SENSORY RECEPTORS &
THE SPECIAL SENSES
Leonardo Da Vinci, The Mona Lisa
Human Anatomy
Sonya Schuh-Huerta, Ph.D.
Peripheral Sensory Receptors
• 2 main categories of sensory receptors
– Free nerve endings of sensory neurons
• Monitor general sensory information
– Complete receptor cells  specialized
epithelial cells or small neurons
• Monitor most types of special sensory information
Peripheral Sensory Receptors
• Sensory receptors also classified according to:
– Location
– Type of stimulus detected
– Structure
Classification by Stimulus Detected
• Mechanoreceptors  respond to
mechanical forces
– Touch, pressure, stretch, vibration, & itch
– Baroreceptors monitor blood pressure
(detect pressure/stretch in walls of vessels)
• Thermoreceptors  respond to
temperature changes (heat/cold)
Classification by Stimulus Detected
• Chemoreceptors 
– Respond to chemicals in solution
• Photoreceptors  respond to light
– Located in the eye
• Nociceptors 
– Respond to harmful stimuli that result in pain
Classification by Location
• Exteroceptors  sensitive to stimuli
arising from outside the body
– Located at or near body surfaces
– Include receptors for touch, pressure, pain, &
temperature
Classification by Location
• Interoceptors  receive stimuli from
internal viscera
– Located in digestive tube, bladder, & lungs
– Monitor a variety of stimuli
•
•
•
•
Changes in chemical concentration
Taste stimuli
Stretching of tissues
Temperature
Classification by Structure
• General sensory receptors
– Widely distributed
– Nerve endings of sensory neurons monitor:
•
•
•
•
•
•
•
Touch
Pressure
Vibration
Stretch
Pain
Temperature
Proprioception
Classification by Structure
• General sensory receptors are:
– Divided into 2 groups
• Free nerve endings
• Encapsulated nerve endings
Free Nerve Endings
• Abundant in epithelia & underlying CT
• Respond to pain & temperature (several types)
Free Nerve Endings
• 2 other specialized types of free nerve
endings 
– Merkel discs (= Epithelial tactile complexes)
• Consist of tactile epithelial cell innervated by sensory
nerve ending
• Slowly-adapting receptors for light touch
– Hair follicle receptors  wrap around hair
follicles
• Detect hair-deflection/pulling
• Rapidly-adapting receptors
General Sensory Receptors –
Unencapsulated (Free)
Encapsulated Nerve Endings
• Consist of 1 or more end fibers of sensory
neurons enclosed in connective tissue
• Mechanoreceptors
– Include 4 main types:
•
•
•
•
Tactile (Meissner’s) corpuscles
Pacinian (Lamellar) corpuscles
Bulbous corpuscles (Ruffini endings)
Proprioceptors
Encapsulated Nerve Endings
• Tactile (Meissner’s) corpuscles
– Spiraling nerve ending surrounded by
Schwann cells
– Occur in the dermal papillae  remember this?
– Rapidly-adapting receptors for discriminative
touch (also light pressure, low freq vibration)
– Occur in sensitive, hairless areas of the skin
General Sensory Receptors
Tactile corpuscle
(touch, light pressure)
Epithelial tactile
complexes (light touch)
Free nerve
endings
(pain and
temperature)
Epidermis
We’ll view under
the microscope
Dermis and
hypodermis
Lamellar
corpuscle
(deep pressure)
Bulbous corpuscle
(pressure)
Hair follicle receptor
(hair movement)
Encapsulated Nerve Endings
• Pacinian (Lamellar) Corpuscles
– Single nerve ending surrounded by layers of
flattened Schwann cells
– Occur in the dermis, hypodermis, & CTs,
joints, etc.
