Biology 30 AP - Eyes, Ears, & Senses

What is stimuli detected by?

variety of sensory receptors (nerve endings/cells)

When does sensation occur?

when electrochemical impulses arrive at cerebral cortex

Sensory adaptation

process where repeated, insignificant signals filtered out

Photoreceptors

detect light

e.g. rods, cones

Chemoreceptors

detect chemical signals

e.g. olfactory cells for smell, taste buds, internal pH levels

Mechanoreceptors

detect varying forms of pressure

e.g. sound waves ear hair cells, proprioceptors for posture

Thermoreceptors

differ hot from cold temperatures

Optic nerve

receives impulses from the photoreceptors (rods and cones) and transmits sensory information from the eye to the brain

Posterior (rear) chamber

contains vitreous humour

Vitreous humour

maintains the shape of the eyeball and supports the surrounding cells

jelly-like

lifetime period

Sclera

white

protects and supports the eyeball

Choroid

black

absorbs stray light rays that are not detected by the photoreceptors

contains blood vessels that nourish the eye

middle layer

prevents light scattering

Retina

beige & very thin

contains the photoreceptors for sight

innermost layer

stimulated by light

rods and cones

send sensory info to occipital lobes through the optic nerves

image formed = upside down and inverted (brain must interpret right side up)

Blind spot (optic disc)

no actual function

area where the ganglion cells merge to form the optic nerve

no photoreceptors

incapable of detecting light

each eye has own blind spot → other one compensates

Fovea centralis

provides for acute vision

high density of cones

tiny depression in center of retina

sharpest vision

Anterior (front) chamber

contains aqueous humour

Lens

bends and focuses light rays onto the fovea centralis

transparent

no blood vessels

Accommodation

ciliary muscles & super-sensory ligaments change lens shape → focus light on retina

Ciliary muscles

changes the shape of the lens in order to foucs

Super-sensory ligaments

attach the lens to the ciliary muscles

Aqueous humour

maintains the shape of the cornea and provides oxygen and nutrients for the surrounding cells

continuously produced and drained by ducts

Iris

regulates amount of light entering the eye

pigmented

surrounds pupil

Pupil

provides opening for light to enter the inner eye

Adaptation

iris constricts/dilates to change pupil size depending on the amount of light

Cornea

transparent part of the sclera that bends light rays into the eye

protects the eye

pain neurons

no blood vessels

Rods

dim light and peripheral vision

concentrated on edges of retina

light-absorbing pigment called rhodopsin → splits into retinal and opsin when light is present → stops inhibitory neurotransmitters → triggers nerve impulse

Cones

fine details (acuity) and colour

concentrated in fovea centralis

undergo splitting reaction using light with pigments called photopsins

3 types: red, green, blue

Order of light shining into eye

aqueous humour → pupil → lens → vitreous humour → retina (nerve fibres) → ganglion cells → bipolar cells → rods/cones

Order of nerve impulses to brain

rods/cones → bipolar cells → ganglion cells → nerve fibres → optic nerve → brain (thalamus → occipital lobe)

Bipolar cells

essential sensory neurons that bridge the gap between photoreceptors and ganglion cells

Ganglio cells

type of neuron found in a ganglion (cluster of nerve cell bodies in the peripheral nervous system) or in the retina

crucial for transmitting sensory signals either directly to the brain or to other parts of the nervous system

Binocular vision

2 forward-facing eyes w/ overlapping fields of view allow for good depth perception

Monocular vision

two sideways-facing eyes

used separately

increased field of view

limited depth perception

Nearsightedness

people who can see close objects but cannot see a clear letter 20 feet

myopia

long eye ball → rays focus in front of the retina when viewing distant objects

Farsightedness

people who have no trouble seeing at a 20 foot distance but cannot see close objects

hyperopia

short eyeball → rays focus behind the retina when viewing close objects

Astimagtism

uneven shape of cornea impair ability to focus light properly

2 major functions of the ear

hearing and balance (equilibrium)

