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Cell Theory

The cell is the basic structural and functional unit of life

Organismal activity depends on individual and collective activity of cells

Biochemical activities of cells are dictated by subcellular structure

Continuity of life has a cellular basis

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Structure of a Generalized Cell

Figure 3.2

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Plasma Membrane

Separates intracellular fluids from extracellular fluids

Plays a dynamic role in cellular activity

Glycocalyx is a glycoprotein area abutting the cell that provides highly specific biological markers by which cells recognize one another

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Fluid Mosaic Model

Double bilayer of lipids with imbedded, dispersed proteins

Bilayer consists of phospholipids, cholesterol, and glycolipids

Glycolipids are lipids with bound carbohydrate

Phospholipids have hydrophobic and hydrophilic bipoles

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Fluid Mosaic Model

Figure 3.3

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Functions of Membrane Proteins


Enzymatic activity

Receptors for signal transduction

Figure 3.4.1

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Functions of Membrane Proteins

Figure 3.4.2

Intercellular adhesion

Cell-cell recognition

Attachment to cytoskeleton and extracellular matrix

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Plasma Membrane Surfaces

Differ in the kind and amount of lipids they contain

Glycolipids are found only in the outer membrane surface

20% of all membrane lipid is cholesterol

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Lipid Rafts

Make up 20% of the outer membrane surface

Composed of sphingolipids and cholesterol

Are concentrating platforms for cell-signaling molecules

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Membrane Junctions

Tight junction – impermeable junction that encircles the cell

Desmosome – anchoring junction scattered along the sides of cells

Gap junction – a nexus that allows chemical substances to pass between cells

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Membrane Junctions: Tight Junction

Figure 3.5a

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Membrane Junctions: Desmosome

Figure 3.5b

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Membrane Junctions: Gap Junction

Figure 3.5c

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Membran transport

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Membrane Potential

Difference in electrical potential across a membrane

Electrical driving force

Force that moves charged molecules across a membrane

Electrochemical driving force

Total driving force acting on ions to move them across the cell membrane

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Cell Membrane Transport

Interstitial fluid Surrounds cells Contained within tissues. Part of the extra-cellular water compartment Derived from blood plasma Contains amino acids, vitamins, hormones, salts, waste

products, etc. Cells must be able to transport materials across the cell

membrane Movement of materials is controlled by the plasma membrane,

that is selectively permeable so that it allows only certain materials pass in and out of the cell. Simple Diffusion Mediated Transport

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Factors Affecting the Direction of Transport

The cell membrane is the permeability barrier for the cell. Impermeable to most water-soluble substances (substances that

dissolve in water) Closely controls passage of materials in and out of the cell.

Passive Transport versus Active Transport Passive Transport

Movement of substances through the membrane without the use of energy from the cell is a physical or passive process.

Does not require ATP Includes simple diffusion, facilitated diffusion, osmosis, and

filtration. Active Transport

Movement of material through the membrane that requires metabolic energy (ATP) is called an active physiological process.

Includes Primary and Secondary Active Transport

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Driving Forces Acting on Molecules

Driving forces affect the direction of movement of molecules


A difference in driving force (chemical or electrical energy) across a cell membrane that tends to push molecules in one direction or another

Always from higher to lower energy if allowed to move spontaneously

There are three types of driving forces

Chemical, Electrical, and Electrochemical

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Driving Forces Acting on Molecules

Chemical driving force

Difference in energy due to a concentration gradient that causes a molecule to move from high to low concentration

Electrical driving force

Difference in energy due to a separation of charge that acts to move ions from high energy to low energy

Electrochemical driving force

Sum of the chemical and electrical driving forces

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Passive Membrane Transport: Diffusion

Simple diffusion – nonpolar and lipid-soluble substances

Diffuse directly through the lipid bilayer

Diffuse through channel proteins

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Passive Membrane Transport: Diffusion

Facilitated diffusion

Transport of glucose, amino acids, and ions

Transported substances bind carrier proteins or pass through protein channels

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Are integral transmembrane proteins

Show specificity for certain polar molecules including sugars and amino acids

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Diffusion Through the Plasma Membrane

Figure 3.7

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Passive Membrane Transport: Filtration

The passage of water and solutes through a membrane by hydrostatic pressure

Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area

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Effects of Solutions of Varying Tonicity

Isotonic – solutions with the same solute concentration as that of the cytosol

Hypertonic – solutions having greater solute concentration than that of the cytosol

Hypotonic – solutions having lesser solute concentration than that of the cytosol

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Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP.





