The Special Senses

Photoreceptors

Mechanoreceptors

Chemoreceptors

Photoreceptors

• Photoreceptors = Rods and Cones

• Sense organs - Eyes

Figure 15.1a

Figure 15.1b

Figure 15.2

Figure 15.3

Internal Structure of the Eye (sagittal section)

Figure 15.4a

The Eye

• Consists of 3 parts:

• Eye wall

• Humors ( bodily fluids)

• Lens

Eye Wall• Composed of 3 layers: Fibrous layer; Vascular layer; sensory layer

Fibrous layer: outer layer composed of tough dense fibrous CT

consists of 2 regions – posterior 5/6th is the sclera ( continuous with the dura mater surrounding the brain); and the anterior 1/6th is the cornea.

The sclera maintains the shape of the eye, protects the eye and serves attachments sites for the extrinsic muscles of the eye

Sclera forms the “WHITE” of the eye

The cornea allows light to enter the eye because it’s avascular and transparent

• Vascular layer = Uvea: middle layer that’s highly vascularized

consists of 3 regions: posterior 5/6th is the choroid, middle ciliary body, anterior iris

The choroid provides nutrients to the sclera and the sensory layer

The ciliary body ends in folds called the ciliary processes which contain blood capillaries that secrete the aqueous humor ; string-like structures extending from the ciliary processes called the ciliary zonule ( = suspensory ligament) which hold the lens in upright position in the eye

The iris is seen anteriorly as the “colored” part of the eye; the central opening in the iris called the PUPIL allows light to enter the lens; the iris contains 2 types of smooth muscle that control the size of the pupil; activation of the sympathetic nervous system and viewing distsnt objects result in pupillary dilation; activation of the parasympathetic nervous system and viewing close objects result in pupillary constriction.

Pupillary Constriction and Pupillary Dilation

Sensory layer• Innermost layer of the eye wall• Confined to the posterior wall ending at the ora serrata• Sensory layer consists of the pigmented layer and the neural layer • Pigmented layer – composed of a single layer of cells which provide nutrients to the

neural layer; contain melanin which absorbs light and prevents it from scattering; contain vitamin A required for the synthesis of the light-absorbing pigment called RETINAL

• Neural layer referred to as the R ETINA– extends anteriorly from the pigmented layer– Composed of 3 layers of neurons: PHOTORECEPTORS; Bipolar neurons;

Ganglion cells– PHOTORECEPTORS - abut the pigmented layer; they respond to light and

generate electrical signals; 2 types of photoreceptors –– (i) rods – more numerous ( 150 million); more sensitive to light - used in dim-light

and peripheral vision; provide images in shades of gray not used for color vision– (ii) cones – ( 80 million); operate in bright light, provide high- acuity color vision; – 3 types of cones – blue, green and red cones– According to the TRICHROMATIC THEORY OF VISION - several colors are

seen depending on which/how many of the three types of cones are activated

Bipolar neurons – receive electrical signals from the photoreceptors and conduct the signal to the ganglion cells

Ganglion cells – neurons that receive the electrical signals from the bipolar neurons; only the axons of the ganglion cells generate and transmit action potentials

Neural Layer - Retina

Photoreceptors – Rods and Cones

The Electromagnetic spectrum and photoreceptor sensitivities

Ganglion cells and the Optic Nerve

Ganglion cells and the Optic Nerve• Ganglion cells are the only neurons in the retina that can generate

and transmit action potentials

• Bundle of axons of the ganglion cells = OPTIC NERVE (CN II)

• The Optic Nerve exits the posterior wall of the eye through the OPTIC DISC = BLIND SPOT - because the optic disc lacks photoreceptors; lateral to the optic disc is the MACULA LUTEA and in its center is the FOVEA CENTRALIS; the macula lutea contains mostly cones and the fovea centralis contains only cones and it’s used for hard focus

The Eye

Figure 15.7

Aqueous and Vitreous Humors

The Humors in the Eye• Humors = fluids in the body

• 2 humors in the eye: Aqueous humor; Vitreous humor

• Aqueous humor – filtered from blood capillaries in the ciliary processes into the anterior chamber in front of the lens; drained by the canal of Schlemm; formed and drained continually. If the rate of synthesis exceeds the rate of drainage, intraocular pressure rises causing damage to retina and the optic nerve resulting in GLAUCOMA

• Function - supplies nutrients and oxygen to the lens and cornea; carries away metabolic wastes; maintains intraocular pressure to support the eyeball

• Vitreous humor – gel-like fluid in the posterior segment behind the lens; formed in the embryo and lasts a lifetime.

• Function – supports the posterior surface of the lens; pushes the neural layer against the pigmented layer; maintains the intraocular pressure.

