C. Shevlin1 Chapter 35 The Senses Objectives Introduction: Humans have 5 senses: touch, taste, smell, sight & hearing The senses are based on receptor cells or groups of receptors that form sense organs.

C. Shevlin2 Receptors are specialised to respond to various stimuli such as heat, light, pressure & chemicals. Touch

C. Shevlin3 The skin contains receptors for touch and temperature. These receptors are found in different concentrations around the body. e.g. many temperature receptors at the elbow. Taste Receptors for taste are located in taste buds. They are found at the top and sides of the tongue and in some parts of the lining of the throat.

C. Shevlin4 The flavour of food is a combination of taste, texture & temperature. Smell The roof of the nasal cavity has about 20 million neurons to detect smell (olfactory neurons). These neurons respond to about 50 different chemicals in the gaseous state. Sight Structure of the eye:

C. Shevlin5 Functions of the parts of the eye

C. Shevlin6 Function of each part of the eye NameFunction Fovea Optic Nerve Blind spot Lens Irish Pupil Ciliary Muscle Vitreous Humour

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9 Eye Sight Defects 1.Short Sight (myopia) Condition: can view close objects clearly but distant objects are out of focus. Light rays are focussed short of the retina. Cause: eyeball is too long or the focussing elements of the eye are too strong. Correction: use a concave (divergent) lens to widen the angle over which the light rays have to be refracted. Short Sightedness Object comes into focus in front of the retina. Need to use a concave lens to focus the image onto the retina so the person sees clearly Long Sight (hypermetrpia) Condition: can view distant objects clearly but close objects are out of focus, The focal point is long of the retina i.e. is behind retina. Cause: eyeball is too short or the focussing elements of the eye are too weak. Correction: use a convex (convergent) lens to reduce the angle over which the light rays have to be refracted. Long sightedness – Object comes to a focus behind the retina. Need to use a convex lens to focus the light onto the retina

C. Shevlin10 Hearing Functions of the Ear The functions of the ear are hearing and balance. The ear is composed of 3 sections: Outer ear Middle ear Inner ear The outer and middle ear are filled with air the inner ear is filled with a fluid called lymph.

C. Shevlin11 Functions of the parts of the ear PartFunction Pinna Helps collect & channel vibrations into the auditory canal Auditory Canal Tube that carries vibrations to t he eardrum Eardrum Small tightly stretched membrane that vibrates due to air vibrations reaching it. It separates the outer & middle ear Semi-circular Canals Make up the vestibular apparatus which is responsible for balance. Ossicles 3 tiny bones in the middle ear tat transmit vibrations from the outer ear to the middle ear and amplify (increase) the vibrations. Vestibular Nerve Nerve that carries impulses from the ear to the brain Cochlear (auditory) Nerve Carries impulses from the cochlea to the brain Eustachian Tube Tube that connects the middle ear and pharynx (throat) and equalises pressure on either side of eardrum Cochlea Spiral tube which is responsible for hearing

C. Shevlin12 PartFunction Pinna Auditory Canal Eardrum Semi-circular Canals Ossicles Vestibular Nerve Cochlear Nerve Eustachian Tube Cochlea Functions of the parts of the ear

C. Shevlin13 The Ear & Hearing The pinna collects vibrations The auditory canal carries vibrations to the eardrum The eardrum carries the vibrations to the middle ear. The ossicles (the hammer, anvil & stirrup bones) amplify (increase) the vibrations and pass them onto the oval window.

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C. Shevlin15 The Eustachian tube connects the middle ear with the pharynx and equalises the pressure between the middle and outer ear.

C. Shevlin16 Inner Ear The inner ear contains: 1) The Cochlea 2) The Semi-circular canals

C. Shevlin17 2. The Ear & Balance Balance is largely detected by the vestibular apparatus In the middle ear. Balance is maintained due to vision, Receptors in the muscles, ligament & Tendons, & pressure receptors In the soles of the feet.

C. Shevlin18 A person who damages their vestibular apparatus loses his/her sense of balance but the person may learn to use other receptors to redevelop their sense of balance. The vestibular apparatus Is filled with liquid. Receptors located in different Parts of the vestibular apparatus Detect whether the head is vertical or not. All these receptors send impulses to the cerebellum of the brain through the vestibular nerve.

C. Shevlin19 Corrective Measures for a Hearing Disorder Disorder: Glue Ear Cause: Surplus sticky fluid collecting in the middle ear often due to overproduction of fluid as a result of infection. Correction: Nose drops taken to decongest and unblock the Eustachian Tube In more severe cases small tubes called grommets are inserted into the eardrum. Grommets allow air into the middle ear & this forces fluid down the eustachian tube.

C. Shevlin20 Summary of Sense Organs