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Nervous Coordination
Learning Objectives:
Nervous system

Nerve impulse

Steps involved in nervous coordination

Neurons (Structure and Types)

Transmission of action potential between cells–synapse

Electrical synapses

Chemical synapses

Transmission of nerve impulse across synapse

Hormones

Endocrine glands

Feedback mechanism

Positive feedback mechanism

Negative feedback mechanism

Reflexes and reflex arc

Levels of the spinal cord and its main functions

Parts of the brain with their main function

Nervous Coordination
Introduction:
In nervous coordination specialized cells or neurons are linked together directly or via the central
nervous system, to form network that connects receptors and effectors. Receptors are the cells
or organs which receive stimuli while the effectors are those which carry out actions or
responses.
The neuron has the capacity to generate and conduct impulses which travel across the synapse
and pass from the receptors to the effectors. The neuron brings about the nervous co-ordination.
The elements of nervous system which help in co-ordination are:
1. Receptors
2. Neurons
3. Effectors

1. Recpetors:
The receptor may be a cell, or neuron ending or a receptor organ. Receptors detect changes in
the external and internal environment of the animal, receptor are classified as follows:
(i) Chemo receptors:
These are for smell (nose), taste (tongue) and for blood CO2, oxygen, blood glucose.
amino acids and fatty acids (receptors in the hypothalamus).
(ii) Mechanoreceptors:
These detect stimuli of touch (free nerve endings + expanded tip ending + stray
edgiest) pressure, hearing and equilibrium.
(iii) Photoreceptors:
(Electromagnetic receptors), these respond to stimuli of light for example in eyes.
rods and cones.
(iv) Thermoreceptors:
Show response to cold and warmth.
(v) Nociceptors:
(Undifferentiated endings) which produce the sensation of pain. Modalities of
Sensation. Each principal sensation is modality (like pain, touch, sight, sound etc.)
Sensory Receptors of The Skin:
The receptors in the skin are concerned with at least five different senses which are touch,
pressure, heat, cold and pain. These sensations are detected by 3 different types of modified
sensory neurons haying naked nerve endings or specialized cellular capsules (pressure, hot and
cold receptors).
(i) Naked or Free Nerve Endings:
These are touch and pain receptors (for example situated at the base of hairs).
(ii) Meissner's Corpuscles:
These are touch receptors. These are encapsulated neuron endings. These lie in
papillae which extend into the ridges of the fingertips. The corpuscle consists of spiral
and much-twisted endings, each of which ends in a knob.

(iii) Pacinian Corpuscles:

 These receive deep pressure stimulus. These are also encapsulated neuron endings.
These are present quite deep in the body. The receptors present in the limbs detect
vibration sense. The receptors present. in the joints of terrestrial vertebrates also
detect the vibrations of the ground.
Note: Free Nerve Ending: On some receptor neurons, a finely branched ending that responds to
touch and pressure, to heat and cold, or to pain; produces the sensations of itching and
tickling.
Number and Kinds of Receptors: Pain receptors are nearly 27 times more abundant than
cold receptors. The cold receptors are nearly 10 times more abundant than heat or
temperature receptors. The receptors are not distributed evenly. For example, touch
receptors are more numerous in the finger tips than in the skin of the back.
2. Neurons:
The chief structural and functional unit of the nervous system is neurons. Neuroglia are
the cells that play a vital role in the nutrition of neurons and their protection by myelin
sheath. These are present in the higher animals and in humans and make up as much as
half of the nervous system.
There are three functional types of neurons in mammals:
The sensory neurons
The associative (intermediate/relay) neurons
The motor neurons

Structure of a Neuron:
A neuron has two main parts which are cell body and fibers.
(i) The Cell Body:
The neuron has a cell body or soma containing nucleus and various organelles
embedded in the cytoplasm. The cell body is the main nutritional part of the cell and
is concerned with the biosynthesis of materials necessary for the growth and
maintenance of the neuron. Nissl's granules are present in the cell body. Nissl's
granules are groups of ribosomes associated with rough E.R, and Golgi apparatus. If
the cell body of the neuron remains intact, it can regenerate axons and dandrite
fibers. A mature neuron cannot divide further.
(ii) The Fibers:
The fibers are the protoplasmic processes arising from the cell body.
There are two main types of cytoplasmic processes or fibers:
(a) Dendrites:
These carry impulse towards cell body. If it is a single fiber, then it is called dendron
but if smaller fibers then they are called dendrites (singular: dendrite).
(b) Axons:
The processes conducting impulses away from cell body are called axons. These
may be more than a meter long in some neurons. Microtubules, neurofibrils,
rough endoplasmic reticulum and mitochondria are present throughout the
axoplasm (cytoplasm of axon) of the neuron.
3. Effectors:
These are the structures which respond when they are stimulated by impulse coming via
motor neuron. The principal effectors are muscles and glands. The muscles respond by
contracting while gland responds by secreting.
Reflex Arc
Flow of impulse through the nervous system involves receptor, neurons, and effectors. Reflex arc
is the path way of passage of impulse during a reflex action. Reflex action is a type of involuntary
action. The direction of stimulus is from receptors to sensory neuron to associative t association
/ relay) neuron and then through motor neuron to the effectors.
Reflexes may be monosynaptic or polysynaptic.

