Hearing and Smell

Amplitude

intensity, measured in decibels (dB) and perceived as loudness

Frequency

measured in number of cycles per second, or hertz (Hz), and perceived as pitch

Pure Tone

tone with a single frequency of vibration

What happens to sound within the ear

transduced into neural activity

External ear structures

Pinna - collects sound waves

Ear canal (auditory canals)

Importance of shape of the external ear

modifies the character of sound frequencies that reach the middle ear

Middle ear

concentrates sound energies

Three ossicles (bones) that connect tympanic membrane to the oval window

malleus, incus, and stapes

Two muscles in middle ear and role

Tensor tympani
Stapedius

control volume: when activated muscles stiffen and reduce sound’s effect

Oval window

opening form middle ear to inner ear

Cochlea

spiral-shaped, fluid-filled

converts vibrations into neural activity

Three parallel Cochlea canals

Scala vestibuli (vestibular canal)

Scala media (middle canal)/cochlear duct

Scala tympani (tympanic canal)

Organ of Corti

part of the cochlea that converts sound into neural activity

contains hari cells and nerve fibers

base of the organ of Corti

basilar membrane

Basilar membrane

sound vibrations cause the membrane to ripple

different segments vibrate most at different frequencies

Hair cells

hair cells in the basilar membrane convert sound into neural activity

Hair cells structure

sloping brush of stereocilia, tiny hairs, protrude from each hari cell

stereocilia connected to each other by tip links: tiny fibers that open ion channels when the stereocilia bend

Depolarization of hair cells

stereocilia bends, tiplinks stretch and open multiple ion channels that allow K+ and Ca2+ ions into the cell

opens VGCCs at the base of the cell causing neurotransmitter releas that lead to APs in postsynaptic cells

Two groups of hair cells

Inner hair cells (IHCs): single row near the central axis

Outer hair cells (OHCs): three rows

Vestibucochlear nerve

contacts the bases of the hair cells

Inner hair cells

necessary for the sensation of auditory stimuli

Outer hair cells

cochlear amplifiers, they amplify various frequencies of sound by increasing vibrations of the basilar membrane

Ear to brain

auditory nerve fibers from hair cells temrinate in the cochlear nuclei

information is sent to the superior olivary nuclei

Superior olivary nuclei

first site of binaural processing, each receives input from left and right cochlear nuclei

passes informtaion to inferior collculi

inferior colliculi

primory auditory centers of the midbrain

outputs go to medial geniculate nuclei of the thalamus (pathways thalamus extend to primary auditory cortec)

Primary auditory cortex (A1)

main place of sound activations

speech sounds also activate more specialized auditory areas

Olfaction

the ability to detect smells (odors)

Olfactory epithelium

sense of smell starts with receptor neurons in the nose

Olfactory receptors

each is a complete cell with a long apical dendrite and branches that extend into the mucosal surface

Odorants

inhaled molecules that interact with olfactory receptor proteins on the dendrites

How do olfactory receptor cells differ from neurons of the brain

1. Incredible diversity of subtypes (mainly GPCRs; hundreds of olfactory cell receptor subtypes)

2. Die and are replace in adulthood, adaptive process to compensate for the hazardous environment of the nostril

Olfactory axon

extends into an olfactory bulb of the brain

axon terminates on a specific glomerulus, which receives information from one specific class of odorant receptors

Olfactory information organized within the olfactory bulb

• each glomerulus receives input from neurons with the same type of receptors

• glomeruli are organized liek a map: asjacent glomeruli receive input from receptors that are closely related

• olfactotopic organization is maintained throughout the brain

• axons of mitral cells (projection cells) convey information from the glomeruli to regions of the brain

Smell’s relationship with the thalamus

the only sensory modality that synapses directly onto the cortex wihtout first passing through the thalamus

• biological importance of smell

• olfactory neurons synapse directly onto areas involved in emotion and memory