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CHNB: SPECIAL SENSORY ORGANS

SKIN

Skin can be divided into thin skin and thick skin. The thick skin is found in the sole of the feet & palm of the

hand and remaining all parts contains thin skin. The skin of the sole may be as thick as 1cm, where as the

skin of the upper eye lid as thin as 0.5mm only. Basically

skin consists of two layers. Those are:

Epidermis

Dermis

Epidermis:

The epidermis of the skin develops from the ectoderm

of the germinal layers. This ectoderm, while developing

into the skin, ectoderm also forms nails, hair, sebaceous

glands and sweat glands. These structures are collectively

called as appendages of the skin. The epidermis contains

no blood vessels, its nutrition being derived from the

capillaries of dermis. As the cells move upwards cells

become dead due to the lack of nutrition and fills with

keratin. The epidermis consists of five layers. Those are

Stratum corneum

Stratum lucidum

Stratum granulosum

Stratum spinosum/prickle cell layer

Stratum germinatum/ basale

Stratum germinatum: Cells are columnar and stand on the basement membrane. The basement membrane

stands between the epidermis and dermis. As the name indicates these cells continuously divide and produce

new cells. Melanocytes which are responsible for the synthesis of melanin (i.e. pigment, which is mainly

responsible for color of skin) are present in this layer.

Stratum spinosum: Iit is 3-5 cells thick. The individual cells have a prickly appearance. The prickled

appearance is due to the fact that cytoplasmic processes come out from the individual cells to meet their

fellow cytoplasmic strands of the adjacent cells.

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Stratum granulosum: It is about 5 cells thick and lies superficial to the stratum spinosum. The precursors of

the keratin take the shape of granules which are present within the cells of stratum granulosum.

Stratum lucidum: It is present in thick skins only. The granules of the stratum granulosum are converted

into eleidin. The cells become rather clear and form stratum lucidum. Eleidin is the precursor of keratin.

Stratum corneum: this layer is the most superficial layer and is in contact with the external environment.

The eleidin is now converted into keratin which is a fibrous protein. The cells by now have become dead.

The most superficial layers of stratum corneum are regularly cast off.

Dermis:

The bulk of the dermis is made up of collagenous fibers. These collagenous fibers are responsible the

water holding capacity of the skin. The superficial portion of the dermis is known as papillary layer and

deeper one is known as reticular layer.

The thin superficial papillary layer is areolar connective tissue in which the collagen and elastic fibers form a

loosely woven mat that is heavily invested with blood vessels On the palms of the hands and soles of the feet,

these papillae lie atop larger mounds called dermal ridges, which in turn cause the overlying epidermis to

form epidermal ridges that increase friction and enhance the gripping ability of the fingers and feet.

Epidermal ridge patterns are genetically determined and unique to each of us.

The deeper reticular layer, accounting for about 80% of the thickness of the dermis, is dense irregular

connective tissue. The network of blood vessels that nourishes this layer, the cutaneous plexus, lies between

this layer and the hypodermis.

Hair: These are formed by a downward growth of epidermal cells into the dermis or subcutaneous tissues

called hair follicles. At the base of the follicle, there is a cluster of cells called the papilla or bulb. The part of

the hair above the skin is the shaft and the remainder is root shows hair growing through the skin. The color

of the skin is genetically determined and depends on the amount of melanin present. White hair is the result

of replacement of melanin by tiny air bubbles.

Arrector pili: skeletal muscle fiber present in the skin. These are attached with the hair follicle, when they

contract, the hair stand out and become erect. The contraction of these muscle fibers is due to the sympathetic

stimulation. Excitement and panic can cause erection of hair.

Sebaceous glands: These consist of secretory epithelial cells derived from the same tissue as the hair

follicles. They secrete an oily substance known as sebum into the hair follicles present in skin of the body

except the palms of the hands and the soles of the feet. Sebum consists of free fatty acids, waxes, paraffin,

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sterols and squalene. These prevent evaporation of water from the skin and consequently prevent heat loss

from the body and it keeps the hair soft and pliable and gives shiny appearance. It provides some water

proofing and acts as bactericidal and fungicidal agent, preventing infection. Growth of the sebaceous glands

is stimulated by the hormone dehydroepiandro sterone (DHA) releasing from adrenal cortex. DHA secretion

is maximal in puberty or early youth.

