Respiration, Nervous System, Brain & Eyes | Science & Technology for UPSC CSE PDF Download

Respiration

In vertebrates (higher animals) the respiration is carried out by any of the following three methods:

• Integumentary or cutaneous respiration,
• Gills or bronchial or, Aquatic respiration as in fishes or, aquatic animals. 
• Lungs respiration in terrestrial animals. 

Amphibians (who inhabit on land as well as water) like frog have developed both types of respiration—cutaneous and lungs. Respiratory pigment, which carries oxygen and carbon dioxide, in man is haemoglobin found in RBC. A person breathes about 16 to 20 times per minute while at rest. However, this breathing rate is higher at the time of muscular exercise and in small children.

Respiratory Organs in Man

Respiratory organ consists of nostrils, nasal chamber, larynx (or, voice box), trachea and two bronchi. Each bronchus divides and subdivides into bronchioles which finally lead to air sacs or alveoli. In man the lungs are made up of some 750,000,000 alveoli having a total surface of about 100 sq. metres. Each lung consists of its bronchial tree with its many air sacs and alveoli units together with blood vessels, nerves and pleura. All these are bound by the connective tissue. The respiratory passage performs three important function:

It makes the air pass over an extensive surface through complicated scrolls of the turbinals, so that air is warmed before entering the lungs. It filters the air by holding dust particles in the mucus covering the turbinals. Its internal wall has the sense of smell.

Respiratory Pigments in Animals
Pigment	Site (Locatedat)	Metallic group			Distribution in Animals
Haemoglobin	RBCs and Plasma	Iron	Red	Red	All vertenrates, Annelids and Molluscs.
Haemocyanin	Plasma	Copper	Blue	Colourless	Most Molluscs and Arthropods.
Haemoerythrin	Corpuscles	Iron	Red	Colourless	Some Annelids.
Chlorocruorin	Plasma	Copper	Green	Green	Some Annelids.
Pinnaglobin	Plasma	Manganese	Brown	Brown	Some Molluscs.

Physiology of Respiration

Taking in of oxygen and passing out of carbon dioxide from lungs constitutes external respiration. Cell respiration or internal respiration occurs in the tissue cells. Air in alveoli has about 14% oxygen which diffuses through the wall of alveoli into blood capillaries. Haemoglobin of erythrocytes combines with oxygen to form a loose compound oxyhaemoglobin. Oxyhaemoglobin reaches tissues through blood where it gives up its oxygen. Some oxygen is also taken in dissolved state in the plasma. Enzymatic reactions bring about oxidation of glucose in cells by a series of complicated chemical reactions to produce chemical energy, carbon dioxide and water. The reduced haemoglobin returns with the blood. In the lungs carbon dioxide is liberated as free gas and oxygen enters the erythrocytes and plasma.

Nervous System

Nervous system consists of central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), automatic nervous system (sympathetic and parasympathetic nerves).

Nervous tissue

Consists of nerve cells (neurons), processes of nerve cells (nerve fibres), packing cells (neuroglia) and epithelial (ependymal) cells. Neurons are of various forms in different parts. A neuron has a long cell body (cyton or perikaryon), cytoplasm (neuroplasm), a conspicuous nucleus, two or more extensions (neurofibrils) and small granules (Nissl’s granules).

The process that transmits stimuli to the cell body is the dendrite (branched) and that carries impulses away from it, is the axon (unbranched). Neurons are the longest cells of the body. A ganglion is a group of cytons with nuclei outside the central nervous system. A nerve is formed by a group of fibres bound together by the connective tissue.
Based on the physiological properties, the neurons are of three types:

• Efferent (Motor) neurons—Carry impulses from the central nervous system to the effector organs.
• Afferent (Sensory) neurons—Carry impulses from periphery to centrally located nervous elements.
• Internuncial (Connector) neurons—These are intermediate between afferent and efferent neurons. They are neurosecretory.

Brain

Elonated, flattened, whitish organ, lies in cranial cavity of skull divisible into 3 regions—fore brain (olfactory lobes, cerebral hemispheres, dicephalon), mid brain (optic lobes, crura cerebri), hind brain (cerebellum, medulla oblongata).

