Download 05 Bioelectrical phenomena in nervous cells

Bioelectrical phenomena
in nervous cells
Measurement of the membrane potential of
the nerve fiber using a microelectrode
Basic Concepts
Forces that determine ionic movement
Electrostatic forces
Opposite charges attract
Identical charges repel
Concentration forces
Diffusion – movement of ions through
semipermeable membrane
Osmosis – movement of water from region of
high concentration to low
Basic mechanisms of transport
 Gating
of
protein
channels
provides a
means of
controlling ion
permeability of
the channels.
I. Membrane Resting Potential
A constant potential difference across the resting cell
membrane
Cell’s ability to fire an action potential is due to the
cell’s ability to maintain the cellular resting potential
at approximately –70 mV (-.07 volt)
The basic signaling properties of neurons are
determined by changes in the resting potential
Concept of Resting Potential (RP)


A potential difference across the cell
membrane at the rest stage or when the
cell is not stimulated.
Property:
 It is constant or stable
 It is negative inside relative to the outside
 Resting potentials are different in
different cells.
Resting Membrane Potential
Na+ and Cl- are more concentrated outside the
cell
K+ and organic anions (organic acids and
proteins) are more concentrated inside.
Active Transport



Movement of molecules and ions against
their concentration gradients.
– From lower to higher concentrations.
Requires ATP.
2 Types of Active Transport:
– Primary
– Secondary
Primary Active Transport




ATP directly required for
the function of the
carriers.
Molecule or ion binds to
carrier site.
Binding stimulates
phosphorylation
(breakdown of ATP).
Conformational change
moves molecule to other
side of membrane.
1. Three sodium ions (Na+) and adenosine
triphosphate (ATP) bind to the carrier protein.
Extracellular fluid
+
Cytoplasm Na
Carrier protein
1
ATP
ATP binding site
Na+ 3
2. The ATP breaks down to adenosine
diphosphate (ADP) and a phosphate (P) and
releases energy.
3. The carrier protein changes shape, and the
Na+ are transported across the membrane.
K+
P
Breakdown of ATP
Carrier protein changes 2
shape (requires energy) ADP (releases energy)
4
4. The Na+ diffuse away from the carrier protein.
K+ 5
Na+
5. Two potassium ions (K+) bind to the carrier
protein.
6. The phosphate is released.
7. The carrier protein changes shape,
transporting K+ across the membrane, and the
K+ diffuse away from the carrier protein. The
carrier protein can again bind to Na+ and ATP.
6
P
Carrier protein resumes
original shape
7
K+
Intracellular vs extracellular ion concentrations
Ion
Intracellular
Extracellular
Na+
K+
Mg2+
Ca2+
H+
5-15 mM
140 mM
0.5 mM
10-7 mM
10-7.2 M (pH 7.2)
145 mM
5 mM
1-2 mM
1-2 mM
10-7.4 M (pH 7.4)
Cl-
5-15 mM
110 mM
Resting Membrane Potential
 Potassium ions, concentrated inside the cell tend to move
outward down their concentration gradient through
nongated potassium channels
 But the relative excess of negative charge inside the
membrane tend to push potassium ions out of the cell
Resting Membrane
Potential
Na+ is more concentrated
outside than inside and
therefore tends to flow into the
cell down its concentration
gradient
Na+ is driven into the cell
by the electrical potential
difference across the
membrane.
• But what about sodium?
• Electrostatic and Chemical forces act together on
Na+ ions to drive them into the cell
• The Na+ channel close during the resting state
Resting Potential
Postulated mechanism of the
sodium-potassium pump
The formation of resting potential
depends on:



Concentration difference of K+ across
the membrane
Permeability of Na+ and K+ during the
resting state
Na+-K+ pump
Factors that affect resting
potential



Difference of K+ ion concentration
across the membrane
Permeability of the membrane to Na+
and K+.
Action of Na+ pump
Basic Electrophysiological
Terms I:



Polarization: a state in which membrane is
polarized at rest, negative inside and positive
outside.
Depolarization: the membrane potential
becomes less negative than the resting potential
(close to zero).
Hyperpolarization: the membrane potential is
more negative than the resting level.
Basic Electrophysiological Terms I:

Reverspolarization: a reversal of membrane
potential polarity.
 The inside of a cell becomes positive relative to the
outside.

Repolarization: restoration of normal polarization
state of membrane.
 a process in which the membrane potential returns
toward from depolarized level to the normal resting
membrane value.
Effect of stimuli of increasing
voltages to elicit an action potential
Typical action potential
II Action Potential
Successive Stages:
(2)
(1) Resting Stage
(3)
(2) Depolarization stage
(1)
(4)
(3) Repolarization stage
(4) After-potential stage
The Action Potential

Components/characteristics
– RMP
– Depolarizing stimulus
– Threshold
– Rapid Na+ entry
(depolarization)
– Isopotential
– Overshoot
– Repolarization (K+
moves out)
– Undershoot (afterhyperpolarization)
– Absolute refractory
period
– Relative refractory
period

Propagation of
action
potentials in
both directions
along a
conductive fiber
Saltatory conduction along a myelinated axon.
Flow of electrical current from node to node is
illustrated by the arrows.