THE SENSES



THE SENSES

There are special nerve cells that transmit various sensations to the brain. They are called receptors. There are seven main types:

SOMATIC RECEPTORS: for the sense of touch and temperature. (touch)

PROPRIOCEPTORS measure the position of the body. That’s how you know your legs are crossed before you stand up.

PAIN RECEPTORS of course, sense pain

MECHANORECEPTORS sense movement (hearing)

THERMORECEPTORS sense heat

PHOTORECEPTORS sense light (sight)

CHEMORECEPTORS sense chemicals (smell, taste)

FIVE SPECIAL SENSES: Smell, taste, vision, hearing (and balance), and touch. We will not be saying anything more about the sense of touch.

OLFACTORY SENSE (smell)

Olfactory receptors are CHEMORECEPTORS; a special type of neuron which senses particular chemicals and triggers a nerve signal. There are hundreds of thousands of types, and they can smell a wide variety of substances. They are extremely sensitive, and can detect just a few molecules! That’s what happens when you smell natural gas.

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GUSTATORY SENSE (taste)

Sensed on taste buds, which are located mostly on the tongue surface, but are also on the palate, pharynx, and a few on the lips. Taste buds have specialized cells, which have chemoreceptors. They synapse on sensory neurons, which take the information to the brain. Taste buds are the only parts of the nervous system that can regenerate completely.

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How many different tastes are there? Dozens. Salt, sweet, bitter, and sour are only a few. Where are they located on the tongue? All tastes are located all over the tongue. Pictures that you see in books where they say sweet sensations are on the tip of the tongue, etc, are wrong. They were drawn 120 years ago by an anatomist that knew his drawing was not right; he just wanted to use it as a starting point for further experimentation.

Taste appreciation is also involved in texture (a mealy apple is not as good), temperature (cold pizza tastes different than warm), and smell (perfume or cigarette smoke clog the senses and decrease taste). There are dozens of taste receptors, hundreds of thousands of smell receptors, so the subtly of taste is from smell.

THE EYE

GLANDS OF THE EYE

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1. LACRIMAL GLANDS are the largest set. They are on the superior lateral eyelid (overhead) and they produce tears, which have enzymes to kill bacteria (which thrive in warm, moist conditions). The tears moisten and lubricate the eye surface. They drain out into the LACRIMAL DUCT, which is seen as a small hole in your lower inner eyelid.

2. LACRIMAL CARUNCLE (“little meat”) is the spot on the medial corner of the eye. It makes an oily secretion to lubricate the eye for the eyelids. When the secretion dries, it is called “sand” in the eyes.

3. CILLIARY GLANDS go to only the cilia. When clogged = STY.

THE EYEBALL

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1. CONJUNCTIVA is like a Saran Wrap covering around the eye and under the eyelids. It also has lots of goblet cells to secrete moisture for those areas. Deep to the epithelium is loose connective tissue with lots of small blood vessels, which are not seen unless the conjunctiva becomes inflamed: Blood-shot eyes from being tired, or PINK EYE, which is CONJUNCTIVITIS (from bacteria, very contagious).

2. SCLERA is the white of the eye, and is continuous with the brain. The eye is part of the brain. The sclera protects the eye.

3. CORNEA is normally transparent and avascular (no blood vessels). It has lots of pain receptors, so a scratched cornea is very painful. Its function is to be the main focuser of light for the eye.

If damaged, need a corneal transplant, which is easy because it is avascular, so there is no need to find a donor match; there are no blood cells there with proteins on them that might cause the body to reject the tissue.

4. LENS changes shape to allow you to distinguish close from far. It changes shape because there are muscles pulling on it. When you are looking far away, the muscles are relaxed. When you look up close, the muscles contract. Constantly looking close puts strain on the muscles of the lens = EYE STRAIN.

PROBLEMS WITH THE LENS

PRESBYOPIA: (“old eyes”). Occurs around age 45-50. With age, the lens loses flexibility. It stays in the position for seeing far, so there is trouble focusing on things that are near.

CATARACTS: Clouds in the lens which can completely cloud the eye. Treatment is to remove the lens and replace it with a plastic one.

IRIS (the colored part of the eye)

If there is lots of pigment, eye is brown; a medium amount = green, small amount = blue, no pigment = pink (albino). The iris regulates the size of the pupil (opening) to allow light in.

RETINA

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The retina contains special cells that are PHOTORECEPTORS (sensors for light).

Two types:

1. CONES (red, green, and blue: a combo of Christmas and Hanukah colors!) functions well in bright light, and senses colors well. They have less light sensitivity (poor at night).

2. RODS functions well in dim light, and senses black and white. They can see well at night, but do not see colors well.

BLIND SPOT: The region where the optic nerve and blood vessels goes in and out of the eye has no photoreceptors. Hold your hands out at 45° and that’s the size of the blind spot. You can still see your hands because the other eye sees it. Close one eye and you’ll find the blind spot.

The retina is the only place in the body where you can see blood vessels directly. The doctor can diagnose hypertension. On a clear, bright day, look at the blue sky and you can see the shadow of your own blood vessels on the photoreceptors as criss-cross lines in field of vision. The little moving dots are your blood cells.

