• Hormones are the chemical messenger produced in small amount by endocrine glands, secreted into blood stream to control metabolism and biological activities in target cell or organs.

Characteristics or properties of hormone:

• Low molecular weight
• Small soluble organic molecules
• Rate of diffusion is very high and are readily oxidized but the effect does not remains constant
• It is effective in low concentration
• Travels in blood
• It has its target site different from where it is produce and  is specific to a particular target
• Hormones are non-specific for organisms and may influences body process of other individuals

Functions of hormones

• Regulatory and homeostasis functions
• Maintain consistency of interior of cell
• Permissive functions; movement of substance in and out of cell
• Integrative function; usually balance two system
• Developmental function; helps in development of foetus

Classification of hormone

Hormones are classified

1. On the basis of chemical nature
2. On the basis of mechanism of hormone action
   1. Group I hormone
   1. Group II hormone

A. On the basis of chemical nature:

1. Protein hormones: insulin, glucagon
2. Steroid hormone: sex hormones, glucocorticoids
3. Aminoacids derivatives hormones: epinephrine, nor epinephrine etc

B.  On the basis of mechanism of hormone action

1. Group I hormone (lipophilic hormone):

• These hormones are lipophilic in nature.
• They are mostly derivatives of cholesterol.
• These hormones binds to intracellular receptors
• Example: Steroid hormones, Estrogen, androgen, glucocorticoids, cholcalciferol, thyroxine etc

2. Group II hormones (water soluble hormone):

• These hormones binds to cell surface receptors and stimulates the release of certain molecules (secondary messengers) to perform biochemical functions

On the basis of secondary messengers group II hormones are of 3 types;

i. Secondary messenger is cAMP:

• eg. Adrenocorticotropic hormone, FSH, LH, PTH,ADH, calcitonin, glucagon,

ii. Secondary messenger is phosphotidylinocitol/calcium or both:

• eg. Acetylcholine, vasopressin, cholecystokinin, gastrin, gonadotropin releasing hormone, thyrotropin releasing hormone,
• Insulin, chorynoic somato mamotropin, epidermal growth factors, fibroblast growth factors, GH, prolactin

iii. Secondary messenger is cGMP:

• Atrial natriuretic peptide (ANP)

Hormones- Properties, functions and classification

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• Hormones are the chemical messenger secreted directly in the blood stream by endocrine gland. They are target specific and bind to the specific receptor. On the basis of binding of hormone on their specific receptor, the mechanism of hormonal action is categorized into two group. They are-

1. Fixed membrane receptor mechanism
2. Mobile receptor mechanism

Fixed membrane receptor mechanism

• The hormones that are protein or amines in compositions such as Growth hormone, ADH, oxytocin, Insulin, Adrenaline, FSH, TSH etc shows this mechanism of action.
• These hormones are water soluble and cannot passes through the lipid membrane and they have their target receptor on the cell membrane. The receptor are fixed on the cell membrane, so hormone can bind on the specific receptor.
• Binding of hormone on specific receptor on target cell activates the enzyme Adenylcyclase in the cell membrane and causes production of cyclic AMP (cAMP).
• cAMP act as secondary messenger. It diffuse through the cell membrane and activates (Protein Kinase) various enzymatic reaction to cause biochemical changes. After the target cell responded to the changes, cAMP is deactivated by a group of enzyme Phosphodiesterase
• 

Figure: Fixed membrane receptor mechanism of hormone action( Eg. FSH)

Mobile receptor mechanism

• The lipid soluble hormones such as steroid hormones and Fatty acids hormones can easily passes through the plasma membrane.
• They have their receptor inside the cell, freely floating in the cytoplasm. Binding of hormone to the specific receptor activates the enzymatic activity of the cell for biochemical changes.
• Some hormones (testosterone, progesterone, estrogen, cortisol, thyroxine) have their receptor localized inside the nucleus, the hormone-receptor complex are carried inside the nucleus.
• The hormone-receptor complex initiate transcription of the DNA to form specific mRNA.
• mRNA initiate protein synthesis in the cytoplasm. The protein (enzyme) causes biochemical changes in the cell.Figure: Mobile receptor mechanism of hormone action (Oestrogen)

Mechanism of Hormone action

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• Mixed gland (both endocrine and exocrine function)

Structure:

• Pancreas is elongated 12-15 cm long organ consisting of head, body and tail. It is located posterior to stomach with its head in the curve of duodenum. The body and tail extends laterally. The tail touches the spleen.
• pancreas is considered as mixed gland as it acts as exocrine and endocrine gland.
• As exocrine gland, pancreas secretes digestive enzymes into pancreatic duct.
• As endocrine gland,it secretes hormones into blood.
• Only about 1% of total weight of gland acts as endocrine gland. This portion of pancreas is known as Islet of Langerhans.
• Adult pancreas contsins 200,000-20,00,000 islet of Langerhans.

