Objectives 7 Electrophysiology

A. Basic Principles:

- opposite charges attract each other

- when opposite charges attract each other, energy is released

- thus, separated charges have potential energy

- potential energy is measured as voltage, or in the case of cells, millivoltage

- the voltage difference between two points is called the potential difference or the potential between these two points

- current is the flow of electrical charge from one place to another; electrons flow into positively charged areas

- resistance is any impediment to flow

- Ohm’s law states that the current is directly proportional to the voltage and inversely proportional to the resistance:

Voltage (V)

Current (I) = Resistance (R)

B. Excitable Cells (Neurons and Muscle Cells)

- voltages exist across the membranes of excitable cells when they are at rest; these voltages are called resting membrane potentials and they exist because of the unequal distribution of Na+, K+, Cl- and protein on either side of the plasma membrane

- there are passive ion channels present in the membranes of excitable cells that allow specific ions to diffuse down their concentration gradients; these channels are always open\

- there are more K+ channels than there are Na+ channels

Na+ Cl-

K+ K+ K+

the Na+/K+ ATPase (pump) works constantly, ejecting 3 Na+ for every 2K+ it pumps into the cell

thus, a carefully controlled voltage exists across the membrane at rest

the membrane is said to be polarized; a voltmeter can be used to measure the voltage which, at rest, averages -70mV

- if the resting membrane potential was to change, there are two possibilities

Depolarization Hyperpolarization

C. Neuron Activation - The Graded Potential

- neurons are stimulated in the area of the dendrites and the cell body

Area of Stimulation

- in addition to passive channels, these areas of the neuron also have other ion channels called gated channels, which are closed at rest, but can be stimulated to open under various conditions

Chemically Gated Ion Channels Light Gated Ion Channel

Mechanically Gated Ion Channel

- when these channels are opened, the resting membrane potential changes in a manner depending on the type of ion flowing through the channel and the direction in which it flows

or

- the number of channels that are open determine the degree of change; therefore,the voltage change is said to be graded and changes in the area of dendrites and cell bodies are called graded potentials

OR

- types of graded potentials:

postsynaptic potential: a graded potential produced in a neuron in response to the binding of neurotransmitter and the opening of a chemically graded ion channel; may be excitatory or inhibitory

receptor potential: a graded potential produced at the peripheral endings of afferent neurons

when they are stimulated by light, heat, mechanical energy

pacemaker potential: a spontaneously arising graded potential that occurs in certain specialized cells

- events involved in the generation of an excitatory graded potential:

1. Neurotransmitter binds to chemically gated ion channels on the cellbody and the dendrites of a neuron

2. The binding of the neurotransmitter oopens the channels:

3. Ion movement causes local depolarization

4. The ions begin to spread in each direction in the cell.

5. If a certain amount of positive charge reaches the axon hillock, an action potential will be initiated there

- graded potentials lose strength an die out as they spread in either direction;the graded potential is said to be decremental

- the most effective stimuli are those delivered as close to the axon hillock aspossible

least effective

more effective

D. The Action Potential

- action potentials are all or none depolarizations that occur when excitatorygraded potentials reach the axon hillock

- action potentials depend on the presence of voltage gated ion channels forNa+ and K+ which are scattered along the axon in unmyelinated neurons

- Graph of an Action Potential

There are several phases in an action potential:

- graded potentials reach the axon hillock

- this causes a slight depolarization

- this depolarization opens voltage gated Na+ channels

- if enough channels open, the axon hillock depolarizes to a threshhold level and, at that points, the action potential becomes self sustaining

threshold: a membrane potential (usually 15-20 mV more positivethan the resting membrane potential) at which largernumbers of voltage gated sodium channels open; at thispoint, the action potential becomes self sustaining and itis guaranteed to happen

2. From Threshold to +30 mV - Depolarization

- more and more voltage gated Na+ channels open and Na+ rushes into the neuron

- the neuron becomes positive (+30) inside

- at +30 mV, the voltage gated sodium channels inactivate and Na+ can no longer

enter; also, Na+ begins to be repelled by the positively charged interior

3. From +30 mV to Resting Membrane Potential - Repolarization

- at +30 mV, the voltage gated K+ channels open and K+ diffuses out of the cell

- as a result, the cell becomes negative again on the inside, restoring resting membranepotential

4. Hyperpolarization

- the voltage gated K+ channels close slowly so the membrane potential falls briefly below tthe resting membrane potential (a period called hyperpolarization)

- the Na+/K+ ATPase (pump) restores the Na+ and K+ to their proper sides, restoring ionic balance and the resting membrane potential

- the neuron can now be retimulated

E. Refractory Period

- Absolute Refractory Period: period during during the action potential in which thevoltage gated Na+ channels are open; this neuron cannot

respond to a second stimulus during this time

- Relative Refractory Period: period during the action potential in which the neuronis hyperpolarized; this neuron can be restimulated but it will require a greater amount of stimulus