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Cell
Types of Cell
Cells 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
•
The basic characteristics of cells are similar:
–
–
–
–
require nutrition
oxygen is the one of the major substance to derive energy
reproducibility
intracellular & extracellular fluids
Cell membrane (75-100 Ao)
Nucleus
Nuclear membrane
H2O
Cellular Excitability
•
Cytoplasm
Communication between the cells & organs occurs primarily by two
different mechanisms:
– Transfer of electrical signals along membranes
– Transfer of soluble signaling chemicals from one location to another
Electrical Signaling of the membrane
Substances are transported through the cell membrane by two
major processes: Diffusion & Active Transport
Cell Membrane
75-100 Ao
Intracellular
Extracellular
Na+
12 mmol/L
Na+
145 mmol/L
K+
155 mmol/L
K+
4 mmol/L
Cl-
4 mmol/L
Cl-
120 mmol/L
- 90 mV +
Diffusion
Diffusion Rate = K
Concentration . Cross-Sectional Area . Temperature
Molecular weight . Distance
Diffusion
• 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
Cell Membrane
75-100
Ao
Intracellular
Extracellular
12
Na+
mmol/L
145
Na+
mmol/L
K+
155
mmol/L
K+
Cl-
4 mmol/L
- 90 mV
+
4 mmol/L
120
Clmmol/L
Active Transport
• 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.
Osmosis
Osmosis is the flow of water across the semi
permeable membrane in response to differences of
intracellular concentration.
barrier
semi permeable membrane
Barrier is removed here.
Membrane Potential
• Since the concentration of ions are different in intra and extracellular fluids,
therefore membrane has a potential.
Nernst Equation: E = 61 log10
[K]o
[K]i
-90 mv
≈
RT
Goldman Equation: E =
F
Ln
Pk.[K]o + PNa.[Na]o + PCl.[Cl]o
Pk.[K]i + PNa.[Na]i + PCl.[Cl]i
R = Universal Gas const.
T = Absolute Temperature
F = Faraday’s const.
P = Permeability coefficent of the membrane to an ion
-85 mv
Action Potential
• 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 & Permeability
Action Potential
• 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, usually
excites 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, usually
excites 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”.
Conduction Velocity
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
Saltatory Conduction
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.
Because the impulse “jumps” over areas of myelin,
an impulse travels much faster along a myelinated
neuron compared to that in a non-myelinated
neuron.