Endocrine portion of pancreas is called Islet of Langerhans, which is a group of cells.

• Alpha cell= produce glucagon. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release.
• Beta cell= produce Insulin. Elevated blood glucose levels stimulate the release of insulin.
• Delta cell= produce peptide hormone Somatostatin. Pancreatic somatostatin inhibits the release of both glucagon and insulin.
• * somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines
• F cell (PP cell)= produce pancreatic polypeptide hormone.

Hormones of pancreas

1. Glucagon

• Alpha cells produce, stores and secretes glucagon.
• Glucagon stimulate glycogenolysis and gluconeogenesis; increase blood glucose level

2. Insulin

• Beta cell produce, store and secretes insulin.
• Insulin stimulate glycogenesis; decrease blood glucose level and Store glucose in the form of glycogen in liver and muscles
• Insulin function opposite to glucagon and work to maintain normal glucose level in blood.

disorders

i. Diabetes mellitus: condition caused by destruction or dysfunction of the beta cells of the pancreas or cellular resistance to insulin that results in abnormally high blood glucose levels.

ii. hyperglycemia: abnormally high blood glucose levels

iii. hypoglycemia; abnormally low blood glucose levels.

3. Somatostatin

• inhibit release of both glucagon and insulin

Pancreas: structure, hormones and functions

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• The adrenal gland occurs in pair. each adrenal gland is made up of two separate endocrine gland. the inner portion of each gland is called Medulla and outer portion is called Cortex.
•  Location: each adrenal gland is located on top of kidney as kidney cap
• Size: 4-6 cm long and 2-3 cm wide

# Structure of Adrenal gland

Divided into outer cortex and inner medulla

1. Adrenal cortex:

• It accounts for about 90% of weight of adrenal gland which weight for 5-7 gram.
• the cortex has three distinct zone

i. Zona glomerulosa:

• It lies directly beneath the capsule.
• It supplies cells for all other three zone. it is actually germinal layer.
• It produce mineralocorticoid.

ii. Zona faciculata:

• It lies beneath zona glomerulosa.
• It make up bulk of adrenal cortex.
• It secretes glucocorticoids and small amount of gonadocorticoids.

iii. Zona reticularis:

• It is the deepest layer.
• It consists of similar cell as zona faciculata but cells are arranged irregularily.
• It produce Dehydroepiandrosterone ( an intermediate hormone needed for production of sex hormone Androstenedione).

#Hormones secreted by Adrenal cortex

1. Glucocorticoid

• Regulate blood glucose level
• Stimulate gluconeogenesis
• Enhance synthesis of aminoacids which are substrate for gluconeogenesis
• Promote protein and nucleic acid metabolism
• Regulated by ACTH
• Acts as anti-inflammatory agents

2. Mineralocorticoid

• It is group of Steroid hormone (eg. Aldosterone), regulate concentration of mineral
• Help in maintaining blood pressure
• Help in Na+ reabsorption and cause excretion of K+, H+ and NH4+

3. Gonadocorticoid

• Sex hormone, stimulate gonads
• Regulated by ACTH

2. Adrenal medulla:

• It is the inner portion of adrenal gland.
• Medulla acts as a separate endocrine gland. It secretes separate hormones than cortex. However hormones produced by medulla are not as essential as cortical hormones.
• The secretory cells of adrenal medulla is called as Chromaffin cell because of their tendency to stain dark color.
• chromaffin cell synthesize, store and secrete epinephrine and nor-epinephrine which prepare body for fright, flight and fight.

