ICSE Class 8 Biology Oxford Chapter 03 Control and Coordination

Control and Coordination

Learning Outcomes

Sense organs and their function: eye, ear, nose, skin, and tongue

Coordination

Response to internal stimuli

Endocrine system

In class 7, you must have learned that the human body is made up of ten different organ systems, each of which performs vital functions in the body. Two of these systems (the nervous and endocrine systems) control the functioning of all organ systems in the body.

The nervous system is helped in this by the sense organs. The sense organs connect the external world to our nervous system. There are five sense organs that capture the five senses of vision (eyes), hearing (ears), smell (nose), taste (tongue), and touch (skin).

The Five Sense Organs

Eyes (vision)

Ears (hearing)

Nose (smell)

Tongue (taste)

Skin (touch)

Fact File

Sharks have two additional senses: an electrical sense that helps them detect tiny electrical currents in the water and a vibration sense that enables them to detect low-frequency vibrations.

In this chapter we will explore the functions of each of these sensory organs in detail. We will also discuss the functioning of nervous and endocrine systems. Let us start by learning about different sense organs and their functions.

Eye

Eyes have the ability to detect light, which enables us to see. Each eye is contained in a socket in front of the skull. The eye is nearly spherical in shape.

The wall of the eye consists of three layers. The outer layer is called the sclera. It is a tough, protective layer. The sclera is opaque, but has a transparent bulge called the cornea in the front. The second or the middle layer is the choroid, which forms the iris, the coloured part of the eye. Pupil, which is situated in the middle of the iris, regulates the amount of light entering the eye. Behind the iris is the lens, which is kept in place by suspensory ligament. The third layer is the retina, which has light-sensitive cells called rods and cones.

Rods are sensitive to dim light and cones to bright light and colour. There are three kinds of cones, each responding to a different colour: red, green, or blue. The lens focuses the light on the retina, which stimulates the rods and cones. The rods and cones pass on the light stimulation in the form of an electrical discharge (or impulse) to the optic nerve. The optic nerve carries this impulse to the brain, where it is interpreted. Figure 3.1 shows the internal structure of the human eye.

Fact File

Although the whole of the retina has light- and colour-sensitive cells, an area called the yellow spot present in the centre of the retina has the greatest number of cone cells. Thus, this area has the most accurate vision.

Ear

The ear may be divided into three regions: the external ear, the middle ear, and the internal ear. The external ear consists of the ear lobe or pinna and the auditory canal. The auditory canal carries sound waves to the tympanum or the ear drum.

The middle ear is a small chamber within a bone of the skull. A chain of three bones called ear ossicles connects the tympanum with the membrane covering the oval opening. From the outer end of the chain, these bones are malleus, incus (anvil), and stapes (stirrup).

The internal ear consists of a complex set of membranous tubes and sacs collectively called the membranous labyrinth. It is situated in a bony cavity containing a fluid. The membranous labyrinth consists of two sacs called the utricle and the saccule. Associated with the utricle are three semicircular canals, which help us in maintaining our balance. Associated with the saccule is the cochlea, a spirally coiled tube that resembles a snail's shell. The cochlea is involved in hearing. It contains thousands of hair cells that vibrate when sound waves strike it. These vibrations are converted to electrical signals, which are conducted by the auditory nerve to the brain. The brain interprets the sound and gives it meaning. Figure 3.2 shows the internal structure of the human ear.

Fact File

Dolphins can hear better than human beings. In dolphins, the lower jaw also plays a role in the process of hearing-by conducting sound vibrations to the middle ear. Also, the teeth in the lower jaw are arranged in such a manner that they act as antennae, which receive the incoming sound. This ability, called echolocation, enables dolphins to point out the location of an object.

Nose

The nose is the organ of smell as also of breathing.

The nose opens externally through two holes called the nostrils. The nostrils and the corresponding nasal passages are separated by a wall called the septum. In the middle of the face there is a hollow space called the nasal cavity. The palate separates the nasal cavity from the mouth. The nasal cavity is lined with a type of epithelium called olfactory epithelium. This epithelium has special receptors that catch the molecules of matter floating in the air that passes through the nose. There are different kinds of receptors for different molecules of matter. One type of molecule may stimulate several different kinds of receptors. All of these signals are sent through the olfactory nerve to the brain, which interprets the particular smell. Figure 3.3 shows the internal structure of the human nose.

Tongue

The tongue is the organ of taste. It is covered with a number of taste buds, each taste bud having a number of taste receptor cells. There are specific areas on the tongue for identifying a particular taste. The four primary tastes recognized are: sweet, salty, bitter, and sour (Fig. 3.4).

Skin

The skin forms a protective covering for the entire body and serves as an important sense organ.

It is about 5 mm thick and consists of an outer thin layer of epidermis and an inner thick layer of dermis. The innermost layer of cells in the epidermis is made up of living cells that divide actively to form new cells. They gradually replace the outer dead cells at the surface of the skin that are being constantly rubbed off and lost from the body. The inner cells contain a dark pigment melanin that protects the body from ultraviolet rays and gives a characteristic colour to the body. The darker your skin is, the more melanin you have. The dermis is thicker than the epidermis and is richly supplied with blood vessels and nerves. Sweat glands, oil glands, and hair follicles are also embedded in the dermis (Fig. 3.5).

The sensation of touch is due to the millions of microscopic receptors that the skin has. These receptors are placed at different depths in the skin and have an uneven distribution. At some places, such as the fingertips, they are very close to each other and large in number, so your fingertips are quite sensitive. In some other places they are spaced apart. Whenever you touch something these receptors send electrical signals to the brain which collects all of them and forms an overall picture. There are three main kinds of receptors:

light, touch and pressure receptors that detect movement or pressure;

heat and cold receptors that detect changes in temperature; and

pain receptors that respond to pain. They are the most numerous.

Teacher's Note

When you touch a hot surface, your hand pulls back instantly before you even feel pain - this is a reflex action that protects you from injury.

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