Anatomy and Function of the Ear

• Special senses include hearing, taste, and smell.

Anatomy of the Ear

• The auricle (pinna) is the external part of the human ear.
• The auditory canal (auditory meatus) channels sound to the middle ear.
• Semicircular canals are fluid-filled structures involved in balance.
• The cochlea is the ear structure storing part of the hearing organs.
• The tympanic membrane (ear drum) is a thin skin layer at the end of the auditory canal, adjacent to the malleus.
• Auditory ossicles are a series of 3 bones in the middle ear (hammer, anvil, and stirrup) that transmit sound to the inner ear.

Anatomy & Function of the Ear

• The ear has three parts: the external, middle, and inner ear.
• The external ear collects and channels sound to the middle ear.
• The middle ear transforms sound wave energy into bone vibrations, then into a compressional wave for the inner ear.
• The inner ear transforms compressional wave energy in its fluid into nerve impulses for transmission to the brain.
• The outer ear includes the ear flap and a ~2 cm long ear canal
• The ear flap protects the middle ear and eardrum.
• The middle ear is an air-filled cavity with the eardrum and three interconnected bones: malleus, incus, and stapes.
• The eardrum is a durable, tightly stretched membrane that vibrates when pressure waves reach it.
• The inner ear consists of the cochlea, semicircular canals, and auditory nerve.
• The cochlea and semicircular canals are filled with water-like fluid.
• The fluid and nerve cells in the semicircular canals act as accelerometers, which detect movement and help maintain balance.
• The cochlea is a 3 cm long snail-shaped organ.
• The cochlea's inner surface is lined with over 20,000 hair-like nerve cells, which are critical for hearing.
• When a compressional wave moves from the middle ear's hammer through the oval window of the inner ear, it causes the small hair-like nerve cells to move.
• This motion releases an electrical impulse that passes along the auditory nerve to the brain.

How Hearing Works

• Sounds are collected by the outer ear and sent through the ear canal to the eardrum.
• Three small bones in the middle ear amplify sounds.
• In the inner ear, sounds from the middle ear cause the liquid inside the cochlea to move.
• Receptor cells send impulses to the auditory nerve, which transmits them to the brain where the impulses are interpreted

Why Two Ears

• Two are used because sound hits one a fraction of a second before the other, producing stronger vibrations.
• Two ears assist with determining sound direction.

Connection to Balance

• If the vestibular organs are damaged/diseased, they send too many/too few impulses to the brain.
• These abnormal signals register in the brain as body imbalance.
• The false feeling of motion or dizziness, is called vertigo.

Disorders of Hearing

• Hearing disorders include:
  • Otitis (inflammation of the ear), acute and chronic otitis media, otitis externa
  • acute labyrinthitis
  • otosclerosis (progressive hearing loss)
  • motion sickness
  • deafness.

Taste

• Taste determines the flavor and palatability of food.
• It signals dangerous gases and toxic food.
• Taste buds are small bumps all over the tongue.
• There are four different types of taste buds.
• The liking of sugar is due to a need for carbohydrates.
• Salt cravings occur because sodium chloride is a dietary necessity.
• Bitter and sour tastes cause avoidance, because most poisons are bitter.
• Umami, the savory taste, drives appetite for amino acids and proteins.
• Taste papillae are visible as little red dots or raised bumps, at the front of the tongue, they are called "fungiform" papillae.
• Three other kinds of papillae are foliate, circumvallate, and non-gustatory filiform.
• Taste buds are collections of cells on these papillae and cannot be seen unaided.
• Fungiform papillae: they are on the most anterior part of the tongue, innervated by the chorda tympani branch of the facial (VIlth cranial) nerve, and richly supplied with blood vessels.
• Foliate papillae: situated on the edge of the tongue slightly anterior of the circumvallate line, predominantly sensitive to sour tastes, and innervated by the glossopharyngeal (IX cranial) nerve.
• Circumvallate papillae are sunken papillae.
• They are situated on the circumvallate line and confer a sour/bitter sensitivity to the posterior 2/3 of the tongue and the IX nerve.
• Bitter tastes (like tonic water) are usually sensed towards the back and rear sides of the tongue.
• Salty and sweet tastes (like sugar) are usually tasted at the tip of the tongue.
• Sour tastes (like lemon juice) are usually tasted at the sides of the tongue, the middle, and towards the front.
• Taste-related diseases include:
  • glossitis (inflammation of the tongue),
  • tongue ulcers,
  • fungal infections,
  • tumors.

Smell

• Every time breath is taken, air passes through the nasal cavity
• Turbinates (shelves of bones) direct it down through the back of the mouth into the throat.
• Some air entering the mouth flows passes the olfactory organs.
• Olfactory organs:
  • Olfactory cells-receptors-roof of nasal cavity,
  • olfactory nerves pass through the ethmoid bone,
  • olfactory bulb,
  • olfactory tract to the olfactory area of the temporal lobe.
• Odor molecules in the air get stuck to the mucus in the nose.
• Sensory hairs detect the odor and send messages to the brain.
• Then, the brain identifies the odor.
• Smell receptor cells are located high behind the nose.
• The receptor is sensitive to chemicals in the mucus in the nose.
• Smell-related diseases include:
  • sinusitis (caused by viruses, fungi/molds, allergies)
  • phantom sensations (unpleasant/disordered tastes and smells)
  • anosmia (no smell).