#Hormones secreted by Adrenal Medulla

1. Epinephrine (adrenaline)

• commonly known as Adrenaline
• Increase rate of metabolism, heart rate and blood pressure
• Increase glycogenolysis, respiration rate, O2 consumption

2. Nor-epinephrine (Nor-Adrenaline)

• Increase heart rate, cardiac output, blood pressure
• Enhance lipid metabolism
• Relaxes smooth muscles of GI tract.
• Release free fatty acid from adipose tissue.

Disorder/disease of adrenal gland:

1. addition’s disease:

• bronze color pigmentation of skin
• low Na+ and high K+ level in blood plasma
• decrease resistance to infection

2. Cushing syndrome:

• abnormal obesity
• high Na+ and Low K+ level in blood

3. Aldosteronism:

• High Na+ level and Low K+ level in blood
• high blood pressure and increase blood volume

4. Adrenal virilism:

• male type external character such as beard, moustaches and voice develops in female

Adrenal gland: structure, location and hormones

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Structural anatomy of Parathyroid gland

• Parathyroid glands are tiny, lentil sized gland embeded in posterior of thyroid gland.
• two pairs of parathyroid glands are found.
• each thyroid lobe contains two parathyroid gland.
• The gland is composed of principal cells (secrete parathormone) and Oxyphilic cells (store parathormone)

Location: embedded on posterior of thyroid gland

Size: small lentil sized, 4 in number

Hormones of Parathyroid gland

1. Parathormone

• Regulate the concentration of calcium and phosphate level in blood
• Increase level of Ca++ in blood by reabsorption from intestine and kidney, Osteolysis
• Low Ca++ concentration in blood stimulate parathormone synthesis.
• Promote decalcification and demineralization of bone.
• stimulates Osteoclast to break (Osteolysis) to release Calcium from bone to blood.

* Improper balance of calcium and phosphate in blood causes faulty nerve impulse transmission, destruction of bone tissue, hamper bone growth and muscle tetany.

** when calcium level increases in blood, parathormone stimulates inhibition of reabasorption of calcium in kidney and inhibit osteolysis of bone.

 Disorder of Parathyroid gland:

1. Parathyroid tetany:

• deficiency of parathormone
• muscle spasms, sustained contractions of muscles
• may leads to death

2. Osteoporosis:

• over secretion of parathormone
• decalcification of bones
• bone becomes soft and porous

Parathyroid gland: structure, location and Hormones

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• Thyroid gland is located in neck, anterior to trachea. It consists of two lobe-one on each side of junction between laryngotrachea. The lobes are connected by a thyroid tissue called Isthmus.
• The gland is composed of two types of cells; follicular cell (secrete thyroid hormone) and para-follicular cell (secrete and store calcitonin hormone).
• Thyroid glands consists of hundreds of thousands of thyroid follicules which stores thyroid hormones.
• Follicles are made up of single layer of follicular cells.

Follicular cell:

• these cells are most prevalent cell.
• They secretes thyroid hormones (T3 and T4)

Parafollicular cell:

• cluster of parafollicular cells are found between follicluar cells.
• they are larger than follicular cell.
• parafollicular cells synthesize and secretes calcitonin.

*Thyroid is the only endocrine gland that is able to store its secretion outside the principle cells.

Location: in neck region on either side of trachea

Size: Each lobe is 4-6 cm long, and 1.5 cm thick

Hormones of Thyroid gland

1. Thyroxine (Tri-Iodothyronine and tetra-Iodothyronine)

• Follicular cells secretes thyroxine which contains 4 atoms of Iodine to form tetra-iodothyroxine (T4).  Follicular cells also secretes tri-Iodothyroxine (T3) which contains only 3 atoms of Iodine.
• T3 and T4 collectively known as Thyroid hormone.

Functions of thyroxine hormone:

• Increase basal metabolism rate by increasing O2 comsumption in most tissues except lungs, brain, testis and retina
• Essential for growth of skeletal in children. Thyroxine is regulated by TSH from Pituitary gland.
• Increase carbohydrate metabolism, promote gluconeogenesis
• Effect on lipid metabolism
• Stimulate NA-K ATPase.
• Influence water and electrolyte balance.
• It is regulated by TSH
• At first T4 is synthesized which is converted to T3. T3 is five times more potent than T4.
• Deficiency of thyroxine: Goiter, Hypothyroidism
• Excess of thyroxine: thyrotoxicosis, hyperthyroidism.