Phosphorylation causes the protein to change its shape.

The shape change expels Na+ to the outside, and extracellular K+ binds.

5 Loss of phosphate restores the original conformation of the pump protein. K+ binding triggers

release of the phosphate group.

6 K+ is released and Na+ sites are ready to bind Na+ again; the cycle repeats.

Concentration gradients of K+ and Na+

Extracellular fluid


Sodium-Potassium Pump

Figure 3.10

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Facilitated Diffusion: Passive Transport Through Membrane Proteins

Particles must be helped through the membrane with the use of transmembrane proteins (carriers/transporters, channels/pores).

Requires a concentration gradient Example

Glucose Important substance that is lipid insoluble and is too

large to pass through membrane pores. Glucose molecules combine with a protein carrier

molecule on the surface of the plasma membrane. The carrier changes shape and releases the glucose inside the cell then returns to its original shape to bring in another glucose on the outside of the membrane.

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Transport Proteins in Facilitated Diffusion


A transmembrane protein that binds to a molecule on one side of the membrane

Conformational change

The carrier “flips” to bring the transported molecule to the other side of the membrane

Transport is limited by the number of carriers available on the membrane.

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Channels or pores

A transmembrane protein that acts as an opening through the membrane

Selective for specific molecules, usually ions such as sodium, potassium, and calcium

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Passive Membrane Transport: Osmosis

Occurs when the concentration of a solvent is different on opposite sides of a membrane

Diffusion of water across a semipermeable membrane

Osmolarity – total concentration of solute particles in a solution

Tonicity – how a solution affects cell volume

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Osmosis: Passive Transport of Water Across Membranes

The flow of water across a selectively permeable membrane

Always from an area of high water concentration to an area of low water concentration.

A special case of diffusion of water across a selectively permeable membrane, such as the plasma membrane.

A semi-permeable membrane is freely permeable to water but not to solutes.

It is a very important process because water is found throughout cells and extra-cellular areas of the body.

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Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.8a

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Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.8b

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Osmosis depends on A concentration gradient for water Relative permeability of dissolved solutes Osmosis occurs when:

There is more water and less solute on one side of the membrane A high concentration of water or a low

concentration of solute And less water and more solute on the other side

A low concentration of water or a high concentration of solute

The concentration gradient is for water

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Osmolarity Total solute concentration Unit is osmole (Osm) or milliosmole (mOsm)

Normal osmolarity (concentration) of body fluids is 300 mOsm

Total solute concentration is 300 milliosmoles per liter

Depends on the total concentration of dissolved solutes

Example 150 mOsm NaCl Dissolved in water the molecule separated into

two particles, so osmolarity is doubled, 300 mOsm

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Comparison of solutions Iso-osmotic

Same concentration Hyper-osmotic

Solution has greater solute concentration than the reference solution

Lower water concentration Hypo-osmotic

Solution has lower solute concentration than the reference solution

Higher water concentration

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Osmotic Pressure (π)

The membrane is selectively permeable in that it does not allow the solute to pass, it is not permeable to certain molecules, particles, or solute.

Remember that high solute concentration means low water concentration (requires more water to reach equilibrium) and low solute concentration means high water concentration (requires water to leave to reach equilibrium).

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Osmotic Pressure (π)

Osmosis will continue to occur or the water will continue to move until:

Equilibrium for water is reached so that the concentration of water and solute is equal on each side of the membrane.