Reflex action
Nerve Impulse
Nerve impulse is a wave of electrochemical changes, which travels along the length of the neuron
involving chemical reactions and movement of ions across the cell membrane.
Electrical Potential:
Electrical potential is a measure of the capacity to do electrical work. It represents stored energy
which is due to separation of charges across a barrier.
In the case of neuron, the charges are positive and negative ions, and the charge separating
barrier is the plasma membrane.
Membrane Potential:
The electrical potential that exists across a cell membrane IS known as membrane potential.
Resting Membrane Potential:
A typical neuron at rest is more positive electrically outside than inside the cell membrane. This
net difference in charge between the inner and outer surface of a non-conducting neuron is
called the resting membrane potential.
Factors Involved in Resting Membrane Potential:
The major factors which are involved in resting membrane potential are sodium ions are tenfold
higher in concentration outside than inside the membrane surface. The potassium ions are
twenty times more concentrated inside than outside. This is due to Na+ / K+ pumping system.
The large negative organic ions (such as proteins, organic acids etc.) are much more inside the
membrane than outside, where they are only in negligible concentration. This makes the inside
of neuron membrane more negative. So inside becomes more negative than the outside of the
cell membrane of neuron.
The membrane potential is of approximately -70 m V. A nerve impulse is initiated by an
appropriate stimulus (called threshold stimulus) applied at one end of the neuron and it results
in a localized change in the resting membrane potential. Resting membrane potential disappears
for a brief instant and is replaced by a new potential called active membrane potential, which is
in the form of impulse. Now pumping of Na+rushes in and the inner membrane surface becomes
more positive than the outside. This is Active membrane potential which is 0.05 volts (– 50mv).
This change is so brief (for perhaps a millisecond) that only a portion of the neuron is inside active
membrane potential state. The neuron conducts this impulse in the form of nerve impulse.
Soon after passage of the impulse, the resting membrane potential is restored by the movement
of a small number of ions especially K+ moving out. This neuron now is ready to conduct another
impulse.
In myelinated neurons the impulse jumps from node to node (node of Ranvier). This is lolled
Saltatory impulse.
The normal speed of nerve impulse in humans is 100 meters per second but In myelinated
neurons maximum speed is 120 meters per second.
Synapse:
The axon endings of the neuron are connected to the dendrites of the next neuron. There is no
cytoplasmic connection between the two neurons and microscopic gaps are left between them.
Each of these contact points is known as synapse. A nerve impulse is passed from one neuron to
the other through the synapse.
However, a single impulse does not necessarily pass the synapse. It may take two or three
impulses arriving in rapid series or perhaps simultaneously from two or more fibres to start an
impulse in the next neuron.
The action potential cannot jump from one neuron to the next in line, however, the message is
transmitted across the synapse in the form of chemical messenger called neurotransmitters.
When an impulse reaches a synaptic knob, synaptic vesicles within fuse with the presynaptic
membrane, the neurotransmitter molecules are released into the synaptic cleft. The
neurotransmitter molecules bind to the receptors on the post-synaptic membrane Nerve impulse
starts in this neuron. Neurotransmitters are chemicals, which are released at the axon ending of
t neurons, at synapse. These are: acetylcholine, adrenaline, nor-epinephrine, serotonin and
dopamine.
Acetylcholine is the main transmitter for synapses that lie outside the cent nervous system.
Others are mostly involved in synaptic transmission within the brain and spit cord.

Human Nervous System

Human nervous system is a type of centralized nervous system.

Fig. Classification of the human nervous system

Protection of Central Nervous System (CNS):
The CNS consists of brain and spinal cord, which are both protected in three ways.
(i) Cranium:
It is the part of skull which protects the brain
(ii) Vertebral Column:
The neural arches of the vertebrae of vertebral column protect the spinal cord
(iii) Meninges:
Beneath the cranium (and vertebral column), the brain and spinal cord are protected
by triple layer of meninges.
Cerebrospinal fluid (CSF):
Between the layers of meninges, the cerebrospinal fluid (CSF) is present. It is similar in
composition to blood plasma. It bathes the neurons of brain and spinal cord. It cushions against
the bumps and jolts. Both brain and spinal cord are hollow. The spinal cord has central canal and
brain has many cavities (ventricles) filled by CSF.
Parts of Central Nervous System (CNS):
These are brain & spinal cord.
Parts of Human Brain:
The human brain 'weighs about 1.5 kilograms and is 85 per cent water. Brain is the major part of
CNS and is well protected in the cranium of skull. The brain can be divided into forebrain, mid
brain and hind brain.
Forebrain
It is further divided into three functional parts, the thalamus, the limbic system and the
cerebrum.
1. Thalamus:
The thalamus is a relay center and carries sensory information to the limbic system
and cerebrum.
The information is taken by sensory neurons from auditory and visual pathways, from the
skin and from within the body.

