sensensory receptors and sense organs

1. SOMATIC AND SPECIAL SENSES
BY
SONI KUMARI SHAH

2. Sense Organ
Highly specialized organ of the body which responds to stimuli by conveying
impulse to the central nervous system.
There are five types of special sensation:
1. Eye
2. Ear
3. Nose
4. Tongue
5. Skin

3. Sensory Receptors
Structures specialized to respond to stimuli and changes in the external
environment are sensory receptors. It consists of nerve endings and
specialized cells in close contact with neurons. It acts as transducers,
converting many form of energy into action potentials that the brain can
intercept.
Types Of Sensory Receptors( On The Basis Of Type Of Stimuli)
1. Chemoreceptors: It responds to changes in the concentration of chemicals.
Receptors associated with sense of smell and taste is of this type.
Chemoreceptors in internal organs detect changes in the concentration of
oxygen, hydrogen ions, glucose and other chemicals.

4. 2. Pain receptors (nociceptors): It responds to tissue damage. Triggering
stimuli include exposure to excess mechanical, electrical, thermal, or
chemical energy.
3. Thermoreceptors: It is sensitive to temperature change.
4. Mechanoreceptors: It is of several types. It senses mechanical forces by
detecting changes that deform the receptors. Proprioceptors sense changes
in tension of muscle and tendons. Baroreceptors in certain blood vessels
detects changes in blood pressure. Stretch receptors in the lungs sense
degree of inflation.
5. Photoreceptors: It is present in the eye. It responds to light energy of
sufficient intensity.

5. Different Sensory Organs And Their Receptors
S. N. Modality Stimulus Receptor types Receptors
1 Vision Light Photoreceptors Rods, Cones
2 Audition Sound Mechanoreceptors Hair cells of cochlear duct
3 Balance Head motion
Mechanoreceptors Hair cells of semicircular
canal
4
Somatic
sensation
Mechanical,
Thermal,
Chemical
Mechanoreceptors ,
Thermoreceptors,
Chemoreceptor, Pain receptor
Free nerve endings,
Meissner’s corpuscles
5 Taste Chemical Chemoreceptor Taste buds
6 Smell Chemical Chemoreceptor Olfactory sensory neurons

6. Generation Of Receptor Potential/ Mechanism Of Sensory Perception
Stimulus
Transduction into electrical energy
Receptor potential
Action potential
Function Of Sensory Receptors
1. Detect the stimulus in environment by absorbing energy.
2. Converts the energy of stimulus into electrical energy(transduction).
3. Produces receptor potential that may become an action potential.

7. Somatic Senses
1. Pain
2. Touch
3. Pressure
4. Stretch
5. Temperature

8. Olfactory Organs
Nose is the organ of sense of smell.
The sensory nerve of smell are olfactory nerve(I cranial nerve).

9. Sensation Of Smelling
The receptors of smell are olfactory rods. They
are present in the mucous membrane of upper
part of nasal cavity.
The ends of olfactory rods collect to form the
olfactory nerve.
This nerve passes through the root of nose
and ends in olfactory bulb.
From the olfactory bulb, the sensation are
carried through olfactory tract to olfactory
areas in the temporal lobe of cerebral cortex.
The perception of smell occurs in this area.

10. Gustatory Organs
Tongue is the organ of taste sensation.
Tongue is a muscular organ in the oral cavity.
Structure Of Tongue:

11. Nerve Supply Of Tongue

12. Taste Perception

13. Eye
The eye is the organ of sight.
It is situated in the orbital cavity and supplied by the optic nerve
(2nd cranial nerve).
It is almost spherical in shape and about 2.5 cm in diameter.
The bony walls of the orbit and the fat protect the eye from injury.
Structurally the two eyes are separate but, some of their activities
are coordinated so that they normally function as a pair.

14. Structure Of Eye
There are three layers of tissue in the walls of the eye:
• the outer fibrous layer: sclera and cornea
• the middle vascular layer or uveal tract: consisting of the choroid,
ciliary body and iris
• the inner nervous tissue layer: the retina.
Structures inside the eyeball include the lens, aqueous fluid and
vitreous body.

15. Sclera And Cornea
The sclera, or white of the eye, forms the
outermost layer of the posterior and lateral
aspects of the eyeball and is continuous
anteriorly with the cornea.
It consists of a firm fibrous membrane that
maintains the shape of the eye and gives
attachment to the extrinsic muscles of the
eye.
Anteriorly the sclera continues as a clear
transparent epithelial membrane, the
cornea.
Light rays pass through the cornea to reach

16. Choroid
The choroid lines the posterior
five-sixths of the inner surface of
the sclera.
It is very rich in blood vessels
and is deep chocolate brown in
colour.
Light enters the eye through the
pupil, stimulates the sensory
receptors in the retina and is then
absorbed by the choroid.

