6 Special Senses

The human body experiences its environment by reacting to stimuli that reach the brain via the nervous system.  The somatic or general senses, including touch, temperature perception, pain, and proprioception (the awareness of one’s position and movement in space), use the free nerve endings in the skin, muscles, and membranes of the body to detect change and communicate that information to the CNS.  While it is possible to literally feel the thumping bass at a concert, to truly appreciate the intricacies of sound – – the pitch, melody, timbre, and texture – – you need a specialized sense called hearing.  The human body has five specialized senses, including vision, hearing, balance, taste, and olfaction, and each requires a special sensory organ to be perceived.

The Eye

The eye is the special sensory organ that gives rise to vision.  The eye is a hollow, slightly elongated sphere formed by a three-layer wall.  The three layers, or tunics, of the eye are, counting from outside:

  • the fibrous tunic that comprises the sclera and cornea
  • the vascular tunic that forms the iris, ciliary body, and choroid
  • the nervous tunic, which is also known as the retina
the layers of the eyeball

Inside the eyeball, the lens is suspended in place by zonular fibers (also called the suspensory ligament), which divides the interior of the eye into two regions, the anterior and posterior cavities.  The anterior cavity is further divided into anterior and posterior chambers by the iris, creating three separate but communicating, fluid-filled spaces within the eye:

  • the anterior chamber that extends from the posterior surface of the cornea to the iris
  • the posterior chamber that extends from the iris to the front surface of the lens
  • the vitreous chamber that extends from the posterior surface of the lens to the retina

The anterior and posterior chambers are filled with the aqueous humor, a watery fluid similar to plasma, while the vitreous chamber is filled with the vitreous body, a translucent, gel-like substance that is also called vitreous humor.

cross section of the eyeball and optic nerve

The fibrous tunic is comprised of two parts: the translucent cornea, in the front of the eyeball, and the opaque, white sclera, in the back.  Cornea and sclera differ primarily in the arrangement of their collagen fibers, which causes the difference in their transparency.

The cornea focuses incoming light onto the retina and serves to protect the eye.  The microscopic layers of the cornea, from the outside in, are:

  • stratified squamous epithelium
  • Bowman’s membrane (also called the anterior limiting membrane)
  • stroma
  • Descemet’s membrane
  • simple squamous epithelium
cross section of cornea

On its external, convex surface, the cornea is covered by stratified squamous epithelium.  The innermost layer of the epithelium is columnar and lies on the Bowman’s membrane.  In histological slides stained with H/E, the Bowman’s membrane is a solid pink layer directly under the epithelium.  The stroma, or core of the cornea, is a connective tissue made from parallel layers of collagen fibers that are produced by resident fibroblasts.  In histological slides, the clearly visible collagen bundles of the stroma are stained pink.  The darker spots in the stroma are the nuclei of the fibroblasts.  The internal, concave surface of the cornea is covered by simple squamous epithelium resting on Descemet’s membrane.

The cornea has no blood vessels, but does contain nerve fibers, including those that sense pain.  The cornea and the visible parts of sclera are loosely covered by the conjunctiva, a membrane that extends to the internal surface of the palpebrae, or eyelids.  The conjunctiva lubricates the eye and protects it from drying.  On the slide below, the conjunctiva is visible as the folded membrane on the left and is formed from connective tissue, with stratified squamous epithelium on its surface.  The space between the conjunctiva and cornea is filled with tears, which are produced by the lacrimal apparatus.

conjuctiva and cornea

The sclera is the white part of the fibrous tunic and covers the majority of the eyeball.  The place where the cornea and the sclera join is called the limbus.  The sclera is made of connective tissue composed of bundles of collagen and elastic fibers.  These fibers are less organized than in the cornea, and that contributes to opacity.  The main cells in the scleral stroma are fibroblasts.

In contrast to the cornea, the sclera has blood vessels; it also has sensory nerve fibers.  The visible part of the sclera, in the front of the eye, is covered by the conjunctiva.  In the back, the sclera is penetrated by the optic nerve exiting the eyeball.

