The General and Special Senses Dr. Ali Ebneshahidi, Study notes of Physiology

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The General and Special Senses

Dr. Ali Ebneshahidi

Function of senses

• 1. Detection of sensations allow the human body to be aware of changes (or stimuli) that occur in the environment or inside the body.
• 2. These senses permit the central nervous system to produce reactions for the stimuli and maintain body homeostasis.
• 3. Somatic senses ("soma" means body) detect touch, pain pressure, temperature, and tension on the skin and in internal organs.
• 4. Special senses detect the sensations of taste, smell, hearing, equilibrium, and sight, only in special sense organs in the head region (a phenomenon known as “cephalization").
• 5. All senses are detected by sensory receptors, and after integration and processing being done in the central nervous system, motor nerves produce a response.

• 4. Mechanoreceptors

a) detect mechanical forces. b) in somatic senses, proprioceptors detect muscle tension, pressoreceptors detect blood pressure, and stretch receptors detect lung inflation, stomach distention, and urinary bladder expansion. c) in special senses, mechanoreceptors in the inner ear detect the senses of hearing (in cochlea)and equilibrium (in semicircular canals).

• 5. photoreceptors

a) detect light intensity. b) in special senses, photoreceptors in the eye detect the tones (by rods photoreceptors) and colors (by cones photoreceptors) of visual images

Somatic Senses

1. sensory receptors in the skin, muscles, joints,

and visceral organs detect external and internal

stimuli of the body.

2. Three types of somatic senses:

a) Exteroceptive senses detect changes that

occur at body surface, such as touch, pressure

and temperature.

b) Proprioceptive senses detect changes that

occur in muscles, tendons, ligaments and joint

tissues.

c) Visceroceptive senses detect changes that

occur in internal organs.

• b) corpuscular receptors - endings of dendrites are enclosed in a connective tissue capsule.
  • also are abundant in the dermis and subcutaneous layers of skin.

4. Four kinds of corpuscular receptors:

a) Meissner’s corpuscles: detect low- frequency vibrations and light pressure (touch).

• found in dermal papillae.

b) pacinian corpuscles: detect deep vibration and pressure.

• found in subcutaneous tissues.

c) Krause’s corpuscles: detect cold temperatures (below 68 ºF or 20 ºC).

• found in the dermis, eyes, lips, and mouth.

d) Ruffini’s corpuscles: detect heat (above 77 ºF or 25 ºC).

• found in the dermis.

Special Senses

• 1. The senses of taste and smell

(detected by chemoreceptors),

hearing and equilibrium (detected by

mecanoreceptors ), and vision

(detected by photoreceptors) are the

five special senses.

• 2. Require specialized sensory

receptors within large, complex

sensory organs in the head region.

Sense of Smell

• a) olfactory receptors located in "olfactory organs“ are specialized chemoreceptors in the nasal cavity.
• b) olfactory receptors are bipolar neurons surrounded by ciliated columnar cells. These sensory cells, after being stimulated by olfactory sensations, send nerve impulses along the olfactory (cranial nerve I) to the cerebrum.
• c) gases in the air entering the nasal cavity are dissolved by nasal mucus (secreted by goblet cells in the columnar epithelium ), and the resulting solution stimulates various olfactory receptors. (^) ebneshahidi

• d) These sensory cells, after being stimulated by taste sensations, send nerve impulses through the facial nerve (nerve VII), glossopharyngeal nerve (nerve IX), or vagus nerve (nerve X ) to the cerebrum.

• e) four primary taste sensations:
  • sweet- sensation caused by organic substances such as sugars and amino acids; most sensitive at the tip of tongue.
  • sour – sensation caused by hydrogen ions (H+) from acidic substances; most sensitive at both sides of the tongue. - salty- sensation caused by metal ions (e.g. Mg++) or inorganic salts (e.g. Nacl); most sensitive at the sides and the tip of tongue.
  • bitter – caused by alkaloids (e.g. nicotine, caffeine) and nonalkaloids (e.g. aspirin); most sensitive at the back of tongue. (^) ebneshahidi

Anatomy of The Eye

• The wall of the eyeball consists of 3 layers of tissue: I) Fibrous Tunic: outermost layer, made of fibrous connective tissue with minimal blood vessels.
• Contains 2 regions: sclera (a white area that extends from the back of the eye toward the front) and cornea (a transparent tissue in the front for allowing light to enter the eyeball). (^) ebneshahidi

III) sensory tunic (also called retina):

• innermost layer, made of specialized nerve tissue. It contains 2 layers of tissue: an outer pigmented layer (which absorbs light and stores vitamin A) and an inner neural layer (that detects light using photoreceptors and sends nerve impulses to the occipital lobe of cerebrum through the optic nerves).
• 2 types of photoreceptors are found on the neural layer: rods (detect tones of visual images) and cones (detect colors). These sensory cells, after being stimulated by visual sensations, send nerve impulses through the optic nerve (Nerve II ) to the occipital lobs of cerebrum.