Hearing, seeing, feeling, smelling and tasting
Summary
Introduction
 The eye and sight
 The ear and hearing
 Sense of taste
 Sense of smell
 Cutaneous receptors

Introduction
Our “senses” continually provide us with
information about our surroundings.
 Sense organs are complex organs like
the eye or specialized receptors in areas
such as the nasal mucosa or tongue.

Introduction

Conversion of a stimulus to a sensation:
 Stimuli (light, sound, temperature, etc. are
changed into an electrical signal or nerve
impulse.
 The signal is then transmitted over a ?
neuron to the ?
 The signal is interpreted and we become
consciously aware of a sensation.
The Eye
Contains receptors for vision and a
refracting system that focuses light rays
on the receptors in the retina.
 The eye sits in the orbit formed by the
maxilla, zygomatic, frontal, sphenoid
and ethmoid bones. Extrinsic muscles
attach the surface of the eyeball to
bones.

The Eye

Eyelids – contain skeletal muscle that allow us to
close them and totally cover the exterior eyeball.

Eyelashes – help to keep dust out of our eyes.
Tears

The Eye

Cranial Nerves
 Optic – vision
 Oculomotor, abducens and trochlear – eye
movment

The eye contains 3 layers
The Eye

Structure of the eyeball
 Sclera – tough fibrous tissue.
○ Front surface is the “white” of our eyes and
the cornea. The cornea is transparent,
receives no blood supply and is nourished by
the aqueous humor.
○ Sclera is covered by the conjunctiva in the
front of the eyeball.
The Eye

Structure of the eyeball
 Choroid - contains a dark pigment to prevent
scattering of light that enters the eyeball.
Also contains blood vessels and 2
involuntary muscles.
○ Iris –
○ Ciliary body (muscle) –
The Eye

Structure of the eyeball
 Lens – composed of transparent, elastic
protein; no blood supply, nourished by the
aqueous humor.
 Retina – contains microscopic receptor cells
called rods and cones.
○ Rods –
○ Cones –
○ Fovea –
The Eye

Layer of the eye
 Retina
○ Ganglionic neurons carry impulses generated
by the rods and cones until they converge at
the optic disc. From the optic disc they form
the optic nerve and pass through the wall of
the eyeball to the occipital nerve.
○ Optic disc – also know as the “blind spot”, no
rods or cones; exit to the optic nerve.
○ Occipital lobe of the cerebrum – visual
interpretation.
The Eye

Structure - fluids of the eyeball – 2
types:
 Aqueous humor – watery fluid in front of the
lens (anterior cavity) nourishes the lens and
cornea.
○ Continually formed by the capillaries in the
ciliary body, flow through the pupil and is
reabsorbed in the canal of Schlemm.
○ If drainage is blocked, the internal pressure in
the eye increases and may damage the eye
and lead to blindness = glaucoma.
The Eye

Structure - fluids of the eyeball – 2
types:
 Vitreous humor – jelly-like fluid behind the
lens (posterior cavity). Literally holds the
retina in place and gives structure to the
eyeball.
The Eye

Focusing Problems
 Presbyopia – “old sightedness” or “short arm
syndrome”. Ciliary bodies lose their
elasticity and can no longer change the
shape of the lens to bring near objects into
focus.
 Myopia – nearsightedness, image focuses in
front of the retina rather than on it, eyeball is
elongated. Corrected by glasses, contacts
or radial keratotomy (Lasix).
The Eye

Focusing Problems
 Hyperopia – “farsightedness”, image
focuses behind the retina, produces a fuzzy
image. Corrected by lenses.
 Astigmatism – refraction error – fuzzy image,
irregular curvature of the cornea or lens,
requires special lenses to correct (Toric
lenses) or contacts.
The Ear
Sense organ associated with hearing
and equilibrium and balance.
 3 main parts

 External
 Middle
 Inner
The Ear

External ear –

External Auditory canal – a curving tube about one
inch long; extends into the temporal bone and end
at the tympanic membrane (eardrum).
The ear

