The Force of Diffusion

• In the absence of an external force or barriers, molecules in a random pattern from their region of higher concentration to their region of lower concentration.

• This movement of molecules is termed as diffusion.

• The rate of movement of molecules is proportional to the temperature.

  • The higher the temperature, greater the rate of movement.

  • At a temperature of 0 K, or at the absolute zero, all molecular movement halts.

• When moving around, the molecules collide into each other, pushing each other away.

  • This collision and repulsion cause the molecules to shift or diffuse to their region of lower concentration.

• This results in a concentration gradient.

The Force of Electrostatic Pressure

• Some substances, when dissolved in water, split into two parts charing opposite electrical charges.

• These substances are called electrolytes and their parts are called ions.

  • Positive (+) charge carrying ions are called cations.

  • Negative (-) charge carrying ions are called anions.

• Ions with similar charges repel each other, while ions with dissimilar charges attract each other.

  • Anions repel anions

  • Anions attract cations

  • Cations repel cations

  • Cations attract anions

• The force created by this attraction and repulsion is called electrostatic pressure.

• Similar to the working of the force of diffusion, electrostatic pressure propagates the movement of ions from one place to another.

  • Cations push away other cations from their region of higher concentration to their region of lower concentration.

  • Anions push away other anions from their region of higher concentration to their region of lower concentration.

• Thus, the electrostatic pressure propagates a flow of electrical charge and creates an electrical gradient.

The balance between the electrical gradient and concentration gradient is maintained by the sodium-potassium pump.

What are intracellular and extracellular fluids?

• • Intracellular Fluid: Fluid inside a cell.

  • Extracellular Fluid: Fluid outside a cell.

• Both these fluids contain ions of varying charges, namely,

  • Organic Anions (A-)

    • Products of cellular metabolism

  • Chlorine Ions (Cl-)

  • Sodium Ions (Na+)

  • Potassium Ions (K+)

• A- ions are restricted to the intracellular fluid, giving the cell interior a negative charge, while Cl-, Na+ and K+ are present in both fluids.

• The forces of diffusion and electrostatic work on these ions, in balance, to generate the membrane potential.

• Though Cl-, Na+ and K+ are found in both fluids, their concentrations in either differ.

  • K+ has higher concentration in the intracellular fluid, while

  • Na+ and Cl- have higher concentration in the extracellular fluid.

Movement of Ions Between the Intracellular and Extracellular Fluids

• Due to the Force of Diffusion

  • As mentioned in the previous section, the force of diffusion creates a concentration gradient that sends molecules from their region of higher concentration, to their region of lower concentration.

  • As a results,

    • K+ ions in the intracellular fluid, tend to be pushed outside the cell.

    • Na+ ions and Cl- ions in the extracellular fluid, tend to be pushed into the cell.

• Due to the Force of Electrostatic Pressure

  • As mentioned in the previous section, the force of electrostatic pressure creates an electrical gradient that pushes away similarly charged ions and attracts oppositely charged ions.

  • The exterior of the cell carries a positive charge.

    • Though K+ ions are pushed outside the cell by the force of diffusion, the electrical gradient pushes the ions back into the cell.

  • The interior of the cell carries a negative charge due to the presence of A- ions.

    • Though Cl- ions tend to be pushed into the cell by the force of diffusion,the electrical gradient pushes the Cl- ions out of the cell.

    • The Na+ ions, on the other hand are attracted by the negative interior and pulled into the cell.

• Thus, the electrical gradient balances the effect of concentration gradient on the K+ and Cl- ions, while enhancing its effect on the Na+ ions.

• The cell membrane is selectively permeable, meaning, certain chemicals pass through the membrane more freely than others.

• While oxygen, urea, carbon dioxide and water have free movement across the membrane, most large ions are not afforded this freedom.

• Biologically important ions, like Ca+, Na+, K+ and Cl- ions, cross the membrane through special gated channels.

• The K+ ions outside the cell are attracted by the negatively charged interior, but as K+ ions have higher concentration side the cell, the concentration gradient pushes K+ ions outside.

  • Thus, K+ ions keep seeming out of the partially closed K channels on the membrane at rest.

• The Na+ ions outside the cell are attracted to the negative charged interior and the the lower concentration of Na+ ions inside the cell adds to pull of Na+ ions into the cell.

  • The movement of Na+ ions inside the cell is halted by the closed Na channels on the membrane at rest.

• The sodium-potassium pump is a gated channel that works through active transportation, by using energy (in the form of ATP or adenosine triphosphate).

