The document summarizes the anatomy and physiology of the olfactory and auditory systems. It describes the main structures involved in smell and hearing, including the olfactory epithelium containing olfactory sensory neurons that detect odors, and the inner ear structures like the cochlea and vestibular apparatus that are responsible for hearing and balance. It also outlines the pathways that transmit signals from these sensory receptors to the brain regions involved in processing smell and sound. Finally, it discusses some common abnormalities and disorders that can impact the sense of smell or hearing.

1. • Sense of smell is also called olfaction
• Olfactory sensory neurons or receptors embedded within the
olfactory epithelium in the nasal cavity, are responsible for
olfaction

2. • There are three cell types at the base of the epithelium.
1. Olfactory Sensory Neurons:
• 10 to 20 million bipolar olfactory sensory neurons are
located in a specialized portion of the nasal mucosa.
• Each neuron has a short, thick dendrite that projects into
the nasal cavity where it terminates in a knob containing
10 to 20 cilia
• The cilia are unmyelinated processes about 2 m long and
0.1 m in diameter and contain specific receptors for
odorants (odorant receptors).
• A single axon projects from each neuron to the olfactory
bulb.
• Odorants bind to specific odorant receptors on the cilia
and initiate a cascade of events leading to generation of
action potentials in the sensory axon.
2. Supporting Cells
3. Basal Stem Cells

3. Structure of the olfactory epithelium.

4. The axons of the olfactory sensory neurons pass through the
cribriform plate of the ethmoid bone and enter the olfactory
bulbs.
In the olfactory bulbs, the axons of the olfactory nerve contact
the primary dendrites of the mitral cells and tufted cells to form
anatomically discrete synaptic units called olfactory glomeruli.
The tufted cells are smaller than the mitral cells and have thinner
axons, but both types send axons into the olfactory cortex.
In addition to mitral and tufted cells, the olfactory bulbs contain
• Periglomerular cells, which are inhibitory neurons connecting
one glomerulus to another,
• Granule cells, which have no axons and make reciprocal
synapses with the lateral dendrites of the mitral and tufted
cells.

5. The axons of the mitral and tufted cells pass posteriorly through
the lateral olfactory stria to terminate on apical dendrites of
pyramidal cells in the olfactory cortex:
From these regions, information travels directly to the frontal
cortex or via the thalamus to the orbitofrontal cortex.

6.  Anosmia (inability to smell) and hyposmia or hypesthesia
(diminished olfactory sensitivity)
• can result from simple nasal congestion or be a sign of a more
serious problem including damage to the olfactory nerves
due to fractures of the cribriform plate, tumours such as
neuroblastomas or meningioma, or infections (such as
abscesses).
• Alzheimer disease can also damage the olfactory nerves.
• Aging is also associated with abnormalities in smell sensation
 Dysosmia (distorted sense of smell) can be caused by several
disorders including sinus infections, partial damage to the
olfactory nerves, and poor dental hygiene
 Hyperosmia (enhanced olfactory sensitivity) is less common than
loss of smell, but pregnant women commonly become
oversensitive to smell.
Abnormalities in Odour Detection

7. • Inner ear is also called
labyrinth consists of two
structures: bony labyrinth and
membranous labyrinth.
• Membranous labyrinth is
situated inside bony labyrinth.
• The space between two
labyrinths are filled with a fluid
called perilymph.
• The membranous labyrinth is
filled with a fluid called
endolymph and it consists of
two structures:
• Cochlea which is concerned
with sense of hearing
• Vestibular apparatus which
is concerned with balance
and equilibrium.

9. COCHLEA
• It is a coiled structure like a snails shell.
• It consists of a bony axis called modiolus and a bony spiral
canal which winds around the modiolus.
• Two membranous partitions called basilar membrane and
vestibular membrane divide the spiral canal of cochlea in to
three compartments;
• Scala vestibuli lies above scala media. It arises from oval
window. At the apex, it communicates with the scala
tympani through a small canal called helicotremma.
• Scala tympani lies below scala media. It lies parallel to
scala vestibuli and end at the round window.
• Scala media also called cochlear duct. It ends blindly at
apex and at the base of cochlea.
• Scala vestibuli and scala tympani contain perilymph. The
scala media is filled with endolymph.

10. ORGAN OF CORTI

11. • The sensory part of cochlea is called Organ Of Corti which is
situated on the upper surface of basilar membrane.
• It is the receptor organ of hearing. It rest upon the lip of
spiral lamina and basilar membrane.
• The roof is formed by tectorial membrane.
• It is made up of sensory elements called hair cells and
various supporting cells.
• The hair cells in organ of Corti are the receptor of auditory
sensation.
• They are of two types; outer hair cells and inner hair cells.
• The surface of hair cells bear a cuticular plate and a
number of short stiff hairs which are called stereocilia.
• Each hair cell has about 100 stereocilia.
• One of them is larger and it is called kinocilium. Sensory
nerve fibres are distributed around hair cells.

12. • Sound transduction is a type of sensory transduction in the
hair cell by which the energy caused by sound is converted in
to action potentials in the auditory nerve fibre.

