Bioelectricity

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Bioelectricity

Definition

Bioelectricity refers to electrical potentials and currents occurring within or produced by living organisms. It results from the conversion of chemical energy into electrical energy. Bioelectric potentials are generated by a number of different biological processes, and are used by cells to govern metabolism, to conduct impulses along nerve fibers, and to regulate muscular contraction. In most organisms bioelectric potentials vary in strength from one to several hundred millivolts. The most important difference between bioelectric currents in living organisms and the type of electric current used to produce light, heat, or power is that a bioelectrical current is a flow of ions (atoms or molecules carrying an electric charge), while standard electricity is a movement of electrons.

Description

Historical background

Prior to the eighteenth century, European physicians and philosophers generally believed that nervous impulses were conducted to the brain via an organic fluid of some kind. The experiments of two Italians, the physician Luigi Galvani and the physicist Alessandro Volta, demonstrated that the true explanation of nervous conduction is bioelectricity. Impulses within the nervous system are carried by electricity generated directly by organic tissue.

In the nineteenth century, such researchers as Emil du Bois-Reymond invented and refined instruments that were capable of measuring the very small electrical potentials and currents generated by living tissue. One of du Bois-Reymond's students, a German scientist named Julius Bernstein, is generally credited with the hypothesis that nerve and muscle fibers are normally polarized, with positive ions on the outside and negative ions on the inside; and that the current that can be measured results from the reversal of this polarization. In the early part of the twentieth century, several British scientists identified the chemical substances involved in the transfer of information between the nerves and muscles.

Cell membrane potential

Bioelectricity begins with the fact that all animal cells have electrical properties derived from the ability of the cell membrane to maintain unequal charges inside and outside the cell. The cell membrane is semipermeable, which means that it forms a selective barrier to ions, which are electrically charged atoms or atom groups. The semipermeability of the cell membrane allows the cell to maintain concentrations of ions in the cytosol (the fluid portion of cell cytoplasm) that differ from those in the fluid outside the cell. Potassium and chloride ions can diffuse through the membrane relatively easily, while sodium ions cannot diffuse into the cell at all.

Because of the semipermeability of the cell membrane, the concentration of sodium in the fluid outside the cell is higher than in the cytosol; the concentration of potassium is higher inside the cell than outside, and the concentration of chloride is higher outside the cell than inside. There are thus two forms of energy stored across the cell membrane—a chemical force (the differences in ion concentration) and an electrical force. This bioelectric potential across the cell membrane is called the resting potential. In most cells the resting potential is about 50 millivolts.

Diffusion

The most important ions in bioelectrical phenomena are sodium (Na⁺, potassium (K⁺, calcium (Ca₂⁺, and chloride (Cl⁻). The first three types of ions carry a positive charge while the chloride ion carries a negative charge.

Ions can move across the cell membrane in two ways. First, they can move through pores called ion channels. Most ion channels are specific to a particular ion or group of ions. In addition, most ion channels are gated, which means that they require a stimulus to open them. Because ions move passively through the channels, the only direction they can travel via channels is from areas of high concentration to areas of low concentration. This movement from areas of higher to areas of lower concentration is called diffusion.

Active transport

A second kind of transport, which moves ions across the cell membrane against the electrochemical gradient is called active transport. Active transport involves an ion pump, which is sometimes called a sodium/potassium pump. Ion pumps differ from ion channels in that the pumps require energy to move the ions. The energy is derived from adenosine triphosphate, or ATP, which is a nucleotide that is the primary source of energy in all living cells. The sodium/ potassium pump controls the volume of the cell and creates the electrical potential across the cell membrane. For example, the concentration of Na⁺ is approximately 10 times higher outside the cell compared to the inside, and the concentration of K⁺ is about 20 times higher on the inside of the cell. This difference is maintained by the action of the cell's ion pumps, which pump three sodium ions outside the cell for every two potassium ions that are pumped inside, consuming one molecule of ATP in the process. Because ions are charged molecules, a difference in chemical concentration establishes a difference in electrical charge as well. The ion channels and the ion pump work together to maintain this charge difference across the cellular membrane.

Bioelectrical functions

Synapses and synaptic transmission

A neuron, or nerve cell, consists of dendrites (receiving portions), a cell body, an axon, and the axon terminal. The axon is a long appendage that conducts information in the form of action potentials away from the cell body. The site of contact between two neurons is called a synapse. The presynaptic neuron releases a chemical called a neurotransmitter into the synaptic cleft between the two neurons. The neurotransmitter passes on information to the postsynaptic neuron. Although most forms of communication between neurons are mediated by chemicals, some neurons also transmit information by direct electrical communication. Neurons may connect to other neurons, to muscles, or to receptor cells in the skin and other sensory organs.

Chemical or electrochemical stimulation of a neuron results in a temporary change in the permeability of the cell membrane. The membrane becomes more permeable to sodium and potassium ions. The sodium ions enter the cell because of their concentration and electrical gradient, while the potassium ions leave the cell because of their chemical gradient. The result is a depolarization (loss of electrical charge) of the cell. The nerve impulse, or action potential, can be defined as a localized region of depolarization that travels down the nerve fiber with the membrane potential being immediately restored behind it.