– Sensitive to deep pressure  rapidlyadapting receptors
General Sensory Receptors
Tactile corpuscle
(touch, light pressure)
Epithelial tactile
complexes (light touch)
Free nerve
endings
(pain and
temperature)
Epidermis
We’ll also view
these under
the microscope
Dermis and
hypodermis
Lamellar
corpuscle
(deep pressure)
Bulbous corpuscle
(pressure)
Hair follicle receptor
(hair movement)
Encapsulated Nerve Endings
• Bulbous Corpuscles (= Ruffini endings)
– Located in the dermis, hypodermis & joints
– Monitor continuous pressure on the skin 
adapt slowly
General Sensory Receptors
Tactile corpuscle
(touch, light pressure)
Epithelial tactile
complexes (light touch)
Free nerve
endings
(pain and
temperature)
Epidermis
Dermis and
hypodermis
Lamellar
corpuscle
(deep pressure)
Bulbous corpuscle
(pressure)
Hair follicle receptor
(hair movement)
General Sensory Receptors –
Encapsulated
Encapsulated Nerve Endings
• Proprioceptors
– Monitor stretch in locomotory organs 
muscles, tendons, joints, & ligaments
– Send inputs on body movement to the CNS
– 3 types of proprioceptors
3 Types of Proprioceptors
• Muscle spindles  measure the changing
length of a muscle (=muscle stretch)
– Imbedded in the perimysium between muscle fascicles
• Golgi tendon organs  located near the
muscle-tendon junction
– Monitor tension within tendons (= tendon stretch)
• Joint kinesthetic receptors
– Sensory nerve endings within joint capsules
– Monitor stretch & pain/nociception
Structure of Proprioceptors
Secondary sensory
endings (type II fiber)
 Efferent (motor)
fiber to muscle spindle
 Efferent
Primary
sensory
endings
(type Ia
fiber)
(motor) fiber
to extrafusal
muscle fibers
Extrafusal
muscle
fiber
Muscle
spindle
Connective
tissue capsule
Intrafusal
muscle
fibers
Sensory
fiber
Golgi
Tendon
organ
Tendon
General Sensory Receptors –
Proprioceptors
The Special Senses
(Ch 16)
The Special Senses!
• Sight, hearing, balance, taste, & smell
• Special sensory receptors:
– Localized  confined to the head region
– Receptors are not free endings of sensory neurons
– Special receptor cells!
• Neuron-like epithelial cells or small peripheral
neurons
– Transfer sensory information to other neurons in
afferent pathways that lead to the brain
The Eye & Vision
• Visual organ  the eye
– 70% of all of the body’s sensory receptors are
in the eyes
– 40% of the cerebral cortex is involved in
processing visual information!
– Only 1/6 of the eye’s surface is visible
Accessory Structures of the Eye
• Eyebrows  coarse hairs on the
superciliary arches
• Eyelids (palpebrae)  separated by the
palpebral fissure
– Meet at the medial & lateral angles (canthi)
– Lacrimal caruncle  reddish elevation at the
medial canthus
– Tarsal plates  connective tissue within the eyelids
– Tarsal glands  modified sebaceous glands
Accessory Structures of the Eye
• Conjunctiva =
transparent mucous
membrane
Levator palpebrae
superioris muscle
Orbicularis
oculi muscle
Eyebrow
Tarsal plate
Palpebral
conjunctiva
Tarsal glands
Cornea
Palpebral
fissure
Eyelashes
Bulbar
conjunctiva
Conjunctival
sac
Orbicularis
oculi muscle
(b) Lateral view; some structures
shown in sagittal section
Accessory Structures of the Eye
• Lacrimal apparatus
 keeps the surface
of the eye moist
– Lacrimal gland 
produces lacrimal fluid
– Lacrimal sac 
fluid empties into nasal
cavity
– Can you see why,
when you cry you get a
leaky nose?...