Fig: The human brain

The Limbic System:
The limbic system is located in an arc between the thalamus and cerebrum. The limbic system
extends through several brain regions, limbic system work together to produce our most basic
and primitive emotions, drives, and behaviors. It is the center of most unconscious emotional
behaviors such as love, hates, hunger, sexual responses, fear, rage, tranquility, thirst, pleasure,
sexual response & the formation of memories

Fig. The limbic system and thalamus

Parts of Limbic System:

The limbic system consists of hypothalamus, the amygdala, and hippocampus and nearby regions
of cerebrum.
(i) The Hypothalamus:
The hypothalamus is a key area. It receives a huge amount of internal and external
sensory information and acts as a coordinate center between the nervous system and
the endocrine (hormonal) system.
The hypothalamus is responsible for sensations such as hunger and thirst. It also helps
control the autonomic nervous system since it regulates body temperature and the
balance of water and salts in the blood. It is linked directly to the pituitary gland by means
of blood vessels and nerves.
Generally, the hypothalamus controls the release of hormones from the pituitary
including the antidiuretic hormone (which controls water reabsorption in the kidneys)
growth hormone and some reproductive hormones.
(ii) Amygdala:
Amygdala in the amygdala, cluster of neurons produce sensation (feeling) of
(pleasure, punishment, love, hate or sexual arousal) when stimulated.
It is also involved in the feelings of fear and rage.
(iii) Hippocampus:
Hippocampus plays an important role in the formation of long-term memory and thus
us required for learning.
Cerebrum:
Cerebrum is the largest part of the brain and is divided into two halves called cerebral
hemispheres. The left cerebral hemisphere controls the right side of the body and right cerebral
hemisphere controls the left side of the body.
Corpus Callosum:
These halves communicate with each other by means of a large band of axons called corpus
callosum. Tens of billions of neurons are packed into this part, the outer region the cerebral
cortex, forms folds called convolutions which greatly increase its surface area.
Functions of Cerebral Cortex:
(i) It receives sensory information, processes it and stores some in memory for future use.
This is also involved in intelligence, reasoning and judgment.
(ii) It is also responsible for poorly understood process that we call thinking.
(iii) The cerebral cortex contains primary sensory areas. Here the signals from sensory organs
such as eyes and ears are received and converted into subjective impressions such as light
and sound. Nearby association areas interpret this information.
(iv) It also contains sensory area of touch. This area receives and interprets sensations of
touch from all parts of the body.
(v) It is also involved in speech.
(vi) It is also a center for sending impulses to voluntary muscles, controlling voluntary
movements.
Midbrain:
It is reduced in humans and has following functions:
1. Relay Centre:
Hindbrain contains reticular formation, which is a relay center connecting hindbrain
with the forebrain. Reticular formation is very important in screening the input
information before they reach higher brain centers.
It also contains auditory relay center.
Reflex Movements: It controls reflex movement of eyes.
Hindbrain:
It includes the medulla, pons and cerebellum.
1. Medulla:
It controls several automatic functions such as breathing, heart rate, blood pressure
and swallowing.
2. Pons:
It is located above the medulla. Its functions are as follows:
(i) Certain neurons in pons influence transitions between sleep and wakefulness
(ii) It is also involved in the rate and pattern of breathing
The Cerebellum:
It co-ordinates body movements and maintain body position. Therefore, smooth and accurate
motions are possible. The cerebellum is also involved in the learning and memory storage for
behaviors. It is best developed in birds, which engage in the complex activity of flight.
Spinal Cord:
Medulla oblongata narrows down into the spinal cord. Structure and Composition:
Structure and Composition:
It is an oval shaped hollow cylinder, running through the vertebral column. It is made up of a very
large number of neurons, the cell-fibers and cell bodies. These are arranged in a definite pattern.
In a cross section, the spinal cord shows two portions:
(i) Inner Portion:
It is butterfly shaped grey matter containing a central canal. Gray matter consists of
cellbodies and non-myelinated nerve fibers or tracts.
(ii) Outer Portion:
It is composed of white matter. White matter is made lip of myelinated nerve fibers
or tracts.