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Osmotic pressure stops the movement of water. Osmotic pressure is the amount of pressure

required to prevent further water movement.

The ability of osmosis to generate enough pressure to lift a volume of water.

A potential pressure due to the presence of non-diffusible solute particles.

The greater the amount of non-diffusible solute, the greater the gradient attracting water across the membrane and the greater the osmotic pressure produced.

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Example NaCl is a very osmotically active particle

because when it dissociates it produces two ions, or double the osmotic activity

Water movement changes the volume of water in the container or cell.

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Tonicity Refers to the relationships between body cells and

the surrounding fluids. A measure of the ability of a solution to cause a

change in cell tone (volume or pressure) by promoting the osmotic flow of water.

Dependent upon concentration and diffusibility of the dissolved solutes Impermeant solutes

Cannot cross cell membrane Permeant solutes

Can move across cell membrane and add to the total solutes within cell

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A solution that has the same concentration of solute (osmotic pressure) as body fluids.

Fluid surrounding a cell has the same concentration of solute as that inside of the cell.

No osmosis occurs.

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A solution that has a lower concentration of solute (osmotic pressure) than body fluids.

Hypotonic extra-cellular fluid has a lower concentration of solute than the concentration inside cell and causes water to move into the cell following its concentration gradient (more water outside, less inside).

Too much water moving into the cell membrane may cause the cell to burst.

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A solution has a higher concentration of solute (osmotic pressure) than the concentration found in body fluids.

Hypertonic extra-cellular fluid will cause water to leave the cell following its concentration gradient (more water inside, less outside) producing a shrunken or crenated cell.

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Isotonic saline solution

A solution that is .9% saline because the body's red blood cells are .9% salt or NaCl.

Therefore, when an isotonic saline solution is introduced into the body, fluid equilibrium will be maintained.


The key to understanding the above terms and process is to understand that hyper and hypo refer to the solute in the solution, not to the water.

Water will move toward the greater amount of solute because the concentration of water there is less.

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Passive Transport: Filtration

Particles forced through a filter or a membrane by hydrostatic pressure. Hydrostatic pressure

Fluid pressure of the blood generated by the left ventricle Opposed by osmotically active particles in the blood (plasma

proteins). Example

Blood pressure generated by the heart and blood vessels forces tissue fluid out of tiny openings in the capillary wall and leaving larger particles of blood cells and protein molecules inside the capillary.

Coffee filters work by the pressure from weight of the water above the coffee grounds forcing the flavored water through the filter and leaving the large particles of coffee grounds on the filter paper.

Filtration and osmosis are the major processes in the capillaries of tissues and the kidney.

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Active Transport Processes

Movement of particles or solutes against a concentration gradient

Requires energy or cellular action with ATP

Primary Active Transport

Direct transport of substances using ATP

Secondary Active Transport

Movement of substances driven by concentration or electrochemical gradients created by Primary Active Transport mechanisms

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Primary Active Transport

Solute pumping Pump or protein carrier

An enzyme-like protein carrier that pumps or carries solutes such as ions of sodium, potassium, and calcium, into or out of the cell against their concentration gradients.

ATPase The enzyme on the protein carrier or pump that

catalyzes the breakdown or phosphorylation of ATP producing energy that drives the pump.

This action may require up to 40% of a cell’s supply of ATP

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Sodium-potassium pump (Na+/K+ ATPase Pump)

Maintains the resting membrane potential of nerve and muscle cells

Sodium Primary extra-cellular ion that is constantly

“leaking” into cells. Potassium

Primary intracellular ion that is constantly “leaking” out of cells.

The sodium/potassium pump constantly pumps 3 sodium ions out and 2 potassium ions into the cell, maintaining the relative negativity inside the cell.

All cells have a negative charge inside because of this mechanism.

