Week 13 - The Sensory System

free dendrite

receptor for pain or temperature

end organ

modified or encapsulated on the dendrite - for touch or pressure

specialized cell

has distinct structural components such as rods and cones of the eye’s retina

chemoreceptors

detect chemicals in solutions (taste and smell)

photoreceptors

respond to light - located in the retina

thermoreceptors

detect changes in temperature - many located in skin

mechanoreceptors

respond to movement

threshold stimulus

the intensity required for a receptor to respond and generate a nerve impulse

sensory adaption

sensations become less intense with continuous exposure

special sense

localized in a special sense organ - vision, hearing, equilibrium, taste, smell

general sense

widely distributed throughout the body - pressure, heat/cold, pain and touch from the skin and internal organs, sense of position from the muscles, tendons and joints

skull bones

form the orbit of the eye and protect the dorsal part of the eyeball

palpebra

aka eyelids - protect the front of the eye

levator palpebra

attached to the upper eyelid - muscle keeps the eye open

eyelashes and eyebrows

keep foreign matter out of the eye

conjunctiva

thin membrane lining the inner surface of the eyelid and covers visible portion of the sclera; cells produce mucus that aids in lubrication

lacrimal gland

produces tears - tears lubricate and contain an enzyme that protects against infection

nasolacrimal duct

washes tears away

optic nerve, opthalmic branch

what are the sensory nerves of the eye?

optic nerve (CN II)

receives visual information from the rods and cones to brain

optic disk

free of receptors “blind spot”, point of exit; retina → thalamus → occipital cortex

opthalmic branch

carries impulses of pain, temperature, touch sensations from eye and its surroundings to brain

motor nerves

carry impulses to the eye muscles

trochlear nerves (CN IV), abducens nerve (CN VI), oculomotor nerve (CN III)

what are the 3 motor nerves?

sclera

the outermost layer - white of eye, tough connective tissue

cornea

transparent, colourless curved portion of sclera

choroid

second tunic - vascular; delicate network of ct interlaced with blood vessels; pigmented dark brown - prevents scattering of light; iris, ciliary muscle and suspensary ligaments

retina

innermost tunic - neural; actual receptor of eye; contains light sensitive cells known as rods and cones, which generate the nerve impulses associated with the sense of vision; covers posterior surface only

refraction

the bending of light rays as they pass between substances of different densities

cornea, aqueous humor, lens, vitreous body

what are 4 refractive parts of the eye?

cornea

continuation of the sclera; transparent and colourless; bulges forward slightly; main refracting structure of the eye; no blood vessels

aqueous humor

watery fluid that fills the eyeball anterior to the lens; maintains bulge of the cornea; permits passage of light

crystalline lens

biconvex clear, circular structure made of a firm elastic material; separates the vitreous body from the aqueous humor; ability to change shape to permit near and far vision

vitreous body

soft jellylike substance that fills the entire space behind the lens; maintains eyeball shape

accomodation

process of adjusting the lens thickness to allow for vision at near and far distances

ciliary muscle, suspensory ligaments, lens

what are the 3 structures of the vascular tunic?

ciliary muscle

controls the shape of the lens

suspensory ligaments

holds the lens in place

iris

coloured part of the eye; composed of two types of muscles - radial fibers and circular fibers; regulates the amount of light entering the eye

constriction

in bright light, circular muscle fibers of the iris contract, reducing size of the pupil

dilation

dimmed light, radial muscles contract and opening is pulled outward and enlarged

rods

photoreceptors that work best in dim light, detect light in black and white (shades of gray); highly sensitive to light; does not provide sharp images, does not detect colour; present in greater number than cones; innermost layer of retina

rhodopsin

pigment required for function - vitamin A necessary to make pigments; night blindness results from lack of vitamin A

dark adaptation

time required for eyes to adapt to low light intensities

cones

function in bright light; sensitive to colour; give sharp images; located in center of retina

fovea centralis

point of sharpest vision; contained within the macula lutea

L, M, S - red, green, blue light

what are the 3 types of cones?

