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.