• Retinal detachment – the retina detaches from the pigmented layer and the vitreous humor seeps into the space – without their nutrient source, the photoreceptors in the retina die leading to blindness

The Lens

The Lens• Avascular, transparent, biconcave and flexible• Held in an upright position behind the pupil and the iris by the ciliary

zonule• Composed of transparent proteins called crystallins• Function – focuses light on the retina; the flexible lens can change

its shape to precisely focus light on the retina referred to as ACCOMODATION

• Focusing for Distant Vision – the normal eyes are adapted for distant vision and therefore accommodation is not necessary; the far point of vision is the distance beyond which accommodation is not needed = 6m = 20ft (20/20 vision)

• Focusing for Close Vision – less than 20ft; involves accommodation of the lens where the lens bulges to focus a close objects onto the retina; the near point of vision is the distance at which the lens can bulge maximally to focus the object on the retina = 10cm = 4in;

• In addition, pupillary constriction occurs in close vision

Figure 15.13

Problems of Refraction

Problems of Refraction

• Myopia = nearsightedness – occurs when distant objects are focused in front of the retina; eyeball too long.

• Correction – concave lenses to diverge the light before it enters the eye; flattened cornea using LASIK

• Hyperopia = farsightedness – occurs when light from close objects are focused behind the retina; eyeball too short

• Correction – convex lenses to converge the light onto the retina

Refraction – spoon appears to be broken at the water-air interface

Thickening and hardening of the crystallins in the lens leads to the formation of cataracts = clouding of the lens

ct

Path taken by light through the eye: cornea—aqueous humor--pupil—lens—vitreous humor—

ganglion cells—bipolar neurons--photoreceptors

Pathway of light through the Retina: Ganglion cells—bipolar neurons--Photoreceptors

Transmission of electrical signals – light hits the photoreceptors and they generate electrical signals Bipolar neurons-Ganglion cells

Visual pathway to the Brain

Transmission of action potentials – impulse transmission

• Axons of ganglion cells form the optic nerves; generate and transmit impulses via the optic nerves; medial fibers of optic nerves cross over to opposite sides at the OPTIC CHIASMA and continue on as the OPTIC TRACTS.

• Impulses are transmitted to the visual reflex centers in the midbrain called the SUPERIOR COLLICULI

• Impulses are transmitted to the visual relay center in the thalamus called the LATERAL GENICULATE NUCLEUS (LGN)

• Impulses finally relayed to the primary visual cortex located in the occipital lobes

Mechanoreceptors – Hair Cells

Sense organs - Ears

The EAR

The Ear - 3 major regions• External ( outer) Ear = pinna( or auricle) + external auditory canal( external

acoustic meatus) • Middle Ear – contains the 3 ossicles = malleus, incus, stapes; air-filled cavity

TYMPANIC MEMBRANE, cone-shaped membrane that separates the external year from the middle ear; the broad base faces the external auditory canal and the apex abuts the malleus.

• Internal ( inner) Ear = labyrinth = Bony labyrinth and Membranous labyrinth; separated from the middle ear by a bony wall with an oval and round windows; the stapes sits atop the oval window

Bony labyrinth – Vestibule, Semicircular canals, Cochlea; all contain CSF-like fluid called PERILYMPH

Membranous labyrinth – consists on interconnecting sacs and ducts located inside the structures of the bony labyrinth; contains fluid called the ENDOLYMPH

Utricle and Saccule - membranous sacs located inside the vestibule; contain the equilibrium receptors that respond to the pull of gravity and head position

Semicircular ducts - membranous ducts located in the semicircular canals; expanded ends called AMPULLAE house equilibrium receptors that respond to the rotational movements of the head

Cochlear duct – membranous duct located in the cochlea

The EAR

Middle and Inner Ear

The 3 ossicles in the Middle Ear

Bony and Membranous Labyrinth

Anatomy of the Cochlea

The Organ of Corti

• Located in the cochlear duct which contains endolymph • Rests on a flexible membrane called the BASILAR

MEMBRANE• Composed of Supporting cells and HAIR CELLS, the

mechanoreceptors; the apical surfaces of hair cells have stereocilia which are microvilli stiffened by actin; stereocilia are trapped in a gel-like membrane called the TECTORIAL MEMBRANE.