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Solute Pumping to Maintain the Membrane Potential


Transport proteins that use energy from ATP hydrolysis to transport specific molecules against the electrochemical gradient across a membrane

Sodium-Potassium pump (Na+/K+ ATPase Pump)

Transports Na+/K+ ions in opposite directions across cell membranes Move 3 Na+ ions out of the cell for every 2 K+ ions into cell Specific for Na+/K+ and unidirectional Phosphorylation of the pump protein causes a conformational change

that turns the binding sites outward to expel Na+ Also decreases affinity for Na+ and increases its affinity for K+ Critical in maintaining resting membrane potential for nerve and

muscle impulse conduction

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Secondary Active Transport

Movement of a molecule that is coupled to the active transport of another molecule

One substance moves down its electrochemical gradient and releases energy in the process

This energy is then used to drive the movement of another substance against its electrochemical gradient

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Cotransport (Symport)

Movement of 2 substances in the same direction


Sodium-linked glucose transport

Couples the inward flow of sodium with the inward flow of glucose

Sodium movement with its electrochemical gradient releases energy that drives the movement of glucose against its concentration gradient

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Countertransport (Antiport or Exchange)

Movement of 2 substances in opposite directions

Example Sodium proton exchange

Couples the inward flow of sodium with the outward flow of protons (H+)

Energy released from the inward flow of sodium along its electrochemical gradient is used to drive the outward flow of protons against its electrochemical gradient

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Pumps and Leaks

Differences in composition of intra- and extra-cellular fluid are maintained by pumps

Substances are constantly, passively leaking across cell membrane in the opposite direction and at the same rate that they are actively pumped across the cell membrane

Net flux across the cell membrane is zero

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Cell to Cell Communication

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Autocrine & Paracrine: local signaling

Endocrine system: distant, diffuse target


Gap junction: local

Nervous system: fast, specific, distant target

Overview of Cell to Cell Communication:

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Gap Junctions and CAMs

Protein channels - connexin

Direct flow to neighbor

Electrical- ions (charge)

Signal chemicals


Need direct surface contact

Signal chemical

Figure 6-1a, b: Direct and local cell-to-cell communication

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Paracrines and Autocrines

Local communication

Signal chemicals diffuse to target

Example: Cytokines

Autocrine–receptor on same cell

Paracrine–neighboring cells

Figure 6-1c: Direct and local cell-to-cell communication

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Signal Chemicals

Made in endocrine cells

Transported via blood

Receptors on target cells

Long Distance Communication: Hormones

Figure 6-2a: Long distance cell-to-cell communication

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Electrical signal down axon

Signal molecule (neurotransmitter) to target cell


Chemical and electrical signals down axon

Hormone transported via blood to target

Long Distance Communication: Neurons and Neurohormones

Figure 6-2 b: Long distance cell-to-cell communication

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Long Distance Communication: Neurons and Neurohormones

Figure 6-2b, c: Long distance cell-to-cell communication

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Signal Pathways

Signal molecule (ligand)


Intracellular signal

Target protein


Figure 6-3: Signal pathways

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Receptor locations

Cytosolic or Nuclear Lipophilic ligand

enters cell Often activates gene Slower response

Cell membrane Lipophobic ligand can't

enter cell Outer surface receptor Fast response Figure 6-4: Target cell receptors

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Ligand- gated channel

Receptor enzymes



Membrane Receptor Classes

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Membrane Receptor Classes

Figure 6-5: Four classes of membrane receptors

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Signal Transduction

Transforms signal energy

Protein kinase

Second messenger

Activate proteins


Bind calcium

Cell response

Figure 6-8: Biological signal transduction

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Signal Amplification

Small signal produces large cell response

Amplification enzyme


Figure 6-7: Signal amplification

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Receptor Enzymes


Activation cytoplasmic

Side enzyme

Example: Tyrosine kinase

Figure 6-10: Tyrosine kinase, an example of a receptor-enzyme

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G-Protein-coupled Receptors

Hundreds of types

Main signal transducers

Activate enzymes

Open ion channels


adenyl cyclase-cAMP

Activates synthesis

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G-Protein-coupled Receptors

Figure 6-11: The G protein-coupled adenylyl cyclase-cAMP system

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Transduction Reviewed

Figure 6-14: Summary of signal transduction systems

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Novel Signal Molecules

Calcium: muscle contraction

Channel opening

Enzyme activation

Vesicle excytosisNitric Oxide (NO)

