Electrical Signals

Electrical properties of cells result from different concentrations of ions across plasma membrane and from permeability characteristics of plasma membrane.

1.1      Concentration differences across the plasma membrane

•  The Na⁺ – K⁺ pump

              -Through active transport

-moves Na⁺ and K⁺ through the plasma membrane against their concentration gradients.

-Sodium ion transport out of the cell, increasing the concentration of Na⁺ outside the cell.

-Potassium ion transport into the cell,increasing concentration of K⁺ inside the cell.

-For each ATP molecule used, three Na⁺ transport out of cell and two K^* transport into the cell.

•  Permeability characteristic of plasma membrane.

-selectively permeable (allowing some but not all substances to pass through it)

1. Leak channel

Potassium ion leak channels are more numerous than Na⁺ leak channels,thus the plasma membrane is more

permeable to K⁺ when at rest than Na⁺.

    2.  Gated Ion Channel.

                i.    Ligand-gated ion channels

                              -Ligand is a molecule that binds to receptor.

-Receptor has a receptor site to which a ligand can bind.

-Ligand-gated ion channels are receptors that have an extracellular receptor

site and a membrane spanning part that forms an ion channels.

-When a ligand binds to receptor site,the ion channel opens or closes.

-exist for Na⁺ ,K⁺ ,Ca²⁺ and Cl^–.

-common in tissues such as nervous and muscle tissue,as well as glands.

                       ii.    Voltage-gated ion channels

-These channels open and close in response to small voltage changes across the plasma membrane.

-Specific for Na⁺ and K⁺ are most numerous in electrically excitable tissues but voltage-gated Ca²⁺ channels

also important especially in smooth and cardiac muscle cells.

                      iii.   Other gated ion channels.

-Respond to stimuli other than ligands or voltage changes presents in specialized electrically excitable

tissues. (etc: touch receptors and temperature receptors)

1.2           Resting Membrane Potential

• Establishing the resting membrane potential.

-When the cell is in an unstimulated condition,the charge difference across the plasma membrane is called the

resting membrane potential. The inside of the cell is negatively charged compared with outside of the cell.

-The resting membrane potential is due mainly to the tendency of positively charged K⁺ to diffuse out of the

cell,which is opposed by the negative charge that develops inside plasma membrane.

• Changing The resting membrane potential

1.  Depolarization is a decrease in the membrane potential in which the charge difference ,or polarity , across the

plasma membrane decreases.

-can result from a decrease in the K⁺ concentration gradient, a decrease in membrane permeability to K⁺, an

increase in membrane permeability to Na⁺ , an increase in membrane permeability to Ca²⁺ , or a decrease in

extracellular Ca²⁺ condition.

2.   Hyperpolarization is an increase in the membrane potential caused by an increase in the charge difference

across the plasma  membrane.

-can result from an increase in the K⁺ concentration gradient,an increase in membrane permeability to K⁺ , and

increase in membrane permeability to Cl^– ,a decrease in membrane permeability to Na⁺, or an increase in

extracellular Ca²⁺ concentration.

1.3               Graded Potentials

• A change in the resting membrane potential caused by a stimulus applied to the plasma membrane of a cell is called graded potential.
• The potential change can vary from small to large.
• It can result from chemical signals binding to their receptors, voltage changes across plasma membrane, mechanical stimulation, temperature changes or spontaneous changes in membrane permeability.
• It is also can be either a depolarization or hyperpolarization.
• The magnitude of graded potentials can vary from small to large depending on the stimulus strength or on summation. Etc. weak stimulus can cause few gated Na⁺ channels to open while stronger stimulus can cause a greater number of gated Na⁺ channels to open.
• Summation occurs when the effects produced by one graded potential are added onto the effects produced by another graded potential.
• A graded potential decreases in magnitude as the distance from the stimulation increases.

1.4              Action Potential

• An action potential is a larger change in the resting membrane potential that spreads over the entire surface of the cell.
• It is produced when a graded potential reaches threshold.
• Action potential are all-or-none.If action potential occurs,they are of the same magnitude,no matter how strong the stimulus.
• As the inside of the membrane becomes more positive due to Na⁺ diffuse into the cell through voltage-gated ion channels,the depolarization occurs.
• Repolarization is a return of the membrane potential toward the resting membrane potential because voltage-gated Na⁺ channels close and Na⁺ diffusion into the cell slows to resting levels and because voltage –gated K⁺ channels open and K⁺ diffuse out of cell.
• The afterhyperpolarization exists because the voltage-gated K⁺channels remain open for a short time.

1.5               Refractory Period

• Once an action potential is produced at a given point on the plasma membrane,the sensitivity of that area to further stimulation decreases for a time called the refractory period.
• The time during an action potential when a second stimulus ,no matter how strong, cannot initiate another action potential is called the absolute refractory period.
• The relative refractory period follows the absolute refractory period and is the time during which a stronger-than-threshold stimulus can evoke another action potential.

1.6               Action Potential Frequency

• The action potential frequency is number of action potentials produced per unit of time in response to a stimulus.
• Action potential is directly proportional to stimulus strength and to the size of graded potential.
• Subthreshold stimulus-any stimulus not strong enough to produce a graded potential that reaches threshold.
• Threshold stimulus- produce a graded potential that strong enough to reach threshold and cause the production of a single action potential.
• Maximal stimulus- produce a maximum frequency of action potential.
• Submaximal stimulus- include all stimuli between threshold and the maximal stimulus strength.
• Supramaximal stimulus- any stimulus stronger than a maximal stimulus and can not produce a greater frequency of action potential than a maximal stimulus.
• A low frequency of action potentials represent a weaker stimulus than a high frequency.

1.7              Propagation of action potential

• Action Potential Propagation in an unmyelinated axon.

1. Action potentials propagate in one direction along the axon.
2. An action potential (white part of the membrane) generates local currents (black arrows) that tend to depolarize the membrane immediately adjacent to the action potential.
3. When depolarization reaches threshold,a new action potential is produced adjacent to where the original action potential occurred.
4. Action potential propagation occurs in one direction because the absolute refractory period of the previous action potential prevents generation of an action potential in the reverse direction.

• Saltatory conduction

1. An action potential at a node of Ranvier generates local currents and  flow directly to the next node of Ranvier because myelin sheath of Schwann cell insulates the axon of internode.
2. When depolarization caused by local currents reaches threshold at the next node of Ranvier,a new action potential is produced.
3. Action potential propagation in myelinated axons is rapid due to action potentials are produced at successive nodes of Ranvier.