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Extrinsic Eye Muscles
• 6 muscles that control movement of eye
– Originate in the walls of the orbit
– Insert on outer surface of the eyeball
• The 6 extrinsic eye muscles are:
– Lateral rectus & medial rectus
– Superior rectus & inferior rectus
– Superior oblique & inferior oblique
– Yes, you have to know them all
Extrinsic Eye Muscles
Superior oblique muscle
Trochlea
Trochlea
Superior oblique tendon
Superior
rectus
Superior rectus muscle
Lateral
rectus
Superior
oblique
Medial
rectus
Lateral rectus muscle
Inferior
oblique
Common
tendinous
ring
Inferior
rectus
muscle
Inferior
oblique
muscle
(a) Lateral view of the right eye
(b) Anterior view of the right eye
Inferior
rectus
Summary of Muscle Actions
Muscle
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique
Action
Controlling cranial nerve
Moves eye laterally
Moves eye medially
Elevates eye and turns it medially
Depresses eye and turns it medially
Elevates eye and turns it laterally
Depresses eye and turns it laterally
VI (abducens)
III (oculomotor)
III (oculomotor)
III (oculomotor)
III (oculomotor)
IV (trochlear)
(c) Summary of muscle actions and innervating cranial nerves
Anatomy of the Eyeball
• Components of the eye
– Protect & support the photoreceptors
– Gather, focus, & process light into precise images
• Anterior pole  most anterior part of the eye
• Posterior pole  most posterior part of the eye
• External walls  composed of three tunics (=
layers)
• Internal cavity  contains fluids (= humors)
The Fibrous Layer
• Most external layer of the eyeball
– Composed of 2 regions of connective tissue
• Sclera  posterior 5/6 of the tunic
– White, opaque region
– Provides shape & an anchor for eye muscles
• Cornea  Anterior 1/6 of the fibrous tunic
• Limbus  junction between sclera & cornea
• Scleral venous sinus  allows aqueous humor to
drain
Medial View of the Eye
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
Pupil
Anterior
pole
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous
sinus
Posterior segment
(contains vitreous humor)
Central artery
and vein of
the retina
Optic disc
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
The Vascular Layer
• The middle coat of the eyeball
• Composed of choroid, ciliary body, & iris
• Choroid  vascular, darkly pigmented
membrane
– Forms posterior 5/6 of the vascular tunic
– Brown color  from melanocytes
– Prevents scattering of light rays within the eye
• Choroid corresponds to the arachnoid & pia
maters
The Vascular Layer
• Ciliary body  thickened ring of tissue,
which encircles the lens
• Composed of ciliary muscle
– Ciliary processes  posterior surface of the
ciliary body
– Ciliary zonule (= suspensory ligament)
• Attached around entire circumference of the lens
• Suspend the lens & allow it to change shape when
focusing on near & far objects
Medial View of the Eye
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Retina
Macula lutea
Cornea
Iris
Fovea centralis
Pupil
Posterior pole
Anterior
pole
Optic nerve
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous
sinus
Central artery
and vein of
the retina
Posterior segment
(contains vitreous humor)
Optic disc
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
The Iris
• Visible colored part of the eye
–several genes code for its color!
• Attached to the ciliary body
• Composed of smooth muscle
• Pupil  the round, central opening in iris
– Sphincter pupillae muscle
– Dilator pupillae muscle
• Act to vary the size of the pupil
– Pupillary light reflex
• Protective response  pupil constriction when a bright
light is flashed in the eye or you go out into direct
sunlight
The Inner Layer (Retina)
• Retina  the deepest tunic
• Composed of 2 layers
– Pigmented layer  single layer of melanocytes
– Neural layer  sheet of nervous tissue
• Contains 3 main types of neurons:
– Photoreceptor cells
– Bipolar cells
– Ganglion cells
Microscopic Anatomy of the Retina
Photoreceptors
Ganglion
cells
Bipolar
cells
Rod
Cone
Nuclei of
ganglion
cells
Outer segments
of rods and cones
Choroid
Amacrine cell
Horizontal cell
Pathway of signal output
Pathway of light
Pigmented
layer of retina
(b) Cells of the neural layer of the retina
Axons of
ganglion cells
Nuclei
Nuclei of
of bipolar rods and
cells
cones
(c) Photomicrograph of retina
Pigmented
layer of
retina
The Retina
• Photoreceptor cells activate
bipolar cells
• Bipolar cells activate ganglion
cells to generate nerve impulses
• Axons from ganglion cells run
along internal surface of the retina
– Converge posteriorly to form the
optic nerve
Posterior Aspect of the Eyeball
Neural layer of retina
Pigmented
layer of
retina
Pathway of light
Choroid
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
Photoreceptors
• 2 main types
– Rod cells  more sensitive to light
• Allow vision in dim light (black & white)
– Cone cells  operate best in bright light
• Enable high-acuity, color vision
• Considered neurons!
