recap

45.1 recap

Sensory receptor cells have receptor proteins that respond to specific stimuli from the external or internal environment by directly or indirectly opening or closing ion channels, causing the generation of action potentials or neurotransmitter release in sensory neurons. How the resulting information is processed and interpreted depends on the neural circuitry that is activated.

learning outcomes

You should be able to:

  • Describe the roles of receptor potentials and action potentials in sensory systems.

  • Explain how the same membrane event, an action potential, can result in sensations of heat, light, sound, touch, or taste.

  • Describe the various ways that environmental stimuli can generate receptor potentials.

  • Explain the functional significance of sensory adaptation.

Question 1

In the crayfish stretch receptor, what creates the receptor potential? Where and how does it stimulate an action potential?

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Mechanical stimulation of the stretch receptor dendrites by stretch of the muscle they innervate causes graded depolarization of the stretch receptor neuron cell body—the receptor potential. The receptor potential spreads to the axon hillock, and if it depolarizes the membrane of the axon hillock to threshold, a Na+ action potential is fired in the axon.

Question 2

How are we able to perceive action potentials—all of which are essentially the same—as different sensations?

The interpretation of an action potential depends on its target neuron in the brain. If the action potential arrives in a nerve of the optic tract projecting to the visual cortex, it is interpreted as light, but if a similar action potential arrives in an olfactory nerve projecting to the olfactory bulb, it is interpreted as smell.

Question 3

Peppers have a distinctive flavor, but they also can be hot if they contain the molecule capsaicin. Explain the difference in how the flavor versus the hotness of a pepper is sensed.

The molecules responsible for the distinctive flavor of peppers are received through chemoreceptors that activate a G-protein signaling cascade that open cation channels in the receptor membrane. Capsaicin, however, directly activates a TRP protein that also serves as a thermosensor, and that protein itself is a cation channel that opens when activated by its specific stimuli.

Question 4

Why is adaptation or the lack thereof important in different sensory modalities?

Adaptation is the property of a sensory system to stop responding to a constant level of receptor stimulation. Adaptation is important when it is of value to recognize when stimulation changes but it is not necessary to have continuous information. An example is the activation of skin tactile sensors by your clothing. Adaptation would not be advantageous when the information is critical, such as pain sensations, or when constant levels of information are essential, such as from the sensory neurons that signal postural muscle tone.

Now that you have a general view of how sensory systems code and process information, you will next learn how sensory systems gather and filter stimuli, transduce specific stimuli into action potentials, and transmit action potentials to the CNS, which perceives them as different types of information.