Special Senses: The Ear

By the end of this class you should understand: The properties of sound waves as they relate to hearing The three regions of the ear and their functions How the inner ear creates the senses of hearing and balance

Sensory Neurons Recall there are five types of neurons, classified by the type of stimulus they respond to Mechanoreceptor Thermoreceptor Nociceptor (Pain receptor) Chemoreceptor Photoreceptor

Mechanoreceptors Mechanoreceptors depend on mechanically gated ion channels When the cell is deformed, they open and allow sodium into the cell There are many types of mechanoreceptors The type of interest today is called the hair cell

Hair Cell Hair cells are unique in that they do not have an axon The “hair” of the hair cell (also known as stereocilia) bend when there are vibrations The bending allows positively charged ions into the cell This depolarizes the cell membrane causing neurotransmitters to be released

Vibrations The ear is responsible for detecting two different types of movements using hair cells Sound: vibrations of the air Vestibular sense: acceleration of the head Both of these occur in the inner ear

Parts of the Ear

Vestibular Sense Sense of balance or equilibrium Can sense both linear and angular acceleration The vestibule detects linear acceleration The semicircular canals detect angular acceleration

Acceleration Types Linear acceleration is in a straight line Detectable in a car that is braking or gunning the engine Also tells you which way is up Angular acceleration is spinning or rolling Detectable doing somersaults or spinning in a circle In both cases the hair cells are stimulated by the linear or angular movements of heavy crystals called otoliths

Acceleration and Nausea The brain must integrate signals from the different senses A combination of forebrain and midbrain work When these signals do not match the brain struggles to integrate them Nausea (“carsickness” and “seasickness”) may result from feeling acceleration in the vestibular sense but not visually detecting any acceleration

Angular Acceleration Spinning rapidly for an extended period of time can cause the semicircular canals to build up a lot of angular momentum When you stop spinning you experience dizziness because the semicircular canals are still spinning Causes the world to “spin” because your eyes are trying to follow the movement of the canals

Nature of Sound The hair cells for the vestibular sense respond to the movement of heavy crystals The hair cells for the sense of sound respond instead to the vibrations caused by sound To understand this, it is important to understand the nature of sound

Pressure Waves Sound is caused by pressure waves moving through a medium (air, water, etc) A single pressure wave will only sound like a crack or boom, not a tone Back-and-forth differences in air pressure at a certain frequency produce a constant tone The faster the waves of air pressure arrive at the ear, the higher the perceived pitch

Pitch and Volume The bigger the waves (the more energy is carried), the higher the volume Higher amplitude, measured in decibels Its all relative! The closer, the louder the sound The faster the waves arrive, the higher the pitch of the sound Higher frequency, measured in hertz Same unit used for light frequency but light waves have millions of times higher frequencies

Resonance A complication is that any object capable of producing or containing sound waves has a resonance frequency The best example of resonance is making waves in a bathtub slowly bigger and bigger by moving your own body in time with the waves Sound waves can have this happen as well! Note the different lengths on harp/piano strings

Resonance in the Cochlea There is a spiral-shaped section of the temporal bone called the cochlea The cochlea serves as a resonating chamber Vibrations from the ear are transmitted to pressure waves in the cochlea Much like piano strings, there is a high-frequency to low-frequency resonance of the hair cells in the cochlea

Parts of the Ear

Pressure Waves Pressure waves are transmitted through the outer ear Pinna (the part of the ear you can touch) Auditory canal (the part of the ear you can put a Q-tip in) The Tympanic membrane/eardrum forms the border between the outer and middle ear

Middle Ear The middle ear is a cavity with three tiny bones called ossicles that attach to the tympanic membrane at one end and the inner ear at the other The ossicles are unique to mammals When air vibrations cause the eardrum to vibrate, the three bones transmit these vibrations to the inner ear

Middle Ear Ossicles The three ossicles are malleus, incus and stapes Hammer, anvil, and stirrup, named for their shapes Malleus is attached to the eardrum, stapes pounds on the oval window The auditory tube connects this space to the nasal passage If blocked due to illness, can result in headaches/earaches and reduced hearing

Oval Window Just as the tympanic membrane forms the border between the outer and middle ear, the oval window forms the border between the middle and inner ear The oval window is where pressure waves enter the cochlea The faster the vibrations transmitted onto the eardrum, the faster the vibrations transmitted into the oval window

The Cochlea The cochlea is filled with a liquid called perilymph that transmits vibrations from the oval window The hair cells plus the membrane they attach to and the sensory neurons form the Spiral Organ, or Organ of Corti The axons from these sensory neurons bundle together to become the cochlear nerve

The Nerves! The cochlear nerve leaves the cochlea heading for the brain The vestibular organs produce another nerve called the vestibular nerve These two nerves actually join to become the vestibulocochlear nerve which is processed by the thalamus and midbrain before transmitting to the temporal lobe

That’s our show! Next Monday is review day! Bring your questions! One week from today is Exam #2!