Unit Ten: The Nervous System: B. Special Senses

Chapter 53: The Chemical Senses—Taste and Smell

Guyton and Hall, Textbook of Medical Physiology, 12th edition

Sense of Taste

• Primary Sensation of Taste

a. Sourb. Saltyc. Sweetd. Bittere. Umami

Sense of Taste

• Threshold for Taste- much more sensitive to bitter taste than all of the others (important protective function from toxins in food)

• Taste Blindness- some people are taste “blind” for substances; phynylthiocarbamide (15-30% ofpeople); unknown reasons

Sense of Taste

• Taste Buds and Their Function

Fig. 53.1 Taste bud

Sense of Taste

• Taste Bud Location- found on three types of papillae on the tongue

a. Large number on the walls of the troughs thatsurround the circumvallate papillae

b. Moderate numbers on the fungiform papillae overthe flat anterior surface of the tongue

c. Moderate numbers on the foliate papillae on thefolds along the lateral surface of the tongue

d. Also on palate, tonsils, epiglottis, and proximalesophagus

e. Adults have 3000-10000; degenerate with age (45 yrs)

Sense of Taste

• Specificity of Taste Buds- each taste bud respondsmostly to one of the five primary taste stimuliwhen the substance is in low concentration; highconcentrations will stimulate two or more taste budtypes

Sense of Taste

• Mechanism of Stimulation

a. Receptor potential- substance causes the taste hairto depolarize

1. For salty and sour, the receptor opens specific ionchannels

2. For sweet and bitter, a second messenger is activated

b. Generation of nerve impulses by the taste bud

Sense of Taste

• Transmission of Taste Signals into the CNS

Fig. 53.2

Sense of Taste

• Rapid Adaptation of Taste

• Taste Preference- results from a mechanism withinthe CNS; can develop taste aversion or negativetaste preference

Sense of Smell

• Olfactory Membrane

Fig. 53.3

Sense of Smell

• Olfactory Cells

a. bipolar nerve cells; 100 million interspersed between the sustentacular cells

b. Olfactory hairs project into the mucus that coats the inner surface of the nasal cavity

c. Hairs form a dense mat in the mucus, react toodors, and stimulate the olfactory cells

d. Boweman’s glands secrete mucus onto the surfaceof the membrane

Sense of Smell

• Stimulation of the Olfactory Cells

a. Odorant substance binds to receptor proteins inthe membrane of each cilium (hair cell)

b. Receptor proteins folds its way into and out of the membrane about 7 times

c. Inside fold is bound to a G protein which then activates adenyl cyclase to form cAMP

d. cAMP activates gated sodium ion channels to open

Sense of Smell

• Stimulation of the Olfactory Cells

e. Ions pour in and depolarize the membrane

f. Excites the olfactory nerve and transmits Apsinto the CNS

g. Only volatile substances can be smelled

h. Substances must be soluble in water

i. Helpful to be slightly lipid soluble

Sense of Smell

• Rapid Adaptation of Olfactory Sensations- 50% in the first couple of seconds

• Primary Sensations of Smell

a. Camphoraceousb. Muskyc. Florald. Peppermintye. Etherealf. Pungentg. Putrid

Sense of Smell

• Transmission of Smell Signals into the CNS

Fig. 53.5 Neural connections of the olfactory system