HUMAN BIOLOGY

SENSORY SYSTEM

OBJECTIVES

At the end of this lesson, the SPN will be able to:1. Describe the types of sensory receptors and

their pathways.2. Discuss proprioceptors and cutaneous

receptors and their functions.3. Identify the types of cells that are

responsible for taste and smell.

OBJECTIVES CON’T

4. Describe the structures of the eye and their functions and the visual pathways to the brain.

5. Describe the structures of the ear and their functions and the auditory pathway to the brain.

6. Differentiate between rotational and gravitational equilibrium.

The Senses

• 1. Provide information about our surroundings—Seeing, Hearing, Touching, Smelling, Tasting

• 2. Maintain our equilibrium-keeps us upright• 3. Allow us to feel pain

Review—Sensory Pathway

• Receptors-detect changes(stimuli) and generate impulses

• Sensory Neurons-transmit impulses from receptors to the CNS

• Sensory Tracts-transmits impulses to a specific part of the brain.

• Sensory Area-most are in the cerebral cortex. These areas feel and interpret the sensations

Types of Sensory Receptors

• 1.MECHANORECEPTORS-detect forms of mechanical energy: changes in pressure, position, or acceleration

• 2. THERMORECEPTORS-are sensitive to heat and cold• 3. NOCICEPTORS-(pain receptors)-detect damage to

tissues.• 4. CHEMORECEPTORS-detect chemical energy of

substances dissolved in the fluid around them.

Types of Sensory Receptors Con’t

• 5. PHOTORECEPTORS-detect visible light.• 6. OSMORECEPTORS-detect changes in water

volume(solute concentration) in some body fluid.

• ****For the purposes of this lesson, we will be discussing mechanorecptors, thermoreceptors, nociceptors, chemoreceptors, and photoreceptors

Cutaneous Senses

• Contains receptors of touch, pressure, pain and temperature.

CUTANEOUS SENSES

• 1. Meissner’s Corpuscles-are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. They provide these regions with special sensitivity in that they react to fine touch.

• 2. Merkel’s Discs-Found in a capsule of epithelial or connective tissue. They adapt slowly and are the most important receptors for steady touch.

Cutaneous Senses Con’t

• 3. Pacinian Corpuscles-Onion-shaped sense organs located deep within the dermis. They detect heavy pressure and vibrations.

• 4. Ruffini’s Endings-located deep in the dermis. Respond to steady touching and pressure. They are also heat receptors

• 5. Krause End Bulb’s- Detect vibration and proprioception in tendons and joints. They are also cold receptors.

PROPRIOCEPTION

• PROPRIOCEPTION-The awareness of posture, movement and changes in equilibrium and the knowledge of position and weight. An example is drivers would be unable to keep their eyes on the road while driving, as they would need to pay attention to the position of their arms and legs while working the pedals and steering wheel.

NOCICEPTORS• The receptors for pain are free nerve endings. • Free nerve endings are found in internal organs except the brain. • For example, smooth muscle of the small intestine has free nerve endings

that are stimulated by excessive stretching or contraction---the resulting pain is called VISCERAL PAIN.

• Sometimes pain that originates in an internal organ can be felt in a cutaneous area. This is called REFERRED PAIN. For example, pain of a heart attack is often felt in the left arm or shoulder.

• ***Pain is the one sense that does not change with continued stimulation***

CHEMORECEPTORS-Sense of Taste and Smell

• Taste buds and the nose are special sense organs. Special sense organs contain groups of specialized receptors.

• Taste and smell are called chemical senses because the receptors for these senses are sensitive to certain chemical substances found in food and in the air.

• TASTE BUDS-Small elevations that hold our taste receptors.

We have roughly 10,000 taste buds scattered over the tongue, the roof of the mouth, and the throat. These small elevations are visible to the naked eye.

• A taste bud has a pore through which fluids in the mouth contact the surface of receptor cells. Sensory neurons then relay the message to the brain.

• Sense of smell also contributes to our perception of food.• Some medications may interfere with the sense of taste, and

this diminishes as we age.