Fig. The spinal cord

Functions:
(i) The spinal cord is the centre for a large number of reflexes.
(ii) It is a pathway for conduction of impulses to and from different parts of the body and
brain.
Peripheral Nervous System (PNS):
It consists of sensory neurons and motor neurons, which may form ganglia and the Nerves.
Ganglia:
These are the concentrations of cell bodies of neurons.
Nerves:
The nerves are the bundles ofaxons or dendrites, bounded-by connective tissue.
Type of Nerves on the Basis of Conduction of Impulse:
The nerves may be sensory, motor or mixed depending upon the direction of impulse they
conduct.
Types of Nerves on the Basis of Link with Brain or Spinal Cord:
These are of two types:
(i) Cranial Nerves:
In humans, there are 12 pairs of nerves, which arise from the brain, or lead to the
brain. These nerves are called cerebral or cranial nerves. Some of these nerves are
sensory, some motor and some are mixed.
(ii) Spinal Nerves:
From the spinal cord 31 pairs of spinal nerves arise or lead to spinal cord. All these
nerves are mixed having fibres of both sensory and motor neurons. Sensory neurons
carry signals to the CNS' from sensory organs. The motor neurons carry signals from
the CNS that controls the activities of muscles and glands.
Nervous Systems Formed by the Motor Neurons:
Motor neurons form two types of nervous systems:
(i) Somatic Nervous System:
It controls voluntary movements. These movements are under the conscious control
of the body. Skeletal muscles are involved in these movements.
(ii) Autonomic Nervous System:
It controls involuntary responses by influencing organs, glands and smooth muscles.
Types of Autonomic Nervous System:
The motor neurons of autonomic nervous system are divided into sympathetic nervous system
and parasympathetic nervous system. Both of these systems function automatically and
innervate all internal organs. These systems utilize two neurons and one ganglion for each
impulse.
(i) Sympathetic System:
Most ganglia of the sympathetic system arise from the middle portion of the spinal
cord and almost terminate in the ganglia that lie near the cord.
Functions:
(a)This system is important during emergency situations and is associated with "fight or
flight".
(b) This system accelerates the heartbeat, dilates the pupil and inhabits the
digestion of food etc.
(ii) Parasympathetic System:
A few cranial nerves including the vagus nerve together with the nerves from the bottom
portion of the spinal cord, form the parasympathetic nervous system.
Functions: It promotes all the internal responses which are associated with the relaxed
state (maintenance activities). Some examples are contraction of the pupils promotes
digestion of food & retards heartbeat.
Nervous Disorders
Following are some of the common disorders of nervous system in humans:
1. Parkinson's Disease (Paralysis Agitans):
Symptoms:
It is a nervous disorder characterized by involuntary tremors reduced motor power and
rigidity poor balance and speech problems. The mental faculties are not affected.
Causes.
Parkinson's disease is a disease in which the death of a small number of cells in the basal
ganglia leads to an inability to select and initiate patterns of movement, onset of disease
usually in 50's and 60's.
The disease is slowly progressive. Therefore, the patient may live for many years. The
disease may result by head trauma.
Treatment:
Effective drugs are available (such as L-dopa also called Levodopa).
In carefully selected patients, surgical destruction of the portions of globus pallidus or the
ventrolateral nucleus of thalamus has proved highly beneficial. A naturally occurring protein
called glial cell-line derived neurotrophic factor (G DNF) boosts uptake or dopamine when
delivered to lab rats and monkeys.
2. Epilepsy:
Symptoms:
(i) Primary Changes:
Sudden transient alterations in brain function associated with excessive rapid electric
discharge in the gray matter.
(ii) Secondary Changes:
It is a convulsive disorder of nerves. It is characterized by sudden and transient
symptoms of motor sensory, psychic or autonomic nature, associated with changes in
consciousness.
Causes:
The start of epilepsy is usually before age 30. Later age start suggests organic disease. In some
patients, emotional disturbances play a significant “trigger" role.
Diagnosis:
Electroencephalography (EEG) is most important test in the study of epilepsy.
Treatment:
Anticonvulsant drugs are used. Alcohol aggravates epilepsy. Therefore persons suffering from
epilepsy should avoid alcohol.
3. Alzheimer’s disease:
Alzheimer’s disease was first described by alois Alzheimer in 1907.
Symptoms:
There is decline in the brain function (especially with age). Its symptoms are similar to
those diseases that cause dementia (mammary loss).
 Causes:
There is a genetic predisposition (tendency) to the disease in some people. So it runs in
families.There is also evidence that high levels of aluminum may contribute to the onset
of this disease.

