Nervous System
NEURAL NETWORK
1.The neural network of the human nervous system consists of billions of nerve cells known as neurons. They relay information to and from the central nervous system (the brain and spinal cord) in the form of electrical impulses. Neurons can sense changes inside and outside the body. They also control thinking and feeling, and muscles and glands.
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| NEURON STRUCTURE The neuron consists of a central portion called the cell body and extensions called dendrites and axons. Dendrites receive stimuli in the form of electrical impulses. Impulses travel from the cell body along the axons to stimulate other cells in the body--for example, another neuron, a gland cell, or a muscle. A substance called myelin surrounds the axon. Gaps in the sheath are known as nodes of Ranvier. At the tip of the axon is a junction called a synapse. |
TRANSMISSION OF NERVE IMPULSE
When the neuron is at rest, the axon maintains a chemical balance by keeping more potassium ions inside the cell and more sodium ions outside. When a signal is transmitted, the myelin sheath is stimulated at the nodes of Ranvier, allowing the ions to leak through. Potassium and sodium ions change places, creating an electric signal that travels along the axon.
SYNAPSE
The space between two neurons is called a synapse. Neurons communicate through the synapse, usually by means of chemicals known as neurotransmitters. When the impulse reaches the synapse, chemical transmitters ferry the impulse from the presynaptic side to receptors on the postsynaptic side, and the signal reaches the next neuron. The first neuron returns to a resting state, and the impulse travels on.
| I | INTRODUCTION |
Nervous
System, those elements within the animal organism that are concerned with
the reception of stimuli, the transmission of nerve impulses, or the activation
of muscle mechanisms.
| II | ANATOMY AND FUNCTION |
The reception of stimuli is the function of
special sensory cells. The conducting elements of the nervous system are cells
called neurons; these may be capable of only slow and generalized activity, or
they may be highly efficient and rapidly conducting units. The specific response
of the neuron—the nerve impulse—and the capacity of the cell to be stimulated
make this cell a receiving and transmitting unit capable of transferring
information from one part of the body to another.
| A | Nerve Cell |
Each nerve cell consists of a central
portion containing the nucleus, known as the cell body, and one or more
structures referred to as axons and dendrites. The dendrites are rather short
extensions of the cell body and are involved in the reception of stimuli. The
axon, by contrast, is usually a single elongated extension; it is especially
important in the transmission of nerve impulses from the region of the cell body
to other cells. See Neurophysiology.
| B | Simple Systems |
Although all many-celled animals have some
kind of nervous system, the complexity of its organization varies considerably
among different animal types. In simple animals such as jellyfish, the nerve
cells form a network capable of mediating only a relatively stereotyped
response. In more complex animals, such as shellfish, insects, and spiders, the
nervous system is more complicated. The cell bodies of neurons are organized in
clusters called ganglia. These clusters are interconnected by the neuronal
processes to form a ganglionated chain. Such chains are found in all
vertebrates, in which they represent a special part of the nervous system,
related especially to the regulation of the activities of the heart, the glands,
and the involuntary muscles.
| C | Vertebrate Systems |
Vertebrate animals have a bony spine and
skull in which the central part of the nervous system is housed; the peripheral
part extends throughout the remainder of the body. That part of the nervous
system located in the skull is referred to as the brain; that found in the spine
is called the spinal cord. The brain and the spinal cord are continuous through
an opening in the base of the skull; both are also in contact with other parts
of the body through the nerves. The distinction made between the central nervous
system and the peripheral nervous system is based on the different locations of
the two intimately related parts of a single system. Some of the processes of
the cell bodies conduct sense impressions and others conduct muscle responses,
called reflexes, such as those caused by pain (see Reflex).
In the skin are cells of several types
called receptors; each is especially sensitive to particular stimuli. Free nerve
endings are sensitive to pain and are directly activated. The neurons so
activated send impulses into the central nervous system and have junctions with
other cells that have axons extending back into the periphery. Impulses are
carried from processes of these cells to motor endings within the muscles
(see Muscle). These neuromuscular endings excite the muscles, resulting
in muscular contraction and appropriate movement. The pathway taken by the nerve
impulse in mediating this simple response is in the form of a two-neuron arc
that begins and ends in the periphery. Many of the actions of the nervous system
can be explained on the basis of such reflex arcs, which are chains of
interconnected nerve cells, stimulated at one end and capable of bringing about
movement or glandular secretion at the other.
| D | The Nerve Network |
The cranial nerves connect to the brain by
passing through openings in the skull, or cranium. Nerves associated with the
spinal cord pass through openings in the vertebral column and are called spinal
nerves. Both cranial and spinal nerves consist of large numbers of processes
that convey impulses to the central nervous system and also carry messages
outward; the former processes are called afferent, the latter are called
efferent. Afferent impulses are referred to as sensory; efferent impulses are
referred to as either somatic or visceral motor, according to what part of the
body they reach. Most nerves are mixed nerves made up of both sensory and motor
elements.
