Feedback mechanism:

A mechanism or a signal that tends to initiate (or accelerate) or to inhibit (or slow down) a process.

Supplement

An example of positive feedback loop is the onset of contractions in childbirth. When contraction begins, the hormone oxytocin is released into the body to stimulate further contractions. As for the negative feedback loop, an example is the regulation of blood glucose levels. If blood glucose levels continue to rise it may result in diabetes. In fact, there are many biologic processes that use negative feedback to maintain homeostasis or dynamic equilibrium.

Possitive Feedback Mechanism:

A feedback mechanism resulting in the amplification or growth of the output signal.

Example:

One example of biological positive feedback is at the onset of contractions in childbirth. When contraction occurs, oxytocin is released into the body stimulating more contractions. Thus, the result is an increased amplitude and frequency of contractions.

Negative Feed back Mechanism:

A feedback in which the system responds in an opposite direction to the perturbation.

Example:

It is a self-regulatory system in which it feeds back to the input a part of a system’s output so as to reverse the direction of change of the output.

The process reduces the output of a system in order to stabilize or re-establish internal equilibrium. There are several negative feedback in biological system to regulate and maintain homeostasis. Some of which are the regulation of hormone synthesis, blood glucose levels, body temperature, and baroflex in blood pressure.

Human Ear:

The ear, which is the organ of hearing and balance, consists of the outer, middle, and inner ear. The outer, middle, and inner ear function together to convert sound waves into nerve impulses that travel to the brain, where they are perceived as sound. The inner ear also helps to maintain balance.

 Look Inside the Ear

                                       Click here to watch the video about human ear.

Outer Ear

The outer ear consists of the external part of the ear (pinna or auricle) and the ear canal (external auditory meatus). The pinna consists of cartilage covered by skin and is shaped to capture sound waves and funnel them through the ear canal to the eardrum (tympanic membrane), a thin membrane that separates the outer ear from the middle ear.

Middle Ear

The middle ear consists of the eardrum and a small air-filled chamber containing a chain of three tiny bones (ossicles) that connect the eardrum to the inner ear. The ossicles are named for their shapes. The hammer (malleus) is attached to the eardrum. The anvil (incus) is the middle bone between the hammer and the stirrup (stapes), which is attached to the oval window, a thin membrane at the entrance to the inner ear. Vibrations of the eardrum are amplified mechanically by the ossicles and transmitted to the oval window.

The middle ear also contains two tiny muscles. The tensor tympani muscle is attached to the hammer and helps tune and protect the ear. The stapedius muscle is attached to the stirrup. This muscle contracts in response to a loud noise, making the chain of ossicles more rigid so that less sound is transmitted. This response, called the acoustic reflex, helps protect the delicate inner ear from sound damage.

The eustachian tube is a small tube that connects the middle ear to the airway in the back of the nose (nasopharynx). This tube allows outside air to enter the middle ear (behind the eardrum). The eustachian tube, which opens when a person swallows, helps maintain equal air pressure on both sides of the eardrum and prevents fluid from accumulating in the middle ear. If air pressure is not equal, the eardrum may bulge or retract, which can be uncomfortable and distort hearing. Swallowing or voluntary "popping" of the ears can relieve pressure on the eardrum caused by sudden changes in air pressure, as often occurs when flying in an airplane. The eustachian tube's connection with the middle ear explains why upper respiratory infections (such as the common cold), which inflame and block the eustachian tube, can lead to middle ear infections or changes in middle ear pressure, resulting in pain.

Inner Ear

The inner ear (labyrinth) is a complex structure consisting of two major parts: the cochlea, which is the organ of hearing, and the vestibular system, which is the organ of balance.

The Endocrine System

The endocrine system is a set of hormone secreting glands within the body of an animal. The function of the endocrine system is homeostasis, communication and response to stimuli. The endocrine system regulates the internal environment of the animal for growth, survival and reproduction as well as allowing it to respond to changes in its external environment.

The endocrine system’s glands secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands.

Endocrine Glands

There are 10 endocrine glands. As stated previously, other organs such as the stomach, intestines, kidneys, heart, brain, and placenta also make hormones.

The Pituitary Gland

The pituitary gland is often called the master gland. That is because the pituitary gland produces hormones that regulate other endocrine glands. Some hormones produced by the pituitary gland are:

1.                Follicle Stimulating Hormone (FSH): Will be discussed in a later Chapter of the syllabus.       

2.                Luteinising Hormone (LH):

          Will be discussed in a later Chapter of the syllabus.

