Unit Three:
Nervous Control
Chapter Six:
Sensations
Prepared by
Prof. Nabih A. Baeshen
Introductory Biology 2
BIO 202
6.1 Introduction
•
Sensory organs in the peripheral nervous system
(PNS) absorb energy from a stimulus (pressure or
light, for example) and transform it into electrical
energy, a generator or receptor potential ,which in
turn, generates an action potential.
•
The action potential, or nerve impulse, travels along
sensory neurons in nerves from the receptor into the
central nervous system (CNS).
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•
In the CNS, the nerve impulses may travel to the part
of the cerebrum where they will be consciously
experienced and interpreted (perceived), or they may
participate in a reflex pathway that is part of the
autonomic nervous system (ANS), involved in
unconscious operation of the body.
•
Sensory receptors are classified in several ways.
•
They can be classified according to the kind of
stimulus they respond to.
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•
Mechanoreceptors
respond
to
mechanical
stimulation such as stretching or pressure.
•
Chemoreceptors respond to chemicals such as
acids.
•
Nociceptors produce pain when activated.
•
Electroreceptors respond to electricity, and
•
Photoreceptors
respond
to
light.
These
are
summarized in Table(1).
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Table (1)
Type of Receptor
Mechanoreceptors
Classification of Sense Receptors by Type of Stimulus Energy
Sense Organ
Stimulus
Organs that affected by touch,
pressure, movement, gravity,
sound waves
Mechanical
Dynamic
Pacinian corpuscles, Merkel
disks,
Ruffini corpuscles, Meissner
corpuscles
Muscle spindles
Golgi tendon organs
Joints receptors
Muscle contraction
Stretch of tendon
Ligaments movement
Statocysts
Statocysts in invertebrates
Gravity
Lateral line
Lateral line in fish
Waves and currents in the water
External, middle, and inner
ear
Gravity
sound waves
Chemical compounds
Odors
Gases
Tactile
Proprioceptors
Auditory organs
Type of Stimulus Energy
Touch, pressure
Mechanical
Dynamic
Chemoreceptors
Taste buds,
Olfactory epithelium
Thermoreceptors
Specialized organs in insects
and vipers,
Skin and tongue of many
animals
Heat
Thermal
Nociceptors
Free nerve endings as in skin
Heat
Painful sting
Chemicals
Thermal
Mechanical
Chemical
Electricity
Electrical
Light
Light
Electroreceptors
Photoreceptors
Specialized organs in some
fishes
Eyespots,
Ommatidia,
Eye
Prepared by Prof. Nabih A. Baeshen
Chemical
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Table (1)
Classification of Sense Receptors by Type of Stimulus Energy
Type of Receptor
Sense Organ
Stimulus
Mechanoreceptors
Organs that affected
by touch, pressure,
movement, gravity,
sound waves
Mechanical
Dynamic
Pacinian corpuscles,
Merkel disks,
Ruffini corpuscles,
Meissner corpuscles
Touch, pressure
Tactile
Muscle spindles
Golgi tendon organs
Joints receptors
Muscle contraction
Stretch of tendon
Ligaments movement
Statocysts
Statocysts in
invertebrates
Gravity
Lateral line
Lateral line in fish
Waves and currents in
the water
External, middle, and
inner ear
Gravity
sound waves
Proprioceptors
Auditory organs
Prepared by Prof. Nabih A. Baeshen
Type of Stimulus
Energy
Mechanical
Dynamic
6
Table (1)
Continued ….
Sense Organ
Stimulus
Type of Stimulus
Energy
Chemoreceptors
Taste buds,
Olfactory epithelium
Chemical compounds
Odors
Gases
Chemical
Thermoreceptors
Specialized organs
in insects and vipers,
Skin and tongue of
many animals
Heat
Thermal
Nociceptors
Free nerve endings
as in skin
Heat
Painful sting
Chemicals
Thermal
Mechanical
Chemical
Electroreceptors
Specialized organs
in some fishes
Electricity
Electrical
Photoreceptors
Eyespots,
Ommatidia,
Eye
Light
Light
Type of Receptor
Prepared by Prof. Nabih A. Baeshen
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•
The sensory receptors are also classified according to
the source of the stimulus.
•
If it originates outside the body, in the environment, it
is an exteroceptor.
•
If it is coming from the body, it is an interoceptor.
•
Proprioceptors are located within the body and
respond to mechanical stress, so they are both
interoceptors and mechanoreceptors.
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•
These receptors participate in reflex pathways that will not
be perceived.
•
They are located in muscles, tendons, and joints and
monitor the stress on these structures, making automatic
adjustments that ensure optimum movement.