• The afferent fibers of the cochlear nerve, a division of the vestibulocochlear nerve ( CN VIII), wrap around the bases of the hair cells

Organ of Corti

Figure 15.33

Photograph of cochlear Hair Cells with stereocilia

Route of Sound Waves through the Ear

Route of sound waves through the Ear• Pinna External Auditory canal Tympanic membrane

vibrates Malleus Incus Stapes Perilymph + Endolymph moveBasilar Membrane oscillates Hair cells move stereocilia bend electrical signals develop transferred to the cochlear nerve cochlear nerve generates and transmits action potentials via the vestibulocochlear nerve impulse transmitted to the auditory relay center in the thalamus called MEDIAL GENICULATE NUCLEUS (MGN) primary auditory cortex in the Temporal lobes

• Impulses also transmitted to the auditory reflex centers in the midbrain called the INFERIOR COLLICULI

Figure 15.34

Auditory Pathway to the primary auditory cortex in the TEMPORAL LOBES inthe cerebral hemispheres in the Brain

THE CHEMORECEPTORS

1.Olfactory cells in the Olfactory Epithelium

2. Gustatory cells in the taste buds

Olfactory Epithelium

Olfactory Epithelium• Yellowish patch in the roof of the nasal cavity• Composed of Supporting cells, Basal cells and Olfactory

cells• Olfactory cells are CHEMORECEPTORS• Olfactory cells are BIPOLAR NEURONS – their axons

bundle up to form the OLFACTORY NERVE (CN I)• Olfactory cells are unique neurons because they do not

exhibit longevity - olfactory cells are replaced every 60 days by the differentiation of the basal cells

• Dendrites of olfactory cells are ciliated and the cilia are

called olfactory hairs – trapped in a thin coat of mucus

Olfaction – Sense of Smell• Chemicals or the odorants must meet 2 criteria – must be volatile

and soluble in the thin coat of mucus covering the olfactory hairs ( cilia)

• Dissolved chemicals attach to the cilia resulting in depolarization which eventually causes the axons of the olfactory nerve to generate and transmit action potentials ( the impulse)

• Impulse is transmitted to a second type of neurons called MITRAL cells – a bundle of axons of the mitral cells form the OLFACTORY TRACT

• Impulse is transferred from the olfactory tract to the olfactory relay centers in the thalamus and in the mammillary bodies

• Impulses transmitted to the thalamus relayed to the primary olfactory cortex in the frontal lobe

• Impulses transmitted in the mammillary bodies are sent to the temporal lobe, hypothalamus, amygdala responsible for the emotional aspect of odors

Taste buds and Gustatory cells

Taste Buds and the Gustatory Cells• Taste buds are located in peg-like projections of the tongue called

PAPILLAE• 4 types of papillae – fungiform, foliate, vallate and filiform papillae

( the filiform papillae lack taste buds)• Each taste bud consists of 2 types of epithelial cells- basal cells and

Gustatory cells = CHEMORECEPTORS • Gustatory cells have long microvilli called gustatory hairs which

extend to the surface of the tongue through taste pores• Gustatory hairs are bathed in saliva.• Gustatory cells are subjected to tremendous friction hence, they are

replaced every 7 days by the differentiation of the basal cells into new gustatory cells

• Afferent fibers coil around the gustatory cells – 3 types cranial nerves are involved in the gustatory pathway to the brain:

– Chorda tympani, a branch of the facial nerve (CN VII)– Glossopharyngeal nerve( CN IX) – Vagus nerve ( CN X)

Figure 15.24

The Gustatory Pathway to the Primary Gustatory Cortex in the Insula

Gustation = Sense of taste• Activation of the gustatory cells – chemicals to be tasted, the tastant, must

dissolve in saliva bathing the gustatory hairs

• Taste sensations: sweet (sugars), salty(NaCl), sour (H+), bitter (alkaloids), “umami” ( monosodium glutamate =MSG)

• Dissolved chemical binds to gustatory hairs – results in depolarization which is transferred to the afferent fibers coiled around the gustatory cells:

• chorda tympani of the facial nerve (CN VII) generates and transmits action potentials from gustatory cells in the anterior two-thirds of the tongue

• Glossopharyngeal nerve (CN IX) generates and transmits action potentials from the posterior third of the tongue and the superior part of the pharynx

• Vagus nerve (CN X) generates and transmits action potentials from the inferior part of the pharynx

• Impulses from these 3 cranial nerves are transmitted to the SOLITARY NUCLEUS in the medulla oblongata; then to the gustatory relay center in the thalamus called the VENTRAL POSTEROMEDIAL NUCLEUS; impulse finally relayed to the primary gustatory cortex located in the Insula

• Taste is 80% smell – same chemicals activate both types of chemoreceptor = olfactory cells and gustatory cells

www.jhu.edu/jhumag/996web/taste.html

• How come food doesn't taste good when I have a cold? Because most of what we call "taste" is in fact smell, triggered by odor molecules from our food and drink. Some molecules we smell in the air, from the plate or as the fork approaches; others vaporize as we chew, then rise into the nasal passages at the back of the mouth.

• Tastebuds alone can detect only sweet, sour, salty, and bitter. "If you lick a pink ice cream cone," says Donald Leopold, an otolaryngologist at Hopkins's Bayview Medical Center, "your tongue tells you it's cold and sweet and smooth, but your sense of smell tells you it's strawberry. Probably 80 percent of what you eat, you appreciate through your sense of smell." That's why if you have a cold, you could mistake a bite of onion for apple.