17. Ciliary body
The ciliary body is the anterior
continuation of the choroid consisting
of ciliary muscle (smooth muscle
fibres) and secretory epithelial cells.
The lens is attached to the ciliary body
by radiating suspensory ligaments, like
the spokes of a wheel.
Contraction and relaxation of the ciliary
muscle fibres, which are attached to
these ligaments, control the size and
thickness of the lens.

18. Iris
The iris is the visible colored ring at
the front of the eye and extends
anteriorly from the ciliary body, lying
behind the cornea and in front of
the lens.
It divides the anterior segment of
the eye into anterior and posterior
chambers which contain aqueous
fluid secreted by the ciliary body.

19. Lens
The lens is a highly elastic
circular biconvex body, lying
immediately behind the pupil.
It consists of fibres enclosed
within a capsule and is suspended
from the ciliary body by the
suspensory ligament.
Its thickness is controlled by the
ciliary muscle through the
suspensory ligament.

20. Retina
The retina is the innermost lining
of the eye.
It is an extremely delicate
structure and well adapted for
stimulation by light rays.
It is composed of several layers
of nerve cell bodies and their
axons, lying on a pigmented
layer of epithelial cells.
The light-sensitive layer consists
of sensory receptor cells, rods
and cones, which contain
photosensitive pigments that
convert light rays into nerve
impulses.

21. Retina
Near the centre of the posterior part
is the macula lutea, or yellow spot.
In the centre of the yellow spot is a
little depression called the fovea
centralis, consisting of only cones.
Towards the anterior part of the
retina there are fewer cones than
rods.
About 0.5 cm to the nasal side of the
macula lutea all the nerve fibres of
the retina converge to form the optic
nerve.
The small area of retina where the
optic nerve leaves the eye is the

22. Interior Of Eye
The anterior segment of the eye, i.e.
the space between the cornea and the
lens, is incompletely divided into anterior
and posterior chambers by the iris.
Both chambers contain a clear aqueous
fluid secreted into the posterior chamber
by the ciliary glands.
Aqueous fluid supplies nutrients and
removes wastes from the transparent
structures in the front of the eye that
have no blood supply, i.e. the cornea,
lens and lens capsule.

23. Interior Of Eye
Behind the lens and filling the
posterior segment (cavity) of the
eyeball is the vitreous body.
This is a soft, colorless,
transparent, jelly-like substance
composed of 99% water, some
salts and mucoprotein.
It maintains sufficient intraocular
pressure to support the retina
against the choroid and prevent the
eyeball from collapsing.

24. Muscles Of Eye

25. Visual Pathway
Optic nerve
Optic chiasma
Optic tract
Lateral geniculate body
Optic radiation
Visual area in occipital
lobe of cerebral cortex

26. Physiology Of Sight
Light is reflected into the eyes by objects within the field of vision. White light is
a combination of all the colors of the visual spectrum (rainbow), i.e. red, orange,
yellow, green, blue, indigo and violet.
A specific colour is perceived when only one wavelength is reflected by the
object and all the others are absorbed, e.g. an object appears red when it only
reflects red light. Objects appear white when all wavelengths are reflected, and
black when they are all absorbed.
In order to achieve clear vision, light reflected from objects within the visual field
is focused on to the retina of each eye.
The processes involved in producing a clear image are
 refraction of the light rays,
 changing the size of the pupils and
 accommodation (adjustment of the lens for near vision)
Although these may be considered as separate processes, effective vision is
dependent upon their coordination.

27. Refraction Of Light Rays
Light rays reflected from an object are bent (refracted) by the lens
when they enter the eye. Although the image on the retina is
actually upside down The brain adapts to this early in life so that
objects are perceived ‘the right way up’.

28. Size Of The Pupils
Pupil size contributes to clear vision by controlling the amount of light
entering the eye. In bright light the pupils are constricted. In dim light they
are dilated.
If the pupils were dilated in bright light, too much light would enter the eye
and damage the sensitive retina.
 In dim light, if the pupils were constricted, insufficient light would enter the
eye to activate the light-sensitive pigments in the rods and cones, which
stimulate the nerve endings in the retina enabling vision.
The iris consists of one layer of circular and one of radiating smooth
muscle fibres. Contraction of the circular fibres constricts the pupil, and
contraction of the radiating fibres dilates it.
The size of the pupil is controlled by the autonomic nervous system;
sympathetic stimulation dilates the pupils and parasympathetic stimulation
constricts them.