Paracrine: arterioles

Activates cAMP

Brain neurotransmitter

Carbon monoxide (CO)

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Novel Signal Molecules

Figure 6-15: Calcium as an intracellular messenger

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Voltage (V) – measure of potential energy generated by separated charge

Potential difference – voltage measured between two points

Current (I) – the flow of electrical charge between two points

Resistance (R) – hindrance to charge flow

Insulator – substance with high electrical resistance

Conductor – substance with low electrical resistance

Electricity Definitions

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Reflects the flow of ions rather than electrons

There is a potential on either side of membranes when:

The number of ions is different across the membrane

The membrane provides a resistance to ion flow

Electrical Current and the Body

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Plasma membrane of neurons is filled with channels that allow specific ions to cross.

Ion channels fall into 1 of 2 categories:

Passive or leakage channels – usually open and allow specific ions to pass (i.e., K+).

Gated channels – only open when appropriate signal received.

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Gated channels – only open when appropriate signal received.

Chemically (ligand) gated channels – only open when the appropriate chemical or neurotransmitter present.

Voltage gated channels – only open when the membrane voltage is at an appropriate level.

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Types of plasma membrane ion channels:

Passive, or leakage, channels – always open

Chemically gated channels – open with binding of a specific neurotransmitter

Voltage-gated channels – open and close in response to membrane potential

Mechanically gated channels – open and close in response to physical deformation of receptors

Role of Ion Channels

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Example: Na+-K+ gated channel

Closed when a neurotransmitter is not bound to the extracellular receptor

Na+ cannot enter the cell and K+ cannot exit the cell

Open when a neurotransmitter is attached to the receptor

Na+ enters the cell and K+ exits the cell

Operation of a Gated Channel

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Operation of a Gated Channel

Figure 11.6a

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Example: Na+ channel

Closed when the intracellular environment is negative

Na+ cannot enter the cell

Open when the intracellular environment is positive

Na+ can enter the cell

Operation of a Voltage-Gated Channel

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Operation of a Voltage-Gated Channel

Figure 11.6b

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When gated channels are open:

Ions move quickly across the membrane

Movement is along their electrochemical gradients

An electrical current is created

Voltage changes across the membrane

Gated Channels

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Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration

Ions flow along their electrical gradient when they move toward an area of opposite charge

Electrochemical gradient – the electrical and chemical gradients taken together

Electrochemical Gradient

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If we measure voltage between the inside of a neuron and the outside we find that the neuron is more negative inside than outside with a potential of about –70 mV.

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The potential difference (–70 mV) across the membrane of a resting neuron

It is generated by different concentrations of Na+, K+, Cl, and protein anions (A)

Ionic differences are the consequence of:

Differential permeability of the neurilemma to Na+ and K+

Operation of the sodium-potassium pump

Resting Membrane Potential (Vr)

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The resting membrane potential results from the concentrations of ions that are in & out of the cell and the permeability to those ions.

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Resting Membrane Potential (Vr)

Figure 11.8

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Used to integrate, send, and receive information

Membrane potential changes are produced by:

Changes in membrane permeability to ions

Alterations of ion concentrations across the membrane

Types of signals – graded potentials and action potentials

Membrane Potentials: Signals

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Changes are caused by three events

Depolarization – the inside of the membrane becomes less negative

Repolarization – the membrane returns to its resting membrane potential

Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

Changes in Membrane Potential

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Changes in Membrane Potential

Figure 11.9

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Short-lived, local changes in membrane potential