2. Calcitonin

• Parafollicular cells secretes calcitonin. Calccitonin is a polypeptide hormone.
• Secretion of calcitonin is regulated by concentration of calcium in blood but not by feed back mechanism of Pituitary gland.

Functions of calcitonin:

• It lowers calcium level in blood ( antagonistic to parathyroid hormone)
• It acts directly on Osteoclast to reduce the remodelingof bone and increase reabsorption of calcium.
• It increases the movement of Ca++ from blood to bone.

 Disorder of thyroid gland

1. Cretinism:

• deficiency of thyroxine in children
• pot belly, pigeon chest in children with physical and mental retardness
• dwarfism (cretins)

2. Myxedema:

• deficiency of thyroxine in adults
• puffiness of skin
• mental retardness
• low body temperature, BMR and blood pressure

3. Simple goitre:

• deficiency of iodine in diet and hence deficiency of thyroxine
• swelling of thyroid gland, visible in neck region
• low BMR. low Blood pressure and heart rate
• fatigue and sluggishness

4. Exopthalmic goitre (Grave’s disease):

• over stimulation thyroid gland
• goitre
• bulging of eye ball
• increased BMR, blood pressure and heart rate
• restlessness and nervousness

Thyroid gland: Hormones and functions

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• Pituitary gland is also known as hypophysis; situated below hypothalamus
• Size: 1 cm long, 1-1.5 cm wide and 0.5 cm thick
• Location: close to optic chiasma in the depression of sphenoid bone
• Pituitary gland has two lobe-anterior and posterior lobe. In between them there is a small region called pars intermedia or intermediate lobe.

Anterior lobe (Adenohypophysis):

• account for 75% of total weight. It consists of functional secretory cell to produce hormones.

Hormones of adenohypophysis

1. Growth hormone (Somatotropin)

• -associate with growth and growth rate of body
• -secretion of GH is controlled by two hormone produced by hypothalamus- Growth hormone releasing hormone (GHRH) and Growth hormone inhibiting hormone(GHIH))

2. Prolactin

• – In female; stimulate growth of mammary gland and milk production
• – Inhibit most of the time by Prolactin Inhibiting hormone (PIH) from Hypothalamus

3. Thyroid stimulating hormone (TSH)

• – stimulate thyroid gland to produce thyroxine
• -secretion of TSH is controlled by thyrotropin releasing hormone (TRH) and thyrotropin inhibitin hormone (TIH) from hypothalamus

4. Adrenocorticotropic hormone (ACTH)

• -stimulate adrenal cortex to secrete glucocorticoids
• -regulated by corticotropic releasing hormone (CRH) and corticotropic inhibiting hormone (CIH) from hypothalamus

5. Luteinizing hormone (LH)

• – stimulate corpus luteum to secrete progesterone and estrogen
• – In male, LH is known as Interstitial cell stimulating hormone (ICSH),it stimulate Interstitial cell of testis to secrete testosterone

6. Follicle stimulating hormone (FSH)

• – stimulate growth of ovarian follicle and secretion of estrogen
• – In male, FSH stimulate testis to produce sperm

7. Melanocytes stimulating hormone (MSH)

• – stimulate melanocyte formation

 Hormones of Neuro-hypophysis

• It doesnot synthesize hormone, rather it stores the hormones produced by hypothalamus

1. Vasopressin (Anti-diuretic hormone)

• -help in osmoregulation
• – increase permeability of PCT, and stimulate reabsorption of water
• – Inhibit by alcohol

2. Oxytocin

• Stimulus uterus contraction during child birth, milk ejection

Pituitary Gland- Hypophysis- Master Gland

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Synapse: the junction between the axon terminal of one neuron and dendrites, cell body or axon of another neuron is called synapse. The neuromuscular junction is also known as synapse.

Synaptic knob: the swelling terminal of axon or dendrites is known as synaptic knob.

Pre-synaptic neuron: the neuron carrying impulse toward synapse

Post synaptic neuron: the neuron which receive the impulse and carry the impulse away from the synapse.