29. Accommodation
Accommodation is the adjustment of the eye to see either near or distant objects
clearly.
Near Vision
The eye must make the following adjustments to focus on near objects i.e
within 6 metres
• constriction of the pupils
• convergence
• changing the refractory power of the lens
Distant Vision
Objects more than 6 metres away from the eyes are focused on the retina
without adjustment of the lens or convergence of the eyes.

30. Accommodation

31. Accessory Organs Of The Eye
The eye is a delicate organ which is protected by several structures
:
• eyebrows
• eyelids and eyelashes
• lacrimal apparatus.

32. Eyebrows
These are two arched ridges of the supraorbital margins of the
frontal bone.
Numerous hairs (eyebrows) project obliquely from the surface
of the skin.
They protect the eyeball from sweat, dust and other foreign
bodies.

33. Eyelid And Eyelashes
The eyelids are two movable folds of tissue situated above
and below the front of each eye.
On their free edges are short curved hairs, the eyelashes.
The layers of tissue forming the eyelids are:
• a thin covering of skin
• a thin sheet of subcutaneous connective (loose areolar)
tissue
• two muscles – the orbicularis oculi and levator palpebrae
superioris
• a thin sheet of dense connective tissue, the tarsal plate,
larger in the upper than the lower eyelid, which supports the
other structures

34. Conjunctiva
This is a fine transparent membrane that lines the eyelids and the
front of the eyeball.
When the eyelids are closed the conjunctiva becomes a closed
sac.
It protects the delicate cornea and the front of the eye.
The medial and lateral angles of the eye where the upper and
lower lids come together are called respectively the medial canthus
and the lateral canthus.

35. Lacrimal Apparatus
For each eye this consists of the structures that secrete tears and
drain them from the front of the eyeball:
• 1 lacrimal gland and its ducts
• 2 lacrimal canaliculi
• 1 lacrimal sac
• 1 nasolacrimal duct.

36. Functions
The fluid that fills the conjunctival sac is a mixture of tears and the oily
secretion of tarsal glands, which is spread over the cornea by blinking.
The functions of this fluid include:
• provision of oxygen and nutrients to the avascular corneal conjunctiva and
drainage of wastes
• washing away irritating materials, e.g. dust, grit
• the bactericidal enzyme lysozyme prevents microbial infection
• its oiliness delays evaporation and prevents friction or drying of the
conjunctiva.

37. Ear
The ear is the organ of hearing and is also involved in balance.
It is supplied by the 8th cranial nerve, i.e. the cochlear part of
the vestibulocochlear nerve, which is stimulated by vibrations
caused by sound waves.
The normal hearing range of man 20Hz-20KHz.

38. Structure
The ear is divided into three distinct parts:
 The external ear
auricle or pinna
external acoustic meatus
 middle ear (tympanic cavity)
tympanic membrane
tympanic cavity with ear ossicles
 inner ear.
bony labyrinth
membranous labyrinth
The outer ear collects the sound waves and directs them to the middle ear,
which in turn transfers them to the inner ear, where they are converted into
nerve impulses and transmitted to the hearing area in the cerebral cortex.

39. External ear
The external ear consists of the auricle (pinna)
and the external acoustic meatus (auditory
canal).
The auricle (pinna)
The auricle is the visible part of the ear that
projects from the side of the head.
It is composed of fibroelastic cartilage covered
with skin.
It is deeply grooved and ridged; the most
prominent outer ridge is the helix.
The lobule (earlobe) is the soft pliable part at the
lower extremity, composed of fibrous and
adipose tissue richly supplied with blood.

40. External acoustic meatus (auditory canal)
This is a slightly ‘S’-shaped tube about 2.5 cm long extending from the
auricle to the tympanic membrane (eardrum).
The lateral third is embedded in cartilage and the remainder lies within
the temporal bone.
The meatus is lined with skin continuous with that of the auricle.
There are numerous ceruminous glands and hair follicles, with
associated sebaceous glands, in the skin of the lateral third.
Ceruminous glands are modified sweat glands that secrete cerumen
(earwax), a sticky material containing protective substances including
the bacteriocidal enzyme lysozyme and immunoglobulins.
Foreign materials, e.g. dust, insects and microbes, are prevented from
reaching
the tympanic membrane by wax, hairs and the curvature of the meatusral
bone.

41. Middle Ear
Tympanic Membrane
The tympanic membrane (eardrum)
completely separates the external
acoustic meatus from the middle ear. It
is oval-shaped with the slightly broader
edge upwards

42. Tympanic Cavity
This is an irregular-shaped air-filled cavity within
the petrous portion of the temporal bone.
The lateral wall of the middle ear is formed by
the tympanic membrane.
The roof and floor are formed by the temporal
bone.
The posterior wall is formed by the temporal
bone with openings leading to the mastoid
antrum through which air passes to the air cells
within the mastoid process.
The medial wall is a thin layer of temporal bone
in which there are two openings:
• oval window
• round window .