Chemical Coordination
In animal’s chemical coordination is by endocrine system. It consists of endocrine glands which
are present in different parts of the body. These glands secrete hormones.
Endocrine Glands:
The endocrine or ductless glands are (with a few exceptions) discrete group of cells which make
specific chemical compounds called hormones (In Greek hormone mean) exciting or setting in
motion).
Hormone:
Hormones are organic compounds of varying structural complexity. They are poured directly and
are transported by blood to respective target tissues.
Function:
The hormones affect the target cells. To affect the target cells they work in the following. They
do not initiate new biochemical reactions. However they regulate enzymatic and other chemical
reactions, already present. They may either stimulate or inhibit a function. Hormones may also
control some long term changes. For example rate of growth, ate of metabolic activity and sexual
maturity.
Chemical Nature of Hormones:
Chemically hormones may be of following four types:
(i) Proteins (e.g. insulin and glucagon.)
(ii) Amino acids & derivatives (e.g. Thyroxin, epinephrine and norepinephrine)
(iii) Polypeptides (e.g. vasopressin or anti-diuretic hormone and oxytocin) and
Steroids (e.g. estrogens, testosterone and cortisone)
Role of Hypothalamus in Endocrine System:
It is part of the fore brain. Here many of the sensory stimuli of the nervous system are converted
into hormonal responses. Nerve cells in the hypothalamus produce and secrete a variety of
hormones. One of the nerve clusters synthesizes oxytocin and antidiuretic hormone
(ADH)vasopressin. These hormones travel down and are stored in the nerve endings located in
the posterior pituitary. Upon proper stimulation from the hypothalamus, oxytocin and
vasopressin are released into the blood supply of the posterior pituitary. Other nerve clusters in
the hypothalamus produce and secrete a battery of releasing and inhibiting hormones. These
hormones are carried by the blood to the anterior pituitary. There they regulate the secretion u
various tropic hormones, growth hormone, and Prolactin manufactured by the anterior pituitary
cells.
The Pituitary Gland:
In the pituitary gland (hypophysis cerebri) is an ovoid structure. It is about 0.5 gm in the adult
and is connected to brain through a short stalk (the infundibulum). It has three lobes which are
anterior ,median and posterior.
Anterior Lobe
The anterior lobe is also called as the master gland. This is because that in addition to producing
primary hormones it produces the tropic hormones which control the secretion of hormones of
other endocrine glands.Anterior lobe of pituitary secretes the following hormones:
1. Somatotrophin (STH):
Release:
Somatotrophin releasing factor (SRF) is secreted from hypothalamus through nut the life.
Functions:
The main function is growth. When growth has mostly ceased after adolescence, the hormone
continues to promote protein synthesis throughout the body.
Disorders:
(i) If this hormone is produced in excess during early life, it leads to Gigantism as a result
there is abnormal development of hands, feet, jaw etc. (known as Acromegaly).
(ii) If there is under-secretion,Dwarfism results in addition to other symptoms associated
with lack of thyroid and adrenal hormone.
2. Thyroid Stimulating Hormone:
Release:
Release of thryotrophin releasing factor from the hypothalamus is controlled by the levels of
thyroxin in the blood. In the presence of low levels of thyroxin, there is increasing production of
TSH and viceversa.
Functions:
It is secreted throughout life but particularly reaches high levels during the periods of rapid
growth and development. It acts directly on the cells of the thyroid gland increasing both their
numbers and their secretary activity.
3. Adrenocorticotrophic Hormone (ACTH):
Release:
Release of corticotrophin releasing factor from the hypothalamus is controlled by steroid levels
in the blood and by direct nervous stimulation of the hypothalamus as a result of stress e.g. cold,
heat, pain, fright and infections.
Functions:
Excess and deficiency results as for disturbance of normal adrenal functions.
4. Gonadotrophic Hormones:
These are of three types:
(i) Follicle stimulating hormone (FSH)
(ii) Luteinising Hormone (LH) it is also called interstitial cell stimulating hormone (ICSH) in the
male
(iii) Prolactin: It is sometimes inappropriately (improperly) called luteotrophic hormone (LTH)
Release:
A common hypothalamic releasing factor is involved in the secretion of FSH and LH/ICSH.
Prolactin is continuously produced from the pituitary and is inhibited by prolactin inhibiting factor
(PIH) from the hypothalamus.
Functions of FSH: In females FSH stimulates follicle development and secretion of oestrogens
from the ovaries.