3.                Growth Hormone (GH): Causes body cells to absorb amino acids and form protein for growth. The main function is to cause the elongation of bones.

4.                Prolactin: stimulates milk formation by the breast after the birth of the baby.

5.                Oxytocin: stimulates muscle contraction of uterus during birth, stimulates muscle contraction in the milk ducts during breast-feeding.

6.                Antidiuretic Hormone (ADH): causes increased water reabsorption by kidneys.

7.                Thyroid Stimulating Hormone (TSH): Combines with iodine at the thyroid gland to produce thyroxine.

Overproduction of GH causes gigantism and underproduction causes dwarfism.

The Hypothalamus

 

The hypothalamus links the nervous system with the endocrine system. It produces hormones that control the pituitary gland’s responses to messages from the brain and other hormones. Some these hormones, called releasing hormones, stimulate the pituitary gland to make other hormones. Others, called release inhibiting hormones, prevent the production of pituitary hormones.

        An example is growth hormone releasing factor. This causes the production of growth hormone (GH) by the pituitary gland.

The Pineal Gland

This gland is in the brain. One hormone produced there is melatonin. Synthesis and release of melatonin is stimulated by darkness and inhibited by light. But even without visual cues, the level of melatonin in the blood rises and falls on a daily (circadian) cycle with peak levels occurring in the wee hours of the morning. Melatonin is readily available in drug stores and health food stores, and it has become quite popular. Ingesting even modest doses of melatonin raises the melatonin level in the blood to as much as 100 times greater than normal. These levels appear to promote going to sleep and thus help, insomnia to hasten recovery from jet lag, and to not to have dangerous side effects.

The Thyroid Gland

The thyroid gland produces the hormone called thyroxin. Thyroxin controls the rate of all the body’s internal reactions. In other words, thyroxin controls the rate of the body’s metabolism.

        Physical conditions related to abnormal thyroid function are:

Hypothyroidism- Under Production of Thyroxine

1.                Cretinism- Under production of thyroxin in young children. This results in low metabolic rates and results in retarded physical and mental development.

2.                Myxoedema- Under production of thyroxin in adults. Characteristics are tiredness, lack of energy, slow mental and physical activity, and weight gain.

3.                Goitre- Swelling of the thyroid caused by myxoedema.

Goitre

In cases of low production of thyroxine tablets are available to increase the thyroxine in the body. Since thyroxine needs iodine to be produced iodine is also administered to boost thyroxine levels.

Thyroxine Excess (Hyperthyroidism)

Thyroxine secretion is above normal. This causes a raised level of metabolism. Symptoms of over production of thyroxin are bulging eyes, weight loss heat production, nervousness, irritability, and anxiety. This condition is called Grave’s Disease. Corrective measures for Grave’s Disease are:

1.    Drugs to suppress thyroid activity

2.    Surgically remove part of the gland

3.    Use radioactive iodine to destroy some of the gland.

The Parathyroids

    There are 4 parathyroid glands. They are located within the thyroid gland. The hormone they produce is calledparathormone. This hormone stimulates the release of calcium from the bones. That is why we must continue to include calcium in our diet even when our bones are fully grown.

The Thymus Gland

This gland is located behind the breastbone. It produces the hormone thymosin. This hormone causes white blood cells (lymphocytes) to become mature and active. These blood cells, as previously discussed in the Blood web page, are involved in the body’s immune system.

The Adrenal Glands

The adrenal glands are located on top of each kidney. They secrete the hormone called adrenaline (also called epinephrine). This hormone prepares the body for stress and is released when we are frightened or feel stress. It does the following:

1.                Increases blood flow to the heart, muscles, and brain.

2.                Reduces blood flow to the kidneys. This helps reduce blood loss if we are cut. It causes us to get pale.

3.                Opens the bronchioles allowing us to get more air.

4.                Increases glucose levels in the blood.

5.                Increases heartbeat rate.

6.                Increase muscular contraction and strength.

7.                Increases mental alertness.

Pancreas

            

            As discussed in the Human Nutrition web page the pancreas secretes pancreatic juice for the digestive system.

        In addition, the pancreas produces the hormone called insulin. This hormone is produced in groups of cells called Islets of Langerhans. Insulin is needed because it reduces blood glucose levels in the blood. It causes cells, especially fat and muscle cells, to absorb glucose from the blood. The glucose is needed for cellular respiration or converted into glycogen. The glycogen is stored in the liver or the muscles for future use in cellular respiration.

