Electrical Properties of the Nervous System Lundy-Ekman, Chapter 2 D. Allen, Ph.D.

Transmission of Information Local and Action Potentials Electrical charge carried by ions –Positive ions –Negative ions

Resistance inside the Axon Controls speed and distance ions will move down the axon Increased resistance will have which effect on ion movement? –A. Speed up –B. Slow down How can we change resistance – Increased diameter will have what effect on resistance?

Resistance across Cell Membrane Cell membrane is a lipid bilayer – high resistance How can we change the resistance? –Ion channels - proteins

Types of Ion Channels Named depending on the factor that causes the ion channel to open or close –Modality-gated –Ligand-gated –Voltage-gated

Modality-gated ion channels Specific to sensory neurons Located on dendrites Channels open or close in response to some sensory stimulus –Temperature –Light –Specific chemicals (taste, pH, oxygen) –Mechanical force Important for detection of sensory modalities – cause local potentials

Ligand-gated Ion Channels Open in response to neurotransmitter or drug binding Located on post-synaptic terminal –Dendrites and soma Cause local potentials

Voltage-gated Ion Channels Open or close in response to changes in the membrane potential Channels will only let specific ions pass Located on axons and presynaptic terminals Important for: –Action potentials –Release of neurotransmitters from the presynaptic terminal

There is a charge difference between the inside and the outside of the neuron. The inside is usually more negative compared to the outside Where are ions the most? InsideOutside K+K+ Na +, Cl -, Ca ++

What happens when we open an ion channel? Ions will move across the membrane The charge on the inside of the membrane will change Sodium ion channels (Na + ) –Sodium will move inside the nerve cell –The inside will become more positive Depolarizing Excitation

Potassium (K + ) –Potassium will move out –The inside will become more negative Hyperpolarization Inhibition Chloride (Cl - ) –Chloride will move in –The inside will become more negative Hyperpolarization Inhibition

Calcium (Ca ++ ) –Calcium will move in –The inside will become more positive Depolarizing Excitation

What maintains the concentrations of Na+ and K+ Sodium-potassium ATPase (pump) –3 Na+ out –2 K+ in

What can change the membrane potential Occurs when there are changes in ion flow through the gated membrane channels –When a channel opens or closes Due to: –Sensory stimuli –Neurotransmitters

Initial changes in membrane potential Local potentials (graded potentials) –In sensory neurons called receptor potentials Modality-gated ion channels –If generated at post-synaptic membrane are synaptic potentials Ligand-gated ion channels

Local potentials Small – several millivolts Graded – can vary in amplitude and duration Can be either depolarizing (inside of cell more positive) or hyperpolarizing (inside of cell more negative) Spread passively for 1-2 mm –Amplitude decreases with distance

How can we increase local potentials Temporal summation –Sequential local potentials can add together Spatial summation –Different local potentials that occur at the same time can combine –The effect can be additive or subtractive

Action Potentials Larger changes than local potentials Large, brief depolarization that is repeatedly regenerated along the length of an axon Actively propagated, so it travels long distances along the axon without a decrease

Characteristics of Action Potentials Large depolarization: From –70 mV to +50 mV All or none: A full action potential occurs, or no action potential occurs. There are no half-size action potentials Always depolarization (never hyperpolarization)

Characteristics of Action Potentials Spread actively Initiation of an action potential requires a sufficient level of depolarization by local potentials called the threshold level (usually about 15 mV) Action potentials are first generated at the axon hillock

Main components of Action Potential Voltage-gated sodium ion channels Voltage-gated potassium ion channels

Voltage-Gated Sodium Channels Depolarization of membrane quickly opens the channels The channels open and sodium ions enter the neuron (becomes positive inside) Channels spontaneously close, even if the membrane remains depolarized (inactivation) It takes time for the channels to be able to be opened again

Voltage-gated Potassium Channels Depolarization of the membrane slowly opens the channels The channels open and potassium ions leave the cell (becomes negative inside) The channels remain open as long as the membrane is depolarized.

Time course of action potential 1.Sodium channels quickly open – depolarize 2.Sodium channels start spontaneously closing – decrease depolarization 3.Potassium channels start opening – hyperpolarize 4.# 2 and 3 continue simulator.htm

5.At end of action potential, sodium channels are all inactivated 6.Potassium channels still open, so the membrane is more hyperpolarized than at rest 7.Potassium channels close, and the membrane potential returns to resting levels

Refractory Periods Immediately after the action potential, the sodium channels are inactivated Absolute refractory period –No stimulus, no matter how strong, can produce a second action potential Relative refractory period –A larger than normal stimulus can produce a second action potential

At end of Action Potential Sodium ion channels are closed, but are able to open –Ready for new action potential Normal resting ion concentrations restored by the Na+/K+ pump

To increase the speed of action potentials, how should we change these electrical properties of a neuron? Resistance inside the neuron Decrease Membrane resistance (across membrane) Increase

How do we change these parameters? Increase axon or dendrite diameter –Decrease intracellular resistance Myelination –Increase membrane resistance

Myelination

Myelin sheath covers myelinated axons Interruptions every 1-2 mm –Nodes of Ranvier High concentration of voltage-gated sodium ion channels

Sodium Channel Density Area of neuronDensity (#/square micron) Soma (Cell body)50-75 Axon Hillock Unmyelinated axon110 Node of Ranvier2,000-12,000 Internode< 25

Saltatory Conduction Action potentials travel quickly through the myelinated region Slow at the Nodes – the action potential is regenerated here Action potentials appear to jump between nodes – saltatory conduction