Decrease in intensity with distance

Their magnitude varies directly with the strength of the stimulus

Sufficiently strong graded potentials can initiate action potentials

Graded Potentials

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Graded Potentials

Figure 11.10

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Graded Potentials

Voltage changes in graded potentials are decremental

Current is quickly dissipated due to the leaky plasma membrane

Can only travel over short distances

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Graded Potentials

Figure 11.11

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A brief reversal of membrane potential with a total amplitude of 100 mV

Action potentials are only generated by muscle cells and neurons

They do not decrease in strength over distance

They are the principal means of neural communication

An action potential in the axon of a neuron is a nerve impulse

Action Potentials (APs)

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Na+ and K+ channels are closed

Leakage accounts for small movements of Na+ and K+

Each Na+ channel has two voltage-regulated gates

Activation gates – closed in the resting state

Inactivation gates – open in the resting state

Action Potential: Resting State

Figure 11.12.1

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Na+ permeability increases; membrane potential reverses

Na+ gates are opened; K+ gates are closed

Threshold – a critical level of depolarization (-55 to -50 mV)

At threshold, depolarization becomes self-generating

Action Potential: Depolarization Phase

Figure 11.12.2

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Sodium inactivation gates close

Membrane permeability to Na+ declines to resting levels

As sodium gates close, voltage-sensitive K+ gates open

K+ exits the cell and internal negativity of the resting neuron is restored

Action Potential: Repolarization Phase

Figure 11.12.3

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Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive

efflux of K+

This efflux causes hyperpolarization of the membrane (undershoot)

The neuron is insensitive to stimulus and depolarization during this time

Figure 11.12.4

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Restores the resting electrical conditions of the neuron

Does not restore the resting ionic conditions

Ionic redistribution back to resting conditions is restored by the sodium-potassium pump

Action Potential: Role of the Sodium-Potassium Pump

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Phases of the Action Potential

1 – resting state

2 – depolarization phase

3 – repolarization phase

4 – hyperpolarization

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Na+ influx causes a patch of the axonal membrane to depolarize

Positive ions in the axoplasm move toward the polarized (negative) portion of the membrane

Sodium gates are shown as closing, open, or closed

Propagation of an Action Potential (Time = 0ms)

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Propagation of an Action Potential (Time = 0ms)

Figure 11.13a

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Ions of the extracellular fluid move toward the area of greatest negative charge

A current is created that depolarizes the adjacent membrane in a forward direction

The impulse propagates away from its point of origin

Propagation of an Action Potential (Time = 1ms)

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Propagation of an Action Potential (Time = 1ms)

Figure 11.13b

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The action potential moves away from the stimulus

Where sodium gates are closing, potassium gates are open and create a current flow

Propagation of an Action Potential (Time = 2ms)

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Propagation of an Action Potential (Time = 2ms)

Figure 11.13c

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Threshold – membrane is depolarized by 15 to 20 mV

Established by the total amount of current flowing through the membrane

Weak (subthreshold) stimuli are not relayed into action potentials

Strong (threshold) stimuli are relayed into action potentials

All-or-none phenomenon – action potentials either happen completely, or not at all

Threshold and Action Potentials

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All action potentials are alike and are independent of stimulus intensity

Strong stimuli can generate an action potential more often than weaker stimuli

The CNS determines stimulus intensity by the frequency of impulse transmission

Coding for Stimulus Intensity

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Coding for Stimulus Intensity

Upward arrows – stimulus applied

Downward arrows – stimulus stopped

Figure 11.14

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Coding for Stimulus Intensity

Length of arrows – strength of stimulus

Action potentials – vertical lines

Figure 11.14

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Time from the opening of the Na+ activation gates until the closing of inactivation gates

The absolute refractory period:

Prevents the neuron from generating an action potential

Ensures that each action potential is separate

Enforces one-way transmission of nerve impulses

Absolute Refractory Period

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Absolute Refractory Period

Figure 11.15

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The interval following the absolute refractory period when:

Sodium gates are closed

Potassium gates are open

Repolarization is occurring

The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events

Relative Refractory Period

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Physiologic Response To Electrical Current