Two theories have been put forward to explain the conduction of nerve impulse across the synapse. They are;

1. Electrical transmission theory
2. Chemical transmission theory

Electrical transmission theory:

Impulse transmission through synapse is accomplished by electric current. When the impulse reaches the pre synaptic knob, the impulse itself act as stimulus for the post synaptic neuron causing depolarization. Now the action potential generate in second neuron.

Chemical transmission theory:

Nerve impulse are conducted across the synapse with the help of chemical substances called neurotransmitter. The process of chemical transmission was discovered by Henry (1936).

Mechanism

Figure: Nerve impulse transmission through Synapse

• When nerve impulse reaches the pre-synaptic knob, it depolarized the presynaptic membrane and causes the opening of voltage gated calcium channel.
• Diffusion of Ca++ ion in the presynaptic knob causes movement of synaptic vesicle to the surface of the knob. Synaptic vesicle carries the neurotransmitter.
• Synaptic vesicles then fused with the presynaptic membrane and get rupture to discharge its content ie. Neurotransmitter (Acetlcholine) into synaptic cleft.
• Synaptic vesicles then return to the cytoplasm of pre-synaptic knob for refilling.
• Some of the released neurotransmitter binds with the protein receptor present on the post synaptic membrane of another neuron and change the membrane potential.
• Other unbound neurotransmitter immediately get lost from the synaptic cleft.
• The depolarization of the post synaptic membrane opens the Sodium channel causing influx of Na+ ion. Thus causing depolarization and generate action potential. In this way, the impulse get transmitted to next neuron along the synapse.

Nerve Impulse Transmission across Synapse

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• Nerve impulse: Nerve impulse is an overall physiological changes that occur in a neuron due mechanical, chemical or electrical disturbance created by a stimulus. It propagation through axon, synapse and neuromuscular junction is called Nerve Impulse conduction.

Nerve Impulse transmission along Neuron

Transmission of nerve impulse along nerve fibre can be summarized in three steps

1. Polarization (Resting Potential)
2. Depolarization (Action Potential)
3. Repolarization

                               Figure: nerve impulse transmission along neuron

Polarization (Resting potetential):

• A neuron at resting is electrically charged but not conducting.
• The Axoplasm or plasma membrane of a resting neuron is negatively charged as compared to the interstitial fluid.
• The potential difference measured at this stage is called resting potential which is about -70mV. The interstitial fluid has high concentration of Na+ ion which is about 16 times higher outside the neuron than inside neuron. Similarly, the axoplasm has high concentration of K+ ion which is about 25 times higher inside than in outer interstitial fluids.
• Due to difference in concentration of ions, Na+ ion tends to diffuse into the axoplasm and K+ ion tends to diffuse outside the axoplasm.
• The membrane of neuron at resting is more permeable to K+ ion than Na+ ion. So, K+ leaves the neuron faster than Na+ enter the neuron.
• The difference in permeability results in accumulation of high concentration of cation (+ve charged ion) outside the neuron compared to the concentration of cation inside.
• This state of resting neuron is called Polarized state and it is electro-negatively charged.

Depolarization (Action Potential):

• Any stimulus beyond the threshold can initiate an impulse.
• When such stimulus is applied in the resting neuron, it opens the sodium channel. Now the permeability of Na+ ion suddenly increases at the point of stimulus causing depolarization.
• The diffusion of Na+ ion increases by 10 times from outside to inside. As a result the axoplasm become positively charges, which is exact opposite to polarized state, so called as depolarized state or reverse polarized state.
• The depolarization of the membrane stimulates the adjacent voltage channel, so the action potential passes as a wave along the length of neuron.

Repolarization:

• When the concentration of Na+ ion inside axoplasm increases, the permeability to Na+ decreases and the sodium channel starts to close.
• The Na-K pump activates, so that Na+ are pumped out and K+ inside until the original resting potential is restored. The process is known as repolarization and it starts from the same point from where depolarization starts.
• The entire process of polarization, depolarization and repolarization occur within fraction of seconds. Now, again the neuron is read for another impulse.

Saltatory conduction:

• Transmission of nerve impulses is very rapid. However, nerve impulse conduction along unmyelinated neuron is slow than that of myelinated neuron. It is because, the myelin sheath act as insulator, so that the impulse have to jump from one node of Raniver to another.
• This speed up the conduction process, and this type of conduction is known as Saltatory conduction. 

Nerve Impulse Conduction

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