Chapter #10

Somatic and Special Senses

10.1 Introduction

 Our senses are not only necessary for us to enjoy life, but to survive.

 Sensory receptors detect changes in the environment and stimulate neurons to send nerve impulses to the brain.

 Sensory receptors vary greatly, but fall into two major categories.

2 major categories

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Receptors associated with somatic senses.

Receptors associated with specialized sensory organs.

Receptors assoc. w/ somatic senses

 Receptors associated with the somatic senses of touch, pressure, temperature, and pain form one group.

 These receptors are widely distributed throughout the skin, muscles, joints and deeper tissues.

 They are structurally simple.

Receptors associated with specialized sensory organs

 Special senses of smell, taste, hearing, equilibrium, and vision.

10.2 Receptors and Sensations

 Each receptor is more sensitive to a specific kind of environmental change but is less sensitive to others.  This selective response distinguishes the senses.

Type of Receptors

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Chemoreceptors- are stimulated by changes in the chemical concentration of substances. (Taste and smell) Pain receptors- by tissue damage.

Thermoreceptors- by changes in temperature.

Mechanoreceptors- by changes in pressure or movement.

Photoreceptors- by light energy.

Sensations

 are feelings that occur when the brain interprets sensory impulses.

 All nerve impulses that travel away from sensory receptors into the central nervous system are alike, the resulting sensation depends on which region of the brain is stimulated.

Projection

 Because the brain projects the sensation back to its apparent source.  Projection allows a person to pinpoint the location of stimulation ; thus the eyes seem to see, and the ears seem to hear.

Sensory adaptation

 When sensory receptors are continuously stimulated, many of them undergo an adjustment called sensory adaptation.

 As receptors adapt, impulses leave them at decreasing rates, until finally these receptors may stop sending signals.  Example a person walks into a room with a strong smell the longer the person is in the room the less noticeable the smell is.

10.3 Somatic Senses

 Receptors associated with the skin, muscles, joints, and viscera make up the somatic senses.

Pressure & Touch Figure 10.1

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Three types of receptors detect touch and pressure .

Free ends of sensory nerve fibers in the epithelial tissues are associated with touch and pressure.

Meissner’s corpuscles are flattened connective tissue sheaths surrounding two or more nerve . Meissner’s fibers and are abundant in hairless areas that are very sensitive to touch, like the lips.

Pacinian corpuscles are large structures of connective tissue and cells that resemble the layers of an onion. They function to detect deep pressure.

Skin Cross Section

Temperature Senses

 Temperature receptors include two groups of free nerve endings: Warm receptors and cold receptors which both work best within a range of temperatures.

  Both types of receptors adapt quickly.

Temperatures near 45 o C stimulate pain receptors; temperatures below 10 freezing sensation. o C also stimulate pain receptors and produce a

Pain

  Pain receptors consist of free nerve endings that are distributed throughout the skin and internal tissues, except in the nervous tissue of the brain, which has no pain receptors .

Pain receptors adapt poorly.

Visceral pain receptors are the only receptors in the organs that produce sensations. Stretching or spasm of smooth muscle.

 Referred pain occurs because of the common nerve pathways leading from skin and internal organs. An example would be a heart attack being felt as pain in the arm or as heartburn.

Acute Pain

 Acute pain fibers are relatively thin, myelinated nerve fibers.

 They conduct nerve impulses rapidly and are associated with the sensation of sharp short duration pain.

 Originates from a restricted area of the skin and seldom continues after the pain producing stimulus stops.

Chronic Pain

 Chronic pain fibers are thin, unmyelinated nerve fibers.

 They conduct impulses slowly and produce dull, aching pain that may be diffused and difficult to pinpoint.

Regulation of Pain

   A person becomes aware of pain when impulses reach the thalamus in the brain, but the cerebral cortex judges the intensity and location of the pain.

Other areas of the brain regulate the flow of pain impulses from the spinal cord and can trigger the release of chemicals called neuropeptides which inhibit the release of pain impulses in the spinal cord.

Other chemicals called endorphins released in the brain provide natural pain control.

10.4 Special Senses

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Special senses are those whose sensory receptors are within large, complex organs in the head.

Smell –Olfactory organs Taste – Taste buds Hearing, equilibrium – ears Sight -- eyes

10.5 Sense of Smell

 The sense of smell is associated with complex sensory structures in the upper region of the nasal cavity. (Superior nasal concha)  We usually smell food at the same time we taste it.

 Chemoreceptors

Olfactory Organs

    Contain the olfactory receptors, are yellowish-brown masses that cover the upper parts of the nasal cavity, the superior nasal conchae, and a portion of the nasal septum.

Olfactory receptors cells are bipolar neurons surrounded by columnar epilthelial cells.

Olfactory impulses are interpreted in the temporal lobe of the brain.

Anosmia is the partial or complete loss of smell.

10.6 Sense of Taste

 Taste buds are the special organ of taste.  Taste buds are located primarily on the surface of the tongue but, are also scattered in the roof of the mouth and the walls of the throat.

 Taste buds include a group of modified cells.

 Each Taste bud has 50 to 150 receptor cells.

 Receptor cells are replaced every three days .

 Tiny projections called taste hairs protrude from the outer ends of the taste cells and extend from the taste pore. Taste hairs are the most sensitive structure of the taste bud .

