Olfactory, Taste, & Hearing

Olfaction

Chemical sense, 50 million receptors in nasal cavity

1. Olfactory receptors - bipolar neurons with cilia
   1. Scent molecules bind to membrane proteins of cilia which produce a generator potential that can cause an action potential
   2. Axons extend up through the cribiform plate into the olfactory bulb and synapse with olfactory tract fibers to olfactory cortex of frontal lobe
   3. Does not synapse in thalamus
   4. Receptors are constantly replaced but number decreases with age
2. Can discriminate >10,000 odors with only 1000 different receptor types, odor stimulates a combination of receptor types, higher concentration of odor molecules adds new receptors to the odor profile and changes perception
3. Chemical must be in solution, takes only a few molecules to reach threshold, shows rapid central adaptation

Taste

chemical sense, 10,000 taste buds located on tongue, soft palate, larynx and pharynx

1. Taste buds - receptor hair cell (have microvilli) Figure in class:

   support cell	receptor hair cell
   	dendrites from facial & glossopharyngeal N.

2. Taste buds found on elevations of tongue called papillae, have different shapes
3. Chemical must be in solution, threshold varies, rapid central adaptation
4. Six primary tastes: sour, salty, bitter, sweet, umami (a.a. in beef & chicken)

Hearing

spiral cochlea contains the organ of Corti which has 16,000 hair cell receptors

1. Basic structure of the ear is shown in text Fig. 15-22
2. Function
   1. Sound vibrations cause the tympanic membrane to vibrate
   2. The center of the tympanum is attached to the handle of the malleus & causes the malleus to move
   3. Vibration is transmitted through the articulated malleus, incus and stapes of middle ear to the oval window of cochlea.  This lever system converts sound waves into mechanical motion & amplifies the force of movement.
   4. Vibration of the oval window is transmitted through the perilymph as a series of pressure waves causing the wall of the scala vestibuli to deform.  This causes the basilar membrane to move.
   5. The bases of the hair cells are embedded in the basilar membrane and their stereocilia are in contact with the overlying tectorial membrane.  Movement of the basilar membrane bends the stereocilia against the tectorial membrane. 
   6. Bending the stereocilia cause depolarization of hair cells, which release neurotransmitter producing action potentials in the cochlear branch of the vestibulocochlear nerve (cranial nerve VIII).
   7. Action potentials travel to cochlear nucleus in brainstem to medial geniculate nucleus of thalamus to auditory cortex in temporal lobe,
   8. Hair cells near the base of the cochlea respond to high frequency sounds (pitch), near apex to low frequency.  Loudness is determined by amplitude (decibels) of the vibrations.
   9. Pressure waves in cochlear duct cause scala tympani to deform producing pressure waves in perilymph which are damped at round window

Equilibrium

saccule, utricle and semicircular canals contain hair cell receptors

1. Static equilibrium - maintenance of body position relative to gravitational force
   1. Otolithic organs in utricle and saccule
      1. Hair cells extend into a gelatinous mass topped by a layer of calcium carbonate called statoconia
      2. Body tilt causes mass to move and bend cilia caausing action potentials
2. Dynamic equilibrium - maintenance of body position in response to sudden movements
   1. Can detect linear and rotational acceleration or deceleration
   2. Otolithic organs in utricle and saccule and ampullae in semicircular canals
      1. Ampullae - hair cells embedded in gelatinous cupula in endolymph of canal
      2. Change in motion causes endolymph flow which moves cupula and bends cilia
   3. Impulses are carried by the vestibular branch of the vestibulocochlear nerve (cranial nerve VIII)