The cranial and spinal nerves are paired;
the number in humans are 12 and 31, respectively. Cranial nerves are distributed
to the head and neck regions of the body, with one conspicuous exception: the
tenth cranial nerve, called the vagus. In addition to supplying structures in
the neck, the vagus is distributed to structures located in the chest and
abdomen. Vision, auditory and vestibular sensation, and taste are mediated by
the second, eighth, and seventh cranial nerves, respectively. Cranial nerves
also mediate motor functions of the head, the eyes, the face, the tongue, and
the larynx, as well as the muscles that function in chewing and swallowing.
Spinal nerves, after they exit from the vertebrae, are distributed in a bandlike
fashion to regions of the trunk and to the limbs. They interconnect extensively,
thereby forming the brachial plexus, which runs to the upper extremities; and
the lumbar plexus, which passes to the lower limbs.
| E | Autonomic Nervous System |
Among the motor fibers may be found groups
that carry impulses to viscera. These fibers are designated by the special name
of autonomic nervous system. That system consists of two divisions, more or less
antagonistic in function, that emerge from the central nervous system at
different points of origin. One division, the sympathetic, arises from the
middle portion of the spinal cord, joins the sympathetic ganglionated chain,
courses through the spinal nerves, and is widely distributed throughout the
body. The other division, the parasympathetic, arises both above and below the
sympathetic, that is, from the brain and from the lower part of the spinal cord.
These two divisions control the functions of the respiratory, circulatory,
digestive, and urogenital systems.
| III | DISORDERS OF THE NERVOUS SYSTEM |
Consideration of disorders of the nervous
system is the province of neurology; psychiatry deals with behavioral
disturbances of a functional nature. The division between these two medical
specialties cannot be sharply defined, because neurological disorders often
manifest both organic and mental symptoms. For a discussion of functional mental
illness, Mental Illness.
Diseases of the nervous system include
genetic malformations, poisonings, metabolic defects, vascular disorders,
inflammations, degeneration, and tumors, and they involve either nerve cells or
their supporting elements. Vascular disorders, such as cerebral hemorrhage or
other forms of stroke, are among the most common causes of paralysis and other
neurologic complications. Some diseases exhibit peculiar geographic and age
distribution. In temperate zones, multiple sclerosis is a common degenerative
disease of the nervous system, but it is rare in the Tropics.
The nervous system is subject to infection
by a great variety of bacteria, parasites, and viruses. For example, meningitis,
or infection of the meninges investing the brain and spinal cord, can be caused
by many different agents. On the other hand, one specific virus causes rabies.
Some viruses causing neurological ills affect only certain parts of the nervous
system. For example, the virus causing poliomyelitis commonly affects the spinal
cord; viruses causing encephalitis attack the brain.
Inflammations of the nervous system are
named according to the part affected. Myelitis is an inflammation of the spinal
cord; neuritis is an inflammation of a nerve. It may be caused not only by
infection but also by poisoning, alcoholism, or injury. Tumors originating in
the nervous system usually are composed of meningeal tissue or neuroglia
(supporting tissue) cells, depending on the specific part of the nervous system
affected, but other types of tumor may metastasize to or invade the nervous
system (see Cancer). In certain disorders of the nervous system, such as
neuralgia, migraine, and epilepsy, no evidence may exist of organic damage.
Another disorder, cerebral palsy, is associated with birth defects.
Receptors
Movement may occur also in direct response to an outside stimulus; thus, a tap
on the knee causes a jerk, and a light shone into the eye makes the pupil
contract. These involuntary responses are called reflexes. Various nerve
terminals called receptors constantly send impulses into the central nervous
system. These are of three classes: exteroceptors, which are sensitive to pain,
temperature, touch, and pressure; interoceptors, which react to changes in the
internal environment; and proprioceptors, which respond to variations in
movement, position, and tension. These impulses terminate in special areas of
the brain, as do those of special receptors concerned with sight, hearing,
smell, and taste.