Taste Sensations

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  Sweet Sour Salty Bitter Are the 4 primary taste sensations.

Saliva enhances the taste of food by dissolving the chemicals that cause taste.

10.7 Sense of Hearing

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External ear consists of 2 parts. The outer, funnel like structure called the auricle.

The S-shapted tube called the external auditory meatus.

Cross Section of the Ear

Middle Ear

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includes an air-filled space, an eardrum, and three small bones.

Tympanic cavity is an air-filled space in the temporal bone.

Eardrum is semitransparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside.

3. Auditory ossicles (bones) are the malleus, incus, and stapes. The three smallest bones in the body. The main function of the bone is to transmit and intensify the vibrations transmitted to the inner ear.

4. Oval Window- opening in the wall of the tympanic cavity. The ossicle (Bone) that sits in the oval window is the stapes.

5. Auditory Tube connects the middle ear to the throat.

 The main function of the auditory tube is to equalize the pressure on both sides of the eardrum.

Inner Ear

    The inner ear is a complex system of communicating chambers and tubes called the labyrinth .

The labyrinth includes 3 Semicircular canals provide a sense of equilibrium and a cochlea which which functions in hearing.

The apex of the cochlea to a membrane-covered opening in the wall of the inner ear called the round window . Hearing receptor cells are located in the organ of Corti.

10.8 Sense if Equilibrium

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Static equilibrium senses the position of the head, maintaining stability, and posture when the head and body are still.

Dynamic Equilibrium senses the motion of the head and aid in balancing the head and body during sudden movement. Semicircular canals detect motion.

are the organs that

10.9 Sense of Sight

 Eyelid has four layers. Skin, muscle, connective tissue, and conjunctiva.

 The skin of the eyelid is the thinnest skin of the body.

 The eyelids are moved by the orbicularis oculi muscle.

 Conjunctiva is a mucous membrane that lines the inner sufarces of the eyelids and folds back to cover the eyeball, except for the cornea.

Figure 10.16 & Table 10.2

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Lacrimal gland which secretes tears.

6 extrinsic muscles move the eye in a various directions.

Superior Rectus move the eye upward.

Inferior Rectus move the eye downward.

Lateral Rectus move the eye outward.

Medial Rectus move the eye inward.

Structure of the Eye

 The eye is hollow.

 The eye has 3 distinct layers.

 Outer Tunic  Middle Tunic  Inner Tunic

Outer Tunic

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Cornea which is the window of the eye and helps focus entering light rays.

The transparency of the cornea is due to the small number of cells and the lack of blood vessels.

Sclera white portion of the eye. Optic nerve

Middle Tunic

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 Choroid coat contains pigment-producing cells that help keep the inside of the eye dark.

Ciliary body which is the thickest part of the middle tunic.

Lens is clear, membrane like structure.

Accommodation allows the shape of the lens to change as the eye focuses on a close object.

Middle Tunic Continued…..

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Iris colored portion of the eye.

Aqueous humor watery fluid that helps the eye maintain shape .

Pupil circular opening in the center of the iris.

Inner Tunic

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Retina which contains the visual receptor cells (photoreceptors). It is a sheet of tissue in the back of the eye.

Macula lutea is a yellowish spot on the retina.

Fovea centrailis is a depression in the center of the macula lutea. This is the region that produces the sharpest vision .

Inner Tunic Continued…..

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Optic disc is on the retina it lacks receptor cells, it is commonly known as the blind spot .

Vitreous humor supports the internal parts of the eye and helps maintain its shape.

Visual Receptors

 Rods long, thin projections found in the retina. Colorless vision, in dim light .

 Cones short, blunt projects found in the retina. Detect color, provide sharp images .

 A Human eye has 125 million rods and 7 million cones.

 A Human eye has 150,000 cones in the fovea centralis and bird eye has 1 million cones.

   “Skin Cross Section”. March 27, 2007. http://www.owlnet.rice.edu/~psyc351/Images/Wolfe-Fig-12-02-0.jpg

“Human Ear.” March 27, 2007. http://images.google.com/imgres?imgurl=http://www.infj.ulst.ac.uk/~pni c/HumanEar/Andy%27s%2520Stuff/MScProject/workingcode_Local/h umanear.jpg&imgrefurl=http://www.infj.ulst.ac.uk/~pnic/HumanEar/And y%27s%2520Stuff/MScProject/workingcode_Local/EarChapter.html&h =380&w=566&sz=58&hl=en&start=1&tbnid=OBMiWPdvlJ96uM:&tbnh =90&tbnw=134&prev=/images%3Fq%3Dhuman%2Bear%26gbv%3D2 %26svnum%3D10%26hl%3Den “Eye”. March 27, 2007. http://images.google.com/imgres?imgurl=http://library.thinkquest.org/J0 02508/images/eyeball.gif&imgrefurl=http://library.thinkquest.org/J0025 08/eyeballs.htm&h=251&w=392&sz=19&hl=en&start=1&tbnid=Rm4w O8m6xYd5aM:&tbnh=79&tbnw=123&prev=/images%3Fq%3Dhuman %2Beye%2Bball%26gbv%3D2%26svnum%3D10%26hl%3Den