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PROBLEMS WITH VISION

HYPEROPIA (far-sighted) eyes are too short; can’t see up close

MYOPIA (nearsighted) eyes are too long; can’t see far away.

Normal eyes are perfect spheres.

Myopic eyes are elongated (overhead projector is in focus, but move it backward, gets fuzzy. Even badly nearsighted eyes are only 1mm from normal. Treatments are glasses or Lasix, which is laser surgery on cornea, when it’s shaved so it focuses light farther back to reach the retina. Contact lenses were invented by Leonardo da Vinci in 1508.

ASTIGMATISM is when the cornea has an irregular shape. Part of the field of view is out of focus.

They eyeball changes shape until age 24.

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ASTIGMATISM TEST

RETINAL DETACHMENT

The retina separates from the underlying blood supply. Looses oxygen, cells die. Usually caused by an injury like a baseball, punch, or airbag to the eye. Treatment is lasering to spot-weld it back. Manifests as a shimmering light. Needs immediate treatment. Those who are most vulnerable are those who are nearsighted.

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MACULAR DEGENERATION

The size of the macula is the size of the printed letter “O” in 14 pt font. When the macula degenerates, you lose a lot of sight. This is the most common cause of blindness in the US.

It’s due to bleeding in the eye, causing scar tissue. The retina does not get enough oxygen, and the cells die. Macular degeneration allows vision in the periphery, but they can’t read or drive.

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DIABETIC RETINOPATHY

This is when the high sugar levels destroy the photoreceptors in the retina. The blood vessels also swell and rupture and the clots block vision. Some of this damage can be repaired by using a laser to evaporate the blood clots, but any damage to the photoreceptors is permanent. It can lead to blindness.

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NORMAL RETINA DIABETIC RETINOPATHY

NOTES

If a child is blind until age 4-5, and then you restore the sight, he will still be blind because the brain doesn’t form properly. With kids who have astigmatism or weak eye muscles, one eye stops seeing (or sees double). The thalamus in the brain will shut off all the signals from the bad eye.

INTERNAL STRUCTURES OF THE EYE

There are two cavities

1. ANTERIOR CAVITY is anterior to the lens, and is filled with AQUEOUS HUMOR, similar to plasma, supplies nutrients to the cornea and lens.

GLAUCOMA is increased pressure within the anterior chamber of the eye. It leads to blindness. This form of blindness is more common in third-world countries because we have tests to detect it and treat it. The test measures how much pressure there is here by seeing how easily the cornea is deformed, either with air or direct pressure. How many of you have had this test?

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2. POSTERIOR CAVITY is filled with VITREOUS HUMOR, which is jelly-like, and helps give shape to the eyeball. It leaks out from a cut, you’ll go blind because the body can’t replace it.

The four main things that cause blindness are macular degeneration, cataracts, glaucoma, and diabetic retinopathy.

THE EAR

The sensory functions of the ear include not only hearing, but balance, too.

1. OUTER EAR

A. EXTERNAL AUDITORY CANAL.

B. PINNA: The cartilage around your ear. The pinna funnels sound in. If you cup your hands to your ears (do it now), you’ll notice the sound of my voice is louder.

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2. MIDDLE EAR is an air filled space (overhead) with structures.

A. TYMPANIC MEMBRANE (ear drum; be able to identify it on the above picture) vibrates in response to sound. Attached to it are 3 bones called OSSICLES which are the smallest bones in the body

1) MALLEUS (hammer)

2) INCUS (anvil)

3) STAPES (stirrup).

Together, these three bones are only one inch long. Their function is to transmit sound vibrations. The malleus vibrates the incus, which vibrates the stapes.

B. AUDITORY TUBE connects the middle ear to the nasopharynx, and is only the thickness of a pencil lead. If this tube is closed, the ears feel plugged up. Its function is to equalize the pressure of the middle ear and the outside air to allow the bones to vibrate freely. Tubes are put in the tympanic membrane to drain fluids in kids.

3. INNER EAR is a complex structure. The main structure of the inner ear is the COCHLEA (“snail shell”). Instead of drawing the cochlea curled up, I’m going to draw it laying out straight.

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When the stapes vibrates, it causes the fluid in the cochlea to vibrate, which triggers HAIR CELLS which are mechanoreceptors that sense vibrations and send the information to the brain. This portion of the inner ear is responsible for hearing.

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SEMI-CIRCULAR CANALS are the portion of the inner ear that is responsible for balance. There are three of them, and they determine movement in three planes (be able to identify them in the picture from the book above. Within each canal is fluid and hair cells (mechanoreceptors), which connect to nerves that go to the brain.

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When you move in one direction, like sliding across the room, the fluid sloshes like a cup of coffee, and it makes the hair cells move. Within the endolymph here are OTOLITHS (“ear rocks”) which are calcium deposits. When you stand perfectly upright, these otoliths fall directly down and bend the hairs on the lower cells. When you tip your head to the side, they will stimulate the hairs on the opposite side. These stimulate the nerves to tell you what position your head is in. Therefore, the sense of balance uses mechanoreceptors.

VERTIGO (dizziness): Inflammation of the semi-circular canals gives you a sense of motion when you’re not moving. This can be debilitating. Sometimes only one canal is affected, so you only get dizzy if you turn your head one way.

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