Cell

Types of CellCells are the basic structural and functional units of the human body & there are many different types of cells (e.g., muscle, nerve, blood, and so on)

Cells

• Communication between the cells & organs occurs primarily by twodifferent mechanisms:– Transfer of electrical signals along membranes– Transfer of soluble signaling chemicals from one location to another

Cellular Excitability

Cell membrane (75-100 Ao)

Nucleus

Nuclear membrane

H2OCytoplasm

• The basic characteristics of cells are similar:– require nutrition– oxygen is the one of the major substance to derive energy– reproducibility– intracellular & extracellular fluids

Electrical Signaling of the membrane

Substances are transported through the cell membrane by two major processes: Diffusion & Active Transport

Diffusion

Diffusion Rate = K Concentration . Cross-Sectional Area . Temperature

Molecular weight . Distance

Intracellular Extracellular

Na+ 12 mmol/L

K+ 155 mmol/L

Cl- 4 mmol/L

Na+ 145 mmol/L

K+ 4 mmol/L

Cl- 120 mmol/L

75-100 AoCell Membrane

- 90 mV +

Diffusion

Intracellular Extracellular

Na+ 12 mmol/L

K+ 155 mmol/L

Cl- 4 mmol/L

Na+ 145 mmol/L

K+ 4 mmol/L

Cl- 120 mmol/L

75-100 Ao

Cell Membrane

- 90 mV +

• Smaller ions diffuse easier through the pores.• Positive ions diffuse with extreme difficulty.• Negative ions pass through membrane much more easily.• Permeability of the membrane is different to different ions.

So what effects diffusion?• Permeability• Concentration• Electrical Potential Difference• Pressure difference

• No substance can diffuse against a concentration difference, or as is often said “up hill”.

• Energy is required to move a substance up hill.

• Moving a substance up hill by using energy is called “Active Transport”.

• In a normal cell, positive sodium ions are pumped out of the cells and positive potassium ions are pumped into the cells by Active Transport. It is believed for every 3 Na+ being pumped out, 2 K+ are pumped in.

Active Transport

The existence and functionality of Na+ pump is very important. The non diffusible substances tend to cause osmosis of water to the inside the cell all the time. Also electrolytes tend to leak along with the water to the inside. If there weren't any mechanism to oppose this, the cell would eventually swell until it burst. But Sodium pump initiates and opposite osmotic tendency to move water out of the cell. That's why when the ATP is not available for Na+ pump to work, the cell swell immediately.

Active Transport

barrier

semi permeable membrane Barrier is removed here.

Osmosis

Osmosis is the flow of water across the semi permeable membrane in response to differences of intracellular concentration.

• Since the concentration of ions are different in intra and extracellular fluids, therefore membrane has a potential.

Nernst Equation: E = 61 log10

Membrane Potential

Goldman Equation: E =

R = Universal Gas const.T = Absolute TemperatureF = Faraday’s const.P = Permeability coefficent of the membrane to an ion

[K]o[K]i

-90 mv

Pk.[K]o + PNa.[Na]o + PCl.[Cl]o-85 mvRT

FLn

Pk.[K]i + PNa.[Na]i + PCl.[Cl]i

≈≈

• At rest, the membrane potential remains undisturbed about -85 mv. However, if one spot receives a stimuli, the membrane potential go through a rapid change called “Action Potential”.

Action Potential

As a result of stimulus, the permeability of membrane to Na+

increases up to 1000 times folds. Therefore, the Na+ rush into the cell carrying enough positive charges to change the membrane potential. This is called “Depolarization”.

Action Potential

As a result of stimulus, the permeability of membrane to Na+

increases up to 1000 times folds. Therefore, the Na+ rush into the cell carrying enough positive charges to change the membrane potential. This is called “Depolarization”.

Action Potential

Almost immediately after depolarization, the pores of membrane again become almost impermeable to Na+ and the membrane potential goes back to its resting state. This is called “Repolarization”.

Action Potential

Action Potential

Action Potential & Permeability

• During absolute refractory period (about 1 ms for a nerve axon), the membrane cannot respond to any stimulus, no matter how intense it is, However, during relative refractory period, it can respond to intense stimulus.

Action Potential

An action potential elicited at any point on a membrane, usuallyexcites adjacent portions of the membrane, resulting propagation of the action potential in any direction. The action potential moves and depolarizes through the entire membrane or it fails to travel at all. This is called “all-or-none law”.

Action Potential

An action potential elicited at any point on a membrane, usuallyexcites adjacent portions of the membrane, resulting propagation of the action potential in any direction. The action potential moves and depolarizes through the entire membrane or it fails to travel at all. This is called “all-or-none law”.

Action Potential

Impulses typically travel along neurons at a speed of anywhere from 1 to 120 m/s.The speed of conduction can be influenced by:

•The diameter of a fiber•The temperature•The presence or absence of myelin

Conduction Velocity

Between areas of myelin are non-myelinated areas called the nodes of Ranvier. Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse. So, in a myelinated neuron, action potentials only occur along the nodes and therefore, impulses “jump” over the areas of myelin – going from node to node in a process called saltatory conduction.

Saltatory Conduction

Because the impulse “jumps” over areas of myelin, an impulse travels much faster along a myelinatedneuron compared to that in a non-myelinatedneuron.