the peripheral nervous system · 2020-03-24 · middle ear tympanic cavity–an air-filled cavity...

THE SENSES

The Peripheral Nervous System:

Types of Sensory Receptors:

Chemoreceptors

Nocireceptors

Thermoreceptors

Mechanoreceptors

Photoreceptors

Sensation

Feeling that occurs when sensory messages

have been interpreted by the brain.

Projection

Phenomenon allowing us to know the source

of sensation

Occurs when the cerebral cortex has

interpreted sensory input

Sensory adaptation

Occurs when a receptor becomes “used to” a stimulus

and no longer notices the stimulus.

Example:

A person entering a room with a horrible odor smells

the odor at first, but the longer they are in the room,

the less noticeable the odor becomes.

The chemoreceptors of their nose (olfaction/smell)

have adapted.

The Senses

Touch and pressure

Temperature

Pain

Special senses

Smell

Taste

Hearing

Equilibrium

Sight

Touch and Pressure Senses

Controlled by sensory nerve fibers in the epithelium

Uses:

Meissner’s corpuscles in the hairless skin for light

touch

Pacinian corpuscles in deeper subcutaneous skin

layers for deep touch and heavy pressures

Temperature Senses

Adapt quickly

Two types:

Heat receptors

Detect temperatures between 77◦F (25

◦C) and 113

◦F (45

◦C)

Cold receptors

Detect temperatures between 50◦F (10

◦C) and 68

◦F (20

◦C)

Temperatures above 113◦F and below 50

◦F are not detectable by

these receptors and are considered pain because they would cause tissue damage.

Pain Sense

Serves as a protective function

Is stimulated by chemicals released from damaged tissues

Awareness occurs when the hypothalamus gets the impulse, the cerebral cortex judges intensity, locates source, and causes the emotional or motor response.

Referred pain is the transfer of pain from a visceral organ to some other body part through nerve pathways that conduct impulses from both parts.

Heart attacks are felt in the left arm and shoulder, the chin, etc. as well as the left chest where the heart is actually located because all of those locations share a common sensory nerve.

Pain Fibers

Two main types:

Acute pain fibers

• Myelinated neurons

• Conduct quickly

• Stop when stimulus discontinues

Chronic pain fibers

• Unmyelinated neurons

• Conduct slowly

• Responsible for dull aching pains that are hard to pinpoint the exact

location.

Special Senses

Smell picked up by the olfactory nerve

a chemoreceptor

called olfaction.

Taste picked up by the tastebuds, chemoreceptors located on the tongue

Primary tastes are sour, sweet, salty, bitter, and umami

Minor tastes: alkaline and metallic.

Hearing picked up by the vestibulocochlear nerve; a mechanoreceptor

Equilibrium picked up by the vestibulocochlear nerve.

Sight controlled by the photoreceptors of the eye and the optic nerve.

The Ear

Has three divisions:

1. Outer Ear

2. Middle Ear

3. Inner Ear

Outer Ear

Channels sound to the middle ear

Two structures

Auricle/pinna—outer flap; catches

sound waves

External auditory meatus– (ear

canal)

Middle Ear

Tympanic cavity– an air-filled cavity

Tympanic membrane—eardrum; vibrated sound

waves that enter the ear

Ossicles—tiny bones that transmit sound from the

eardrum to the oval window

Malleus—hammer

Incus—anvil

Stapes--stirrups

Eustacian tube—connects the middle ear to the throat

to equalize air pressure on both sides of the eardrum

Inner Ear

Semicircular canals—3 fluid-filled hoops at right

angles to each other; contain hair cells that aid in

equilibrium

Cochlea—snail-shell shaped, bony organ that

contains the oval window, round window, and the

organ of corti.

Oval window—struck by the stapes; vibrates fluid

inside the inner ear and stimulates hearing receptors

Organ of Corti—hearing receptors

Vestibule—contains the utricle and saccule; used in

static equilibrium

Hearing and Equilibrium

Equilibrium Dynamic equilibrium

Determined by fluid movement in the semi-circular canals

Deals with orientation in regards to a horizontal plane

Static equilibrium

Determined by hairs in the vestibule of the inner ear

Deals with orientation in relation to the ground

Sound Travels as waves (compressions and rarefactions)

Speed is 346 m/sec or 775 mi/hr

Characteristics:

Loudness—amplitude, decibels

Pitch –waves/sec; hertz (Hz)