43. Auditory ossicles
These are three very small bones
only a few millimeters in size that
extends across the middle ear from
the tympanic membrane to the oval
window.
They form a series of movable joints
with each other and with the medial
wall of the cavity at the oval window.
The ossicles are held in place by
fine ligaments and are named
according to their shapes.

44. Auditory ossicles
The malleus. This is the lateral hammer-
shaped bone. The handle is in contact with
the tympanic membrane and the head forms
a movable joint with the incus.
The incus. This is the middle anvil-shaped
bone. Its body articulates with the malleus,
the long process with the stapes, and it is
stabilized by the short process, fixed by
fibrous tissue to the posterior wall of the
tympanic cavity.
The stapes. This is the medial stirrup-
shaped bone. Its head articulates with the
incus and its footplate fits into the oval
window.

45. Inner Ear
The inner ear or labyrinth (meaning
‘maze’) contains the organs of hearing
and balance.
It is described in two parts, the bony
labyrinth and the membranous
labyrinth and is divided into three main
regions:
• the vestibule, containing the utricle and
saccule
• three semicircular canals with
semicircular ducts
• the cochlea with cochlear duct.

46. Internal Ear
The bony labyrinth. This is lined with periosteum. Within the bony
labyrinth, the membranous labyrinth is suspended in a watery fluid called
perilymph.
The membranous labyrinth. This is filled with endolymph.

47. The Vestibule
This is the expanded part nearest
the middle ear.
The oval and round windows are
located in its lateral wall.
It contains two membranous sacs,
the utricle and the saccule, which
are important in balance.

48. The Vestibule
The utricle is a membranous sac
which is part of the vestibule and the
three membranous ducts open into it
at their dilated ends, the ampullae.
The saccule is a part of the vestibule
and communicates with the utricle
and the cochlea.
In the walls of the utricle, saccule
and ampullae are fine, specialized
epithelial cells with minute
projections, called hair cells.
Amongst the hair cells there are
receptors on sensory nerve endings,
which combine forming the
vestibulocochlear nerve.

49. The Cochlea
This resembles a snail’s shell. It has
a broad base where it is continuous
with the vestibule and a narrow
apex, and it spirals round a central
bony column.
A cross-section of the cochlea
contains three compartments:
• the scala vestibuli
• the scala media, or cochlear duct
• the scala tympani.

50. Organ Of Corti
The organ of corti is the sensitive
element in the inner ear and can be
thought of as the body’s microphone. It
is situated on the basilar membrane in
one of the three compartment of
cochlea.it contains hair cells, basal cells,
cells of Hensen, etc. the sensory hair
project from the scala media and inner
end of the cells of nerve fibres which
unite to form cochlear nerve. The
tectorial membrane overhangs the
sensory hair in scala media. It is
stimulated by sound waves or nerve
impulse of hearing starts from organ of
corti.

51. The Semicircular Canals And Duct
These are three tubes arranged so that
one is situated in each of the three
planes of space.
They are continuous with the vestibule
and are also important in balance
The semicircular canals have no
auditory function although they are
closely associated with the cochlea.
Instead they provide information about
the position of the head in space,
contributing to maintenance of posture
and balance.

52. The Semicircular Canals And Duct
There are three semicircular canals,
one lying in each of the three planes of
space. They are situated above,
beside and behind the vestibule of the
inner ear and open into it.
The semicircular canals, like the
cochlea, are composed of an outer
bony wall and inner membranous
tubes or ducts.
The membranous ducts contain
endolymph and are separated from the
bony wall by perilymph.

53. Mechanism Of Balance
Movement of head or alteration in its position produces movement of
endolymph present in the semicircular canals.
The movement of endolymph stimulates that nerve endings in ampullae.
The impulses are carried to brain through the vestibular portion of 8th
cranial nerve.
The impulses produce sensations which makes us conscious about the
position of the head. If the position of the head is disoriented, we can
then adjust it so as to maintain balance and equilibrium.

54. Mechanism Of Hearing
Sound waves in air are collected
by pinna.
The external auditory meatus
directs these waves to the
tympanic membrane which then
vibrates.
The vibrations are transmitted by
malleus, incus and stapes to the
membrane covering fenestra
ovalis.

55. Mechanism Of Hearing
From the inner surface of this
membrane, vibrations are
transmitted to organ of corti through
perilymph and endolymph.
From the organ of corti, the
impulses(produced by vibration) are
carried to brainstem through the
cochlear portion of 8th cranial nerve.
The fibers are then carried to
auditory center of brain which is
present in the temporal lobe of
opposite side.