Mader Ch 18-Senses

7/31/2019 Mader Ch 18-Senses

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Chapter 18: Senses

Sensory Receptors and Sensations

Sensory receptors are specialized to detect certaintypes ofstimuli.

Each type of sensory receptor responds to a

particular kind of stimulus.

Exteroceptors (hearing, sight receptors, for

example) detect stimuli from outside the body.

Interoceptors receive stimuli from inside the body;

they are directly involved in homeostasis.

Types of Sensory Receptors

Chemoreceptors respond to chemical substances,

such as changes in pH, or the senses of taste and

smell.Pain receptors are chemoreceptors that respond to

chemicals from damaged tissues.

Mechanoreceptors respond to mechanical forces.

The senses of hearing and balance both involve

mechanoreceptors.


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Proprioceptors (mechanoreceptors) in tendons

around joints make us aware of position;

pressoreceptors in arteries detect blood pressure

changes, and stretch receptors in lungs detectdegree of inflation.

Thermoreceptors respond to temperature changes;

there are both warm receptors and cold receptors.

Photoreceptors respond to light energy.

Special photoreceptors called rods result in black-

and-white vision, while cones detect color.

How Sensation Occurs

Sensation occurs when nerve impulses reach the

cerebral cortex.

Perception is an interpretation of the meaning ofsensations.

The sensation that results depends on the part of

the brain receiving the impulses.

Receptors may integrate signals before sending

nerve impulses.

Sensory adaptation occurs when a stimulus

continues but the receptor decreases its response.


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Proprioceptors and Cutaneous Receptors

Proprioceptors

Proprioceptors help us know the position of ourlimbs in space.

Proprioceptors include muscle spindles that are

stimulated when muscle fibers stretch; a reflex is

initiated and the muscle tightens in proportion to

the degree of stretch.

These proprioceptors allow the muscles to

maintain the proper length and tension, or muscle

tone.

The knee-jerk reflexinvolves muscle spindles.

Signals to the CNS from muscle spindles help

maintain balance and posture.

Golgi tendon organs are proprioceptors with the

opposite effect.

Cutaneous Receptors

The dermis of the skin contains sensory receptors

for touch, pressure, pain, and temperature(warmth and cold).


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Three types of cutaneous receptors are sensitive to

fine touch:

1) Meissner corpuscles are concentrated in finger

tips, lips, tongue, nipples, and genital areas;2) Merkel discs are found where the epidermis

meets the dermis; and

3) free nerve endings (root hair plexus) around

hair follicles all detect touch.

Three different types of pressure receptors are

Pacinian corpuscles,Ruffini endings, andKrause

end bulbs.

Temperature receptors are simply free nerve

endings in the epidermis; some are responsive to

cold and others are responsive to warmth,

although there are no structural differencesbetween them.

Pain Receptors:

Nociceptors are pain receptors on internal organs

and may be sensitive to temperature, pressure, or

chemicals.

Referred pain occurs when stimulation of internalpain receptors is felt as pain from the skin.

Referred pain most likely happens because of

shared nerve pathways between the skin and

internal organs.


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Chemical Senses:

Chemoreceptors in the carotid arteries and aorta

respond to the pH of the blood and communicate

with the medulla oblongata to change breathingrate.

For example, when blood pH drops, these

chemoreceptors signal the medulla respiratory

center that triggers breathing rate to increase;

expiration of CO2 raises the pH of the blood to

normal.

Taste and smell are chemical senses.

Sense of Taste:

The taste buds located in papillae on the tongue

contain taste cells that communicate with sensory

nerve fibers.Microvillion taste cells contain receptor proteins

that match chemicals in food.

The brain determines the taste according to a

weighted average of incoming impulses from

taste buds sensitive to either sweet, sour, salty, or

bitter tastes.


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Sense of Smell

Olfactory cells (modified neurons) are located in

epithelium in the roof of the nasal cavity.

After molecules bind to receptor proteins on thevaried cilia of olfactory cells, nerve impulses lead

to olfactory areas of the cerebral cortex.

The perceived odor is determined by the

combination of olfactory cells stimulated.

The effects of smell and taste combine.

Olfactory cell location and anatomy

Sense of Vision:

Vision is dependent on the eye and the visual areas

of the cerebral cortex.

It is estimated that at least one-third of the

cerebral cortex is involved in processing visual

information.

Anatomy of the Eye:

The eye has three layers.

The sclera is the outer layer seen as the white of

the eye and includes the transparent bulge in the

front of the eye called the cornea.

The choroidis the middle, darkly pigmented layerthat absorbs stray light rays; it also becomes the

iris that regulates the size of thepupil.


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Behind the iris, the choroid thickens and forms

the ciliary body.

The ciliary body contains the ciliary muscle, whichcontrols the shape of the lens for near and far

vision.

The lens divides the eye into two compartments:

the anterior compartment(containing aqueous

humor) and theposterior compartment(containing

vitreous humor).

Rod cells and cone cells are located in the retina

that forms the inner layer.

