The SensesBiology II

The Eye

• Of all the senses, the eye is has the most sensory neurons and axons. • 70% of all sensory receptors in the entire body are in the eyes!

Eye Muscles

• Six muscles attach to the outer surface of the eye.• These

muscles provide the range of motion needed by the eyes.

Eye Muscles

• The muscles of the eye are controlled by a combination of the abducens, oculomotor, and trochlear cranial nerves.•The lateral and medial rectus move the eye side-to-side.•The superior and inferior rectus raise and lower the eye vertically.•The inferior and superior oblique raise and lower the eye diagonally.

External Accessory Structures of the Eye

• Lacrimal glands produce tears that moisten the eyes and clear foreign material.• This fluid is drained

from the eyes through lacrimal canals, which eventually empty into the nasal cavity.

Accessory Structures of the Eye

Internal Accessory Structures of the Eye

• Sclera•Outermost layer of tough connective tissue.• “White of the eye”

• Cornea•Transparent, allows light to pass through.•Serves as protection for the front of the eye; can easily repair itself.•Can be transplanted easily without rejection.

Internal Accessory Structures of the Eye

• The choroid is a thin, dark layer of tissue that prevents light from scattering inside the eye.• On the front of the eye, the choroid is

modified into two different structures:•The ciliary body, a smooth muscle attached to the lens.•The iris, a pigmented layer that can expand and contract to control the amount of light that enters the eye.• The transparent center of the iris is the pupil.

The Choroid

• The choroid is responsible for the red eye effect in photography, when the pupils are large enough, the flash exposes the redness of the blood vessels in the choroid.

Internal Accessory Structures of the Eye

• The innermost layer of the eye, the retina, contains all the receptor neurons.•Rods – Neurons that detect light.•Cones – Neurons that detect color.

Eye Humours

• There are two types of fluid found in the eyes.•Aqueous humour is found between the cornea and lens. This fluid provides nutrients to the cells of the cornea and ensures the cornea is fully inflated.•Vitreous humour is a thicker gel found between the lens and the retina. This mass keeps the main body of the eye inflated and holds the retinal tissue in place.• The collagen fibers may start to clump with age,

causing “floaters” in vision.

Pathway of Vision

• Light reflects off objects and enters the eye through the cornea and pupil.• As the light hits the lens, it is

refracted.• The ciliary body can contract and

expand the lens, allowing focus on objects close or far away.

Pathway of Vision

• As the image passes through the biconvex lens and is projected on the retina, it is inverted and shrunk.

• The brain must re-interpret this image so we perceive it as the correct size and upright.

Pathway of Vision• The color and light of the

image stimulates both rods and cones. This signal is sent down the axons of the neurons, through the optic nerve.• The optic nerve from each eye

cross at the optic chiasma and pass into the thalamus.• The signals finally reach the

occipital lobe on the opposite side of the brain, where they are interpreted and perceived.

Optics from a Distance

• Rays of light from a distant object enter the lens nearly parallel to each other.• The stretched lens

causes the light to refract to a central point, the fovea centralis.

Optics up Close

• Rays of light from a nearby object enter the pupil at different angles. • The compressed

lens refracts these rays of light to the fovea centralis.

Differences in Animals

• Animals have a shiny coating on their choroid, called the tapetum lucidum. • This allows for additional reflection of

light, enhancing night vision.

Differences in Membranes

• Some animals have a nictitating membrane, a transparent eyelid that can be drawn across the eye to moisten it and protect it.

Colorblindness• Three different types of

cones, neurons that detect color, are present in the retina.• Each type is sensitive to a

different wavelength of light.• Colorblindness is an X-

chromosome recessive trait that causes a lack of one or more types of cones.

Colorblindness

• Colorblindness is tested by presenting a patient with a pattern of colored circles. This is the Ishihara Test. • The inability to see certain numbers

in the pattern indicates a type of colorblindness.

Colorblindness

• 29, 45, -- , 26•Normal vision

• 70, --, 5, --•Red/Green deficient

• 70, --, 5, 6•Red deficient.

• 70, --, 5, 2•Green deficient.

Vision Disorders

• Cataracts are caused by a hardening of the lens with age. This causes hazy vision, and eventually, blindness.

• Glaucoma is caused by too much pressure in the aqueous humor region.

Vision Disorders

• Myopia, or near-sightedness, is caused by an eyeball that is too long. •Distant objects appear blurry, as the rays of light from an object converge before reaching the retina.

