Special Sensory Reception

Equilibrium (balance) and Hearing

Externalacousticmeatus

Auricle(pinna)

Helix

Lobule

Pharyngotympanic(auditory) tube

Tympanicmembrane

Externalear

Middleear

Internal ear(labyrinth)

Anatomy of the Ear

• Three parts of the ear:1. External (outer) ear• Includes the auricle (pinna) which surroundes the entrance to

the external acoustic meatus (ear canal)– The auricle protects the ear canal and collects and funnels sound into

the ear canal

• Ceruminous glands are along the external acoustic meatus and secrete a waxy material that helps prevent the entry of foreign objects and insects

• The external acoustic meatus ends at the tympanic membrane (eardrum) which separates the inner and middle ear

Anatomy of the Ear

2. Middle ear (tympanic cavity)• Air-filled chamber• Communicates with the superior portion of the

pharynx (nasopharynx). This connection is called the auditory tube (pharyngotympanic tube or Eustachian tube)– Enables equilization of pressure on either side of the

eardrum

• Contains the auditory ossicles (3 tiny bones together)– Connect the tympamun with the inner ear– The 3 bones are the malleus, incus, stapes– Act as levers that conduct the vibrations to the inner ear

Pharyngotympanic(auditory) tube

Auditoryossicles

Entrance to mastoidantrum in the epitympanic recess

Tympanic membrane

Semicircularcanals

Cochlea

Cochlearnerve

Vestibularnerve

Oval window(deep to stapes)

Round window

Incus(anvil)

Malleus(hammer)

Stapes(stirrup)

Vestibule

Anatomy of the Ear

3. Internal (inner) ear• Provides the senses of hearing and equilibrium• Protected by a bony labyrinth

– Surrounds and protects the membranous labyrinth– Subdivided into 3 parts:

» Vestibule – contains sacs that provide sensations of gravity and linear acceleration

» Semicircular canals – receptors here are stimulated by rotation of the head

» Cochlea – receptors here provide the sense of hearing

• The receptors of the inner ear are the hair cells– Communicate with sensory neurons by continually releasing small

quantities of neurotransmitters

Anterior

Semicircularducts insemicircularcanals

PosteriorLateral

Cristae ampullaresin the membranousampullae

Utricle investibule

Saccule investibule Stapes in

oval window

Temporalbone

Facial nerve

Vestibularnerve

Superior vestibular ganglion

Inferior vestibular ganglion

Cochlearnerve

Maculae

Spiral organ(of Corti)Cochlearductin cochlea

Roundwindow

Equilibrium

• 2 aspects of equilibrium:– Dynamic equilibrium

• Aids us in maintaining our balance when the head and body are moved suddenly

– Static equilibrium• Maintains our posture and stability when the body is motionless

• All equilibrium sensations are provided by hair cells

Hearing• Receptors of the cochlear duct provide us with the

sense of hearing– The receptor responsible are hair cells similar to those in

the vestibule and semicircular canals– Sound energy is converted in air to pressure pulses which

stimulate hair cells along the cochlear spiral– The frequency (pitch) of the percieved sound is

determined by which part of the cochlear duct is stimulated

– The intensity (volume) is determined by how many hair cells at that location are stimulated

Steps in Hearing

1. Sound waves arrive at the tympanic membrane2. Movement of the tympanic membrane causes displacement

of the auditory ossicles3. The movement of the stapes at the oval window establishes

pressure waves in the preilymph of the vestibular duct4. The pressure waves distort the basilar membrane on their

way to the round window of the tympanic duct5. Vibration of the basilar membrane causes vibration of hair

cells against the tectorial membrane6. Information about the region and intensity of stimulation is

relayed to the CNS over the cochlear branch of the cranial nerve VIII