Reflex action
1. Voluntary Action
Turning the button of the stove is a voluntary action: the hand carries out the action after receiving an order to do so from the brain.
2. Reflex Action (Involuntary Action)
A reflex is an involuntary and automatic action. The body moves in response to a stimulus (for example, pulling the hand back after having burned it on the edge of a hot pan), and this is carried out even before the brain has become aware of it. As the brain has no information on which to act, the reflex is faster than a voluntary movement.
3. Mechanism of Reflex Action
During a voluntary action, a nerve message is sent from the brain. This message travels down the spinal cord and is transmitted to the part of the body concerned (here the muscle of the lower arm) via a motor neuron (nerve cell)..
4. Stimulus
The first stage of a reflex is the perception of the stimulus, which here is the burning of the finger on the edge of the hot pan. The stimulus is perceived by touch receptors on the skin and generates a message that is transmitted to the spinal cord via a sensory neuron.
5.Reaction
The first stage of a reflex is the perception of the stimulus, which here is the burning of the finger on the edge of the hot pan. The stimulus is perceived by touch receptors on the skin and generates a message that is transmitted to the spinal cord via a sensory neuron.
6. Brain Information
At the same time as it sends a message to the effectors, the spinal cord also sends a message to the brain. When the brain has processed the data, the hand is already far away from the hot pan. The advantages of reflexes are that they are automatic and fast, and occur without having had to be thought about. Reflexes play a very important part in protecting our body from external danger. However, it is vital that the data are also processed by the brain, as this is what makes it possible to learn and to avoid certain dangers thereafter.
7. Reflex Arc
At the same time as it sends a message to the effectors, the spinal cord also sends a message to the brain. When the brain has processed the data, the hand is already far away from the hot pan. The advantages of reflexes are that they are automatic and fast, and occur without having had to be thought about. Reflexes play a very important part in protecting our body from external danger. However, it is vital that the data are also processed by the brain, as this is what makes it possible to learn and to avoid certain dangers thereafter.
HORMONES
Reflex action
1. Voluntary Action
Turning the button of the stove is a voluntary action: the hand carries out the action after receiving an order to do so from the brain.
2. Reflex Action (Involuntary Action)
A reflex is an involuntary and automatic action. The body moves in response to a stimulus (for example, pulling the hand back after having burned it on the edge of a hot pan), and this is carried out even before the brain has become aware of it. As the brain has no information on which to act, the reflex is faster than a voluntary movement.
3. Mechanism of Reflex Action
During a voluntary action, a nerve message is sent from the brain. This message travels down the spinal cord and is transmitted to the part of the body concerned (here the muscle of the lower arm) via a motor neuron (nerve cell)..
4. Stimulus
The first stage of a reflex is the perception of the stimulus, which here is the burning of the finger on the edge of the hot pan. The stimulus is perceived by touch receptors on the skin and generates a message that is transmitted to the spinal cord via a sensory neuron.
5.Reaction
The first stage of a reflex is the perception of the stimulus, which here is the burning of the finger on the edge of the hot pan. The stimulus is perceived by touch receptors on the skin and generates a message that is transmitted to the spinal cord via a sensory neuron.
6. Brain Information
At the same time as it sends a message to the effectors, the spinal cord also sends a message to the brain. When the brain has processed the data, the hand is already far away from the hot pan. The advantages of reflexes are that they are automatic and fast, and occur without having had to be thought about. Reflexes play a very important part in protecting our body from external danger. However, it is vital that the data are also processed by the brain, as this is what makes it possible to learn and to avoid certain dangers thereafter.
7. Reflex Arc
At the same time as it sends a message to the effectors, the spinal cord also sends a message to the brain. When the brain has processed the data, the hand is already far away from the hot pan. The advantages of reflexes are that they are automatic and fast, and occur without having had to be thought about. Reflexes play a very important part in protecting our body from external danger. However, it is vital that the data are also processed by the brain, as this is what makes it possible to learn and to avoid certain dangers thereafter.
HORMONES
| I | INTRODUCTION |
Hormone, chemical that transfers information and
instructions between cells in animals and plants. Often described as the body’s
chemical messengers, hormones regulate growth and development, control the
function of various tissues, support reproductive functions, and regulate
metabolism (the process used to break down food to create energy). Unlike
information sent by the nervous system, which is transmitted via electronic
impulses that travel quickly and have an almost immediate and short-term effect,
hormones act more slowly, and their effects typically are maintained over a
longer period of time.