In males, FSH stimulates development of the germinal epithelium of the testis and sperm
production.
Functions of LH: LH works with FSH to stimulate oestrogen secretion and rupture of mature
follicles to release egg or ovum. LH also causes the lutenisation (lit. "turning yellow”) of the
ruptured follicle and work with prolactin to maintain the corpus luteum (and hence it secretes
progesterone).
ICSH in the male stimulates the interstitial cells of the testis to secrete testosterone.
Functions of Prolactin:
Prolactin stimulates milk production (and acts with LH as described above).
Median Lobe
Median lobe secretes the following hormone:
Melanophore Stimulating Hormone:
Release:
External light governs its secretion, more secretion in pregnancy.
Inhibition:
Its inhibition of secretion is controlled by hypothalamus.
Functions:
Stimulates melanocytes in skin to produce brown pigment, melanin which darkens the skin.
Disorder:
Excess MSH is secreted in Addison's disease. One of the symptoms of which is darkening of the
skin.
Posterior Lobe
The posterior lobe of the pituitary gland secretes the following hormones:
1. Antidiuretic Hormone (ADH) or Vasopressin:
Release:
Secretions caused by decrease in blood pressure, blood volume, and osmotic pressure of the
blood which is detected by osmoreceptors in the hypothalamus. External sensory stimuli also
influence hypothalamic neurosecretory cells.
Functions:
Increased levels cause increased water reabsorption in distal parts of kidney.
Disorder:
Lack of this hormone produces Diabetes insipidus. As a result there is the production of large
quantities of dilute urine and great thirst. Oxytocin:
Release:
Its release is stimulated by distension of cervix, decrease in progesterone level in blood and
neural stimuli during parturition and suckling.
Functions:
Primary action is on smooth muscles, particularly in the uterus during childbirth and also causes
milk ejection from mammary glands.
Thyroid Gland
Location:
In mammals thyroid gland consist of two lobes situated below the larynx. In mammals the thyroid
gland consists of two lobes.
Hormones:
It produces:
(i) Thyroxin (or tetraiodo-thryonine: T4) tri-iodothyronine or T3 (which has a structure
similar to thyroxin with 3 iodine atoms and not 4).
(ii) Calcitonin hormone.
Functions:
The thyroid is active continuously but produces higher levels of secretions during periods of rapid
growth and sexual maturation and in stress situations such as cold and hunger.
(i) Functions of Thyroxin and Tri-iodothyronine:
These two hormones act in the same way:
(a) They act on the basal metabolic rate by stimulating the breakdown of glucose and
release of heat and generation of ATP.
(b) They also act in combination with somatotropin in bringing about growth, and act
directly on brain cells causing them to differentiate.
(c) In amphibians, they affect the process of metamorphosis. If secretion of thyroid is
deficient, tadpole larvae of frog does not metamorphose to develop into frog, but
instead grow to a large sized tadpole.
(ii) Effects of Over Secretion (Grave's Disease):
Excess thyroxin produces a condition called Grave's disease, with exophthalmic goiter and
increase in the basal metabolic rate.
This can lead to cardiac failure if prolonged. The cause of Graves' disease is the production
of an abnormal body protein. This protein continuously stimulates the thyroid for
excessive secretion.
Effects of Under Secretion:
(i) Cretinism:
If congenitally 'deficient, the lack of thyroxin causes cretinism, where the individual
fails to develop normally. They are small, have coarse scanty hair, thick yellowish scaly
skin and mentally retarded they also fail to develop sexually.
(ii) Goiter / Myxoedema (Occurs Later in Life):
Individuals with iodine-deficient diets may have goiter, a condition in which the
thyroid becomes greatly enlarged.
It produces a swelling of the neck (goiter) and may lead to lying down of excess fat and.
weight is increased. The condition is known as myxoedema. In the myxoedema, the
puffiness of hands and skin is produced. Reduced metabolism, body temperature and
pulse rate results. All bodily and mental processes are retarded. Table salt with iodine is
recommended so that there is no deficiency of iodine and thus of thyroxin in the body.
Functions of Calcitonin:
High Ca++ ions concentration in the blood causes stimulation of the synthesis and release of
calcitonin. Low levels of Ca++ ions suppress its manufacture.
Functions:
Its function is antagonistic to parathormone (from the parathyroid glands) and prevents removal
of Ca++ ions from the bones.
Oversecretion / Undersecretion:
Excess or deficiency leads to a disturbance of calcium metabolism. Disturbance in calcium
metabolism seriously affects nerves; skeleton, muscle and blood etc.