The vascular tunic, also known as the uvea, is the layer of the eye situated between the outer fibrous tunic and the inner nervous tunic.  The vascular tunic is comprised of three parts:

  • the iris
  • the ciliary body
  • the choroid

The iris is the anterior portion of the vascular tunic and is visible through the cornea.  The iris divides the anterior cavity of the eye into the anterior and posterior chambers. The iris has a muscle located in its stroma that, by constricting and relaxing, changes the size of the pupil, limiting or increasing the amount of light that enters the eye and projects onto the retina.

The color of the iris depends on the presence of melanocytes in the stroma and in the epithelium that covers the iris.  In brown eyes, pigment is present both in the stroma and the epithelium; in blue eyes, pigment is only present in the epithelium.  In albino eyes, melanin is absent in both the stroma and the epithelium.  The irises appear red, however, because the blood vessels of the inner eye can be seen through these translucent irises.

the vascular tunic

The ciliary body is the part of the vascular tunic that is adjacent lens.  It consists of a ring of ciliary processes – – finger-like projections of vascular tissue covered by the ciliary epithelium that secretes the aqueous humor which fills the anterior and posterior chambers of the eye.  The ciliary body also includes a layer of muscle underneath the ciliary processes that controls the shape of the lens.  This is different than the muscle that controls the size of the pupil.  On the slide above, the ciliary body is circled; the image below provides a magnified view.

detail of the ciliary body

The choroid is the largest part of the vascular tunic, extending from the ciliary body to the optic disc where the optic nerve leaves the eyeball.  The choroid is made of capillaries and is responsible for the nutrition of the outer parts of the retina.  As such, it is tightly attached to the underlying retinal pigment epithelium, but loosely attached to the overlying sclera.

detail of the choroid and the optic nerve

The nervous tunic, or retina, is neuronal tissue that contains the rods and cones, photoreceptors that sense and process light in the visible range.  In the retina, the energy of visible light is changed into electrical impulses that travel to the brain and are interpreted by neurons in the visual cortex to provide vision.

The cells in the retina are organized into five layers, going from the choroid inward to the vitreous body:

  • rods and cones
  • horizontal cells
  • bipolar cells
  • amacrine cells
  • ganglion cells

Counterintuitively, this means that light shines through the transparent horizontal, bipolar, amacrine, and ganglion cells to strike the rods and cones!

Only three of the five layers are in the direct pathway of the signal transduction.  From rods and cones, the signal is passed to bipolar cells and then to ganglion cells.  Horizontal cells synapse with the rods and cones, while amacrine cells synapse with ganglion cells.  However, all cells are necessary for visual processing, and defects in any of the cell types result in blindness.  Axons of ganglion cells from the entire retina gather in the optic disk and continue as the optic nerve, also known as cranial nerve II, or CN II.

In histological slides, the retina appears as a ten-layered structure whose layers are defined by morphological characteristics.  These include the presence or absence of pink-stained fibers or of cell nuclei, which are visible as purple circles.  Some cells might form two or even three neighboring layers. Based on how it appears in the microscope, retina has the following layers, proceeding from the chorion to the vitreous body:

  • retinal pigment epithelium
  • layer of photoreceptor cells (rods and cones)
  • outer limiting membrane
  • outer nuclear layer
  • outer plexiform layer
  • inner nuclear layer
  • inner plexiform layer
  • ganglion cell layer
  • nerve fiber layer
  • inner limiting membrane
    layers of the retina

Adjacent to the choroid, the outermost layer of the retina is the retinal pigment epithelium, a cuboidal epithelium that contains melanin and provides nourishment to the retina.  Projections of these cells, especially rich in melanin granules, extend between the outer segments of rods.