Hormones were first identified in 1902 by
British physiologists William Bayliss and Ernest Starling. These researchers
showed that a substance taken from the lining of the intestine could be injected
into a dog to stimulate the pancreas to secrete fluid. They called the substance
secretin and coined the term hormone from the Greek word hormo,
which means “to set in motion.” Today more than 100 hormones have been
identified.
Hormones are made by specialized glands or
tissues that manufacture and secrete these chemicals as the body needs them. The
majority of hormones are produced by the glands of the endocrine system, such as
the pituitary, thyroid, adrenal glands, and the ovaries or testes. These
endocrine glands produce and secrete hormones directly into the bloodstream.
However, not all hormones are produced by endocrine glands. The mucous membranes
of the small intestine secrete hormones that stimulate secretion of digestive
juices from the pancreas. Other hormones are produced in the placenta, an organ
formed during pregnancy, to regulate some aspects of fetal development.
Hormones are classified into two basic types
based on their chemical makeup. The majority of hormones are peptides, or amino
acid derivatives that include the hormones produced by the anterior pituitary,
thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically
produced as larger proteins. When they are called into action, these peptides
are broken down into biologically active hormones and secreted into the blood to
be circulated throughout the body. The second type of hormones is steroid
hormones, which include those hormones secreted by the adrenal glands and
ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty
substance produced by the body) and modified by a series of chemical reactions
to form a hormone ready for immediate action.
| II | HOW HORMONES WORK |
Most hormones are released directly into the
bloodstream, where they circulate throughout the body in very low
concentrations. Some hormones travel intact in the bloodstream. Others require a
carrier substance, such as a protein molecule, to keep them dissolved in the
blood. These carriers also serve as a hormone reservoir, keeping hormone
concentrations constant and protecting the bound hormone from chemical breakdown
over time.
Hormones travel in the bloodstream until
they reach their target tissue, where they activate a series of chemical
changes. To achieve its intended result, a hormone must be recognized by a
specialized protein in the cells of the target tissue called a receptor.
Typically, hormones that are water-soluble use a receptor located on the cell
membrane surface of the target tissues. A series of special molecules within the
cell, known as second messengers, transport the hormone’s information into the
cell. Fat-soluble hormones, such as steroid hormones, pass through the cell
membrane and bind to receptors found in the cytoplasm. When a receptor and a
hormone bind together, both the receptor and hormone molecules undergo
structural changes that activate mechanisms within the cell. These mechanisms
produce the special effects induced by the hormone.
Receptors on the cell membrane surface are
in constant turnover. New receptors are produced by the cell and inserted into
the cell wall, and receptors that have reacted with hormones are broken down or
recycled. The cell can respond, if necessary, to irregular hormone
concentrations in the blood by decreasing or increasing the number of receptors
on its surface. If the concentration of a hormone in the blood increases, the
number of receptors in the cell wall may go down to maintain the same level of
hormonal interaction in the cell. This is known as downregulation. If
concentrations of hormones in the blood decrease, upregulation increases the
number of receptors in the cell wall.
Some hormones are delivered directly to the
target tissues instead of circulating throughout the entire bloodstream. For
example, hormones from the hypothalamus, a portion of the brain that controls
the endocrine system, are delivered directly to the adjacent pituitary gland,
where their concentrations are several hundred times higher than in the
circulatory system.
| III | HORMONAL EFFECTS |
Hormonal effects are complex, but their
functions can be divided into three broad categories. Some hormones change the
permeability of the cell membrane. Other hormones can alter enzyme activity, and
some hormones stimulate the release of other hormones.
Recent studies have shown that the more
lasting effects of hormones ultimately result in the activation of specific
genes. For example, when a steroid hormone enters a cell, it binds to a receptor
in the cell’s cytoplasm. The receptor becomes activated and enters the cell’s
nucleus, where it binds to specific sites in the deoxyribonucleic acid (DNA),
the long molecules that contain individual genes. This activates some genes and
inactivates others, altering the cell’s activity. Hormones have also been shown
to regulate ribonucleic acids (RNA) in protein synthesis.