Parathyroids:
Location:
In man the glands are found embedded in the posterior part of the lateral lobes of the thyroid.
Hormone:
These produce a hormone called Parathormone.
Functions:
Low levels of blood Ca++ ions stimulate the parathyroid directly to increase parathormone
production. The high levels of Ca++ ions suppress its release.
Under secretion /Over secretion:
Under-activity causes a drop in blood Ca + ions which in turn leads to muscular tetany. Over-
activity would lead to a progressive demineralization of the bones similar to rickets. Similarly
there is the formation of massive kidney stones. Both conditions may be fatal.
Islets of Langerhans (Pancreas)
Location:
These are present in the pancreas.
Control:
These are under the control of pituitary trophic hormones STH and ACTH and responds directly
to the level of blood glucose.
Cell Types:
The islets contain two types of cells:
(i) β-cells: These are larger in number and secrete insulin.
(ii) α-cells: These are smaller in number and secrete glucagon.
Functions of Insulin:
Insulin decreases blood glucose levels in many ways which include:
• Increasing glycogen synthesis.
• Increasing cell utilization of glucose.
• Conversion of glucose into lipid and protein.
• Insulin inhibits the hydrolysis of glycogen in the liver and the muscles.
Disorders Due to Undersecretion or Oversecretion of Insulin:
Diabetes Mellitus:
Failure to produce insulin leads to a condition called diabetes mellitus.
Symptoms:
(a) High level of blood sugar
(b) Sugar in the urine,
(c) A disturbance of the body's osmotic equilibrium
(d) derangement of the nervous system.
(e) Toxic metabolites from fat (which need 'glucose energy' for their oxidation) also
accumulate and are only lost from the kidney with valuable metal cations.
(f) The body becomes dehydrated.
Hypoglycaemia:
If excess insulin is produced, the utilization of sugar is too great and its level falls in the blood
(hypogIycaemia) which upsets nerve and muscle functioning.
Functions of Glucagon:
Glucagon is antagonistic to insulin and increases the blood glucose levels. It does this mainly by
promoting breakdown of glycogen to glucose in the liver and muscles.
It also increases the rate of breakdown of fats.
Disorders:
Glucagon abnormalities are rare as endocrine disorders. Tumors on the a-cells will cause excess
glucagon secretions. The result is high blood glucose levels. This in turn damages the a-cells.
Adrenals
Location:
A pair of adrenals is present, one on the top of each kidney.
Structure:
Its outer layer is called as adrenal cortex while the inner layer is called as adrenal medulla.
The Hormones of Adrenal Medulla:
The medulla produces the hormones adrenaline and noradrenaline.
Release:
Both adrenaline and noradrenaline are secreted in stress situations. In rats whose adrenal
medulla has been removed surgically the ability to withstand any stress situation (such as cold)
is reduced.
Functions:
Adrenaline: It dilates blood vessels In certain parts of the body (such as the skeletal muscles) and
increases the heart's output.
Noradrenaline:
It constricts blood vessels in certain areas (such as the gut).
Combined Effects:
(i) Both hormones are involved in raising blood pressure
(ii) Both promote the release of glucose from liver glycogen
(iii) They also reinforce the effects of the sympathetic system
Cortical Hormones:
The adrenal cortex secretes Cortisol, Corticosterone and aldosterone hormones. ACTH from the
pituitary stimulates secretion of these hormones. The adrenal cortex is active at all times but
especially active in a shock or stress situations and infections.
Functions:
Cortisol: It increases blood glucose level mainly by its production from protein and b)
antagonizing the action of insulin.
Corticosterone:
It increases blood glucose levels and regulates mineral ion balance. Therefore it is both a
glucocorticoid and a mineralocorticoid.
Aldosterone:
The adrenal cortex mainly secretes aldosterone. It conserves the level of N a + ions in the body
by preventing their loss from the kidney tubules.
Disorders:
(i) Under Secretions:
The destruction of the adrenal cortex (such as occurs in Addison's disease) will lead to
general metabolic disturbance. In this case there will be weakness of muscle action
and loss of salts.
Stress situation such as cold which would normally be overcome lead to collapse and
death.
(ii) Over Secretions:
The reverse of this is found in Cushing’s disease where too much cortical hormone is
produced. Symptoms are an excessive protein breakdown resulting muscular and
bone weakness. The high blood sugar disturbs the metabolism as in diabetes.
Androgens (e.g. Testosterone):
Very small amounts of androgens are secreted in both male and female by adrenal glands.
Androgens cause development of the secondary male characteristics.
Disorder:
A tumor on the inner part of the adrenal cortex in a female can cause excess of androgens to be
produced. As a result there is the development of certain male characteristics. However; such
cases are very rare.
Gut
Many parts of the gut function as endocrine tissue. The important hormones produced are
Gastrin and secretin:
(i) Gastrin:
It is the hormone produced by mucosa of the pyloric region of the stomach. It
stimulates the secretion of gastric juice.
(ii) Functions:
It is produced' under the influence of protein food in the stomach after it is partially
digested.
(iii) Secretin:
It is produced from the duodenum when acid food touches its lining.
Functions:
It affects the pancreas to produce and release pancreatic juice. It also affects the rate of
bile production in the liver.
Gonads
These are ovaries and testes. The ovaries are inside the body while the testes are outside
Ovary:
Following hormones are secreted by the ovary:
Oestrogens:
Oestrogens are secreted by ripening follicles (and, in many species. by interstitial cells the ovary).
The development of the follicle was initiated by FSH from the pituitary.
Functions:
(i) Oestrogens bring about the development of the secondary sexual characters in the
female.
(ii) Cause thickening of the uterine wall and
(iii) At a point during the oestrous or menstrual cycle, Oestrogens exert a positive feedback
As a result; there is a sharp rise in LH output by the pituitary
(iv) It also aids in healing and repair of uterine wall after menstruation
(v) Under the influence of oestrogens, some of cells of uterine wall become glandular and
start secreting proteinaceous secretions which are taken up by the embryo when in its
early stages of development.
Under secretion:
Deficiency of the sex hormones leads in the young to failure to mature sexually and sterility in
the adult.
Progesterone:
Release:
Progesterone is produced by the ruptured follicle in response to LH from the pituitary.
Functions:
(i) Progesterone inhibits further FSH secretion from the pituitary. In this way it prevents any
more follicles from ripening.
(ii) It also affects the uterus, causing further thickening and vascularization of its wall, and
other areas of the female body, preparing it for the maintaining state of pregnancy.
(iii) It suppresses ovulation. That is why it is a major component of birth control pill.
Testes
Structure:
The testes consist of many coiled seminiferous tubules. Here the spermatozoa develops between
the tubules are the regions of interstitial cells.
Hormones:
The interstitial cells produce gonadal hormones called testosterone and 17 β-
Hydroxytestosterone.
Release of Hormones:
After the initiation of the sex organs in the foetus, the level rises fairly consistently until puberty.
After puberty the supply of LH, and therefore the level of testosterone, remains constant.
Functions:
(i) In the foetus it initiates the development of the sex organs
(ii) At puberty it brings about development of the male secondary characteristics and
promotes the sex drive
(iii) The castrated male fails to develop secondary sexual characteristics and his body is more
like an immature female.
Feedback Mechanism
It is a type of interaction in which a controlling mechanism is itself controlled by the products of
reactions it is controlling.
Explanation:
For proper body functions two opposing systems are needed, if there are accelerators, there
must be inhibitors. If one hormone in the body promotes or stimulates a reaction, another will
check it. This occurs due to feedback mechanism.
Let us take an example:

