becomes fully repolarized.

This depolarization / repolarization starts at one point on the membrane, then spreads to nearby regions of the membrane, causing them to depolarize / repolarize. This, in turn, stimulates regions a little further out to depolarize / repolarize, and these events spread away from the original location

This movement of depolarization & repolarization Is an action potential which travels along the plasma membrane of the neuron.

While some neurons use this continuous type of action potentials to carry their electrical signals, most neurons use a more efficient method of carrying action potentials called saltatory conduction. This is much more rapid and requires much less energy.

Saltatory conduction can only occur on myelinated neuron processes. The action potential (depolarization then repolarization) occurs only at nodes of Ranvier, so the action potential skips from node to node to node.....

Whether the action potential travels along an axon by continuous or saltatory conduction, it eventually spreads along telodendria and reaches the axon terminals. From here, the signal can be passed to another cell at a synapse

Two types of synapses: a) Ions can pass directly from one neuron to another if their plasma membranes are connected by gap junctions, thus starting a new action potential on the second cell. This is an electrical synapse; it is rare. b)The action potential can cause the axon terminal of the first neuron to release a neurotransmitter, which binds to the plasma membrane of the second cell and stimulates a new action potential on it. This is a chemical synapse; it is very common

Chemical Synapse

More Definitions Presynaptic Neuron: The neuron which secretes the neurotransmitter at a synapse. Postsynaptic Neuron: The neuron to which this neurotransmitter binds, thus creating a new action potential on its plasma membrane.

Notice that the same neuron can be the postsynaptic neuron at one synapse and the presynaptic neuron at the next synapse.

There are dozens of different chemicals which act as neurotransmitters, some of which are listed in this table from Saladin.

However: any neuron can only secrete one type of neurotransmitter from all of its axon terminals

Additionally, at each synapse there must be a perfect match between neurotransmitter and receptor: The postsynaptic cell must have receptors which are specific for the neurotransmitter which is secreted by the presynaptic cell: For example: If the presynaptic neuron secretes acetylcholine, the postsynaptic neuron must have acetylcholine receptors. If the presynaptic neuron secretes serotonin the postsynaptic neuron must have serotonin receptors etc,

Recall: When resting, the plasma membrane of a neuron is polarized because it has more positively charged ions on the outside and more negatively charged ions on the inside.

This polarization of the membrane is measured as its voltage. This can be increased or decreased by changing how many ions are separated. A voltage of “0” means that positive and negative ions are mixed together and not separated from each other… Millivolts No separation of + and - ions

This polarization of the membrane is measured as its voltage. This can be increased or decreased by changing how many ions are separated. A voltage of “0” means that positive and negative ions are mixed together and not separated from each other. As positive and negative ions get separated from each other across the membrane, the further away from “0” the voltage will move Millivolts No separation of + and - ions + and – ions separated

Millivolts A normal resting voltage for a neuron (that is, how much are the positive ions and negative ions separated across its plasma membrane) is between -60 and -70 millivolts. The “ - ” means that more positive ions are on the outside and more negative ions are on the inside of the membrane.

A normal resting voltage for a neuron is between -60 and -70 millivolts. As Na + flows into the cell (the membrane depolarizes), the voltage moves closer to “0” and will go past it (more positive ions on the inside) As K + flows out of the cell (the membrane repolarizes), the voltage returns toward its resting value (more positive ions on the outside)

In reality, the plasma membrane of a neuron does not depolarize (lots of Na + flowing in) as quickly as those earlier diagrams imply since not all of the sodium gated channels open at the same time. Instead, a few Na + gates open first, then a few more, and a few more…. Each time this raises the voltage of the membrane a little bit, until the voltage reaches a point, called the threshold voltage, which causes all of the remaining Na + gates to open as well, causing rapid depolarization.

Those small depolarizations are called Excitatory Postsynaptic Potentials (EPSP) Each of these raises the voltage of the neuron’s plasma membrane closer to its threshold voltage If enough EPSPs occur in a short period of time, the full action potential begins.

Inhibitory Postsynaptic Potentials (IPSPs) can also occur, which INCREASE in the separation of positive and negative ions across the plasma membrane of the Neuron (making it even more polarized.) These IPSPs make it less likely that the plasma membrane of the neuron will reach threshold voltage.

Remember when we discussed excitatory synapses and inhibitory synapses affecting the axon hillock? Those synapses produce EPSPs and IPSPs. The dendrites (and body) of every neuron are constantly receiving both stimulatory signals which move its membrane voltage closer to threshold (EPSPs), and inhibitory signals which move it membrane voltage further away from threshold (IPSPs). Those travel along the membrane until they reach the axon hillock, and if there are enough more EPSPs than IPSPs to reach threshold voltage, it will depolarize.

Inhibitory Postsynaptic Potentials can also block the synapse of a neuron. If an action potential (depolarization/repolarization) moving along the plasma membrane of that neuron’s axon and telodendria reaches a place where another neuron is creating an IPSP, the membrane will no longer be able to reach threshold voltage at that point. The action potential will not reach the axon terminal, so it will not be able to release neurotransmitter.