Nails: They protect the tips of fingers and toes. The root of the nail is embedded in the skin and covered by

the cuticle which forms the hemispherical pale area called the lunula. The nail plate is the exposed part that

has grown out from the germinative zone of the epidermis called the nail bed. Finger nails grow more

quickly than toe nails and growth is quicker when the environmental temperature is high.

Sweat glands: they are formed the epithelial cells the bodies of glands lie coiled in the subcutaneous tissue

and opens through a duct on to the surface of the skin. These are widely distributed throughout the skin and

are most numerous in the palms of the hands, soles of the feet, axillae and groins. Sweat glands are of two

types. Those are: 1. Eccrine glands 2. Epocrine glands.

S.No. Eccrine Gland Epocrine Gland

1 Function from the birth. Activate at the period of puberty.

2 Works up to the death. Functioning will decrease with age.

3 It secretes watery clear fluid. It secretes thick and milky secretions.

4 Useful in temperature regulation andemotional

conditions.

Useful in emotional conditions only.

5 Present all over the body. Present only in axillae, pubic region and

around the nipples of the breast.

6 Functions under the control of nervous system. Functions under the control of endocrine

system.

7 Stimulated by vitamin-D3. Stimulated by DHA.

Functions of the skin:

1. Protection:

Protects from bacteria or toxic substances due to the presence of langerhans cells (specialized

immune cells).

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Protects from U.V. rays or light by the combined activity of stratum corneum and melanin.

Protects from mechanical injury, invasion by microorganisms, chemicals, physical agents and

dehydration.

2. Sensory perception: Nerve endings present in the dermis useful for the sensory perception like touch,

pain, pressure, temperature…

3. Balance of salts: To prevent the excess salt loss and dehydration.

4. Excretory functions: Skin excretes traces of urea, salts and fatty substances in the form of sebum.

5. Absorptive functions: Absorbs the drugs which are applied topically.

6. Regulation of body temperature: Under normal resting conditions, and as long as the environmental

temperature is below 31–32°C (88–90°F), sweat glands continuously secrete unnoticeable amounts of

sweat [about 500 ml (0.5 L) of sweat per day]. When body temperature rises, dermal blood vessels dilate

and the sweat glands are stimulated into vigorous secretory activity. Sweat becomes noticeable and can

account for the loss of up to 12 L of body water in one day. Evaporation of sweat from the skin surface

dissipates body heat and efficiently cools the body, thus preventing overheating.

7. When the external environment is cold, dermal blood vessels constrict. This causes the warm blood to

bypass the skin temporarily and allows skin temperature to drop to that of the external environment. Once

this has happened, passive heat loss from the body is slowed, thus conserving body heat.

8. Secretory function

9. Water balance.

10. Synthesis of vit-D3: it occurs by the conversion of 7-dehydro cholesterol into vitamin-D3 in the presence

of light.

OLFACTION, the sense of smell

It involves olfactory receptors responding to chemical stimuli. The sense of smell, called olfaction; These

organs are located in the nasal cavity. The olfactory organs are made up of two layers: the olfactory

epithelium and the lamina propia. The olfactory epithelium contains the olfactory receptor cells, supporting

cells. This epithelium covers the inferior surface of the cibriform plate. The underlying lamina propia

consists of areolar tissue, numerous blood vessels, and nerves. This layer also contains olfactory glands,

whose form thick, pigmented mucus.

When you inhale through your nose, the air swirls within the nasal cavity, and this turbulence brings airborne

compounds to your olfactory organs. Sniffing repeatedly increases the flow of air across the olfactory

epithelium, intensifying the stimulation of the olfactory receptors. However, only the molecules of water-

soluble and lipid-soluble materials that can diffuse into the mucus can stimulate olfactory receptors.

