Chapter 3 Flashcards
Resting membrane potential
the potential energy residing in a non-conducting membrane
- Typical value for excitable cells= -75 to -85 mV
- Inside of the cell is negative with respect to the outside
voltage
a measure of the quantity of charges in a given location referenced to another location
current
When two areas of different charge density are connected by a conductor, current will flow from high charge density to low charge density. **Unit=ampere (Amp) **I=E/R I=current E=voltage R=Resistance
resistance
property of a conductor due to which it opposes the flow of current through it; hindrance to charge flow.
**Unit=ohm
diffusion potential
develops as a result of the unequal diffusion of ions in opposite directions
electrogenic pump
a membrane pump that moves unequal quantities of like charges in opposite directions across the membrane; participates in the membrane potential
electroneutral pump
moves equal quantities of like charges in opposite directions across a membrane
electrochemical equilibrium
if the work required to move an ion against a chemical gradient is equal and opposite to the work required to move the ion against an electrical gradient, then that ion is said to be in electrochemical equilibrium.
Nernst Equation
used to calculate hypothetical transmembrane voltages or potentials that would have to exist to offset the tendency for any given ion to move due to a concentration gradient.
work
a force does work and results in movement
potential
difference in charge density between two locations
-unit of measure for potential=volt; potential difference that causes one coulomb to do one joule of work.
Excitable membranes
specialized for transmitting “electrical messages” from one part of a cell to another.
- *Nerve cells
- *Muscle cells
Action potential
changes in transmembrane potential that occur sequentially along the surface of an excitable cell.
- most action potentials in neurons originate in the cell body and move in one direction through the remainder of the cell.
- Action potentials in skeletal muscle cells originate at the neuromuscular junction and move away from it in all directions.
propagation
the “movement” of an electrical impulse during action potential.
Synapse
a structure that allows neurons to communicate with other neurons.
-electrochemical alterations at this site insures that an electrical change in one cell is properly communicated to the next cell.
The main reason that neuronal “messages” leave the central nervous system is to evoke activity in…..?
Effectors, i.e., smooth muscle, cardiac muscle, glands, or skeletal muscle.
Neuromuscular Junction
Translates activity of neuron into physical work
Common features of neuromuscular junction
- release of chemical transmitter presynaptically
- diffusion of transmitter across synapse
- attachment of transmitter to receptor on postsynaptic membrane
- creation of new potential in postsynaptic membrane
Receptors
specialized cells that are capable of tranducing chemical, mechanical, thermal, electromagnetic, or electrical energy to the language of the nervous system (current flow and AP)
Reflexive response
reception of environmental perturbations gives rise to stereotyyped motor response
Work
W=FX
F= force (electromotive force or voltage); measure of the quantity of charges in a given location referenced to another location
X=distance
Unit of force= volt (mV)
Conductor
- resistance to current flow is low
* Extracellular & Intracellular fluids
Insulator
- Resistance to current flow is high
* Lipid molecules
Conductance
Movement of ions through membranes
- Reciprocal of resistance
- Unit= mho (opposite of resistance, ohm)
Measurement of transmembrane potentials
- heated glass pipets filled with good conductor (KCl)
- Electrode passed through cell membrane and connected to volmeter
- Interior of cell is then referenced to exterior
Diffusion potential
develops as a result of unequal diffusion of ions in different directions across a membrane
Diffusion potential results from
- Sodium/ Potassium concentrations
- K higher con. inside, Na higher con. outside
- K will efflux, Na influx - Membrane Permeability
- 50-70 x more permeable to K than to Na; more K ions efflux than Na ions influx - Non-diffusible anions
- Large proteins that can’t diffuse out of the cell are negatively charged
Since the inside of the membrane is negatively charged, what must be done to pump Cl- into the cell & K+ and Na+ out of the cell
Work must be done to move against an electrical gradient. -We=ZiFEm Zi= valence and charge of ion F= Faraday's constant Em= transmembrane potential
Formula for work that must be done to move against a concentration (chemical) gradient
Wc= RT (logc [ion]i - logc [ion]o
R=gas constant
T=absolute temperature
**since Na is more concentrated outside the cell, work must be done for it to efflux
Electrochemical equilibrium
Happens when work required to move an ion against a chemical gradient is equal and opposite to work required to move it against an electrical gradient.
- *We + Wc=0
- this indicates that there is no net force acting on that ion and if the membrane were to become more permeable to that ion, the net flux would be zero
The Nernst Equation
Can use this equation to calculate hypothetical transmembrane potentials that would have to exist to offset the tendency for any given ion to move due to a concentration gradient.
- this is done by setting the sum of We + Wc to zero and solving for the theoretical transmembrane potential, Em.
- Substitute in this equation the actual measured concentrations for a given ion.
Nernst Equation valuees for K+, Na+, Cl-
- K+= -97 mV
* there is a net force operating on K+ and if the permeability of the membrane were to increase to K+ alone it would efflux - Na+= +65 mV
* Indicated a strong tendency for sodium to influx (both chemical and electrical gradients) - Cl-= -90 mV
* close to normal Em
* indicated that chloride is passively distributed in resting cell
* Sudden increase in permeability would not result in much Cl- movement
