EXCITABLE TISSUES& ACTION POTENTIALDr. Tamara Alqudahtamara.alqudah@ju.edu.jo

Ion Channels• When ion channels are open, they allow specific ions to

move across the plasma membrane, down theirelectrochemical gradient.

• Ion channels open and close due to the presence of“gates.” The gate is a part of the channel protein that canseal the channel pore shut or move aside to open thepore.

• There are four types of ion channels:1. Leak channels2. Ligand-gated channels3. Mechanically-gated channels4. Voltage-gated channels

Leak channels• Randomly alternate between open and closed positions.• Most important are Na+ & K+ channels• Found in nearly all cells

Ligand-gated channels• Opens and closes in response to the binding of a ligand

(neurotransmitters, hormones and particular ions)stimulus.

• Located in neurons.

Mechanically-gated channel• Opens or closes in response to mechanical stimulation in

the form of vibration, touch, pressure, or tissue stretching.• Located in receptors that monitor stretching of internal

organs.

Voltage-gated channel• Opens in response to a change in membrane potential

(voltage) .• Voltage-gated channels participate in the generation and

conduction of action potentials in the axons of all types ofneurons.

Resting Membrane Potential• The resting membrane potential exists because of a

small build-up of negative ions in the cytosol along theinside of the membrane, and an equal build-up of positiveions in the extracellular fluid along the outside surface ofthe membrane.

• In neurons, the resting membrane potential ranges from(-40 to -90) mV. A typical value is -70 mV.

• The minus sign indicates that the inside of the cell isnegative relative to the outside.

Resting membrane potential• A membrane potential, an electrical potential difference

(voltage) across the membrane. In excitable cells, thisvoltage is termed the resting membrane potential.

• Resting= the membrane isn’tStimulated

Factors that contribute to thedevelopment of resting membranepotential

Unequal distribution of ions in the ECF &cytosol

• The ECF has high concentration of Na+ and Cl– ions.• The cytosol has high concentration of K+, and the two

dominant anions are phosphates attached to molecules, suchas ATP, and amino acids in proteins

• The membrane’s permeability to K+ is much higher than itspermeability to Na+.

• Plasma membranes have many more K+ ions leak channels than Na+

leak channels• K+ ions leak channels are leakier than the Na+ ion leak channels.

• The number of K+ that diffuse down their concentrationgradient out of the cell into the ECF is greater than thenumber of Na+ that diffuse down their concentration gradientfrom the ECF into the cell.

Inability of most anions to leave the cell• Most anions inside the cell are not free to leave

Electrogenic nature of the Na+– K+ ATPases

• Na+– K+ ATPases remove more positive charges from thecell than they bring into the cell, they are electrogenic,which means they contribute to the negativity of theresting membrane potential.

• Na+– K+ ATPases help maintain the resting membranepotential by pumping out Na+ as fast as it leaks in.

Excitable tissues• Neurons and muscle fibers are considered excitable cells.

• They have the ability to respond to certain stimuli byproducing electrical signals such as action potentials.

• An action potential that occurs in a neuron causes therelease of neurotransmitters.

• An action potential that occurs in a muscle fiber, causescontraction in the muscle fiber.

Electrical Signals In Neurons• Neurons (nerve cells) are electrically excitable.• They communicate with one another using two types of

electrical signals:1. Graded potentials are used for short- distance

communication only.2. Action potentials allow communication over long distances

within the body.

• The production of graded potentials and action potentialsdepends on two basic features of the plasma membrane ofneurons

1. The existence of a resting membrane potential2. The presence of specific types of ion channels that open or

close in response to specific stimuli

Graded Potentials• Is a small deviation from the resting membrane potential

that makes the membrane either more polarized (insidemore negative) or less polarized (inside less negative).

• Hyperpolarizing graded potential: when the membranebecomes more polarized

• Depolarizing graded potential: when the membranebecomes less polarized

• Graded potentials occur mainly in the dendrites and cellbody of a neuron. By the action of mechanically- gatedchannels and ligand-gated channels.

• Graded means that they vary in amplitude (size),depending on the strength of the stimulus.

• They are larger or smaller depending on how manyligand-gated or mechanically-gated channels haveopened (or closed) and how long each remains open.

• They are useful for short-distance communication only,because they die out within a few millimeters of theirpoint of origin. This called decremental conduction.

Generation of Action Potentials• A sequence of rapidly occurring events that decrease

and reverse the membrane potential and then eventuallyrestore it to the resting state.

• An action potential has two main phases: a depolarizingphase and a repolarizing phase.

• It occurs in the membrane of the axon of a neuron whendepolarization reaches a certain level termed thethreshold (about - 55 mV in many neurons)

• An action potential will occur in response to a thresholdstimulus, a stimulus that is just strong enough todepolarize the membrane to threshold.

All-or-none principle

• The amplitude (height) of an action potential is alwaysthe same and does not depend on stimulus intensity.

• The greater the stimulus strength above threshold, thegreater the frequency (number) of the action potentialsuntil a maximum frequency is reached as determined bythe absolute refractory period

Depolarizing Phase• When a depolarizing graded potential or some other

stimulus causes the membrane of the axon to depolarizeto threshold.

• voltage-gated Na+ channels open rapidly. Both theelectrical and the chemical gradients favor inwardmovement of Na+.

• As more channels open, Na+ inflow increases, themembrane depolarizes further, and more Na+ channelsopen. This is an example of a positive feedbackmechanism.

• The inflow of Na+ changes the membrane potential from-55 mV to +30 mV

Repolarizing Phase• The voltage-gated Na+ channel becomes inactivated

because the inactivation gates close. This decreases theNa+ inflow.

• Voltage -gated K+ channels also open. This increases K+

outflow.• The membrane potential changes from +30 mV to -70 mV

After-hyperpolarizing Phase• During this phase, the voltage-gated K+ channels remain

open and the membrane potential becomes even morenegative (about - 90 mV).

• As the voltage - gated K+ channels close, the membranepotential returns to the resting level of - 70 mV.

Refractory Period• The period of time after an action potential begins during

which the neuron can’t generate another action potentialin response to a normal threshold stimulus is called therefractory period.

• Absolute refractory period, even a very strong stimuluscan’t initiate a second action potential.

• Relative refractory period is the period of time duringwhich a second action potential can be initiated, but onlyby a larger than normal stimulus.

Propagation of Action Potentials• An action potential keeps its strength as it spreads along

the membrane. This mode of conduction is calledpropagation and it depends on positive feedback.

• This is essential to communicate information from onepart of the body to another.

• It can propagate from the cell body toward the axonterminal, it can’t propagate backward.

Stimulus

Continuous and SaltatoryConduction

• Continuous conduction, involves step-by stepdepolarization and repolarization of each adjacentsegment of the plasma membrane✔ Occurs in unmyelinated axons and in muscle fibers.

• Saltatory conduction, current carried by Na+and K+

flows across the membrane mainly at the nodes ofRanvier.✔ More rapid conduction✔ Opening a smaller number of channels only at the nodes rather

than the whole membrane, means less energy consumption.✔ Occurs In myelinated axons

Factors That Affect the Speed ofPropagation

1. Amount of myelination• Action potentials propagate more rapidly along myelinated axons

2. Axon diameter• Larger diameter axons propagate action potentials faster

3. Temperature• Axons propagate action potentials at higher speeds when their

temperature increases