5 TYPES OF TASTE• The taste buds for each type of taste are

concentrated on the tongue in particular regions.• 1. Bitter-back of the tongue• 2. Sour-sides of the tongue• 3. Salty-tip and sides of the tongue• 4. Sweet-tip of the tongue• 5. Umami (oo-mommy)- described as “meaty”,

“savory” or “broth-like” taste buds were identified six years ago. These buds are located in the middle center of the tongue.

SENSE OF SMELL• The receptors for smell are chemoreceptors which

detect vaporized chemicals that have been sniffed into the upper nasal cavities.

• The receptor cells for smell end in a tuft of 6-8 cilia, and when chemicals bind to these cilia, nerve impulses that are generated are transmitted directly to the olfactory bulb of the cerebral cortex.

• Sensory Adaptation-occurs quickly with odors. If you smell a strong odor, the odor quickly fades away.

HUNGER AND THIRST (visceral sensations) triggered by internal changes

• THIRST-mechanism necessary to regulate concentration of body fluids

• Originates in the hypothalmus which will result in increased intake of water

• Localized in tongue, mouth, and pharynx• Continues until relief or death• Polydipsia-increased or excessive thirst

HUNGER

• Is an intermittent sensation• Located in the region of the stomach due

partially to contractions of stomach muscles• Not continuous, decreases if starving• Appetite-desire for food, no relationship to

need food• Anorexia-loss of appetite

PHOTORECEPTORS

• Photoreceptors are in the eyes. The eyes are located in the orbits.

EXTERNAL STRUCTURES OF THE EYES

• Muscles-6 of them for gross movement• EYELIDS and EYELASHES• Eyelid-The thin skin of an eyelid covers muscle and

connective tissue. • Lining the inner surface is a transparent mucous

membrane, called the Conjunctiva. The conjunctiva folds back to cover the anterior of the eye, except for the cornea, and prevents tears from entering the orbits.

EYELDS and EYELASHES Con’t

• The eyelids have eyelashes that can trap a tiny particle of grit, causing the eyes to close immediately.

• Sebaceous glands associated with each eyelash produce an oily secretion that lubricates the eye. Inflammation of one of these glands is called a sty.

TEARS• Tears are produced by the lacrimal glands which are located at

the upper, outer corner of the eyeball within the orbit.• A lacrimal apparatus consists of the lacrimal gland and the

lacrimal sac with its ducts. The lacrimal gland produces tears that flow over the eye when the eyelids are blinked.

• Tears are mostly water and contain lysozyme, an enzyme that inhibits the growth of most bacteria on the wet, warm surface of the eye. Tears drain into the nose by way of the nasolacrimal duct. The more you cry, the more congestion.

INTERNAL STRUCTURES OF THE EYE

• The eye is an elongated sphere about 1 inch in diameter, has three layers or coats

1. outer layer is the SCLERA or “whites” of the eye

2. CHOROID-the middle layer3. RETINA- the inner layer

The Outer Sclera

• The outer sclera is a white fibrous layer except for the transparent CORNEA. The cornea is the window of the eye. The cornea covers the front of the eyes; it bends light rays.

The Choroid• Is the middle, thin, dark brown layer that contains

many blood vessels and absorbs stray light rays.• Toward the front, the choroid thickens and forms the

ring-shaped ciliary body containing the ciliary muscle, which controls the shape of the lens for near and far vision.

• Finally, the choroid becomes a thin, circular, muscular, and pigmented diaphragm, called the IRIS (colored part of the eye), which regulates the size of the pupil, the hole through which light enters the eyeball.

LENS

• The lens is attached to the ciliary body by ligaments and divides the cavity of the eye into two smaller cavities----the posterior cavity and the anterior cavity.

POSTERIOR CAVITY

• The posterior cavity is filled with vitreous humor, a viscous, gelatinous material, that helps retain the shape of the eyeball by keeping the retina in place.

• The posterior cavity is found between the lens and the retina.