•
Other interoceptors respond to changes in pressure, pH,
body temperature, and in the concentration of chemicals in
the blood. These operate automatically (again, without
perception)
to
maintain
homeostasis
(dynamic
equilibrium).
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6.2 Mechanoreceptors: Tactile Sensation
•
Many exteroceptors are present in the skin, these
relay tactile sensations such as touch, pressure,
and temperature to the cerebrum where they will be
perceived and localized (Fig. 6-1).
•
In
both
invertebrates
and
vertebrates,
the
thermoreceptors are important for locating the host
or prey from distances.
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Fig. 6-1. Tactile Sensation Receptors in
The Human Skin
Prepared by Prof. Nabih A. Baeshen
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6.3 Mechanoreceptors: Gravity Sensation
•
Another type of mechanoreceptors is a gravity receptor
known as a statocyst in invertebrates (Fig. 6-2) .
•
Gravity sensing is performed by hair cells in vertebrates.
•
These are referred to as the lateral line in fish (Fig. 6-3).
•
Hair cells are present also in the sensory apparatus in the
inner ear of humans in ❶ the vestibule and semicircular
canals where they sense gravity, equilibrium, and
rotational motion, and ❷ in the cochlea where they
respond to sound waves (Fig. 6-4).
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Statolith
Sensory hairs
Sensory neurons
Fig. 6-2. Statocyst
Prepared by Prof. Nabih A. Baeshen
Fig. 6-3. Lateral Line
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sound waves  pinna  auditory canal  tympanic membrane 
bones of the middle ear  oval and round windows  inner ears 
hair cells in vestibular system and cochlea  auditory nerve  brain
Prepared by Prof. Nabih A. Baeshen
Fig. 6-4. Anatomy of The Human Ear
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6.4 Chemoreceptors: Taste and Smell
•
Taste and smell in humans are conferred by
chemoreceptors present in taste buds in the tongue
and olfactory patches in the nose, respectively.
•
Taste cells within the buds respond to chemicals from
food or drink that are dissolved in the saliva.
•
These will induce impulses that will be relayed to the
cerebrum and will be perceived as sweet, bitter, salty,
or acidic. (Fig. 6-5)
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Human tongue
Scanning Electron Microscopy photo
graph of Taste buds
Locations of Main Taste Types
Regions on the Tongue
Microscopic Structure of
Taste Bud
Tasty molecules (sweet) link to Microvilli in the taste bud then enter through the taste or stimulating
taste receptor cells initiating receptor potential that travel to the brain by sensory neurons
Prepared by Prof. Nabih A. Baeshen
Fig. 6-5. Taste Receptors in The Human Tongue
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•
Smell (Olfactory) receptors
o
Some of the ingested chemicals (odor molecules)
in food or drink will vaporize and pass through the
internal nasal opening into the nasal cavity,
stimulating olfactory cells there.
o
Generated impulses in these cells will again travel
via olfactory nerve to the brain for perception.
Thus, the sense of smell complements and
enhances the sense of taste (Fig. 6-6).
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ingested chemicals (odor molecules)  stimulate olfactory cilia of the olfactory cells 
generating impulses  travel along the olfactory nerve  to olfactory bulb in the brain  where
perception of the smell is taking place
Prepared by Prof. Nabih A. Baeshen
Fig. 6-6. Olfactory Organ in Human Nose
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6.5 Nociceptors
•
Nociceptors the pain receptors are free nerve
endings that are present in all tissues, especially the
skin.
•
These can respond to temperature, mechanical
influences (such as being stung by a pin or an insect),
or chemicals.
•
Pain is considered a useful, though unpleasant,
warning to the organism to avoid potentially harmful
situations.
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•
Tissue injuries hurts blood vessels and cells which
lead to release K+ , enzymes and many other
chemicals.
•
They gathered in the pain region and stimulate the
pain receptors which is free nerve endings (Fig. 6-7).
•
Nerve impulses are generated and travel to the spinal
cord and from there to the thalamus in the brain and
from there to some other regions in the cerebrum.
•
The brain respond by releasing natural pain relieving
chemicals called endorphins (Fig. 6-8).
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Fig. 6-7. Nociceptors
Fig. 6-8. Brain Response to Nerve
Impulses Generated by Pain Receptors
Tissue injury  release of many chemicals  stimulate pain receptors 
generation of nerve impulses (Fig. 6-7)  spinal cord  thalamus  cerebrum
 releasing natural pain relieving substance (analgesics) (Fig. 6-8.).