Vision Disorders

• Hyperopia, or far-sightedness, is caused by an eyeball that is too short.•Nearby objects appear blurry, as the rays of light from the object do not converge before getting to the retina.

Olfaction

• All olfactory nerve receptors are located at the roof of the nasal cavity.•These neurons have long cilia and are coated with mucus.•Airbourne chemicals must dissolve in the mucus to be detected.

• Impulses are sent to the temporal lobe of the brain through the olfactory nerve.

Olfactory Structures

Taste

• Receptor neurons for taste are located on taste buds.•Most of these are located on the tongue, although some are on the soft palate and cheek.

Taste

• The tongue is covered in projections called papillae. •Taste buds are found on the sides of these papillae.•Long microvilli (tiny hairs) are found at the end of each neuron. Chemicals dissolved in saliva activate the microvilli.

Taste

• Taste is not divided into separate regions.• There are only subtle

differences over the surface of the tongue.• There are, however, five

different types of taste buds that are activated by different molecules.

Taste

• Each taste is believed to be an evolutionary mechanism for detecting different nutrients.•Bitterness is thought to recognize different poisons.•Sweet recognizes sugars and carbohydrates.•Salty recognizes compounds that contain electrolyte ions (Sodium, potassium, etc)•Sourness tends to be found in foods with high amounts of vitamins (citrus fruit)•Umami tends to be activated by high protein foods, such as soy sauce or cheese.

The Ear

• The ear actually houses two separate organs, each with a different sense:•Hearing (detection of sound waves)•Equilibrium (balance)

The Ear

External Ear

• The external structures of the ear are involved in the sense of hearing only.•The pinna collects sound waves, like a funnel.•Animals with larger pinna are able to detect subtle sounds, such as the burrowing of underground prey.

External Ear

• The auditory canal is a narrow passageway through the temporal bone of the skull.• The canal is lined with ceruminous glands

that produce ear wax.•Slightly acidic and antibacterial.•Ear wax is gradually moved towards the outer part of the canal, carrying any trapped dirt and dust with it.

• The tympanic membrane, or ear drum, ends the auditory canal.

Middle Ear

• The structures of the middle ear are also involved in hearing only.• The cavity of the middle ear is filled

with three bones:•Malleus• Incus •Stapes

• These bones convey vibrations from sound waves into the inner ear.

Middle Ear

• The eustachian tube connects the middle ear chamber to the throat.•Equalizes pressure in the middle ear when yawning or chewing.• “Popping” sensation in the ear.

Inner Ear

• The inner ear contains organs for both hearing and balance.•The cochlea is for hearing.•The semicircular canals are for balance.

Inner Ear - Cochlea

• Sound waves enter a spiral shaped organ called the cochlea through the oval window.• The sound waves vibrate through a fluid

inside the cochlea, stimulating tiny hair cells lined throughout.• Each hair cell is stimulated by a different frequency.•Higher frequencies are detected by the outermost region, lower frequencies by the innermost region of the cochlea.

• The round window bends out during this time, allowing for fluid movement inside the cochlea.

• The human ear can perceive, at best, 12Hz to 20,000 Hz. • This range shrinks later in life.•Most can detect sounds as low as 5dB in amplitude and as high as 75dB safely.

Hearing Damage

• Tinnitis, or ringing of the ears, can be caused when the cilia are constantly stimulated.• Hearing loss can be caused by a

single exposure to a very intense sound (>150dB) or repeated exposures to loud sounds (>75dB)

Inner Ear - Semicircular Canals

• Three canals situated at right angles to each other (think 3D – X, Y, and Z)• Each canal is filled with fluid called

endolymph.• At the base of each canal is a

separate chamber that contains a cupula, with its own fluid and cilia.

Head Rotation

• At rest, there is no movement in the endolymph or the cupula.• When the head moves,

the cupula is deflected but the endolymph lags.• As motion continues, the

endolymph begins to move and the cupula is no longer deflected.

Head Rotation

• When motion is stopped, the endolymph keeps moving due to momentum.•Even though motion is stopped, the cilia still perceive it due to the endolymph.•Dizziness!

Elevation and Lateral Movement

• There are two chambers in the semicircular canals that detect upward and lateral movement – the saccule and utricle. • When the head

moves vertically or laterally, small calcium stones move, causing the cilia to bend.