ANTERIOR CAVITY• The anterior cavity is found between the cornea and lens.• Is filled with aqueous humor, a watery solution secreted by the ciliary

body.• A small amount of aqueous humor is produced each day. Normally,

aqueous humor enters the anterior chamber by the pupil and is then reabsorbed by way of tiny ducts located where the iris meets the cornea.The ducts are known as the Canal of Schlemm

• The presence of aqueous humor in the anterior cavity of the eye creates a pressure called intraocular pressure. If this circulation is blocked it increases the intraocular pressure which causes glaucoma.

RETINA• Is the inner layer of the eye. Posterior 2/3 of the eyeball. • Contains the visual receptors, the rods and the cones.• RODS-detect only the presence of light• CONES-detect colors. Are most abundant in the center of the

retina.• There are no rods or cones where the optic nerve passes

through the retina. This is the spot where the number of blood vessels enter the eye and where the optic nerve exits the eye is called the blind spot or optic disc

RETINA CON’T• The retina contains a very special region called the

macula lutea. • The macula lutea is an oval, yellowish area with a

depression called the fovea centralis. In this region, vision is most acute because there is a great concentration of cone cells.

• In summary, the retina contains the rods and cones which are the sense receptors for sight. When either is stimulated, nerve impulses begin and are transmitted via the optic nerve to the brain.

RODS• In dim light, the iris causes the pupil to enlarge so that more

light rays can enter the eye. • The rods do not detect fine detail or color, so at night, all

objects appear to be blurred and have a shade of gray. • Because of their abundance and position in the eyes, rods do

detect even the slightest motion.• Rods contain rhodopsin, a protein (derived from Vit. A) that

gives the outer segments of the rods a purple color and adapts the eye to low density light. This makes black and white vision possible.

• Rods are predominately outside the fovea centralis.

CONES

• Are located primarily in the fovea centralis• Function in bright light to detect fine detail and color.• To perceive depth, as well as to see color, we turn

our eyes so that reflected light from the object strikes the fovea centralis.

• Color vision depends on 3 kinds of cones, one kind for each of three colors: blue, green, and red. The colors we see depend on which of these cones are activated.

Color Blindness• Complete color blindness is extremely rare.• In most instances, a particular type of cone is lacking or

deficient in number.• The lack of either red or green cones is the most common

type of color blindness, affecting about 5% of the American population.

• If the eye lacks red cones, the green colors become accentuated, and vice versa.

• Night blindness is the inability to see well in dim light or at night is caused by a deficiency of Vitamin A.

PHYSIOLOGY OF VISION• When we look at an object, light rays are bent

(refracted) and focused on the retina.• The pathway for vision is:

1. CORNEA-BENDS LIGHT RAYS2. AQUEOUS HUMOR3. LENS4. VITREOUS HUMOR5. RETINA6. OPTIC NERVE7. OCCIPITAL LOBE IN THE BRAIN

ACCOMMODATION

• Lens shape is controlled by the ciliary muscle within the ciliary body

• Although the lens remains flat (ciliary muscle is relaxed) when we view distant objects, it rounds up (ciliary muscle contracts) when we view close objects. This is a process called accommodation.

ACCOMMODATION CON’T

• When the eyeball is too short, accommodation by the lens may not be sufficient to bring an object into focus.

• After the age of 40, the lens loses some of its elasticity and is unable to accommodate as well. Then a person will need glasses

• Normal vision acuity is referred to as 20/20. This means that the eye should see an object clearly at 20 feet away.

MYOPIA

• Near sightedness is referred to as myopia. • This means that the eye sees near objects but not

distant ones. For example, if an eye has 20/80 vision; it means what a normal eye can see at 80 feet away, the nearsighted eye can only see the object when it is 20 feet away.

• Correction of nearsightedness requires a CONCAVE lens to spread out light rays before they strike the eye.

HYPEROPIA and PRESBYOPIA

• Farsightedness (hyperopia) means that the eye sees distant objects well because the eyeball is too short or the lens is too thin.