Prepared by Prof. Nabih A. Baeshen
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6.6 Electroreceptors
•
Electroreceptors are present in some fish.
•
They permit the fish to detect electrical currents in the
water to locate the prey or identify migration routes to
breeding areas.
•
They are hair cells found in the lateral line system in
fishes like sharks (Fig. 6-9).
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Lateral line where electroreceptors hair
cells are located
Fig. 6-9. Electroreceptors in the
Lateral Line in Fishes
Electroreceptors detect electrical fields generated in water by the activity of
muscles of the prey and therefor, the prey can be located by predators that have
these electroreceptors.
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6.7 Photoreceptors
•
The simplest kind of photoreceptor is the eyespots of
some invertebrates, which merely allows assessment of
illumination intensity without image formation (Fig. 6-10).
•
The compound eyes in insects and crustaceans consist of
many ommatidia that form a mosaic image (Fig. 6-11).
•
Each ommatidium is a lens covered by a cornea, which
refracts (bends) the light passing through it, focuses it on
a crystalline cone, and directs it to the photoreceptor (this
is similar to the retina in humans).
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Fig. 6-10. Simple Eyespot in Planaria
 The eyespots are very primitive eyes as the one in palanaria (a flat worm).
 Pigment cells stimulates the sensory cells when exposed to light.
 Thus detecting the light without forming an image, this make the animal goes
back to dark and moist places to hide from predators.
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a.
b.
c.
Prepared by Prof. Nabih A. Baeshen
Fig. 6-11. Compound
Eye: (a) Scanning
Electron Micrograph
of an Insect, Showing
a Compound Eye on
Each Side of the
Head.
(b) Scanning Electron
Micrograph Showing
that Each Eye is
Made
Up
of
Ommatidia. (c) Each
Ommatidium
Has
Many Photoreceptor
Cells Covered by a
Lens and Surrounded
by Pigmented Cells
that Prevent Light
From Passing Out to
Neighboring
Ommatidium.
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•
The human eye receives reflected light as an
inverted image (Fig. 6-12).
•
The light travels through the cornea, the aqueous
humour, pupil, lens, and the vitreous humour
before it is focused on the retina, where it is
processed and sent to the brain via the optic nerve
(Fig. 6-13).
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Prepared by Prof. Nabih A. Baeshen
Fig. 6-12. Human Eye Parts and Their
Functions
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❼
❶
❷
❸
❹
❺
❻
Aqueous
Humor
Vitreous
Humor
❶ Reflected light of the object  ❷ cornea refracts it  aqueous humor  ❸ pupil
 ❹ lens further refracts it  vitreous humor  ❺ inverted image on retina (rods
& cones)  impulses  ❻ optical nerve  ❼ brain  normal image
Fig. 6-13. Path of Light Through the Eye Forming
an Inverted Image of The Object
Prepared by Prof. Nabih A. Baeshen
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•
The photoreceptors in the retina are called rods and cones.
•
Rods are sensitive to quite dim light and do not respond to color,
while the cones respond to color but are sensitive only to intense
light.
•
When light is received by the rods, it interacts with retinal, a
cofactor of the protein opsin.
•
Together these form the pigment rhodopsin:
retinal + opsin
•
dim
light
rhodopsin
The interaction induces a series of reactions inside the rods to
produce nerve impulses, which are transmitted by the optic
nerve to the brain for perception.
•
Vision disorders as accounted for in (Fig. 6-14) .
