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Cell
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
Structure of a Generalized Cell
Figure 3.2
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
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
Fluid Mosaic Model
Figure 3.3
Functions of Membrane Proteins
Transport
Enzymatic activity
Receptors for signal transduction
Figure 3.4.1
Functions of Membrane Proteins
Figure 3.4.2
Intercellular adhesion
Cell-cell recognition
Attachment to cytoskeleton and extracellular matrix
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
Lipid Rafts
Make up 20% of the outer membrane surface
Composed of sphingolipids and cholesterol
Are concentrating platforms for cell-signaling molecules
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
Membrane Junctions: Tight Junction
Figure 3.5a
Membrane Junctions: Desmosome
Figure 3.5b
Membrane Junctions: Gap Junction
Figure 3.5c
Membran transport
Definitions
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
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
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
Driving Forces Acting on Molecules
Driving forces affect the direction of movement of molecules
Gradient
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
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
Passive Membrane Transport: Diffusion
Simple diffusion – nonpolar and lipid-soluble substances
Diffuse directly through the lipid bilayer
Diffuse through channel proteins
Passive Membrane Transport: Diffusion
Facilitated diffusion
Transport of glucose, amino acids, and ions
Transported substances bind carrier proteins or pass through protein channels
Carriers
Are integral transmembrane proteins
Show specificity for certain polar molecules including sugars and amino acids
Diffusion Through the Plasma Membrane
Figure 3.7
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
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
Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP.
1
2
3
4
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
Cytoplasm
Sodium-Potassium Pump
Figure 3.10
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.
Transport Proteins in Facilitated Diffusion
Carriers
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.
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
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
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.
Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8a
Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8b
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
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
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
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).
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.
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.
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.
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
Isotonic
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.
Hypotonic
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.
Hypertonic
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.
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.
Remember:
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.
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.
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
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
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.
Solute Pumping to Maintain the Membrane Potential
Pumps
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
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
Cotransport (Symport)
Movement of 2 substances in the same direction
Example
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
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
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
Cell to Cell Communication
Chemical
Autocrine & Paracrine: local signaling
Endocrine system: distant, diffuse target
Electrical
Gap junction: local
Nervous system: fast, specific, distant target
Overview of Cell to Cell Communication:
Gap Junctions and CAMs
Protein channels - connexin
Direct flow to neighbor
Electrical- ions (charge)
Signal chemicals
CAMs
Need direct surface contact
Signal chemical
Figure 6-1a, b: Direct and local cell-to-cell communication
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
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
Neurons
Electrical signal down axon
Signal molecule (neurotransmitter) to target cell
Neurohormones
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
Long Distance Communication: Neurons and Neurohormones
Figure 6-2b, c: Long distance cell-to-cell communication
Signal Pathways
Signal molecule (ligand)
Receptor
Intracellular signal
Target protein
Response
Figure 6-3: Signal pathways
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
Ligand- gated channel
Receptor enzymes
G-protein-coupled
Integrin
Membrane Receptor Classes
Membrane Receptor Classes
Figure 6-5: Four classes of membrane receptors
Signal Transduction
Transforms signal energy
Protein kinase
Second messenger
Activate proteins
Phosporylation
Bind calcium
Cell response
Figure 6-8: Biological signal transduction
Signal Amplification
Small signal produces large cell response
Amplification enzyme
Cascade
Figure 6-7: Signal amplification
Receptor Enzymes
Transduction
Activation cytoplasmic
Side enzyme
Example: Tyrosine kinase
Figure 6-10: Tyrosine kinase, an example of a receptor-enzyme
G-Protein-coupled Receptors
Hundreds of types
Main signal transducers
Activate enzymes
Open ion channels
Amplify:
adenyl cyclase-cAMP
Activates synthesis
G-Protein-coupled Receptors
Figure 6-11: The G protein-coupled adenylyl cyclase-cAMP system
Transduction Reviewed
Figure 6-14: Summary of signal transduction systems
Novel Signal Molecules
Calcium: muscle contraction
Channel opening
Enzyme activation
Vesicle excytosisNitric Oxide (NO)
Paracrine: arterioles
Activates cAMP
Brain neurotransmitter
Carbon monoxide (CO)
Novel Signal Molecules
Figure 6-15: Calcium as an intracellular messenger
Bioelectrics
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
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
THE ROLE OF MEMBRANE ION CHANNELS
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.
THE ROLE OF MEMBRANE ION CHANNELS
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.
TYPES OF ION CHANNELS
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
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
Operation of a Gated Channel
Figure 11.6a
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
Operation of a Voltage-Gated Channel
Figure 11.6b
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
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
THE RESTING MEMBRANE POTENTIAL
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.
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)
SOURCE OF THE RESTING MEMBRANE POTENTIAL
The resting membrane potential results from the concentrations of ions that are in & out of the cell and the permeability to those ions.
Resting Membrane Potential (Vr)
Figure 11.8
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
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
Changes in Membrane Potential
Figure 11.9
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
Graded Potentials
Figure 11.10
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
Graded Potentials
Figure 11.11
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)
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
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
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
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
Repolarization
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
Phases of the Action Potential
1 – resting state
2 – depolarization phase
3 – repolarization phase
4 – hyperpolarization
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)
Propagation of an Action Potential (Time = 0ms)
Figure 11.13a
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)
Propagation of an Action Potential (Time = 1ms)
Figure 11.13b
THE PROPAGATION OF AN
ACTION POTENTIAL
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)
Propagation of an Action Potential (Time = 2ms)
Figure 11.13c
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
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
Coding for Stimulus Intensity
Upward arrows – stimulus applied
Downward arrows – stimulus stopped
Figure 11.14
Coding for Stimulus Intensity
Length of arrows – strength of stimulus
Action potentials – vertical lines
Figure 11.14
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
Absolute Refractory Period
Figure 11.15
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
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
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
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
Effects at Cellular Level
Excitation of nerve cells Changes in cell membrane permeability Protein synthesis Stimulation of fibrobloast, osteoblast Modification of microcirculation
Effects at Tissue Level
Skeletal muscle contraction Smooth muscle contraction Tissue regeneration
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
Effects at Segmental Level
Transcutaneous electrical stimulation cannot directly stimulate lymph smooth muscle, or the autonomic nervous system without also stimulating a motor nerve
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
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
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
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
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
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
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
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.
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
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
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
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
Strength - Duration Curves
Rheobase describes minimum intensity of current necessary to cause tissue excitation when applied for a maximum duration
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