Diabetes is a serious condition that results from 1 of 2 causes. In type 1 diabetes, the pancreas no longer makes insulin and therefore blood glucose cannot enter the cells to be used for energy. In type 2 diabetes, either the pancreas does not make enough insulin or the body is unable to use insulin correctly. Symptoms of diabetes are high glucose levels in the blood and urine, the production of large amounts of urine, severe thirst, loss of weight, and tiredness.

Injections of insulin, which are taken daily, the control of carbohydrate intake, exercise, and weight control treat diabetes.

Anabolic Steroids

Anabolic steroids are hormone supplements that habe been used. They build up muscle, speed up recovery of muscle from injury, and help strengthen bones. There are many serious side effects such as liver and adrenal gland failure, infertility, impotence, and the development of male characteristics in females that can result if they are misused. They are also, sometimes given to animals to promote increased lean muscle (meat) production. This practice is banned in the EU.

Control of Thyroxine Level

          Control of thyroxine level as well as many other hormones is done by negative feedback. If the thyroxine level is normal the pituitary gland is inhibited from releasing thyroid stimulating hormone (TSH). As a result, no further thyroxine is produced. When thyroxine levels are low the pituitary gland produces TSH. This causes more thyroxine to be produced by the thyroid gland.

An Example of negative feedback in the role of the thyroid in maintaining body temperature at 37°C.:

1. The hypothalamus of the brain detects a drop in blood temperature.
2. The hypothalamus stimulates the pituitary to secrete TSH (thyroid-stimulating hormone).
3. This hormone stimulates the thyroid to increase its secretion of thyroxine.
4. The higher concentration of thyroxine increases metabolism and heat production increases.
5. The blood is warmed back to normal temperature.

OR:

1. Hypothalamus detecting raised blood temperature and reduces its stimulation of the pituitary.
2. High thyroxine levels inhibiting the release of TSH from the pituitary.
3. The increased level of thyroxine leads to the limitation or reduction of its secretion.
4. Body’s metabolism slows down as a result of less thyroxin. The body’s temperature goes down.

Review Chart  of Major Hormonal Glands

Where the Hormone is Produced	Hormone(s) Secreted	Hormone Function
Adrenal Glands	Adrenalin	Causes Emergency Responses (fight/flight)
Pituitary Gland	Growth hormone	Affects growth and development; stimulates protein production
Pancreas	Insulin	Lowers blood sugar levels; stimulates metabolism of glucose, protein, and fat
Hypothalamus	Growth Hormone Releasing Factor	Causes growth hormone to be made
Pineal Gland	Melatonin	Controls body rhythms
Parathyroid Glands	Parathyroid hormone (Parathormone)	Affects bone formation and excretion of calcium and phosphorus
Thyroid	Thyroxine	Controls Metabolism
Thymus	Thymosin	Matures white blood cells

 Defects of Human Eye:

There are many eye related problems and defects of the eye, the main few are briefly discussed below:

  Defects of the eye:

Myopia: (nearsightedness) This is a defect of vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses/contacts with concave lenses these help to focus the image on the retina.

Hyperopia: (farsightedness) This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.

Human Eye:

The anatomy and physiology of the human eye is an important part of many courses 

(e.g. in biology, human biology, physics, and practical courses in medicine, nursing, and therapies).
It includes a simple diagram of the eye together with definitions of the parts of the eye labelled in the illustration

This is a very simple introduction the subjects of "The Eye" and "Visual Optics".

                                            Click here to watch the video about Anatomy of Eye.