Rods and cones are photoreceptive cells located in the deepest layer of the retina that can convert visible light to electrical impulses.  Both rods and cones have two segments: the outer segment, where the photoreceptors are located, and the inner segment, which is comprised of the cell body and a short axon.  Rod cells are highly sensitive to light, but provide minimal detail and contribute little to color vision.  Rods are prevalent in the outer areas of the retina.  Rod cells are responsible for peripheral vision and night vision.  Cone cells function best in bright light, and provide detailed, color vision.  Cones are found primarily in the fovea, a densely packed, rod-free area that provides acute vision for activities where detail is essential.  The outer segments of rods and cones form the photoreceptor outer segments layer.  The inner segments of the rods and cones, containing the cell bodies (and thus the nuclei), form the outer nuclear layer.

The outer plexiform layer is formed by the axons of rods and cones synapsing with the dendrites of the bipolar cells, and by horizontal cells that interact with the surrounding photoreceptive and bipolar cells

The inner nuclear layer consists of nuclei of bipolar, horizontal, and amacrine cells.

In the inner plexiform layer, axons of bipolar cells synapse with dendrites of ganglion cells, and with amacrine cells that modulate the surrounding ganglion cells.

The ganglion cell layer contains the cell bodies of ganglion cells and their surrounding neuroglia.

In the nerve fiber layer, axons of ganglion cells extend toward the optic disk to form the optic nerve.

cells of the retina, by layer

The Ear

The ear is the organ that is specialized for hearing and balance.  The ear has three parts, the external ear, the middle ear, and the inner ear.

The external ear is made of the auricle (or pinna) and the external auditory canal, which extends to the tympanic membrane (or eardrum).  The auricle collects sound waves and directs them to the auditory canal.  The auricle is composed of an internal layer of elastic cartilage that is covered by fat and skin.  The image below shows a cross section of the auricle with the internal elastic cartilage visible as a dark-stained layer.

cross section of the auricle

The middle ear is a cavity in the temporal bone and extends from the tympanic membrane to the oval window in the bony labyrinth.  The middle ear contains three tiny bones or auditory ossicles: the malleus, incus, and stapes.  The function of the middle ear is to amplify incoming sound waves, passing them to the inner ear.

The inner ear lies in the temporal bone, and it consists of interconnected canals filled with a fluid called endolymph.  The cochlea is a spiral canal within the bone that makes three turns around a central bony column called the modiolus.  This canal is partitioned by membranes into three separate spaces, the scala vestibuli, the scala tympani, and the scala media, or cochlear duct.  Located within the cochlear duct is the organ of Corti.  Sound waves traveling through the endolymph vibrate the hair cells of the organ of Corti.  These receptor cells translate their movement into nerve impulses that travel via the vestibulocochlear nerve (CN VIII) to the brain, where they are perceived as sound.

The images below show a cross section of the cochlea.  The scala vestibuli (labeled “SV”) is separated by the vestibular membrane (VM) from the cochlear duct (CD).  The scala tympani (ST) is a separate channel separated from the cochlear duct by the basilar membrane that contains the organ of Corti (OC).  A higher magnification image of the organ of Corti is on the right.

cross section of the cochlea
detail of a cross section of the cochlea

The inner ear also houses the vestibular system, which provides the special sense of balance.  The vestibular system is comprised of the semicircular canals, which indicate rotational movement and aid in orientation, and the otolith organs, which indicate linear acceleration.

The semicircular canals are three fluid-filled loops that sit within the bony labyrinth at right angles to one another.  As the head rotates in various directions, the movement of the fluid within the canals pushes on the cupula within each loop.  The cupula is a structure that contains hair cells which transduce rotational movement (i.e., turning around, nodding one’s head, or performing a cartwheel) into neural signals.

The two otolith organs, the utricle and the saccule, are located at the conjunction of the semicircular canals.  Both the utricle and the saccule contain a macula, which detects linear acceleration and consists of three layers.  The bottom layer is comprised of sensory hair cells, each with 40-70 stereocilia surrounding a large kinocilium.  The ends of these cilia are embedded in the otolithic membrane, itself weighed down by heavy calcium carbonate crystals called otoliths, statoconia, otoconia, or statoliths.  When the head is upright, the otolithic membrane pushes directly down on the hair cells, and this minimal stimulation signals to the brain that the head is stabilized.  When the head is tipped, gravity pulls on the otoliths embedded within the otolithic membrane, thus bending the cilia and stimulating the hair cells.  The brain distinguishes whether the head alone or the entire body is tilting by combining visual cues with input from the otolith organs and stretch receptors in the neck.

