Biology 211 Anatomy & Physiology I Electrophysiology

Recall: A neuron carries an electrical signal produced by the movement of ions across its plasma membrane

Any atom or molecule carrying an electrical charge (+ or -) The movement or flow of electrical charges from one place to another Potential energy which exists whenever opposite electrical charges are separated

Condition when ions are separated across a biological membrane (it has a voltage) Condition when ions are again separated (it regains its voltage) Condition when this separation of ions is lost (it loses its voltage) Movement along a membrane of a short segment of depolarization immediately followed by repolarization (a current)

When resting, the plasma membrane of a neuron is polarized. Sodium ions are concentrated on its outer surface & potassium ions are concentrated on its inner surface. Large negative ions (proteins, phosphate, sulfate, etc) are also concentrated on the inner surface. Sodium channels and potassium channels are closed.

An action potential begins when the sodium gates (or "gated channels") open on one section of the membrane. For now, don't worry about what causes this to happen. Sodium ions, carrying their positive charges, flow into the cell, making the inner surface of the plasma membrane more positive. The plasma membrane has begun to depolarize.

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 begins to pump 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 at one point on the membrane spreads to nearby regions of the membrane, causing them to depolarize then repolarize. This, in turn, stimulates regions a little further out to depolarize and repolarize, so these events spread away from the original location. This movement of depolarization and repolarization is the action potential which travels along the plasma membrane of the neuron.

While some neurons carry action potentials along their plasma membranes in this continuous fashion, most of them 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 depolarization and 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) The current (flow of electric charges carried by ions) can pass directly from the axon terminal to the second cell if their plasma membranes are connected by gap junctions which allow ions to flow between the cells. This is an electrical synapse; it is rare. b) The action potential can cause the axon terminal to release a chemical, called 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

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, measured as its voltage, can be increased or decreased by changing how many ions are separated. A greater voltage means that more positive and negative ions are separated; a lower voltage means that fewer positive and negative ions are separated

More Electrophysiology Terms to Know: The level of polarization (separation of + and - ions across the plasma membrane) at which ion channels rapidly open and the membrane rapidly depolarizes

Excitatory Postsynaptic Potential (EPSP): A DECREASE in the separation of ions across the plasma membrane of the postsynaptic cell. It is less polarized. Thus, EPSPs raise the voltage closer to the threshold voltage

Inhibitory Postsynaptic Potential (IPSP): An INCREASE in the separation of ions across the plasma membrane of the postsynaptic cell. It is more polarized. Thus, IPSPs make it less likely that the membrane voltage will reach the threshold voltage

Excitatory synapsesInhibitory synapses Axon Hillock Remember when we discussed excitatory synapses and inhibitory synapses affecting the axon hillock? Those are EPSPs and IPSPs

IPSPs can also turn other synapses "off" or "on"