14. AUDITORY PATHWAY
RECEPTORS
• The outer and inner hair cells in organ or Corti are the
receptors of the auditory sensation.
FIRST ORDER NEURONS
• The first order neurons of the auditory pathway are the
dendrites of the bipolar cells are distributed around the
hair cells of organ of Corti.
• Their axons leave ear as cochlear nerve fibres and enter
medulla oblongata.
• The fibres are then divided in to two groups and end in
dorsal and ventral cochlear nuclei of medulla oblongata

15. SECOND ORDER NEURONS
• Second order neurons arises from the dorsal and ventral
cochlear nuclei of medulla oblongata. The axons of second
order neurons run in different directions:
1. Some of the fibres cross the midline and terminate in
superior olivary nucleus and lateral lemniscus of
opposite side.
2. Some fibres terminate in superior olivary nucleus and
lateral lemniscus of the same side.

16. CORTICAL AUDITORY CENTRES
• Are situated in temporal lobe.
• The auditory areas are area 41, 42 and Wernicke's area.
• Area 41 and 42 are primary auditory area and is concerned
with perception of auditory impulses.
• Wernicke's area is responsible for analysis and interpretation
of sound with the help of auditorypshychic area.
THIRD ORDER NEURONS
• Third order neurons arises from the superior olivary
nucleus and nucleus of lateral lemniscus and terminate in
medial geniculate body which forms the subcortical
auditory centre.
• From medial geniculate body fibres reaches the temporal
cortex via auditory radiation.

17. DEAFNESS
 Hearing loss is the most common sensory defect in humans
 Deafness can be divided into two major categories: conductive
(or conduction) and sensorineural hearing loss.
 CONDUCTIVE DEAFNESS
• It refers to impaired sound transmission in the external or
middle ear and impacts all sound frequencies.
• Among the causes of conduction deafness are
• Obstruction of the external auditory canals with wax
(cerumen) or foreign bodies,
• Otitis externa (inflammation of the outer ear,
"swimmer's ear") and otitis media (inflammation of
the middle ear) causing fluid accumulation
• Perforation of the eardrum, and
• Osteosclerosis in which bone is resorbed and replaced
with sclerotic bone that grows over the oval window.

18.  SENSORINEURAL DEAFNESS OR NERVE DEAFNESS
• It is caused by damage of any structure in cochlea.
Most commonly the result of loss of cochlear hair cells
but can also be due to damage in basilar membrane or
cochlear duct or the lesion in auditory pathways
• It often impairs the ability to hear certain pitches while
others are unaffected.
• Causes are
• Antibiotics such as streptomycin and gentamicin
obstruct the mechanosensitive channels in the
stereocilia of hair cells.
• Damage to the outer hair cells by prolonged
exposure to noise is associated with hearing loss.
• Other causes include tumours of the eighth cranial
nerve and vascular damage in the medulla.

19. VESTIBULAR APPARATUS
• It is formed by three semicircular canals and otolith organ or
vestibule.
• The semicircular canals are named as anterior, posterior
and lateral semicircular canals.
• Each semicircular canal has one narrow end and an
enlarged end, which is called ampulla. Ampulla contains
the receptor organ crista ampullaris. All the semicircular
canals open in to utricle, which then opens in to saccule.
• The receptors of semicircular canals give response to
rotatory movements or angular acceleration of the
head.
• Otolith organ or vestibule is formed by utricle and saccule.
• The receptor organ in otolith organ is called macula and
it contain proprioreceptors.
• The receptors of vestibule give response to linear
acceleration of head.

20. CRISTAE AMPULLARIS
• It is the receptor organ situated inside the semicircular canals
• The receptors of semicircular canals give response to rotatory
movements of the head.
• The crest is formed by a receptor epithelium which consists of
hair cells and supporting cells.
• Hair cells are the proprioreceptor cells of cristae ampullaris.
• They are of two types; type I and type II hair cells.
• Type I are flask shaped.
• The type II are cylindrical in shape.
• The apex of each hair cell has a cuticular plate from which 40-60
cilia called stereocilia arise. one of them is taller and called
kinocilium.
• From the cristae ampullaris, a dome shaped gelatinous structure
extends up to the roof of the ampulla. It is known as cupula and
it encloses the cilia of hair cells.

22. MACULA
• It is also formed by neuroepithelium and supporting cells.
• The neuroepithelium made up of two type of cells; type I and
type II.
• Macula is also covered by a gelatinous membrane called
otolith membrane. It is a flat structure and not dome shaped
like cupula.
• The stereocilium and kinocilium of each hair cell are
embedded in otolith membrane.
• It also contain some crystals called otoconia or statoconia or
ear stones. The otoconia are usually calcium carbonate
crystals.

23. Mechanotransduction
• It is a type of sensory transduction in the hair cell by which
mechanical energy caused by stimulus is converted to action
potentials in vestibular nerve fibre.
• The RMP of hair cell is about -60 mv.
• The movement of stereocilia towards kinocilium causes
development of mild depolarization in hair cells up to -50 mv
which is called receptor potential.
• The receptor potential causes the generation of action potentials
in nerve fibre.
• Movement of kinocilium in opposite direction causes
hyperpolarization of hair cells and cessation of generation of
action potential occurs.