  • Despite the electrical and concentration gradients, the pump pushes out three Na+ ions out of the cell while pulling in two K+ ions, while the membrane is at rest.

• The Na+ ions that are pumped out cannot re-enter as the Na channels are closed, but the K+ ions leaking out through the partially closed K channels carry a positive charge across the electrical gradient.

• The inside of the neuron is negative and is applied with a positive charge.

• This takes away some of the electrical charge across the membrane (near the electrode), reducing the membrane potential.

• This change in the cell’s membrane potential which reduces its polarisation to zero is called depolarisation.

• The voltage level that incites the action potential is called the Threshold of Excitation.

• The membrane potential reverses itself and the inside of the stimuli becomes positive which the outside becomes negative.

• This electrical activity is quick and is followed by the next phase - re-polarisation.

• The membrane potential drops from positive to negative but before it restores normalcy (-70mV), it overshoots and becomes hyper-polarised.

• Hyper polarised refers to being more polarised than usual and refers to a change in the cells membrane potential that makes the membrane potential more negative than before.

• This state lasts for a very short period of time before the membrane potential is back to being at its resting state.

• All three of these phases last for only 2 milliseconds.

• This very rapid reversal of the membrane potential is known as the Action Potential. They are electrical messages carried by the axon from the cell body to the terminal buttons.

Movements of ions through the membrane during the action potential happens in the following steps:

1. The threshold of excitation is reached which causes the Na channels in the membrane to open. The opening of Na channels is triggered by depolarisation.
   Because these channels are opened by changes in the membrane potential, they are called voltage-dependent ion channels.

2. The influx of positively charged sodium ions produces a rapid change in the membrane potential, from -70 mV to +40 mV.
   The axonal membrane contains two types of voltage dependent channels
   a) Voltage dependent Na channels
   b) Voltage dependent K channels
   The potassium channels however require greater level of depolarisation before they open.

3. At the peak of the action potential, the Na Chanels enter a refractory period which renders the Na channels inactive, hence temporarily stopping the flow of Na ions into the cell.

4. While the Na channels are inactive, the voltage-dependent K channels open, letting K ions move freely through the membrane. At this time, the inside of the axon is positively charged, so K is driven out of the cell. The outflow of cations causes the membrane potential to return toward its normal value. As it does so, the potassium channels begin to close again.

5. Before restoring its potential at resting period (-70mV), the membrane overshoots and becomes hyper-polarised (more negative than usual). The Na-K pump enables the exchange of Na And K ions in order to restore the resting potential.

6. Once the membrane potential returns to normal, the sodium channels reset so that another depolarisation can cause them to open again.

All or none law

• This law states that an action potential either occurs or does not occur; and, once triggered, it is transmitted down the axon to its end.

• An action potential always remains the same size, without growing or diminishing.

• When an action potential reaches a point where the axon branches, it splits but does not diminish in size.

• An axon will transmit an action potential in either direction, or even in both directions, if it is started in the middle of the axon’s length.

The rate law

• Consider these facts:

  • The strength of a muscular contraction can vary from very weak to very forceful, and the strength of a stimulus can vary from barely detectable to very intense.

  • The occurrence of action potentials in axons controls the strength of muscular contractions and represents the intensity of a physical stimulus.

But if the action potential is an all-or-none event, how can it represent information that can vary in a continuous fashion?

• The all-or-none law is supplemented by the rate law which means that variable information is represented by an axon’s rate of firing.

• A high rate of firing causes a strong muscular contraction, and a strong stimulus causes a high rate of firing in axons that serve the eyes.

Saltatory conduction

• The smallest axons in mammalian nervous systems are myelinated.

• Conduction of an action potential in a myelinated axon is somewhat different from conduction in an un myelinated axon.

• In the myelinated areas there can be no inward flow of Na1 when the sodium channels open because there is no extracellular sodium.

• The axon conducts the electrical disturbance passively from the action potential to the next node of Ranvier.

• The disturbance gets smaller as it passes down the axon, but it is still large enough to trigger a new action potential at the next node.

• The action potential gets re-triggered, or repeated, at each node of Ranvier, and the electrical disturbance that results is conducted decrementally along the myelinated area to the next node.

• Transmission of this message, hopping from node to node, is called saltatory conduction

How is Saltatory Conduction useful?

1. It is economic. Myelinated axons expend much less energy to maintain their sodium balance.

2. It is fast. Conduction of an action potential is faster in a myelinated axon because the transmission between the nodes is very efficient and speedy, enabling the animal to react faster and think faster.