A single hormone may affect one tissue in a
different way than it affects another tissue, because tissue cells are
programmed to respond differently to the same hormone. A single hormone may also
have different effects on the same tissue at different times in life. To add to
this complexity, some hormone-induced effects require the action of more than
one hormone. This complex control system provides safety controls so that if one
hormone is deficient, others will compensate.
| IV | TYPES OF HORMONES |
Hormones exist in mammals, including humans,
as well as in invertebrates and plants. The hormones of humans, mammals, and
other vertebrates are nearly identical in chemical structure and function in the
body. They are generally characterized by their effect on specific tissues.
| A | Human Hormones |
Human hormones significantly affect the
activity of every cell in the body. They influence mental acuity, physical
agility, and body build and stature. Growth hormone is a hormone produced by the
pituitary gland. It regulates growth by stimulating the formation of bone and
the uptake of amino acids, molecules vital to building muscle and other
tissue.
Sex hormones regulate the development of
sexual organs, sexual behavior, reproduction, and pregnancy. For example,
gonadotropins, also secreted by the pituitary gland, are sex hormones that
stimulate egg and sperm production. The gonadotropin that stimulates production
of sperm in men and formation of ovary follicles in women is called a
follicle-stimulating hormone. When a follicle-stimulating hormone binds to an
ovary cell, it stimulates the enzymes needed for the synthesis of estradiol, a
female sex hormone. Another gonadotropin called luteinizing hormone regulates
the production of eggs in women and the production of the male sex hormone
testosterone. Produced in the male gonads, or testes, testosterone regulates
changes to the male body during puberty, influences sexual behavior, and plays a
role in growth. The female sex hormones, called estrogens, regulate female
sexual development and behavior as well as some aspects of pregnancy.
Progesterone, a female hormone secreted in the ovaries, regulates menstruation
and stimulates lactation in humans and other mammals.
Other hormones regulate metabolism. For
example, thyroxine, a hormone secreted by the thyroid gland, regulates rates of
body metabolism. Glucagon and insulin, secreted in the pancreas, control levels
of glucose in the blood and the availability of energy for the muscles. A number
of hormones, including insulin, glucagon, cortisol, growth hormone, epinephrine,
and norepinephrine, maintain glucose levels in the blood. While insulin lowers
the blood glucose, all the other hormones raise it. In addition, several other
hormones participate indirectly in the regulation. A protein called somatostatin
blocks the release of insulin, glucagon, and growth hormone, while another
hormone, gastric inhibitory polypeptide, enhances insulin release in response to
glucose absorption. This complex system permits blood glucose concentration to
remain within a very narrow range, despite external conditions that may vary to
extremes.
Hormones also regulate blood pressure and
other involuntary body functions. Epinephrine, also called adrenaline, is a
hormone secreted in the adrenal gland. During periods of stress, epinephrine
prepares the body for physical exertion by increasing the heart rate, raising
the blood pressure, and releasing sugar stored in the liver for quick energy.
Hormones are sometimes used to treat
medical problems, particularly diseases of the endocrine system. In people with
diabetes mellitus type 1, for example, the pancreas secretes little or no
insulin. Regular injections of insulin help maintain normal blood glucose
levels. Sometimes, an illness or injury not directly related to the endocrine
system can be helped by a dose of a particular hormone. Steroid hormones are
often used as anti-inflammatory agents to treat the symptoms of various
diseases, including cancer, asthma, or rheumatoid arthritis. Oral
contraceptives, or birth control pills, use small, regular doses of female sex
hormones to prevent pregnancy.
Initially, hormones used in medicine were
collected from extracts of glands taken from humans or animals. For example,
pituitary growth hormone was collected from the pituitary glands of dead human
bodies, or cadavers, and insulin was extracted from cattle and hogs. As
technology advanced, insulin molecules collected from animals were altered to
produce the human form of insulin.
With improvements in biochemical
technology, many hormones are now made in laboratories from basic chemical
compounds. This eliminates the risk of transferring contaminating agents
sometimes found in the human and animal sources. Advances in genetic engineering
even enable scientists to introduce a gene of a specific protein hormone into a
living cell, such as a bacterium, which causes the cell to secrete excess
amounts of a desired hormone. This technique, known as recombinant DNA
technology, has vastly improved the availability of hormones.
Recombinant DNA has been especially useful
in producing growth hormone, once only available in limited supply from the
pituitary glands of human cadavers. Treatments using the hormone were far from
ideal because the cadaver hormone was often in short supply. Moreover, some of
the pituitary glands used to make growth hormone were contaminated with
particles called prions, which could cause diseases such as Creutzfeldt-Jakob
disease, a fatal brain disorder. The advent of recombinant technology made
growth hormone widely available for safe and effective therapy.
| B | Invertebrate Hormones |
In invertebrates, hormones regulate
metamorphosis (the process in which many insects, crustaceans, and mollusks
transform from eggs, to larva, to pupa, and finally to mature adults). A hormone
called ecdysone triggers the insect molting process, in which these animals
periodically shed their outer covering, or exoskeletons, and grow new ones. The
molting process is delayed by juvenile hormone, which inhibits secretion of
ecdysone. As an insect larva grows, secretion of juvenile hormone declines
steadily until its concentrations are too low to prevent the secretion of
ecdysone. When this happens, ecdysone concentrations increase until they are
high enough to trigger the metamorphic molt.
