biopotential generation
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BIOPOTENTIAL GENERATION,
BY
UMAR SHU’AIBU
9/25/2014
Biological systems frequently have electrical activity associated with them.
This activity can be a constant dc electric field, constant flux of charge-
carrying particles , or time-varying electric field. Bioelectric phenomena
are associated with the distribution of ions or charged molecules in a
biological structure and the changes in this distribution resulting from specific
processes. These changes can occur as a result of biochemical reactions.
Biopotential is a compound word, consisting of bio, and potential, bio means life,
or a living matter while potential is the voltage difference between two point.
So literally, bio potential is the voltage difference between two points in living
cell, tissue, organ, or system or organism.
Biopotentials result from the electrochemical activity of a certain class of
cells, known as excitable or active cells which are part of nervous,
muscular or glandular tissue.Within the human body, active cells are
surrounded by body fluids having a high Cl concentration. An active cell
acts as a constant current source when stimulated, and creates an ionic
current within the body fluid. This current induces electrical potentials
within the human body. These biopotentials decrease in amplitude with
increasing distance from the active cell. The detection of biopotentials is
therefore closely related to the detection of the ionic current created by the
active cells.
Electrical activity is explained by differences in ion concentrations within the
body. A potential difference (voltage) occurs between two points with
different ionic concentrations Cell membranes at rest are more permeable
to some ions (e.g. K+, Cl–) than others (e.g. Na+), Na+ ions are pumped
out of the cells, while K+ ions are pumped in
The excitable cell exhibit two types of potential(i.e resting and active
potential) as can be describe below.
The individual excitable cell maintains a steady electrical potential difference
between its internal and external environments. This resting potential of the
internal medium lies in the range 40 to 90 mV, relative to the external
medium. All cells have a membrane that serves as a barrier between the fluid
outside of them and that inside, and to control what types of particles can
go in and out of the cell. Resting membrane potential is the difference in
voltage of the fluids inside a cell and outside a cell, which is usually
between -70 to -80 millivolts (mV). All cells have this difference, but it is
particularly important in relation to nerve and muscle cells, since any
stimulus that changes the voltage and makes it different from the resting
membrane potential is what allows the cells to transmit electrical signals .
This creates a concentration gradient for Na+. As Na+ accumulates on the
outside of the neuron, it tends to leak back in.
Na+ must pass through proteins channels to leak back through the
hydrophobic plasma membrane. These channels restrict the amount of
Na+ that can leak back in.
This maintains a strong positive charge on the outside of the neuron
The K+ inside the neuron also tends to follow its concentration gradient and
leak out of the cell. The protein channels allow K+ to leak out of the cell
more easily. As a result of this movement in Na+ and K+ ions, a net
positive charge builds up outside the neuron and a net negative charge
builds up inside as can be shown below
This difference in charge between the outside and the inside of the
neuron cell is called the Resting Potential. The resting potential
is about –70 mV. When the neuron is at rest, it is polarized
A change in the environment ( pressure, heat, sound, light) is detected by the
receptor and changes the shape of the channel proteins in part of the
neuron .
The Na+ channels open completely and Na+ ions flood into the neuron. The
K+ channel close completely at the same time and K+ ions can no longer
leak out of the neuron in that particular area. This can be shown in the
diagram below
The interior of the neuron in that area becomes positive relative to the outside
of the neuron. This depolarization causes the electrical potential to
change from –70 mV to + 40 mV . The Na+ channels remain open for
about 0.5 milliseconds then they close as the proteins enter an inactive
state. The total change between the resting state (-70 mV) and the peak
positive voltage ( +40mV) is the action potential ( about 110 mV). The
spike in voltage causes the K+ pumps to open completely and K+ ions rush
out of the neuron. The inside becomes negative again. This is
repolarization.
So many K+ ions get out that the charge goes below the resting potential.
While the neuron is in this state it cannot react to additional stimuli.
The Refractory period lasts from 0.5 to 2 milliseconds.
During this time, the Na-K-ATPase pump reestablishes the resting potential.
The sequence of depolarization and repolarization generates a small electrical
current in this localized area. The current affects the nearby protein
channels for Na+ and causes them to open. When the adjacent channels
open, Na+ions flood into that area of the neuron and an action potential
occurs. This in turn will affect the areas next to it and the impulse passes
along the entire neuron. The electric current passes outward over the
membrane in all directions ,but the area to one side is still in the refractory
period and is not sensitive to the current. Therefore the impulse moves
from the dendrites toward the axon. The AP remains constant as it travels
down the neuron. Its amplitude is always the same because it corresponds
to wide open Na+ channels.
The frequency of the AP can change. Below is the diagram showing the flow
of current.
Action Potentials occur maximally or not at all. In other words, there's no
such thing as a partial or weak action potential. Either the threshold
potential is reached and an action potential occurs, or it isn't reached and no
action potential occurs.
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