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Olfactory receptors are highly modified neurons. The exposed tip of each receptor cell forms a prominent

knob that projects beyond the epithelial surface. The knob provides a base for up to 20 cilia that extend into

the surrounding mucus. These cilia lie parallel to the epithelial surface, exposing their considerable surface

area to dissolved compounds called odorants. Between 10 to 20 million olfactory receptors are packed into

an area of roughly 5 cm2. A German shepherd dog sniffing for smuggled drugs or explosives has an olfactory

receptor surface 72 times greater than that of the nearby customs inspector!

Olfactory Pathways

The olfactory system is very sensitive. As few as four odorant molecules can activate an olfactory receptor.

Axons leaving the olfactory epithelium collect into 20 or more bundles that penetrate the cribriform plate of

the ethmoid to reach the olfactory bulbs of the cerebrum.

Olfactory Discrimination

The olfactory system can make subtle distinctions among 2000–4000 chemical stimuli. No apparent

structural differences exist among the olfactory cells, but the epithelium as a whole contains receptor

populations with distinct sensitivities. It appears likely that the CNS interprets each smell on the basis of the

overall pattern of receptor activity.

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GUSTATION, the sense of taste,

Gustation, or taste, provides information about the foods and liquids we eat and drink. Taste receptors, or

gustatory receptors, are distributed over the superior surface of the tongue and adjacent portions of the

pharynx and larynx. The most important taste receptors are on the tongue. Taste receptors and specialized

epithelial cells form sensory structures called taste buds. An adult has about 5000 taste buds. The superior

surface of the tongue has epithelial projections called lingual papillae. The human tongue has three types of

lingual papillae: (1) filiform papillae, (2) fungiform papillae, and (3) circumvallate papillae. Filiform papillae

provide friction that helps the tongue move objects around in the mouth, but do not contain taste buds. Each

small fungiform papilla contains about five taste buds; each large circumvallate papilla contains as many as

100 taste buds. The circumvallate papillae form a V near the posterior margin of the tongue.

Taste Receptors

Taste buds are recessed into the surrounding epithelium, isolated from the unprocessed contents of the

mouth. Each taste bud contains about 40–100 receptor cells and many small stem cells, called basal cells.

Each gustatory cell extends microvilli, sometimes called taste hairs, into the surrounding fluids through the

taste pore, a narrow opening. A typical gustatory cell survives for only about 10 days before it is replaced.

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Gustatory Pathways

Taste buds are innervated by cranial nerves VII (facial), IX

(glossopharyngeal), and X (vagus). The facial nerve innervates

all the taste buds located on the anterior two-thirds of the

tongue. The circumvallate papillae and the posterior one-third

of the tongue are innervated by the glossopharyngeal nerve.

The vagus nerve innervates taste buds scattered on the surface

of the epiglottis. The sensory afferent fibers carried by these

cranial nerves to the medulla oblongata, and the axons of the

postsynaptic neurons enter thalamus, the information is

projected to the taste area.

Gustatory Discrimination

Primary taste sensations are sweet, salty, sour, and bitter. There

is some evidence for differences in sensitivity to tastes along

the axis of the tongue, with greatest sensitivity to salty–sweet

anteriorly and sour–bitter posteriorly. However, there are no

differences in the structure of the taste buds, and taste buds in

all portions of the tongue provide all four primary taste sensations. Our tasting abilities change with age. We

begin life with more than 10,000 taste buds, but the number begins declining dramatically by age 50. The

sensory loss becomes especially significant because, as we have already noted, aging individuals also

experience a decline in the number of olfactory receptors. As a result, many elderly people find that their

food tastes bland and unappetizing, whereas children tend to find the same foods too spicy.