Photoreceptors
• Rods & cones have an inner & outer
segment
– Outer segments are receptor regions
• Light absorbing pigments are present
• Are continuously shed & replaced over time
• Light particles (photons) modify the visual
pigment, which causes a change in the
flow of ions in/out of the cell, & generates
a nerve impulse in the photoreceptor
Photoreceptors
Process
of bipolar
cell
Synaptic
terminals
Inner
fibers
Mitochondria
Outer
fiber
Connecting
cilia
Outer segment
Inner segment
– rhodopsin (in rods)
– 3 opsins in cones
detect green, red, blue
– primates are trichromats
what does this mean?
Nuclei
Cone
cell
body
Pigmented layer
• Visual pigments in
the rods & cones that
allow you to see:
Rod cell
body
Rod
cell
body
Apical
microvillus
Discs
containing
visual pigments
Discs being
phagocytized
Melanin
granules
Pigment
cell
nucleus
Basal lamina
(border with
choroid)
Photoreceptors
• Photoreceptors
– Vulnerable to damage by light or heat
– Cannot regenerate if destroyed
– But continuously renew & replace their outer
segments
– Look closely at the retina where exactly are
the photoreceptors (which layer of cells are
they?)
Regional Specializations of the
Retina
• Ora serrata retinae
– Neural layer ends at the posterior margin of the ciliary
body
– Pigmented layer covers ciliary body & posterior
surface of the iris
• Macula lutea  contains mostly cones
• Fovea centralis  contains only cones
– Region of highest visual acuity!!!
• Optic disc  where optic nerve emerges
from retina – blind spot!
Medial View of the Eye
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Cornea
Iris
Pupil
Fovea centralis
Anterior
pole
Optic nerve
Retina
Macula lutea
Posterior pole
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous
sinus
Central artery
and vein of
the retina
Posterior segment
(contains vitreous humor)
Optic disc
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
Blood Supply of the Retina
• Retina receives
blood from 2
sources
– Outer 1/3 of the
retina  supplied
by capillaries in
the choroid
– Inner 2/3 of the
retina 
supplied by central
artery & vein of the
retina
Central artery
and vein
emerging
from the
optic disc
Macula
lutea
Optic disc
Retina
Internal Chambers & Fluids
• The lens & ciliary zonules divide eye
• Posterior segment (cavity)
– Filled with vitreous humor
•
•
•
•
Clear, jelly-like substance
Transmits light
Supports the posterior surface of the lens
Helps maintain intraocular pressure
Internal Chambers & Fluids
• Anterior segment
– Divided into anterior & posterior chambers
• Anterior chamber  between the cornea & iris
• Posterior chamber  between the iris & lens
• Filled with aqueous humor
– Renewed continuously
– Formed as a blood filtrate
– Supplies nutrients to the lens & cornea
Internal Chambers & Fluids
Cornea
Lens
Posterior
segment
(contains
vitreous
humor)
Iris
Lens epithelium
Lens
Cornea
2
Corneal epithelium
Corneal endothelium
Aqueous humor
Anterior chamber
1 Aqueous humor is
formed by filtration from
the capillaries in the ciliary
processes.
