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Biomedical Physiology - Coggle Diagram
Biomedical Physiology
Lecture 1
Cell Membrane Potential
Learning Objectives
LO1:
Describe how electrical and chemical forces affect solute movement and apply your understanding to decide which way solutes will move across a membrane at a given membrane potential in a normal mammalian cell
Ions in and around a normal mammalian cell
as a result of the
selective movement of ions across cell membranes
, the
ICF
and
ECF
have different
concentrations
of particular ions
High in
ECF
Na+
Cl-
Ca2+
High in
ICF
K+
Cell Membrane Potential (Vm)
ICF
and
ECF
have different solute concentration
unequal distribution of ion charges inside and outside of cells
Membrane Potential (Vm)
difference in charge (potential difference) between the inside and outside of a cell at any point in time
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Cell Membrane Currents
each ion has a concentration gradient across the cell membrane
in the presence of open ion channels or carriers, ions will move across the membrane
down
their concentration gradient
if ions move from one side of the membrane to the other, their charge moves with them
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Forces driving ion currents in solution
movement of ions (and therefore current flow) in solutions is determined by two forces
Chemical Gradients
ions move from
high concentration to low concentration
The chemical gradient an only be worked out for a singe solute
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Electrical Gradient
opposite charges
attract
, like charges
repel
Electrical gradient depends on interaction between charge on the ion (
valance
) and charge on the inside of the cell (
membrane potential, Vm
)
Electrochemical Gradient
overall force on an ion due to combination of
chemical
and
electrical
driving forces
to determine the net movement of a particular ion at a particular membrane potential, we need to know its
equilibrium potential (Ex)
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LO2:
Determine how changes in membrane permeability will alter the membrane potential in a normal mammalian cell
If a cell membrane is only permeable to one type of ion
assuming channels are open, net ion movement will continue until that ion is at equilibrium, after which there will be no net ion movement
Vm will be equal to the equilibrium potential for that ion
this situation never happens in a real cell
If a cell membrane is permeable to more than one type of ion
Different ions have a 'tug-of-war' over Vm
Vm will approach the equilibrium potential of the ion that has the highest permeability (greatest number of open ion channels)
The Goldman Equation
The Goldman equation essentially 'adds together' the Nernst equation for lots of different ions to get a numerical value for the membrane potential (Vm)
Accounts for
Concentration gradients of all ions
across the membrane ([x]i, [x]o)
differing
permeability
of membrane to these ions (number of open ion channels, Px)
LO3:
Describe the ion movements and driving forces that give rise to the resting membrane potential (RMP) of cells
Resting membrane potential
Resting membrane potential
the overall voltage across the cell membrane when the cell is not transmitting an electrical signal
In most cells Vm stays constant for long periods of time at a value called the
resting membrane potential
RMP is observed in all cells
excitable
non-excitable
RMP is usually
negative inside
relative to outside
Negative RMP is because at rest the membrane is most
permeable to K+ ions
Vm is dominated by
K+ leak channels
at rest,
25x more
open K+ channels than Na+ channels
compared to other ions, there is more movement of K+ across the membrane, so RMP is close to the
negative K+ equilibrium potential (-90mV)
solutes other than K+ and Na+ don't contribute much to the RMP because:
Proteins can't cross the cell membrane
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Ca2+ concentration gradient is relatively weak
Cl- equilibrium potential is close to normal RMP of cells, so little net movement
concentration gradient for K+ is
slightly bigger
than for Na+
RMP is usually ~ -70mV
approaching Ek (-90mV)
overall;
much more K+ efflux than Na+ influx down the chemical gradient, due to high number of open K+ leak channels
Electrochemical Gradients acting at RMP
Large K+ efflux (down chemical gradient) leaves a
net negative charge
on the inside of the cell
This creates an electrochemical gradient for
both Na+ and K+
influx
K+ continues to leave the cell down its chemical gradient, creating an increasingly strong electrical gradient for Na+ and K+ to move into the cell
at a particular value of Vm, K+ efflux (chemical gradient) will be roughly opposite and equal to Na+ influx (chemical and electrical gradient) and K+ influx (electrical gradient)
The membrane will be relatively stable at this value of Vm, called the
resting membrane potential (RMP)
LO4:
Outline how the Na+/K+ ATPase pump works and explain how it is important in maintaining stable fluid composition and membrane potential in cells
Maintaining the status quo:
membrane pumps
at RMP, electrochemical forces lead to a
net efflux of K+
and a
net influx of Na+
over time, this could lead to
a change in the composition of the ICF and ECF
run down of the cell membrane potential
can't solve this problem with more channels - channels can only move ions
with the electrochemical gradient
need pumps, which use energy to move ions
against their electrochemical gradient
Membrane Pumps
carriers with special enzymatic activity that can use energy from ATP hydrolysis to move ions against their electrochemical gradient
RMP is stabilized by
active transport
Passive transport
ions move
down electrochemical gradient
(channels/carriers)
Active Transport
ions move
against electrochemical gradient using energy
from hydrolysis of ATP (pumps)
all cells have
Na+/K+ ATPase pumps
use energy from ATP to move Na+ and K+
against electrochemical gradient
Introduction to the
Nervous System
The nervous system acts to
sense
,
integrate
and
respond
to changes in the body's
environment
The nervous system consists of billions of
neurons
organised into
networks
The neuron is the main
signaling
unit of the nervous system
Neuron signaling involves
generation
and
propagation
of
electrical signals
Electrical signals
require the
presence
or
movement
of
charge
Charge is a
physical property
of matter
There are two types of charges
Positive +
Negative -
Biological Electrical signals are carried by
ions
Cations
Anions
Movement of ions is restricted by the
Cell Membrane
Separates
Intracellular fluid (ICF)
from
extracellular fluid (ECF)
Extracellular Fluid
plasma + interstitial fluid
Phospholipid bilayer
with proteins and lipids inserted into it
ions cannot diffuse across the phospholipid bilayer
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The Nervous system is a complex
electrical circuit
Neurons process information in the form of
electrical events occurring across their cell membranes
Potential Difference
measured in volts (V)
Current
Net movement of charge between two places
Measured in Amperes (amps, I)
Current will flow if two places with a potential difference are connected by a conductor