THE EYE

ANATOMY OF EYE:

The eyes are extremely sophisticated visual instruments. Each eye is a slightly irregular spheroid with an

average diameter of 24 mm and a weight of about 8 g. Within the orbit, the eyeball shares space with the

extrinsic eye muscles, the lacrimal gland, and the cranial nerves and blood vessels that supply the eye and

adjacent portions of the orbit and face. Orbital fat cushions and insulates the eye. The wall of the eye

contains three distinct layers, formerly called tunics:

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1. An outer fibrous layer (Sclera and cornea)

2. An intermediate vascular layer (Choroid)

3. A deep inner layer (retina).

The visual receptors, or photoreceptors, are located in the inner layer. The eyeball itself is hollow, and its

interior can be divided into two cavities, anterior and posterior. The anterior cavity is divided into two

chambers, the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris

and the lens). The posterior cavity, or vitreous chamber, contains jelly-like vitreous humor. The shape of the

eye is stabilized in part by the vitreous body and a clear, watery fluid called aqueous humor, which fills the

entire anterior cavity.

The Fibrous Layer

The fibrous layer, the outermost layer of the eye, consists of the sclera and the cornea. The fibrous layer (1)

supports and protects, (2) serves as an attachment site for the extrinsic eye muscles, and (3) contains

structures that assist in the focusing process.

Most of the ocular surface is covered by the sclera, or “white of the eye,” consisting of a dense fibrous

connective tissue containing both collagen and elastic fibers. This layer is thickest over the posterior surface

of the eye, near the exit of the optic nerve, and thinnest over the anterior surface. The transparent cornea is

structurally continuous with the sclera. The cornea has no blood vessels; the superficial epithelial cells must

obtain oxygen and nutrients from the tears that flow across their free surfaces. Corneal damage may cause

blindness even though the functional components of the eye-including the photoreceptors are perfectly

normal.

The Vascular Layer

The vascular layer, is a pigmented region that includes the iris, ciliary body, and choroid. It contains

numerous blood vessels, lymphatic vessels, and the intrinsic (smooth) muscles of the eye. The functions of

this middle layer include (1) providing a route for blood vessels and lymphatics that supply tissues of the

eye; (2) regulating the amount of light that enters the eye; (3) secreting and reabsorbing the aqueous humor

that circulates within the chambers of the eye; and (4) controlling the shape of the lens, an essential part of

the focusing process.

The Iris

The iris, which is visible through the transparent corneal surface, contains blood vessels, pigment cells, and

two layers of smooth muscle fibers, those are radial and circular muscles. When these muscles contract, they

change the diameter of the pupil. There are two types of papillary muscles: dilators and constrictors. Both

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muscle groups are controlled by the autonomic nervous system. For example, parasympathetic activation in

response to bright light causes the pupils to constrict and sympathetic activation in response to dim light

causes the pupils to dilate. Eye color is determined by genes that influence the density and distribution of

melanocytes on the anterior surface and interior of the iris, as well as by the density of the pigmented

epithelium. When the connective tissue of the iris contains few melanocytes, light passes through it and

bounces off the pigmented epithelium. The eye then appears blue. Individuals with green, brown, or black

eyes have increasing numbers of melanocytes in the body and on the surface of the iris. The eyes of human

albinos appear a very pale gray or blue-gray.

The CiliaryBody

At its periphery, the iris attaches to the anterior portion of the ciliary body, a thickened region that begins

deep to the junction between the cornea and the sclera. The bulk of the ciliary body consists of the ciliary

muscle. The epithelium covering this muscle has numerous folds called ciliary processes. The suspensory

ligaments of the lens attach to the tips of these processes. The connective tissue fibers of these ligaments hold

the lens posterior to the iris and centered on the pupil. As a result, any light passing through the pupil will

also pass through the lens.

The Choroid

The choroid is a vascular layer that separates the fibrous layer and the inner layer. The choroid contains an

extensive capillary network that delivers oxygen and nutrients to the retina.