Where are the sensory receptors (mechanoreceptors) for the ear located?

innermost part (inner ear)

Pinna

external part

funnels sound to auditory canal

Auditory canal

lined with cerbuminous glands (make wax)

directs the sound to the tympanic membrane

Tympanic membrane (eardrum)

vibrates with sounds → causes ossicles to vibrate

Ossicles

three small bones which transmit the vibrations to the oval window → magnifies the sound

malleus (hammer) → incus (anvil) → stapes (stirrup)

Eustachian tube

air-filled tube that allows equalization of pressure in the middle-ear (connects the ear to the back of the nose to maintain pressure on both sides of the eardrum)

Semicircular canals

fluid-filled structure providing information about 3D body movement and position

contains receptor cells for position

Cochlea

coiled tube that identifies sounds and converts them to nerve impulses

contains receptor cells

Vestibule

a chamber at the base of the semicircular canals

important in balance

contains the utricle and saccule → two small sacs that establish 2D head position

Auditory nerve

transmits messages to brain

Scala vestibuli & scala tympani

chambers inside the cochlea

filled with perilymph (fluid) that moves with vibrations from the oval window

Organ of Corti

located in the cochlea

consists of stereocilia (mechanosensing organelles of hair cells that are like microvilli) lying on the basilar membrane which will move in response to movements in the fluid

How does hearing work?

sound waves in air are funnelled into the pinna → auditory canal

sound vibrates the solid tympanic membrane (eardrum) → vibrates the ossicle → vibrates the oval window (sound amplified by the 3 bones)

oval window bulges inwards → round window bulges outward → maintains pressure → transmits pressure change through fluid-filled cochlea

movement of fluid back and forth in cochlea → bends hair-like receptors in the Organ of Corti in cochlea

movement of hair cells → stimulates sensory nerves in the basilar membrane → sends signal via auditory nerve to the thalamus to the temporal lobe

What is the cochlea sensitive to?

different areas sensitive to different pitches/frequencies of sound → brain interprets the pitch based on area stimulated

tip = sensitive to low pitches

base = sensitive to high pitches

How is loudness interpreted?

by the number of sensory neurons that respond to the stimulus

2 types of equilibrium

2D = static = positional = gravitational

3D = dynamic = rotational

Static equilibrium (2D)

movement along one plane

maintained by the utricle and saccule → two jelly-filled sacs in the vestibule

• contain small calcium carbonate stones (otoliths) that move in the jelly when the head is moved → hair cells bend → message sent to thalamus and cerebellum and proprioceptors (linked to muscles that control posture)

Dynamic equilibrium (3D)

maintains balance

maintained by the 3 semi-circular canals each filled with jelly

arranged so one canal in each 3Ds of space

hair cells in the ampullae (base of each canal) are bent when head moved → sends information to thalamus and cerebellum and proprioceptors (linked to muscles that control posture)

Touch

skin has 4 million sensory receptors (not evenly distributed)

different receptors for light, touch, pressure, pain, and temperatures

pain activated by either mechanical pressure or chemical signals

skin tissue damaged → nociceptors release chemicals that trigger pain receptors to send pain signals to the brain

Smell

humans can sense ~10 000 different odours

each odour particle = lock and key into specific chemoreceptors called olfactory cells located in the upper nasal cavity

• particles bind to cells → ion channels open and action potentials triggered

olfactory chemoreceptors linked directly to the olfactory bulb in the brain → impulses move to emotional centers of the brain and the frontal lobe where perception occurs

What is sense of smell closely linked to?

sense of taste

~90% of what we taste is due to sense of smell

animals release pheromones responsible for mate recognition and attraction

• hormone-like molecules detected by a nose structure called vomeronasal organ

Taste

taste buds = thousands of chemoreceptors

evolved to avoid eating poisonous foods

four/five tastes: sweet, sour, bitter, salty, and savoury

saliva dissolves food → food molecules enter papillae → taste buds send impulses to brain stem, thalamus, and end at the gustatory centre of the parietal lobe