Quality—smoothness of a wave

The Eye

Accessory structures

Eyelid

Conjunctiva

Lacrimal apparatus

Lacrimal gland

Lacrimal duct

Tears--lysozyme

Eyebrows

Eyelashes

The Eye -- continued

Structures of the eye

Cornea

Sclera

Optic Nerve

Lens

Pupil

Iris

Retina

Rods

Cones

Vision

Occurs due to the physical properties of light

Light travels at a speed of 300,000 km/sec

Refraction

Bending of light as it passes through a more

dense medium

Depends on the curvature of the medium

Concave Lenses—diverge the light

Convex Lenses—converge light

OTHER IMPORTANT

TERMS AND CONCEPTS

RELATED TO VISION AND

SIGHT

Convergence Movement of the eyes inward to adjust focus on the retina

Crossed eyes if objects are brought too close to the eyes

Accomodation Ability of the lens to achieve sharp focus

Field of Vision What you can see without moving the eyes

Peripheral Vision Wide view

Monocular Vision Vision with one eye

Binocular vision Ability to focus what is seen with two eyes

Depth perception Stereoscopic vision

Seeing in 3 dimensions—lateral, vertical, and horizontal

Dark and Light Adaptation

Dark Adaptation

The ability to distinguish objects when moving from light to dark

Conversion speed equals the speed at which rhodopsin is reformed from opsin

and retinene after being struck by light

Purkinje Shift—

Shift from cone reception to rod reception at dusk

Night Blindness

Slow return to dark adaptation due to lack of vitamin A

Light Adaptation

Adaption moving from dark to light

Rod reception changes to cone reception

Color Vision

Determined by the cones in your eyes

What we can see as the visible spectrum of light (ROY G. BIV)

Longer wavelengths than visible are infrared (heat); shorter ones are ultraviolet

or UV

Afterimage

Afterimage is the carryover of a visual impression

Two types:

Positive

Seen the same as the original stimulus

Negative

Seen in a complimentary color of the orginal

stimulus

Due to cone fatigue

Visual Acuity

Visual acuity is the sharpness or degree of detail

Cones are responsible

Influence by brightness, size, color, and retinal area

stimulated

Measured with the Snellen Eye Chart (“E” chart)

20/20 vision—means the eye can see from 20 feet what a normal eye

can see from 20 feet

20/10—means the eye can see from 20 feet what the normal eye

should see from 10 feet

20/100-means the eye can see from 20 feet what a normal eye can

see from 100 feet.

Ear disorders and clinical terms

Deafness

Can be complete or partial

Sensory—disturbed inner ear circulation or fluid pressure and aging are

causes; rarely leads to total deafness

Conductive

Due to blockage, perforated eardrum, middle ear infection, or ossicle

disease

Usually can be corrected by cleaning the ear , surgical reconstruction,

or hearing aids

Otosclerosis

hereditary cause of partial deafness;

progressive; ossicles lose ability to move

treated with prosthetic stapes replacement

Tympanoplasty

Surgical reconstruction of the middle ear bones

Tinnitus

Head noise or ringing in the ears

Vertigo

Dizziness due to changes in inner ear circulation

Meniere’s disease

Dizziness due to increased inner ear pressure

Otitis media

Middle ear inflammation

Eye Disorders

Myopia

Nearsightedness

Can’t see distant objects clearly

Cornea refracts too much light or eyeball is too long

Corrected by a concave lens

Hypermyopia

Farsightedness

Distant objects seen more clearly

Light rays focus behind retina

Presbyopia

Farsightedness due to age

Lens loses elasticity

Corrected by a convex lens

Astigmatism

A cornea or lens with an imperfect curvature

Symptoms include eyestrain and headache

Corrected by glasses or contacts

Cataract

Clouding of the lens

Usually in elderly

Treated by lens removal , contacts/glasses, or prosthetic

lens implants

Glaucoma

Depression of the optic disc due to an increase in anterior chamber fluid

Causes a restricted field of vision or blindness

Treated by drugs or surgery that cause aqueous humor to drain

Conjuctivitis

Inflamed or irritated conjunctiva membrane of the eye

Also called Pink Eye

If bacterial or viral it is highly contagious

Noncontagious forms can be caused by pollen, smoke, dust, wind, air pollution

Treated by antibiotics and eyedrops

Iritis

Inflammation of the iris

Uveitis

Inflammation of the uvea

Retinoblastoma

Tumor of the retina

Keratitis

Inflammation of the cornea

Color Blindness

Hereditary inablility to distinguish colors

Carried on the X chromosome

Two Kinds:

Dichromatic

Red/green –common form

Blue/yellow--rare

Monochromatic

Extremely rare

Black and white vision

Colorblindness Color Wheel

The outer ring of colors

is the normal color

wheel.

The inner rings show

how colorblind

individuals of various

degrees see the same

colors.

Ishihara’s Color Plates used to diagnose

types and degrees of colorblindness

How a stoplight is perceived if you

have red-green colorblindness

Spherical Aberrations

Results in a series of focal points

Causes a fuzzy perimeter image

Eye corrects by decreasing pupil size

Chromatic Aberrations

Causes a “halo” effect of rainbow colors surrounding an

object