ORGAN OF HEARING, SMELL AND TASTE

CONTENT

• The organ of hearing (Structure of the ear and their functions)
• Mechanism of hearing
• Mechanism of balancing
• Organ of smell (structure and function of nose)
• Organ of taste (structure and functions of the tongue

THE ORGAN OF HEARING

Mammals have two ears on each side of the head.  The greater part of it is enclosed in the skull for protection.  The ear is for hearing as well as for maintaining balance.

STRUCTURE OF THE EAR AND THEIR FUNCTIONS

The mammalian ear is divided into 3 parts.

1. The outer ear.
2. The middle ear.
3. The inner ear.

The outer ear

This is made up of the following from outside the organism:

1. The Pinna: – This is flexible being made up of soft cartilage covered with skin.  The pinna collects sounds, detects their direction and directs them into the canal called external auditory meatus (ear tube).
2. External Auditory Meatus: – This consists of fine hairs and glands which produce wax. Therefore it prevents germs, insects and dusts from entering and affecting the ear. It allows the passage of sound waves from the pinna to the ear drum.
3. The Tympanic Membrane (Ear Drum):- This is a thin membrane which vibrates when sound waves get to it.  It separates the external ear from the middle ear.  Therefore, it transmits sound waves from the outer ear to the middle ear.

The middle ear

This is a small air-filled chamber in the skull, which is made up of three tiny soft bones (ear ossicles) and Eustachian tube.

1. Ear Ossicles: – These include malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles form a bridge linking the outer and inner ear through a membrane covered opening called round window.  The ossicles transmit vibrations across the tympanic membrane to the oval window and the pressure in the window is magnified.
2. Eustachian Tube: – This is a narrow tube in the middle ear connecting it to the pharynx.  This usually opens when yawning, it allows air to enter or leave the middle ear so that the air pressure on both side of the eardrum is equal.

The inner ear

 This is made up of a bony complex passage way called bony labyrinth, which is filled with a fluid called perilymph. Inside the bony labyrinth are membranous sacs and tubes called membranous labyrinth   filled with the fluid, endolymph.

The bony and membranous labyrinth forms the two auditory structures which include the cochlea and the semicircular canals. These two structures are connected to the utriculus and sacculus respectively.

1. The Cochlea: – a snail shell-like structure of hearing is concerned with hearing. It contains sensory hair cells (mechanoreceptors) which synapse with sensory neurones that form the cochlear nerves; they both form the organ of corti.   
2. Semi-circular canal: – the three canals lie at right angle to one another, they have swollen ends called ampullae which contain sensory hair cells and otoliths (ear stones). These structures are concerned with balance and maintaining the posture of the body.

EVALUATION

1. Mention five structural parts of mammalian ear and their functions
2. Mention two sensory cells found in the ear

FUNCTIONS OF THE EAR

The two major functions of the ear is hearing and balancing.

Mechanism of hearing

The pinna detects and collects sound waves in the air and concentrates them and passes them on through the external auditory meatus.  The wave causes the tympanic membrane to vibrate and the vibrations are passed on to the ear ossicles which amplify them.   The oval windows magnify the vibrations, passing them into the inner ear (cochlea) where the perilymph and endolymph vibrate.  The vibrations of the endolymph stimulate the organ of corti in the cochlea to convert the sound vibration to electrical impulse. The impulses then stimulate the auditory nerves of the acoustic branch to the brain for interpretation.

Mechanism of balancing

Head movement in any direction causes the fluid in the semicircular canals and the otoliths in the ampullae to move. Impulses are set up and transmitted through the vestibular branch of the auditory nerves to the brain for interpretation.  The brain relays impulses to the body muscles for balancing the position of the body.

DISEASES OF THE EAR

The major disease of the ear is deafness.  It can be temporary or permanent.  It can be caused by damage to the tympanic membrane, eustachian tube or sensory cells in the cochlea.  Also, wax blocking the ear canal, infection of the ear or sudden very loud or constant noise causes deafness.

Care of the ear

1. Regular use of cotton wool
2. Avoid the use of sharp object in cleaning the ear.
3. Avoid loud noise
4. Avoid being blown on the ear side that can damage the ear drum.
5. Consulting medical doctors in case of any ear problem.

EVALUATION

1. Describe the mechanism of hearing and balancing.
2. Outline five ways of caring for the ear.

ORGAN OF SMELL (NOSE)

The human olfactory organ is the nose.  The epithelia lining of the nasal cavity is rich in sensory nerve ending.  Compared to lower class animals like dog, man has a poor sense of smell.  We detect smell quickly but for a short time.

MECHANISM OF SMELLING

Nose functions well when wet.  The smell receptors are stimulated by chemicals.  Particles of volatile chemical substances in the air dissolve in the mucus layer over the cell receptors in the nostrils.  The stimulation of the receptors gives rise to nerve impulses which travel through the olfactory nerve to the olfactory lobe of the brain.  The brain then interprets the kind of smell.

ORGAN OF TASTE (TONGUE)

The sensory cells for taste are called taste buds located on the tiny swellings on the exposed surface on the tongue.  They are connected by four sensory nerves to the brain which interprets what is tasted.  The tongue therefore is sensitive to four primary tastes:

1. Sweet taste: detected by chemoreceptors at the tip of the tongue.
2. b.Salty taste: detected by chemoreceptors by the side (frontal).
3. Sour taste: detected by chemoreceptors at the side of the tongue (towards the back).
4. Bitter taste – detected by chemoreceptors at the back.

MECHANISM OF TASTING

Chemicals from any substance put in the mouth dissolves in the saliva on the tongue, this stimulates the sensory nerve endings in taste buds which then pass the impulses to the brain for interpretation as sweet, bitter, sour or salty

NOTE: – Both smell and tastes are closely related sensations, hence we smell what we taste.  When we eat and drink, the taste receptors are stimulated, at the same time flavour producing chemical from the food dissolves in the moist air in the mouth and flow into the nasal cavity to stimulate the smell receptors.  The smell sensation is more severe than taste sensation.

EVALUATION

1. Draw the structure of the tongue to show the major sensitive areas 
2. Describe the olfactory functions of the nose

GENERAL EVALUATION

1. State the advantages of having two ears
2. Discuss briefly two types of deafness
3. Draw a well labeled diagram of the tongue to show the areas that are sensitive to primary tastes
4. Describe the mechanism of smelling
5. State four functions of the organ of smell

READING ASSIGNMENT

College Biology, chapter 11 page 229 – 234

WEEKEND ASSIGNMENT

1. The following are parts of the inner ear except a) ampulla b) semi-circular canal            c) cochlea d)  ear ossicles
2. ——– helps to equalize air pressure on either side of the ear drum. a) sacculus                  b) Eustachian tube c) ampulla d) endolymph
3. The following are concerned with hearing except a) organ of corti b) semi circular canals c) cochlea d) acoustic nerve
4. Which part of the ear is responsible for the maintenance of balance a) cochlea                          b) perilymph c) semi-circular canal d) stapes
5. The inner ear is filled with ——– and ——— fluid

THEORY

1. Explain why a person that has been spinning feel dizzy for sometime after stopping.
2. State five causes of deafness