In insects that migrate long distances,
such as the locust, a hormone called octopamine increases the efficiency of
glucose utilization by the muscles, while adipokinetic hormone increases the
burning of fat as an energy source. In these insects, octopamine levels build up
in the first five minutes of flight and then level off as adipokinetic hormone
takes over, triggering the metabolism of fat reserves during long distance
flights.
Hormones also trigger color changes in
invertebrates. Squids, octopuses, and other mollusks, for example, have
hormonally controlled pigment cells that enable the animals to change color to
blend in with their surroundings.
| C | Plant Hormones |
Hormones in plants are called
phytohormones. They regulate most of the life cycle events in plants, such as
germination, cell division and extension, flowering, fruit ripening, seed and
bud dormancy, and death (see Plant: Growth and Differentiation).
Plant biologists believe that hormones exert their effects via specific receptor
sites in target cells, similar to the mechanism found in animals. Five plant
hormones have long been identified: auxin, cytokinin, gibberellin, abscisic
acid, and ethylene. Recent discoveries of other plant hormones include
brassinosteroids, salicylates, and jasmonates.
Auxins are primarily responsible for
protein synthesis and promote the growth of the plant's length. The most common
auxin, indoleacetic acid (IAA), is usually formed near the growing top shoots
and flows downward, causing newly formed leaves to grow longer. Auxins stimulate
growth toward light and root growth.
Gibberellins, which form in the seeds,
young leaves, and roots, are also responsible for protein synthesis, especially
in the main stem of the plant. Unlike auxins, gibberellins move upward from the
roots. Cytokinins form in the roots and move up to the leaves and fruit to
maintain growth, cell differentiation, and cell division. Among the growth
inhibitors is abscisic acid, which promotes abscission, or leaf fall; dormancy
in buds; and the formation of bulbs or tubers, possibly by preventing the
synthesis of protein. Ethylene, another inhibitor, also causes abscission,
perhaps by its destructive effect on auxins, and it also stimulates the ripening
of fruit.
Brassinosteroids act with auxins to
encourage leaf elongation and inhibit root growth. Brassinosteroids also protect
plants from some insects because they work against some of the hormones that
regulate insect molting. Salicylates stimulate flowering and cause disease
resistance in some plants. Jasmonates regulate growth, germination, and flower
bud formation. They also stimulate the formation of proteins that protect the
plant against environmental stresses, such as temperature changes or
droughts.
| V | COMMERCIAL USE OF HORMONES |
Hormones are used for a variety of commercial
purposes. In the livestock industry, for example, growth hormones increase the
amount of lean (non-fatty) meat in both cattle and hogs to produce bigger, less
fatty animals. The cattle hormone bovine somatotropin (BST) increases milk
production in dairy cows. Hormones are also used in animal husbandry to increase
the success rates of artificial insemination and speed maturation of eggs.
The United States Food and Drug
Administration (FDA) approved the use of BST in November 1993. However, the
safety and ethics of BST use are disputed by several consumer groups, which
object to the production of milk using artificial stimulation. They claim that
after a regular course of injections, cows show symptoms of production stress.
Reanalysis of industry data by several scientists pointed to increased incidence
of mastitis and other health problems in BST-treated cows. Concerns about the
side effects of BST prompted authorities in Canada and the European Union to
prohibit its use.
In plants, auxins are used as herbicides, to
induce fruit development without pollination, and to induce root formation in
cuttings. Cytokinins are used to maintain the greenness of plant parts, such as
cut flowers. Gibberellins are used to increase fruit size, increase cluster size
in grapes, delay ripening of citrus fruits, speed up flowering of strawberries,
and stimulate starch break down in barley used in beer making.
In addition, ethylene is used to control
fruit ripening, which allows hard fruit to be transported without much bruising.
The fruit is allowed to ripen after it is delivered to market. Genetic
engineering also has produced fruits unable to form ethylene naturally. These
fruits will ripen only if exposed to ethylene, allowing for extended shipping
and storage of produce.