The retina lines the back half of the eye and has

cone cells densely packed in one area called thefovea centralis.

Sensory fibers from the retina form the optic nerve

leading to the brain.

Focusing:

The cornea and the lens focus light rays on theretina.

To see a close object, the ciliary muscles change the

lens shape to provide visual accommodation.


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After age 40, the lens is less able to accommodate

and near vision is less acute.

Cataracts occur when the lens becomes opaque;sun exposure might be a factor in developing

cataracts.

Photoreceptors:Both rod cells and cone cells have an outersegment with membranous disks containing

embedded pigments.

Rods contain a deep purple pigment called

rhodopsin that is composed ofretinal(made from

vitamin A) and the protein opsin.

Rods are numerous and provide peripheral vision,perception of motion, and vision in dim light at

night.

When a rod absorbs light, rhodosin splits into

opsin and retinal, leading to a cascade of reactions

and the closing of rod membrane ion channels.Inhibitory neurotransmitters are no longer

released from the rod.


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Breakdown of rhodopsin in rods thus initiates

nerve impulses.

Cones have three different pigments (red, greenand blue) made from retinal and opsin; opsin

varies between the three.

Integration of Visual Signals in the Retina:

The retina has three layers of neurons: rods and

cones are near the retina, bipolar cells are in the

middle, and the innermost layer containsganglion

cells that carry impulses to the optic nerve.

The rod and cones synapse with the bipolar cells,

which in turn synapse with ganglion cells that

initiate nerve impulses.

As signals pass from one layer to the next,

integration occurs because each layer contains

fewer cells than the previous layer.

However each cone connects directly to one

ganglion cell, while a hundred rods may synapsewith only one ganglion cell.

It is likely that much processing occurs in the

retina before impulses are sent to the brain.


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There are no rods and cones where the optic nerve

exits the retina; this is the blind spot.

Integration of Visual Signals in the Brain:

The visual pathway begins with the retina and

passes through the thalamus before reaching the

cerebral cortex.

The visual pathway and the visual cortex split the

visual field apart, but the visual association areas

rebuild it so we correctly perceive the entire visual

field.

Optic chiasma

Abnormalities of the Eye:

Color Blindness

The most common abnormality is a lack of red

and/or green cones.

Distance Vision

Nearsightedindividuals (elongated eyeball) cannot

see distant objects; this is corrected by a concavelens.

Farsightedindividuals (shortened eyeball) see

distant objects well but not up close; this is

corrected by a convex lens.


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Astigmatism occurs with an uneven cornea or lens.

Common abnormalities of the eye

Sense of Hearing:

The ear has two sensory functions: hearing and

balance.

The sensory receptors for both senses are located

in the inner ear, and both use a type of

mechanoreceptor consisting ofhair cells with

stereocilia (long microvilli).

Anatomy of the Ear:

The ear is divided into three parts.

1 The outer earconsists of thepinna and theauditory canal, which direct sound waves to the

middle ear.

2 The middle earbegins at the tympanic membrane

(eardrum) and contains the ossicles: the malleus,

incus, and stapes that amplify sound waves.

The malleus is attached to the tympanicmembrane, and the stapes is attached to the oval

window, which is covered by membrane.


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When the stereocilia of the hair cells bend, nerve

impulses are generated in the cochlear nerve and

are carried to the brain.

Sense of Equilibrium

Rotational EquilibriumRotational equilibrium depends on the stimulation

of hair cells within the ampullae of the semicircular

canals.

Continuous movement of fluid within the canals

can cause motion sickness.

Vertigo is dizziness from a sensation of spinning.

Gravitational Equilibrium:

Stimulation of hair cells within the utricle and thesaccule, two sacs located in the vestibule, by the

slippage of calcium carbonate granules or otoliths,

provide impulses that tell the brain the direction

of movement of the head.

The movement of the otoliths provides a sense ofgravitational equilibrium.


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Chapter Summary:

Sensory receptors respond to specific

environmental stimuli.Sensation occurs in the brain when sensory

receptors send nerve impulses to the brain.

Senses are divided into exteroceptors that detect

stimuli from outside the body, and interoceptors

that receive stimuli from inside the body.

Proprioceptors in muscles and joints help the

body maintain balance and posture.

Cutaneous receptors in the skin respond to touch,

pressure, pain, and temperature (both warmth

and cold).

In the mouth, the microvilli of taste cells havemembrane protein receptors that respond to

certain molecules.

Olfactory cells within the olfactory epithelium

respond to molecules and result in a sense of

smell.

Photoreceptors for sight contain visual pigments,which respond to light rays.

Some integration occurs in the retina of the eye

before nerve impulses are sent to the brain.


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Sensory receptors for hearing are hair cells in the

cochlea of the inner ear that respond to pressure

waves.Sensory receptors for balance are hair cells in the

vestibule and semicircular canals of the inner ear

that respond to the tilt of the head and to the

movement of the body, respectively.

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Mader Ch 18-Senses