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Normal Eye
Correction
Disorder
Nearsighted
Eye
Object focused on
front of retina
Correction by concave lens
that focus object on retina
Farsighted
Eye
Object focused
behind retina
Correction by convex lens
that focus object on retina
Astigmatism
Object not in focus
Prepared by Prof. Nabih A. Baeshen
Correction by Cylindrical lens
that focus object on retina
Fig. 6-14. Human Vision
Disorders an their Corrections
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APPENDIX
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Key Terms
Aqueous humor
Astigmatism
‫السائل (الخِلط) المائي‬
‫الالبؤرية‬
Auditory canal
‫القناة السمعية‬
Auditory nerve
‫العصب السمعي‬
Blind spot
Cylindrical lens
Chemoreceptors
Choroid
Ciliary muscles
Cochlea
Compound eyes
Concave lens
Prepared by Prof. Nabih A. Baeshen
‫البقعة العمياء‬
‫عدسة اسطوانية‬
‫مستقبالت كيمائية‬
‫مشيمة‬
‫عضالت ھُدبية‬
‫قوقعة‬
‫عيون مُركبة‬
‫عدسة محدبة‬
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Cones
‫مخاریط‬
Conjunctiva
‫الملتحمة‬
Convex lens
‫عدسة مقعرة‬
Cornea
‫القرنية‬
Crystalline cone
‫مخروط بلوري‬
Electroreceptors
‫مستقبالت كهربائية‬
Endorphin
Equilibrium
Exteroceptors
Eyelid
Eyespots
Farsighted eye
Fovea
Free nerve endings
Prepared by Prof. Nabih A. Baeshen
)‫إندورفين (داخلية المورفين‬
‫توازن‬
‫مستقبالت خارجية‬
‫جفن العين‬
‫البقع العينية‬
‫طول النظر‬
)‫نقرة مركزیة (حُفيرة‬
‫نهايات عصبية حرة‬
34
Generator
Gravity receptors
Hair cells
Homeostasis
Inner ear
Interoceptors
Iris
Lateral line
Lens
Mechanoreceptors
Microvilli
Middle ear
Nasal cavity
Nearsighted eye
Prepared by Prof. Nabih A. Baeshen
‫مولد‬
‫مستقبالت الجاذبية‬
‫الخاليا الشعرية‬
‫اتزان حيوي‬
‫األذن الداخلية‬
‫المستقبالت الداخلية‬
‫القزحية‬
‫الخط الجانبي‬
‫عدسة‬
‫المستقبالت الميكانيكية‬
‫زغيبة‬
‫األذن الوسطى‬
‫تجويف األنف‬
‫قصر النظر‬
35
Nociceptors
Odor molecules
)‫ األلم‬،‫مستقبالت اإلیذاء (الضرر‬
‫جزيئات الروائح‬
Olfactory bulb
‫بصيلة الشم‬
Olfactory cells
‫خاليا الشم‬
Olfactory cilia
Olfactory epithelium
‫أھداب شمية‬
‫الطالئية الشمية‬
Olfactory nerve
‫عصب الشم‬
Olfactory organ
‫عضو الشم‬
Olfactory patches
Ommatidia
Ommatidium
Outer ear
Opsin
Optic nerve
Prepared by Prof. Nabih A. Baeshen
‫بقع شمية‬
‫عوینات‬
‫عوینة‬
‫األذن الخارجية‬
)‫أُوبسين (بروتين غشائي‬
‫عصب بصري‬
36
Oval window
Pain receptors
Photoreceptors
‫النافذة البيضاوية‬
‫مستقبالت األلم‬
‫مستقبالت ضوئية‬
Pigment cells
‫خاليا صبغية‬
Pinna
‫صيوان األذن‬
Planaria
Perceived
‫دودة مفلطحة‬
‫المدركة‬
Predator
‫المفترس‬
Pressure
‫ضغط‬
Prey
Primitive
Proprioceptors
Pupil
Receptor potential
Prepared by Prof. Nabih A. Baeshen
‫الفریسة‬
)‫أولي (بدائي‬
‫المستقبالت الذاتية‬
)‫حدقة (بؤبؤ‬
‫فرق جهد المستقبل‬
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Reflex pathways
‫مسارات انعكاسية‬
Refract
‫انكسار‬
Retina
‫شبكية‬
Retinal
Rhodopsin
Rods
Rotational motion
Round window
)‫صبغة الشبكية (ریتينال‬
)‫رودوبسين (صبغة بصریة‬
‫العصویات‬
‫حركات دائرية‬
‫نافذة مستدیرة‬
Saliva
‫اللعاب‬
Sclera
‫الصُلبة‬
Semicircular canals
Sensory cells
Sensory organs
Sensory receptors
Prepared by Prof. Nabih A. Baeshen
‫القنوات الهاللية‬
‫خاليا حسية‬
‫أعضاء حسية‬
‫مستقبالت حسية‬
38
Smell
Smell receptors
Statocyst
Stimulus
Suspensory ligaments
Tactile sensation
Taste
Taste bud
Taste receptors
Temperature
Thermoreceptors
Touch
Transform
Tympanic membrane
Prepared by Prof. Nabih A. Baeshen
‫الشم‬
‫مستقبالت الشم‬
‫كيس التوازن‬
‫منبه‬
‫الرباط المعالقي‬
‫حاسة اللمس‬
‫التذوق‬
‫براعم التذوق‬
‫مستقبالت التذوق‬
‫درجة الحرارة‬
‫مستقبالت حرارية‬
‫اللمس‬
‫تحويل‬
‫غشاء الطبلة‬
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Unconscious
Vestibule
‫الالشعوري‬
‫الدھليز‬
Vision disorder
‫عيوب اإلبصار‬
Vitreous humor
‫سائل (خلط) زجاجي‬
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