Creating muscle contraction through nerve or muscle stimulation

Stimulating sensory nerves to help in treating pain

Creating an electrical field in biologic tissues to stimulate or alter the healing process

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Physiologic Response To Electrical Current

Creating an electrical field on the skin surface to drive ions beneficial to the healing process into or through the skin

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Physiologic Response To Electrical Current

As electricity moves through the body's conductive medium, changes in the physiologic functioning can occur at various levels Cellular Tissue Segmental Systematic

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Effects at Cellular Level

Excitation of nerve cells Changes in cell membrane permeability Protein synthesis Stimulation of fibrobloast, osteoblast Modification of microcirculation

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Effects at Tissue Level

Skeletal muscle contraction Smooth muscle contraction Tissue regeneration

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Effects at Segmental Level

Modification of joint mobility Muscle pumping action to change

circulation and lymphatic activity Alteration of the microvascular system

not associated with muscle pumping Increased movement of charged proteins

into the lymphatic channels

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Effects at Segmental Level

Transcutaneous electrical stimulation cannot directly stimulate lymph smooth muscle, or the autonomic nervous system without also stimulating a motor nerve

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Systematic Effects

Analgesic effects as endongenous pain suppressors are released and act at different levels to control pain

Analgesic effects from the stimulation of certain neurotransmitters to control neural activity in the presence of pain stimuli

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Physiologic Response To Electrical Current

Effects may be direct or indirect Direct effects occur along lines of current

flow and under electrodes Indirect effects occur remote to area of

current flow and are usually the result of stimulating a natural physiologic event to occur

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Muscle and Nerve Responses

Excitability dependent on cell membrane's voltage sensitive permeability

Produces unequal distribution of charged ions on each side of the membranecreates a potential difference between the charge of the interior of cell and exterior of cell

Potential difference is known as resting potential because cell tries to maintain electrochemical gradient as its normal homeostatic environment

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Muscle and Nerve Responses

Using active transport mechanism-cell continually moves Na+ from inside cell to outside and balances this positive charge movement by moving K+ to the inside Produces an electrical gradient with + charges outside

and - charges inside

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Nerve Depolarization

To create transmission of an impulse in nerve, resting membrane potential must be reduced below threshold level

Changes in membrane's permeability may then occur creating an action potential that propagates impulse along nerve in both directions causing depolarization of membrane

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Nerve Depolarization

Stimulus must have adequate intensity and last long enough to equal or exceed membrane's basic threshold for excitation

Stimulus must alter membrane so that a number of ions are pushed across membrane exceeding ability of the active transport pumps to maintain the resting potentials thus forcing membrane to depolarize resulting in an action potential

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Depolarization Propagation

Difference in electrical potential between depolarized region and neighboring inactive regions causes the current to flow from depolarized region intercellular material to the inactive membrane

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Depolarization Propagation

Current also flows through extracellular materials, back to the depolarized area, and finally into cell again

Makes depolarization self propagating as process is repeated all along fiber in each direction from depolarization site.

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Depolarization Effects

As nerve impulse reaches effector organ or another nerve cell, impulse is transferred between the two at a motor end plate or a synapse

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Depolarization Effects

At this junction, a transmitter substance is released from nerve

Transmitter substance causes the other excitable tissue to discharge causing a twitch muscle contraction

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Strength - Duration Curves

Represents The Threshold for Depolarization of a Nerve Fiber

Muscle and nerve respond in an all-or-none fashion and there is no gradation of response

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Strength - Duration Curves

Shape of the curve relates intensity of electrical stimulus (strength) and length of time (duration) necessary to cause the tissue to depolarize

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Strength - Duration Curves

Rheobase describes minimum intensity of current necessary to cause tissue excitation when applied for a maximum duration

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Strength - Duration Curves

Chronaxie describes length of time (duration) required for a current of twice the intensity of the rheobase current to produce tissue excitation