Middle ear – tiny epithelium lined cavity
which is hollowed out of the temporal
bone.
 Tympanic membrane – separates the
external and middle ear and vibrates when
sound waves strike it.
 3 tiny bones called ossicles (bones) transmit
sound waves.
The Ear

Middle Ear
 Bones
○ Malleus –
○ Incus –
○ Stapes –
The Ear

Middle Ear
 Oval Window separates the middle ear from
the inner ear.
 Eustachian tube – connects the throat with
the middle ear; allows air to enter and leave
the middle ear which equalizes pressure.
Why do throat and ear infections occur
together?
The Ear

Middle Ear - Hearing Sequence
 Sound waves cause the eardrum to vibrate,
and this movement is transmitted and
amplified by the ear ossicles.
 Movement of the stapes against the oval
window causes movement of fluid in the
inner ear which generates electrical
impulses.
The Ear

Inner Ear – contains mechanoreceptors
that are activated by vibration and
generate nerve impulses that result in
hearing and equilibrium. The 3 spaces
are called the bony labyrinth and contain
fluids called perilymph and endolymph.
 Vestibule – membranous sacs (utricle and
saccule) adjacent to the oval window and
between the semicircular canals. Contains
receptors for equilibrium.
The Ear

Inner Ear
 Cochlea – snail shell; contains the Organ of
Corti which holds the receptors for hearing
(hair cells). As the hairs bend (vibration)
they generate an electrical impulse.
 Semicircular Canals – contain the crista
ampularis which is a specialized receptor
that generates a nerve impulse when you
move your head. Receptors for equilibrium.
Sense of Taste
Taste buds – chemical receptors that
generate nervous impulses resulting in
the sense of taste. There are about
10,000 microscopic taste buds located
on the papillae of the tongue.
 Gustatory cells –

Sense of Taste

Taste Sensations
 Sweet, sour, bitter, salty, and Umami
(=savory).
 Other flavors results from a combination of
taste bud stimulations and olfactory receptor
stimulation. i.e. our taste sensations include
odors as well.
Sense of Smell



Olfactory receptors – chemical receptors
responsible for the sense of smell are located in
the upper part of the nasal cavity.
Olfactory receptors are stimulated by chemicals
dissolved in the watery mucus that lines the nasal
cavity. We detect about 10,000 different scents.
Olfactory receptors are easily fatigued – many
odors are not noticeable after a time.
Hunger and Thirst

Visceral sensations – receptors are
located in the hypothalamus; the
stimulus is a change in the body’s
water/salt content and levels of nutrients
in the blood.
Cutaneous Sensations

Receptors of the general sense organs
are found in almost every part of the
body.
 Encapsulated nerve endings – located in the
dermis; touch and pressure.
 Free nerve endings – mainly in the dermis of
the skin, mucosa, internal organs. They
sense pain or crude touch. *Referred pain.
Cutaneous Sensations

Receptors
 Meissner’s corpuscles – skin, fingertips and
lips; sens of fine touch and vibration.
 Ruffini’s corpuscles – skin and sq tissue of
the fingers; touch and pressure.
 Pacinian corpuscles – subcutaneous; deep
pressure and vibration.
Cutaneous Sensations

Receptors
 Krause’ end bulbs – skin and sq; touch and
maybe cold.
 Muscle spindles – skeletal muscle;
proprioception. Proprioception is the sense
of position and movement in various parts of
the body.
Characteristics of Sensations

Projection – sensation seems to come
from the area where the receptors were
stimulated; in reality they are being “felt”
via the cerebral cortex.
 Phantom pain – receptors are removed with
amputated limbs but severed nerve endings
continue to send impulses to the brain.
Characteristics of Sensations
Intensity – the intensity of a sensation is
related to the strength of the stimulus
and/or number of receptors stimulated.
 Contrast – effect of a previous sensation
on a current sensation; brain compares
a new sensation to a previous one.

Characteristics of Sensations
Adaptation – you become unaware of a
continuous stimulus.
 After image – sensation remains in the
consciousness even after the stimulus is
gone – flash from a camera.