Term Definition / Description Aqueous Humour The aqueous humour is a jelly-like substance located in the anterior chamber of the eye. ... more about the Aqueous Humour Choroid The choroid layer is located behind the retina and absorbs unused radiation. ... more about the Choroid Ciliary Muscle The ciliary muscle is a ring-shaped muscle attached to the iris. It is important because contraction and relaxation of the ciliary muscle controls the shape of the lens. ... more about the Ciliary Muscle Cornea The cornea is a strong clear bulge located at the front of the eye (where it replaces the sclera - that forms the outside surface of the rest of the eye). The front surface of the adult cornea has a radius of approximately 8mm. The cornea contributes to the image-forming process by refracting light entering the eye. ... more about the Cornea Fovea The fovea is a small depression (approx. 1.5 mm in diameter) in the retina. This is the part of the retina in which high-resolution vision of fine detail is possible. ... more about the Fovea Hyaloid The hyaloid diaphragm divides the aqueous humour from the vitreous humour. ... more about the Hyaloid Membrane Iris The iris is a diaphragm of variable size whose function is to adjust the size of the pupil to regulate the amount of light admitted into the eye. The iris is the coloured part of the eye (illustrated in blue above but in nature may be any of many shades of blue, green, brown, hazel, or grey). ... more about the Iris Lens The lens of the eye is a flexible unit that consists of layers of tissue enclosed in a tough capsule. It is suspended from the ciliary muscles by the zonule fibers. ... more about the Lens Optic Nerve The optic nerve is the second cranial nerve and is responsible for vision. Each nerve contains approx. one million fibres transmitting information from the rod and cone cells of the retina. ... more about the Optic Nerve Papilla The papilla is also known as the "blind spot" and is located at the position from which the optic nerve leaves the retina. ... more about the Optic Papilla Pupil The pupil is the aperture through which light - and hence the images we "see" and "perceive" - enters the eye. This is formed by the iris. As the size of the iris increases (or decreases) the size of the pupil decreases (or increases) correspondingly. ... more about the Pupil Retina The retina may be described as the "screen" on which an image is formed by light that has passed into the eye via the cornea, aqueous humour, pupil, lens, then the hyaloid and finally the vitreous humour before reaching the retina. The retina contains photosensitive elements (called rods and cones) that convert the light they detect into nerve impulses that are then sent onto the brain along the optic nerve. ... more about the Retina Sclera The sclera is a tough white sheath around the outside of the eye-ball. This is the part of the eye that is referred to by the colloquial terms "white of the eye". ... more about the Sclera Visual Axis A simple definition of the "visual axis" is "a straight line that passes through both the centre of the pupil and the centre of the fovea". However, there is also a stricter definition (in terms of nodal points) which is important for specialists in optics and related subjects. ... more about the Visual Axis Vitreous Humour The vitreous humour (also known as the "vitreous body") is a jelly-like substance. ... more about the Vitreous Humour Zonules The zonules (or "zonule fibers") attach the lens to the ciliary muscles. ... more about the Zonules

Reflex Action:

Reflex actions:

It is defined as an involuntary action performed by muscles under the direction of spinal cord in response to the stimulus. The best example of reflex action is pulling of hand when we touch hot or very cold objects and contraction of pupil when light is shown in the eye.

Reflex arc:

The path which is followed by impulses during reflex action is called reflex arc. It consists of an afferent (or sensory) nerve, usually one or more interneurons within the central nervous system, and an efferent (motor, secretory, or secreto-motor) nerve

.                                                                     Click here to watch the video about reflex action

Two situations are invoved in reflex action.

Voluntory coordination

Involuntry coordination

Voluntry Coordination:

This process occur with the thinking process involved e.g making a choice of questions in the examination paper.Walking,talking,eating and speaking are some other examples.

Involuntry co-ordination:

these processes occur without thining process being involved.Some involuntry coordinations continue to occur without our being aware of them e.g working of heart, lungs and others.This part of nervous system is called autonomous nervous system.i.e it works all by itself.Many other responses are instantaneous in nature.They occur within fractions of seconds. They are called reflex Actions.

Peripheral Nervous System-Control and Coordination:

Peripheral Nervous System (PNS)

It includes the nerves that arise from the brain and spinal cord. There are twelve pairs of nerves which directly arise from the brain and called cranial nerve. Similarly there are 31 pairs of nerves that arise from the spinal cord and called spinal nerve. They connect the organ system to the brain.

Autonomic Nervous system (ANS)

Autonomic nervous system has two types.
Sympathetic nervous system
Parasympathetic Nervous system.

It is the system of ganglion and sub-ganglion. The local swelling of nerves in organs are called ganglion and further swelling of nerves to various part of the organ is called sub-ganglia. It is divided into sympathetic and parasympathetic nervous system.The function of ANS is to maintain body steady state rapidly.

Cranial Nerves: They arise from brain and terminate inside it except the Vague. There are 12 pairs of cranial nerves in human beings. They control the activities of eyes ears and tongue etc.

Spinal nerves: They arise from the spinal cord and spread to different parts of the body. There are 31 pairs of spinal nerves in the human beings. Out of them 8 are cervical 12 are thoracic 5 pairs of lumbar 5 pairs of sacral and 1 pair coccygeal nerves