colour blindness

lack of one or more types of cones - most are inherited

red-green colour blindness

lacks green light sensitive cones - can’t distinguish green from red; x-linked genetic condition

hyperopia

farsightedness; usually results from an abnormally short eyeball; light rays focus behind the retina

myopia

nearsightedness; usually results from a too long eyeball

astigmatism

caused by irregularity in the curvature of the cornea or lens

presbyopia

age-related loss of elasticity in the lens; can’t see close up

strabismis

the two eyes do not move together resulting from lack of coordination of the extrinsic eye muscles; improper alignment of eyes - can result in blindness

amblyopia

loss of vision in a healthy eye bc it cannot work properly with the other eye - from persistent strabismus

conjunctivitis

inflammation of conjunctiva

pink eye

cocci or bacilli; acute eye infection

inclusion conjunctivitis

an acute eye infection caused by chlamydia trachomatis

trachoma

chronic infection from chlamydia, can cause blindness

ophthalmia neonatorum

neonatal conjunctivitis; introduced at birth - chlamydia, gonococcus or some other sexually transmitted organism; acute

injuries

scratch of the cornea - most common

cataract

an opacity of the lens or the outer covering of the lens; initially - loss of visual acuity (sharpness), untreated leads to complete blindness

glaucoma

a condition characterized by excess pressure of the aqueous humor; increased intraocular pressure causes retinal degeneration; loss of peripheral vision initially; can lead to blindness if left untreated

diabetic retinopathy

retina is damaged by blood vessel hemorrhages; optic atrophy - diabetes related

retinal detachment

separates from underlying layer of the eye as a result of head trauma, tumors or aging - can lead to blindness

nyctalopia

inability to see in dim light - caused by retinal degeneration or lack of vit a

macular degeneration

material accumulates on the retina; distorts the center of the vision; associated with smoking, high-cholesterol diet, exposure to sunlight and genetics

age related macular degeneration, wet, dry

what are the 3 forms of macular degeneration?

age related macular degeneration

progressive degeneration

wet

abnormal blood vessels grow under the retina causing it to detach

dry

material accumulates on the retina causing gradual vision loss

ear

sense organ for both hearing and equilibrium

outer ear

includes an outer projection “pinna” and a canal

middle ear

an air space containing 3 small bones - ossicles; amplifies sound waves and transmits sounds to the inner ear

inner ear

most complex and contains the sensory receptors for hearing and equilibrium

pinna

visible portion of the ear “auricle”; heals direct sound waves into the ear

external auditory canal (meatus)

extends medially, ends at the tympanic membrane; lining is thin and contains ceruminous glands which produces cerumen; channels sound waves into the middle ear

tympanic membrane

at the end of meatus (ear drum); vibrates in response to soundwaves - moves ossicles

malleus, incus, stapes

what are the small bones of ossicles?

auditory tube - eustacian tube

connects middle ear with the throat; opens to equalize pressure (between atmosphere and middle ear); valve can be forced open by swallowing hard, yawning or blowing with the nose and mouth sealed

vestibule, semicircular canal, cochlea

what are the 3 divisions of the inner ear?

vestibule

equilibrium, two bony structures

semicircular canal

equilibrium, 3 bony tubes in 3 planes

cochlea

hearing, coiled like a snail

organ of corti

organ of hearing within the cochlea - spiral organ; composed of hair cells located inside cochlear duct

equilibrium

sensory receptors within the vestibule and semicircular canals - ciliated; nerve supply via vestibular nerve

maculae, cristae

what are two types of equilibrium?

maculae

vestibule; senses the position of the head relative to the force of gravity and to acceleration

cristae

3 semicircular canals; detect rotational acceleration

vertigo

abnormal sensation of spinning

ostitis externa

swimmers ear; inflammation of the external auditory canal

otitis media

infection of the middle ear

sensorineural hearing loss

damage to the auditory nerves or cochlea

presbycusis

age related to hearing loss

conductive hearing loss

problems with the structures that conduct the sound wave to the inner ear; as a result of ear wax accumulation, otosclerosis or damage to the tympanic membrane or ossicles

cochlear implant

processes sound waves and directly activates the vestibulocochlear nerve; enables deaf individuals to perceive sounds, particularly speech

sense of taste - gustation

involves receptors in the tongue and two different nerves facial and glossopharyngeal cranial nerves

taste buds

gustatory sensory organs, on superior surface of tongue; most are located at the back of the tongue - circumvallate papillae

olfactory receptors

helps to detect gases and other harmful substances; warns us of spoiled food; smell can trigger memories and other psychological responses; located in upper part of the nasal cavity; deteriorate with age

sense of touch

receptors tactile corpuscles - meissner’s corpuscle; found mostly in the dermis of the skin and around the hair follicles (fingertips and toes, lips and tip of tongue)