Chemical Mechanical Electromagnetic Temperature Pain Gustation, Olfaction, pH, oxygen Hearing, Balance, Position, Touch Vision, Electric, Magnetic Receptors Act as Transducers
Receptors Act as Amplifiers • One photon of light contains 3 x 10 –19 joules of energy, the action potential it produces has 5 x 10-14 joules of energy , so energy is increased 1.7 x 10 5 • Receptor thresholds; • Eye – 10 photons • Olfaction – 1 molecule in 25,000 • Taste – 3 x 10-5 M quinine as bitter
Ion Channels and Senses • Hearing and Stretch – mechanical stretching of the membrane and channel • Temperature – temperature dependent Na/K channel • Taste – Phosporylation mediated Na/K channel • Olfaction – cAMP mediated Na/K channel • Sight – GMP mediated Na+ channel • Electricty – Voltage gated Na+ channel
Receptors Relay Information Proportional to the Log of the Stimulus Strength
Receptors are more sensitive in the lower range of their perceptual field
Receptors Present Relative Information • Receptor types may be phasic or tonic • Some receptors require biochemical regeneration • Receptors have different thresholds • Receptors my have different fields • Receptors can be laterally inhibited • Information may be interpreted differently by the CNS
Sensitivity and sensation is modified by the number of neurons serving an area.
Information Presented • Modality • Location of Stimulus • Intensity and Duration of Stimulus
Umami – the taste of deliciousness also involves a G protein complex but its effectors is not known This receptor is triggered by the the amino acid glutamate. Like the NMDA receptor involved in long term potentiation, the receptor allows Ca++ and Na+ to enter and depolarize the cell– making it more sensitive. A much lower amount of another molecule will be required to initiate firing. Glutamate – monosodium glutamate is a primary ingredient of soy sauce.
Thresholds in Humans • .05 M NaCl • .17 M Sucrose • .001 H+ for sour • 3.0 x 10-5 quinine for bitter
General Taste Receptors • Most organisms show a general chemical sense. Protozoa respond positively to amino acids and sugars and are repelled by acids and salts. Glycine and taurine are generally food attractants. Frogs whip their legs to remove acids and salts. These receptors are found on the skin or external surface.
Internal Chemical Receptors • There are internal receptors for H+, salts, sugars and oxygen distributed in the digestive, respiratory and excretory tracts.
Contact Receptors • In flies, taste receptors on the feet have 4-5 neurons. One responds to touch, one responds to water, one responds to sugar and one to salts. The last cell responds to fatty acids – pheromones used as sexual attractants. When the fly feeds the crop swells and neurons from the crop reduce the sensitivity of the taste receptors.