2 Aqueous humor flows
from the posterior chamber
through the pupil into the
anterior chamber. Some also
flows through the vitreous
humor (not shown).
Anterior
segment Posterior chamber
(contains
aqueous
humor)
Ciliary zonule
(suspensory
ligament)
3
Scleral venous
sinus
Ciliary
processes
Corneoscleral
junction
Ciliary
muscle
Bulbar
conjunctiva
Sclera
3 Aqueous humor is
reabsorbed into the venous
blood by the scleral venous
sinus.
1
Ciliary
body
The Lens
• A thick, transparent, biconvex disc
– Held in place by its ciliary zonule
• Lens epithelium  covers anterior surface
of the lens
• Lens fibers form the bulk of the lens
– New lens fibers are continuously added
– Lens enlarges throughout life
The Eye as an Optical Device
• Structures in the eye bend light rays
• Light rays converge on the retina at a single focal point
• Light bending structures (refractory media) are:
– lens, cornea, & humors
• Accommodation  curvature of the lens is adjustable
– Allows for focusing on nearby objects
The Eye as an Optical Device
Sympathetic activation
Nearly parallel rays
from distant object
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
(a) Lens is flattened for distant vision. Sympathetic input
relaxes the ciliary muscle, tightening the ciliary zonule,
and flattening the lens.
Parasympathetic activation
Divergent rays
from close object
Inverted
image
(b) Lens bulges for close vision. Parasympathetic input
contracts the ciliary muscle, loosening the ciliary zonule,
allowing the lens to bulge.
The Eye as an Optical Device
Myopic eye (nearsighted)
Hyperopic eye (farsighted)
Focal point
Eyeball
too long
Uncorrected
Focal point is in front of retina.
Corrected
Concave lens moves focal
point further back.
Eyeball
too short
Uncorrected
Focal point is behind retina.
Corrected
Convex lens moves focal
point forward.
Visual Pathways
• Most visual information travels to the
cerebral cortex
• Responsible for conscious “seeing”
• Other pathways travel to nuclei in the midbrain &
diencephalon
Visual Pathways to the Cerebral
Cortex
• Pathway begins at the retina
– Light activates photoreceptors
– Photoreceptors signal bipolar cells
– Bipolar cells signal ganglion cells
– Axons of ganglion cells exit the eye in the
optic nerve
– Some axons cross over to the opposite side
of the brain – they run through the optic
chiasma (remember this?)
Visual Pathways to the Cerebral
Cortex
• Then optic tracts send axons to:
– Lateral geniculate nucleus of the thalamus
• Synapse with thalamic neurons
• Fibers of the optic radiation reach the primary
visual cortex
Visual Fields & Visual Pathways to the Brain
Both eyes
Fixation point
Right eye
Left eye
Optic nerve
Suprachiasmatic
nucleus
Pretectal
nucleus
Lateral
geniculate
nucleus of
thalamus
Optic chiasma
Optic tract
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Optic
radiation
Occipital
lobe
(primary visual
cortex)
(a) The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
Superior
colliculus
Visual Pathways to Other Parts
of the Brain
• Some axons from the optic tracts
– Branch to midbrain
• Superior colliculi (remember this on the brain?)
• Pretectal nuclei
Disorders of the Eye & Vision
• Age-related macular degeneration (AMD)
– The macula begins to degenerate or lift off retina
– Involves the build-up of visual pigments in retina
– Blurred vision to blindness
• Retinopathy of prematurity
– Blood vessels grow within the eyes of premature infants
– Vessels have weak walls  cause hemorrhaging & blindness
• Trachoma  contagious infection of the conjunctiva
• Blindness – Congenital or acquired; Causes: cataracts,
glaucoma, AMD, corneal opacity, diabetic retinopathy,
childhood blindness, trachoma, etc.
Questions…?
What’s Next?
Lab: PNS & Special Senses Histology
& Eye Models
Mon Lecture: Special Senses II
Mon Lab: Cow Eye Dissection!