The Inner Layer

The inner layer, containing the retina and optic nerve, is the innermost layer of the eye. It consists of a thin,

outer layer called the pigmented part, and a thick inner layer called the neural part. The pigmented part of

the retina absorbs light that passes through the neural part, preventing light from bouncing back through the

neural part. In addition to light receptors, the neural part of the retina contains supporting cells and neurons

that perform preliminary processing and integration of visual information

Organization of the Retina

In sectional view, the retina contains several layers of cells. The outermost layer, closest to the pigmented

part of the retina, contains the photoreceptors, or cells that detect light. The eye has two types of

photoreceptors: rods and cones. Rods do not discriminate among colors of light. Highly sensitive to light,

they enable us to see in dimly lit rooms. Cones provide us with color vision. Three types of cones are present,

and their stimulation in various combinations provides the perception of different colors. Cones give us

sharper, clearer images than rods do, but cones require more intense light.

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Rods and cones are not evenly distributed across the outer surface of the retina. Approximately 125 million

rods form a broad band around the periphery of the retina. As you move away from the periphery, toward the

center of the retina, the density of rods gradually decreases. In contrast, most of the roughly 6 million cones

are concentrated in the area where a visual image arrives after it passes through the cornea and lens. This

region, which is known as the macula, contains no rods. The very highest concentration of cones occurs at

the center of the macula, an area called the fovea centralis. The fovea is the site of sharpest vision: When you

look directly at an object, its image falls on this portion of the retina.

The Optic Disc. The optic disc is the origin of the optic nerve. From this point, the axons turn, penetrate the

wall of the eye, and proceed toward the diencephalon. The optic disc has no photoreceptors or other

structures typical of the rest of the retina. The optic disc is commonly called the blind spot.

The Lens

The lens lies posterior to the cornea, held in place by the suspensory ligaments that originate on the ciliary

body of the choroid. The primary function of the lens is to focus the visual image on the photoreceptors. The

lens does so by changing its shape. The lens consists of concentric layers of cells that are precisely organized.

Crystallins, are responsible for both the clarity and the focusing power of the lens.

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PHYSIOLOGY OF VISION:

For us, to see, light rays must be focused on the retina, and the resulting nerve impulses must be transmitted

to the visual areas of the cerebral cortex in the brain.

Refraction: Refraction of light rays is the deflection or bending of a ray of light as it passes through one

object and into another object of greater or lesser density. The refraction of light within the eye takes place in

the following pathway of structures the cornea, aqueous humour, lens and vitreous humour.

Lens: Lens is the only adjustable part of the refraction system. The distance between the center of the lens

and its focal point is the focal distance of the lens. The focal distance is determined by two factors:

1. The Distance of the Object from the Lens: The closer an object is to the lens, the greater the focal

distance.

2. The Shape of the Lens: The rounder the lens, the more refraction occurs, so a very round lens has a

shorter focal distance than a flatter one.

Accommodation: Accommodation is the automatic adjustment of the eye to give us clear vision. During

accommodation, the lens becomes rounder to focus the image of a nearby object on the retina and the pupil

constricts to block at peripheral light rays that would otherwise blur the image. The lens flattens when we

focus on a distant object.

Dark and night adaptation: When light rays strike the retina, they stimulate chemical reaction in the rods

and cones. In the rods, the chemical rhodopsin breaking down to form scotopsin and retinol. This chemical

reaction generates an electrical impulse. In cones, cone pigment breaking down to form retinol and

photopsin.

Adaptation to darkness: While going outside at night, takes a little while because being in a well lit area

has broken down most of the rhodopsin in the rod and resynthesis of rhodopsin is slow. The opposite

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situation perhaps being suddenly awakened by a bright light can see almost painful. What happens is, in

darkness the rods has resynthasised a fully supply of rhodopsin. And the sudden bright light breakdown all

the rhodopsin at the same time. The barrage of impulse generated is very intense and the brain may interpret

any intense sensation as pain. A few minutes later the bright light seems fine because rods are recycling their

rhodopsin slowly and it is not breaking down all at once.

Rods and Cones:

The rods and cones of the retina are called photoreceptors because they detect photons, basic units of visible

light. Our eyes are sensitive to wavelengths of 700–400 nm, the spectrum of visible light. Rods provide the

central nervous system with information about the presence or absence of photons, with little regard to their

wavelength. Cones provide information about the wavelength of arriving photons, giving us the perception of

color.

