Chapter 29 The Senses

I. Sensory Reception

1. Sensory receptors convert stimulus energy to action potentials (AP).

2. Sensory receptor cell is a specialized cell or neuron that detects specific stimuli from an organismꞌs external or internal environment and sends information to the central nervous system (CNS).

3. Sensory organs, such as eyes and taste buds, contain sensory receptors.

4. Sensory transduction is the conversion of a stimulus signal to an electrical signal by a sensory receptor.

5. Receptor potential is the electrical signal (membrane potential) produced by sensory transduction. In contrast to action potentials with all-or-none law, receptor potentials vary; the stronger the stimulus, the greater the receptor potential.

6. Sensory adaptation is the tendency of sensory neurons to become less sensitive, when they are stimulated repeatedly. For example, a prominent smell becomes unnoticeable over time.

7. Sensory receptors detect 5 categories of stimuli:

(1)  Thermoreceptors: detect either heat or cold. Other temperature sensors located deep in the body monitor the temperature of the body. For example, the hypothalamus acts as the bodyꞌs thermostat.

(2)  Mechanoreceptors: Mechanical energy, such as touch, pressure, stretching, motion, and sound, stimulate mechanoreceptors to bend or stretch, resulting in tension on the plasma membrane. Hair cell is a type of mechanoreceptor that detects sound waves and other forms of movement in air or water.

(3)  Pain receptors: In humans and most mammals, the skin has the highest density of pain receptors. Pain receptors may respond to excess heat or pressure or to chemicals released from damaged or inflamed tissues.

(4)  Chemoreceptors: are sensory receptors that detect both energy and carbon from organic compounds, such as nose and taste buds. Arteries have internal chemoreceptors can detect changes in the amount of oxygen in the blood. Osmoreceptors in the brain detect changes in the total solute concentration of the blood. One of the most sensitive chemoreceptors in the animal kingdom is inside nares of sharks.

(5)  Electromagnetic receptors: are sensory receptors that detect energy of different wavelengths, such as electricity, magnetism, and light. The most common electromagnetic receptors are photoreceptors that detect the electromagnetic energy of light in the visible or ultraviolet part of the electromagnetic spectrum.

II. Vision

1. Several types of eyes have evolved among animals.

2. A simple light-detecting organ is the invertebrate eyecup.

3. The compound eye (an image-forming eye) consists of up to several thousand light-detectors ommatidia in insects and crustaceans.

(1)  Every ommatidium has its own light-focusing lens and several photoreceptor cells.

(2)  One ommatidium picks up light from only a tiny portion of the field of view.

(3)  The animalꞌs brain then forms a mosaic visual image by assembling the data from all the ommatidia.

4. The single-lens eye also is an image-forming eye found in some invertebrates and all vertebrates.

(1)  Light enters the single-lens eye through the pupil.

(2)  Iris changes the diameter of the pupil to let in more or less light.

(3)  Light passes through a single lens that focuses light onto the retina, which contains many photoreceptor cells.

(4)  Fovea is the retinaꞌs center of focus and the place on the retina where photoreceptors are highly concentrated.

5. Many vision problems can be corrected with artificial lenses or surgery. Three of the most common vision problems are nearsightedness, farsightedness, and astigmatism.

(1) Nearsightedness (myopia): a nearsighted eyeball is longer than normal, which can be corrected by concave lens.

(2) Farsightedness (hyperopia): a farsighted eyeball is shorter than normal, which can be corrected by convex lens.

(3) Astigmatism is blurred vision caused by a misshapen lens or cornea, which can be corrected by asymmetric lens.

(4)  A cataract is a clouding of the lens caused by protein clump with age.

(5)  Glaucoma is mainly caused by an excess of aqueous humor, leading to increased pressure in the eye and resulting in damage to the optic nerve.