Functions of Brain

The olfactory lobes are concerned with the organs of smell. The cerebral hemispheres are the dominating parts of the brain, they receive impulses from the nose, eyes, ears, and tactile receptors and control their functioning. The hemispheres are also the seat of voluntary actions and they co-ordinate the activities of the animal. Moreover, the cerebral hemispheres are the seat of memory, consciousness, associations, imagination, will power and experiences. Diencephalon relays impulses from the posterior regions to the cerebral hemispheres; it recognizes sensations of heat, cold, pain, and body movements, and it regulates somatic activities of the autonomic nervous system.
The anterior optic lobes along with the cerebral hemispheres control vision, the posterior optic lobes are auditory in function.

The cerebellum is concerned with balancing and co-ordination of muscles, responses started by cerebral hemispheres are carried out by the cerebellum, and it makes necessary adjustments for the orders of the cerebral hemispheres. The medulla oblongata controls heat, respiration, taste, blood pressure and secretions of digestive glands; it transmits impulses from the cerebral hemispheres to the spinal cord and also in the opposite direction, thus it regulates complex muscular movements of the body. Four functional divisions of the spinal cord, namely, the somatic sensory, somatic motor, visceral motor and visceral sensory are extremely well developed in mammals.

Reflex action and Reflex arc

A nervous reflex is an involuntary act brought about by stimulation of a receptor. A reflex may be defined as an immediate and rapid response given without our awareness by an effector organ on the arrival of some external or interval stimulus.

Reflexes may be of two types.

Simple reflex: It is an inborn, inherited or unlearned response to a stimulus e.g. knee-jerk reflex.

Conditioned reflex: It is the response acquired as a result of training or experience to a stimulus that originally failed to evoke the reaction.

The structural and functional basis of reflex action is called reflex arc.

Functions of Sympathetic Nervous System: (on stimulation)

(a) Contraction of muscles.

(b) Secretion of sweat glands.

(c) Constriction of blood vessels of the skin.

(d) Dilation of bronchi.

(e) Contraction of the heart.

(f) Sudden increase in the blood pressure.

(g) Contraction of urinary bladder muscles.

(h) Sudden fall in number of RBC in blood.

(i) Rapid coagulation of blood.

Functions of Parasympa thetic Nervous System (on stimulation) :

(a) Constriction of the pupil.

(b) Dilation of blood vessels.

(c) Contraction of urinary bladder muscles.

(d) Contraction of digestive tract walls.

Eyes

Eyes are situated in the orbits of skull. Each eyeball is protected by upper and lower eyelid having eyelashes at its margin. The third eyelid or nictating membrane is reduced and vestigeal in man and is found at one corner of the eye. On the margin of eyes are small meiboian glands present that secrete an oily substance to lubricate the eyes; and lacrymal or tear glands to produce tears that keep the eye ball moist. Eyes are moved by a set of six eye muscles. In man both eyes are brought forward so that he has binocular vision. Eyeball is formed of following three layers: Sclerotic: It is the outermost layer of the eyeball whose two-third portion is opaque and is in orbit; while one-third is continued in front a transparent cornea. Over the cornea another transparent membrane called conjunctive is present which is an extension of the skin of eyelid. It has got fine blood capillaries. Choroid: It lies next to the selevotic and is made of loose connective tissues. In front it thickens as a ringlike ciliary body. In front of the ciliary body choroid forms Iris which possess a circular aperture in the center called pupil. The presence of lens and iris divides the eye ball into an anterionr aquous chamber, filled by watery aquous humour, and a posterior vitreous chamber filled by gelatinous vitreous humour.

Retina: Retina is a thin delicate membrane which consists of two sub-layers.

(a) an outer layer of pigment cells lying immediately after the choroid; and

(b) an inner sensory layer consisting of two types of nerve cells—rods & cones, which are connected to optic nerves.

Rodes contain the visual pigment rhodopsin which helps the animal to see in dim light. Nocturnal animals like owls have mainly rods in their retina. This enables them to see in dim light. Cones contain different pigments and help the animal to differentiate the three primary colours i.e., red, green and blue. Rods and cones are not evenly distributed in the retina. Some animals like sparrows which are active in day time only have mostly cones in their retina and have very poor night vision. The posterior part of retina contains only cones and has yellow pigment, and is called as yellow spot or area centralise or fovea. There is maximum vision in this region. In the middle of retina a blind spot is present that lack both rods and cones. Usually the optic nerve arises from this spot. 

Working of the Eye

The size of pupil regulates the amount of light entering into the ball. An inverted image of the object is formed on the retina which is conveyed to the brain by the optic nerve.
The image is then re-inverted by the brain. Change in curvature of lens form images of box near and far objects. But in some animals like fishes accomodation for near objects is brought about by increasing the size of eyeball but not of lens.