Rods and cones are named based on their outer segment shapes. These have four segments in their structure:

An outer segment

An inner segment

Nuclear region

Synaptic region

Visual Pigments: The discs of the outer segment in both rods and cones contain special organic compounds

called visual pigments. The absorption of photons by visual pigments is the first key step in the process of

photoreception—the detection of light. Visual pigments are derivatives of the compound rhodopsin (visual

purple), the visual pigment found in rods. Rhodopsin consists of a protein, opsin, bound to the pigment

retinal, which is synthesized from vitamin A. All rods contain the same form of opsin.

The outer segment contains the disc shaped structures which may be several hundreds in number. These discs

are derived from the cell membrane and contain photopigments. The disc degrade continuously and are

replaced by fresh discs .The main function of inner segment is to produce the pigments, which migrate to the

outer segment to store in the discs. The nuclear region and the synaptic region useful to communicate with

the bipolar neurons. It has been estimated that there are all together 120 million rods and 6 million cones in

each retina.

Cones and colour vision Cones are necessary for day light or bright light and colour vision. In the macula

lutea particularia in the fovea centralis there are only cones no rods. The three types of cones are blue cones,

green cones, and red cones. Cones contain the same retinal pigment that rods do, but retinal is attached to

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other forms of opsin (Photopsin). Each type has a different form of opsin and a sensitivity to a different range

of wavelengths. Their stimulation in various combinations is the basis for color vision.

S No Cone type Photopsin Wavelength absorb

1 Blue sensitive cyanolabe 420 to 490nm

2 Green sensitive Chlorolabe 490to575nm

3 Red sensitive erythrolabe 650 to 720 nm

In an individual with normal vision, the cone population consists of 16 percent blue cones, 10 percent green

cones, and 74 percent red cones. Each type is most sensitive to a specific portion of the visual spectrum.

Color discrimination occurs through the integration of information arriving from all three types of cones. For

example, the perception of yellow results from a combination of inputs from highly stimulated green cones,

less strongly stimulated red cones, and relatively unaffected blue cones.

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If all three cone populations are stimulated, we perceive the color as white. Because we also perceive white if

rods, rather than cones, are stimulated, everything appears black-and-white when we enter dimly lit

surroundings or walk by starlight.

Color blindness occurs when one or more classes of cones are nonfunctional. The cones may be absent, or

they may be present but unable to manufacture the necessary visual pigments. In the most common type of

color blindness (red–green color blindness), the red cones are missing, so the individual cannot distinguish

red light from green light.

Anatomy of the Ear

The ear is divided into three anatomical regions: the external ear, the middle ear, and the internal ear. The

external ear—the visible portion of the ear—collects and directs sound waves toward the middle ear, a

chamber located within the petrous portion of the temporal bone. Structures of the middle ear collect sound

waves and transmit them to an appropriate portion of the internal ear, which contains the sensory organs for

hearing and equilibrium.

The External Ear

The external ear includes the outer fleshy and cartilaginous auricle, or pinna. The auricle opens into the

external acoustic meatus, a passageway that leads to the tympanic membrane. The auricle collects sound

waves and directs them toward the external acoustic meatus, which transmits them to the tympanic

membrane. The external acoustic meatus is lined with hairs and ceruminous glands, which produce

cerumen, a modified sebum commonly called earwax. The hairs and cerumen help prevent foreign objects

from reaching the delicate tympanic membrane and cerumen, also slows the growth of microorganisms and

reduces the chances of infection. The tympanic membrane is a thin membrane separating the external ear

from the middle ear. It consists of a thin layer of connective tissue sandwiched between two epithelial layers.

Sound waves reaching the tympanic membrane cause it to vibrate.

The Middle Ear

The middle ear, or tympanic cavity, is an air-filled chamber separated from the external acoustic meatus by

the tympanic membrane. The middle ear communicates with the nasopharynx (the superior portion of the

pharynx), through the auditory tube. The auditory tube is also called the pharyngotympanic tube or the

Eustachian tube; it is about 4 cm long. The auditory tube equalizes pressure on either side of the tympanic

membrane.