Feedback in Thyroid Gland Function:

(i) Low body temperature (or stress) stimulates neurosecretory cells of the hypothalamus.
These cells release hormones.
(ii) These hormones activate the release of thyroid-stimulating hormone (TSH) in the anterior
pituitary.
(iii) The TSH then stimulates the thyroid gland to release thyroxine.
(iv) Thyroxine causes an increase in the metabolic activity of most body cells, generating A TP
energy and heat.
(iv) Both the raised body temperature and higher thyroxine levels in the blood inhibit the
releasing-hormone cells and the TSH-producing cells.

TABLE: Human endocrine glands and their principal secretions.

Glands Hormone Chemical Effect
Structure
Posterior lobe Anti-diuretic Peptide Reduces amount of water lost In urine
hormone (ADH) Raises blood pressure by constricting
arterioles.
Oxytocin Peptide Contraction of smooth muscle during
childbirth. Stimulates secretion of milk
from mammary glands.
Anterior lobe Adrenocorticotrophic Peptide Stimulates production and release of
hormone (ACTH) hormones from adrenal cortex.
Follicle stimulatin Glycoprotein Controls the development of follicles
hormone(FSH) also in the ovary, and sperm cells in the
called ICSH in males testis.
Growth hormone Protein Promotes growth (especially of
skeleton and muscles). Affects body
metabolism.
Luteinising hormone Glycoprotein Stimulates ovulation and formation of
(LH) the corpus luteum (stimulates
testosterone production in males).
Prolactin Protein Stimulates milk production and release
during pregnancy.
 Thyroid stimulating Glycoprotein Stimulates growth of thyroid gland
hormone (TSH) synthesis and production of thyroid
hormones.
Thyroid Thyroxin + Iodine Thyroxin Increases rate of cell
Tri-iodothyronine Containing metabolism, controls aspects of
Amino acids growth and development and controls
basal metabolic rate (BMR)
Parathyroid Parathormone Peptide Raises blood calcium levels by
stimulating release of calcium from
bone
Pancreas (Islets Insulin (Produced Protein Lowers blood glucose levels by making
of Langerhans) By the β-cells) cell membranes more permeable to
glucose increase glycogen storage in
liver.
Glucagon (Produced Peptide Raises blood sugar levels by
by the a cells) stimulating glycogen breakdown in the
liver
Adrenal Cortex Glucocorticoid Steriods In response to stress, raises blood
e.g. cortisol glucose.
Mineralocorticoids, Steriods Concerned with water retention
e.g., aldosterone Increases reabsorption of sodium
chloride in kidneys, so important in
control of blood volume and pressure
Adrenal medulla Adrenaline Modified amino acid

Fear, flight and flight reactions Prepares the body for

heightenedactivity. Mimics effects of the autonomic
nervous system

Noradrenaline Modified amino acid

As adrenaline

Gonads Ovary Oestrogens Steroids Female sex characteristics building of uterus lining
(follicle) after menstruation, inhibits FSH
Ovary (corpus) Progesterone Steroids
Stimulates maturation of uterus lining, maintains
pregnancy inhibits FSH.
Testis Androgens Steroids Support sperm production important the
e.g development ofmad in secondary sexual
testosterone characteristics
Placenta Human Sterioid
Chorionic
Causes corpus luteum to secrete thus maintaining
Gonadotrophin progesterone. Pregnancy
(HCG)