• Correction requires a convex lens to converge light rays before they strike the eye.

• Presbyopia- farsightedness due to age. Usually after age 40. You need glasses for reading.

MECHANORECEPTORS-THE EAR

• The mechanoreceptors are in the ears. The ear accomplishes 2 sensory functions:

• 1. equilibrium(balance)• 2. hearing

The receptors for both of these are located in the inner ear and consists of hair cells with cilia that respond to mechanical stimulation.

Each hair cell has from 30 to 150 cilia. When the cilia of any particular hair cell are displaced in a certain direction, the cell generates a nerve impulse, which is sent along the 8th cranial nerve (vestibulocochlear) to the brain.

The ear has 3 divisions:1. EXTERNAL2. MIDDLE3. INNER

EXTERNAL EAR

• The external ear consists of the pinna (external flap, auricle) and external auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands.

• In the upper wall are ceruminous glands, modified sweat glands that secrete ear wax, which helps guard the ear against entrance of foreign materials.

MIDDLE EAR

• The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall with 2 small openings covered by membranes. These openings are called the oval window and round window.

EAR OSSICLES• Between the tympanic membrane and the oval window are 3

small bones called the ossicles. The ossicles are the malleus, also called the hammer, incus, also called the anvil (blacksmith’s tool), and the stapes, also called the stirrup. The ossicles are named for the objects they resemble.

• The malleus adheres to the tympanic membrane , and the stapes touches the oval window.

• The posterior wall of the middle ear also has an opening that leads to the mastoid sinuses of the skull.

AUDITORY (Eustachian) Tubes• The auditory (eustachian) tubes, which extend from

the middle ear to the nasopharynx, permits equalization of air pressure. Chewing gum, yawning, and swallowing help move air through the auditory tubes during ascent and descent in elevators and airplanes.

• Infections of the middle ear (otitis media) can occur frequently during childhood. In children, the eustachian tube is flattened and more horizontal.

INNER EAR

• The inner ear, anatomically speaking has 3 areas:1. Vestibule-concerned with balance2. Semicircular Canals-concerned with balance

3. Cochlea-shell like structure, concerned with hearing

Rotational Equilibrium• The receptors attuned to rotation are on a ridge of the swollen base of

each semicircular canal.• These receptors are sensory hair cells; their delicate hairs project up into a

jelly-like cupula (“little cap”)• When your head rotates horizontally or vertically or tilts diagonally, fluid in

a canal corresponding to that direction moves in the opposite direction • As the fluid presses against the cupula, the hairs bend. This bending then

sends the impulse via the vestibular nerve to the brain.• As the signals are integrated with information from your muscles and

eyes, the brain orders compensating movements that help you keep you balance when you stand, walk, dance, or move your body in other ways.

Gravitational Equilibrium• A vestibule, or chamber, lies between the semicircular canals

and the cochlea.• It contains 2 small sacs called the utricle and saccule.• Each sac contains an otolith organ which has hair cells

embedded in a jellylike “membrane”. The material also contains hard bits of calcium carbonate granules called otoliths (“ear stones”)

• Movements of the membrane and otoliths signal changes in the head’s orientation relative to gravity.

COCHLEA

• The spirals of the cochlea resembles a snail shell• The cochlea contains the Organ of Corti, which sends

nerve impulses to the brain stem by way of the 8th cranial nerve. Eventually, the nerve impulses are relayed to the temporal lobe of the cerebrum where they are interpreted as sound.

• Therefore, the hearing receptors are contained within the organ of corti.

HEARING

• The process of hearing begins when sound waves enter the auditory canal. Sound travels by the successive vibrations of molecules, (just as ripples travel across the surface of a pond)

PATHWAY FOR HEARING• Pathway for Hearing

1. Soundwaves2. External Canal3. Tympanic Membrane (Sound Vibrations are first picked up here)4. Malleus5. Incus6. Stapes7. Oval Window8. Sensory Receptors in inner ear9. Acoustic Nerve(8th cranial nerve)10. Brain (Temporal Lobe)