Two covered openings, the oval window and the round window on the medial side of the middle ear, connect

the middle ear with the inner ear. The middle ear contains three auditory ossicles: the malleus (hammer),

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incus ( anvil), and stapes (stirrup). These bones transmit vibrations from the tympanic membrane to the oval

window. The malleus is attached to the medial surface of the tympanic membrane. The incus connects the

malleus to the stapes. The base of the stapes is seated in the oval window.

The Internal Ear

The senses of equilibrium and hearing are provided by receptors in the internal ear. The internal ear has two

major divisions: the bony labyrinth and the membranous labyrinth. The bony labyrinth is filled with

perilymph, a fluid similar to cerebrospinal fluid. The membranous labyrinth is suspended in the surrounding

perilymph, and it contains endolymph, which is chemically similar to K+-rich intracellular fluid. Its three

regions are the vestibule, the cochlea, and the semicircular canals. Receptors within the vestibule and

semicircular canals provide the sense of balance and position of head in the air.

The cochlea, from the Latin “snail,” is a spiral, conical bony chamber. Receptors within the cochlear duct

provide the sense of hearing. It extends from the anterior part of the vestibule and coils for about 21/2 turns.

The cochlear duct houses the spiral organ (organ of corti), the receptor organ for hearing. The cochlear duct

and the osseous spiral lamina, together divide the cavity of the bony cochlea into three separate chambers or

scalae. The scala vestibuli, which lies superior in the cochlear duct, is continuous with the scala tympani,

which is the cochlear duct and terminate at the round window. The scala vestibuli and the scala tympani

contain perilymph. Between these two chambers scalamedia is present, filled with endolymph.

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Mechanism of Hearing

Every sound produces sound waves or vibrations in the air, which travel at about 332 meters per second. The

auricle, because of its shape, concentrates the waves and directs them along the auditory meatus causing in-

and-out movement of the tympanic membrane. (The total force of a sound wave applied to the tympanic

membrane is transferred to the oval window, but because the oval window is much smaller than the tympanic

membrane, the force per unit area is increased 15 to 20 times). Tympanic membrane vibrations are

transmitted through the middle ear by movement of the ossicles. At their medial end the footplate of the

stapes rocks to and fro in the oval window, setting up fluid waves in the perilymph. These indent the

membranous labyrinth, causing a wave motion in the endolymph, the hair cells transform the pressure waves

in the cochlea into receptor potentials. Movements of the basilar membrane stimulate the hair cells because

they are attached to the membrane. The nerve impulses generated pass to the brain in the cochlear portion of

the vestibulocochlear nerve (8th cranial nerve). The fluid wave is finally expanded into the middle ear by

vibration of the round window. The vestibulocochlear nerve transmits the impulses to the hearing area in the

cerebrum where sound is perceived and to various nuclei in the pons varolii and the midbrain.

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The Gonads

In males, the interstitial cells of the testes produce the male hormones known as androgens. Testosterone is the

most important androgen. During embryonic development, the production of testosterone affects the

development of CNS structures, including hypothalamic nuclei, which will later influence sexual behaviors.

Nurse cells in the testes support the differentiation and physical maturation of sperm. Under FSH stimulation,

these cells secrete the hormone inhibin. It inhibits the secretion of FSH at the anterior lobe of the pituitary gland

and perhaps suppresses GnRH release at the hypothalamus.

In females, steroid hormones called estrogens are produced in the ovaries under FSH and LH stimulation.

Estradiol is the principal estrogen. Circulating FSH stimulates the secretion of inhibin by ovarian cells, and

inhibin suppresses FSH release through a feedback mechanism comparable to that in males.

At ovulation, follicles in the ovary release an immature gamete, or oocyte. The remaining follicle cells then

reorganize into a corpus luteum, that releases a mixture of estrogens and progestins. Progesterone is the

principal progestin. During pregnancy, the placenta and uterus produce additional hormones that interact with

those produced by the ovaries and the pituitary gland to promote normal fetal development and delivery.

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