Transmission of nerve impulses to muscle

Muscle contraction is the end result of a process similar to the transmission of action potentials from one neuron to another. The neurotransmitter that is released from the presynaptic neuron is a chemical called acetylcholine. The postsynaptic cells on the muscle cell membrane receive the acetylcholine, which increases the permeability of the muscle cell membrane to sodium and potassium ions. As the sodium ions enter the cell, the potassium ions leave, producing a net depolarization of the cell membrane. This electrical signal travels along the muscle fibers. The muscle action potential is conveyed through the movement of calcium ions into actual muscle contraction through the interaction of two types of proteins, actin and myosin.

Role in human health

Bioelectricity is one of the fundamental forms of energy in the human body. In the form of moving action potentials, it is the basis for such central bodily functions as conduction of motor, autonomic, or sensory messages along the nerves; muscle contraction; and brain function. Specifically, motor nerve signals result in muscle contractions. Autonomic nervous signals control such basic functions of the body as breathing and heartbeat. Sensory nerve signals collect input from the outside world, including warnings of damage to the body in the form of pain.

Bioelectrical signals in humans

There are three types of electrical signals in human beings, two of which are routinely monitored or analyzed for diagnostic purposes. The first is the electroencephalogram, which is a relatively weak, fluctuating signal that originates in the brain. The second is the electrocardiogram, which is about 100 times stronger than the electroencephalogram, and is produced by the contractions of the heart muscle. The third type of electrical signal in humans, the surface electrical potential, is about as strong as the electrocardiogram but changes more slowly over time. The origin and significance of the surface electrical potential in humans are not yet known.

Common diseases and disorders

A large number of diseases and disorders are related to disturbances of the bioelectrical system. These conditions can be classified according to the component of the nerve cell/muscle cell group, or motor unit, that is affected. The motor unit can be divided into the motor neuron, the nerve root (paired bundles of nerves coming from the spinal cord ), the nerve plexus (bundles of nerves further removed from the spinal cord), the peripheral nerve, the neuro-muscular junction, and the muscle fiber. Defects in any of these components may disrupt bioelectrical signals.

Defects in the motor neuron can be inherited, such as spinal muscular atrophies; or acquired, such as poliomyelitis or amyotrophic lateral sclerosis (Lou Gehrig's disease). Nerve root problems may result from herniated disks in the spine, metastatic cancer, neurofibroma, or trauma. Diseases of the plexus include acute brachial neuritis (inflammation of the nerves of the arm), damage caused by diabetes mellitus, blood clots, metastatic cancer, and trauma.

The peripheral nerves may be damaged through hereditary, infectious, inflammatory, and metabolic causes. Examples of some hereditary conditions include hereditary motor and sensory neuropathy (HMSN) and some autonomic neuropathies. Infection with diphtheria, herpes, HIV, leprosy, and Lyme disease can all cause types of peripheral neuropathies. Some inflammatory causes of peripheral neuropathy include chronic inflammatory demyelinating polyneuropathy (CIDP), Guillain-Barré syndrome, and vasculitis. Metabolic causes of peripheral nerve damage include amyloidosis, diabetes mellitus, dysproteinemic neuropathy, excessive ethanol intake (alcoholic neuropathy), and renal failure.

Disorders of the neuromuscular junction can result from botulism (severe food poisoning caused by ingesting the neurotoxin made by Clostridium botulinum), congenitial myasthenic syndrome, Eaton-Lambert syndrome, myasthenia gravis, and toxic neuromuscular junction disorders.

Muscle fiber problems can be divided into dystrophies, channelopathies, and congenital, endocrine, and metabolic defects. Dystrophies are diseases characterized by progressive muscular weakness. Channelopathies are diseases caused by defects in the ion channels that control the membrane conduction system. These include familial periodic paralysis and Thomsen's disease. Central core disease, centronuclear myopathy, and nemaline myopathy are three examples of congenital muscle fiber damage. Endocrine disorders that may disrupt electrical signals to muscle tissue include acromegaly, Cushing's syndrome, hypothyroidism, and thyrotoxic myopathy. Metabolic causes of muscle fiber damage include glycogen storage disease and lipid storage disease.

KEY TERMS

Acetylcholine— A short-acting neurotransmitter involved in the process of muscle contraction.

Action potential— The change in electrical potential that occurs between the inside and outside of a nerve or muscle fiber when it is stimulated.

Active transport— The movement of ions across the cellular membrane against the electrochemical gradient by means of the sodium/potassium pump.

Adenosine triphosphate (ATP)— A nucleotide that is the primary source of energy in living tissue. It fuels the sodium/potassium pump.

Axon— The appendage of a neuron that transmits impulses away from the cell body.

Cytosol— The fluid portion of cell protoplasm.

Depolarization— Movement of the resting potential of a cell membrane back toward zero.

Diffusion— The movement of ions across the cell membrane from areas of higher concentration to areas of lower concentration.

Ion— An atom or atom group that acquires an electrical charge by the loss or gain of electrons.

Ion channel— A passive means of moving ions from one side of the cell membrane to another by diffusion.

Myosin— A protein found in muscle tissue that interacts with another protein called actin during muscle contraction.

Neurotransmitter— A chemical released by nerve endings that helps to transmit information from one nerve cell to another nerve cell, or from a nerve to a muscle.

Resting potential— A steady-state condition with no net flow of ions across the cell membrane. The resting potential can be observed on unstimulated nerve or muscle tissue.

Sodium/potassium pump— The mechanism of active transport that moves potassium ions into the cell and sodium ions out, consuming ATP in the process.

Synapse— A region in which nerve impulses are transmitted across a gap from an axon terminal to another axon or the end plate of a muscle.