Taste Buds

The characteristic rough texture of the tongue is due to the presence of lingual papillae, nipple-like bumps that cover the tongue’s surface.  Lingual papillae increase both the surface area and the friction between the tongue and food, allowing for better taste sensation and improved manipulation of the bolus while chewing, respectively.  There are four different types of lingual papillae – – folate, circumvallate, fungiform, and filiform – – and all except the filiform papillae contain taste buds.

Taste buds are the special sensory organs for taste.  They are located primarily in deep crevices called taste pores, which surround lingual papillae, but can also be found on the soft palate, the pharynx, and on the epiglottis.  In the slide below, taste buds are indicated by arrows.  At this magnification, they look like lighter, finger-like projections within the epithelial layer.  Taste buds contain taste receptor cells which respond to the chemical properties of food particles dissolved in saliva, creating neural signals that the brain interprets as the sensation of taste.

There are five basic taste sensations: bitter, salty, sour, sweet, and umami (which can be described as “savory,” and is the taste of broth or cooked meat).  The various taste sensations do not arise from significant physiological differences in taste buds, and specific tastes are not localized to particular areas of the tongue.

cross section of a lingual papillae

Nasal Cavity

Olfaction, the sense of smell, occurs when odorants bind to olfactory receptors located in the nasal cavity.  There are many types of odor receptors, and each responds not to a single odor, but to a number of similarly structured odorants.  This diversity allows for millions of different smells – – even smells never before encountered – – to be discerned.  Signals from the odor receptors are sent to the olfactory bulb in the brain, via the olfactory nerve (cranial nerve I or CN I) where they are interpreted as smell.  Olfaction is not only tied to memory and emotion, but combines with taste to form the sense of flavor in foods.

Vocabulary

amacrine cell

anterior cavity (of the eye)

anterior chamber

aqueous humor

auricle (pinna)

basilar membrane

bipolar cells

Bowman’s membrane (anterior limiting membrane)

choroid

ciliary body

ciliary epithelium

ciliary processes

circumvallate papillae

cochlea

cone

conjunctiva

cornea

cupula

Descemet’s membrane

endolymph

external auditory canal

external ear

fibrous tunic

filiform papillae

foliate papillae

fovea

fungiform papillae

ganglion cell layer

hair cell

horizontal cell

inner ear

inner nuclear layer

inner plexiform layer

iris

kinocilium

lacrimal apparatus

limbus

lingual papillae

macula

middle ear

nerve fiber layer

nervous tunic

olfactory nerve (cranial nerve I, CN I)

optic disk

optic nerve (cranial nerve II, CN II)

organ of Corti

otolith organs

otolithic membrane

otoliths (statoconia, otoconia, statoliths)

outer nuclear layer

outer plexiform layer

oval window

palpebrae

photoreceptor outer segments layer

posterior cavity (of the eye)

posterior chamber

retina

retinal pigment epithelium

rod

saccule

scala media (cochlear duct)

scala tympani

scala vestibuli

sclera

semicircular canals

somatic (general) senses

specialized senses

stroma

taste bud

taste pores

tympanic membrane (eardrum)

utricle

vascular tunic (uvea)

vestibular membrane

vestibular system

vestibulocochlear nerve (cranial nerve VIII, CN VIII)

vitreous body (vitreous humor)

vitreous chamber

zonular fibers (suspensory ligament)

Study Prompts

  • Is the anterior cavity of the eye the same as the the anterior chamber of the eye?  the posterior cavity and the posterior chamber?  What about the anterior cavity and the posterior chamber?

 

  • Can you list the layers of the eyeball?  of the cornea?  of the retina?

 

  • What is the difference between the otolith organs and the otoliths?

License

Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

Share This Book