1. Sense Organs

2. Sense Organs The sense organs are divided into two categories:
General sense organs
Special sense organs
General sense organs sense touch, temperature, pain.
Special sense organs function to produce vision, hearing, balance, taste, and smell.
Both the general and the special sense organs also function to maintain homeostasis

3. Receptors
Distribution of receptors - Receptors for special senses of smell, taste, vision, hearing, and equilibrium are grouped into localized areas or into complex organs - General sense organs of somatic senses are microscopic receptors widely distributed throughout the body in skin, mucosa, connective tissue, muscles, tendons, joints, and viscera

4. Classification of Receptors: By Location
Exteroceptors: detect pressure, touch, pain, and temperature. Located on or near body surfaces.
Visceroceptors: detect pressure, stretch, chemical changes, hunger, and thirst from internal organs.
Proprioceptors: Provide information on body movement, orientation in space, and muscle stretch. Found in skeletal muscle, joint capsules, and tendons.

5. Classification of Receptors: By Stimulus Type
- Mechanoreceptors: pressure receptors.
- Chemoreceptors: activated by chemicals
- Thermoreceptors: temperature receptors
- Nociceptors: pain receptors
- Photoreceptors: light receptors of the eye
Osmoreceptors: receptors for electrolytes in extracellular fluids
Classification by structure
1- Free nerve endings
2- Encapsulated nerve endings

6. Free nerve endings - Most widely distributed type of sensory receptor
- Include both exteroceptors and visceroceptors
- Called nociceptors: receptors for pain
- Also detect itching, tickling, touch, movement, and mechanical stretching
- Primary receptors for heat and cold
- Two types of nerve fibers carry pain impulses from nociceptors to the brain:
- Acute (A) fibers: mediate sharp, intense, localized pain
- Chronic (B) fibers: mediate less intense, but more persistent, dull or aching pain
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7. Free nerve endings
Other free nerve ending receptors - Root hair plexuses: weblike arrangements of free nerve endings around hair follicles - Merkel discs: mediate sensations of discriminative touch

8. Encapsulated nerve endings
- Meissner’s corpuscle: Tactile disks involved in touch. - Krause’s end bulbs: detect low-frequency vibrations Carry cold impulses. - Ruffini’s corpuscles: mediate crude and persistent touch; may be secondary temperature receptors for heat (85° to 120° F). - Pacinian corpuscles: mechanoreceptors that respond quickly to sensations of deep pressure, high-frequency vibration, and stretch.
The number of Meissner’s corpuscles decreases between ages 12 and 50 by about 4X in each square millimeter of the skin on the fingertips. This correlates with gradual loss of sensations for small probes as a person ages (Thornbury and Mistretta, 1981). Note added by Mohamad Termos © 2009

9. Stretch Receptors
There are two types of stretch receptors: 1- Muscle spindles 2- Golgi tendon These receptors operate to provide body with information concerning length and strength of muscle contraction
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10. Special Senses
Characterized by receptors grouped closely together or grouped in specialized organs; senses of smell, taste, hearing, equilibrium, and vision

11. Taste
Your taste buds have receptors for different kinds of chemicals: sugar, salt, sours, and bitters. This diagram shows how a sugar molecule can enter a taste bud and bind to an ion channel in the membrane of a receptor cell. The receptor cell then sends neurotransmitters to activate the sensory neuron that goes to the brain.
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12. Taste
Each taste bud has only one kind of taste receptor, either sweet, salty, sour, or bitter. Each taste bud sends this information into the brain, where it is integrated. Increasing taste increases the frequency of action potential to the brain.
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13. Sense of Smell
Olfaction works similarly to taste. Receptors in the nose can bind to a certain kind of chemical, which triggers the release of neurotransmitters to the nerves that send action potentials to the brain. Again, stronger, more pungent smells send more frequent potentials.

14. Olfactory pathway
When level of odor-producing chemicals reaches a threshold level, the following occurs:
Receptor potential, and then action potential, is generated and passed to the olfactory nerves in the olfactory bulb
The impulse then passes through the olfactory tract and into the thalamic and olfactory centers of brain for interpretation, integration, and memory storage
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15. The Ear: Organ of Hearing and Balance

16. Hearing and balance
The ear is not only responsible for hearing, but also for balance! It is an organ that can distinguish between volume and pitch of sounds, as well as rotation in all three planes. The outer ear, or pinna, is responsible for channeling sound into the auditory canal. The tympanic membrane, commonly referred to as the “eardrum” receives the sound pressure waves in the ear, and converts them into mechanical vibrations. These vibrations are passed along through three small middle ear bones: the malleus, incus, and stapes.

17. Hearing and balance
The stapes then hits another membrane called the oval window. The oval window is the opening to the fluid filled, spiral shaped cochlea. Vibrations are sent along the cochlear fluid and are sensed by the movement of “reed-like” cells on the basilar membrane. The basilar membrane motion receptors send the information to the brain via sensory neurons. The further the wave moves along the cochlea, the lower pitch of sound we hear

18. Hearing and balance
There is another part of the inner ear, above the cochlea, that senses position and balance of the head. This region contains the semi-circular canals. These canals are three fluid-filled, half circle shaped canals oriented orthogonally to each other. When the head moves in the other direction (just as if you quickly jolt a glass of water forward, it might splash backwards towards your hand). The fluid motion moves hairs on the membrane (like on the skin), which stimulate nerve fibers to send action potentials to the brain to let you know which way you are moving.

19. The Eye: The organ of vision
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20. Vision
The eye is a very interesting organ that is able to sense a fairly large spectrum of light. The eye is filled mostly with water (aqueous humor and vitreous humor). Light is initially refracted by the cornea, and enters the eye through the pupil. The lens then focuses the light and targets it to the retina: the location of light sensors in the back of the eye.

21. Vision
In the retina, there are sensory cells called rods and cones which can sense light. Rods are used for night vision, whereas cones can sense color and detail.
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22. Vision
The fovea centralis is in the center of the retina and contains only cones for the most detailed color vision. The rods and cones activate visual pigments according to different wavelengths of light, which stimulate the nerves to fire action potentials into the brain. It is interesting that the rods and cones are actually in the deepest layer; so light must cross the neurons to reach the photoreceptor cells, before the signal is transmitted in the opposite direction.

23. Vision: The Eye Neuronal pathway of vision
- Fibers that conduct impulses from rods and cones reach the visual cortex in occipital lobes via optic nerves, optic chiasma, optic tracts, and optic radiations
- Optic nerve contains fibers from only one retina, but optic chiasma contains fibers from the nasal portion of both retinas; these anatomical facts explain peculiar visual abnormalities that sometimes occur
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