The increase in threshold of a receptor when a series of stimuli of subthreshold intensity are applied in succession is called:
**Core Concept**
The concept being tested is known as "habituation," which refers to a decrease in the response to a stimulus that is repeated at a subthreshold intensity. This phenomenon is related to the adaptation of sensory neurons and is an important aspect of sensory processing.
**Why the Correct Answer is Right**
When a series of stimuli of subthreshold intensity are applied in succession, the sensory neurons adapt by decreasing their excitability. This leads to an increase in the threshold of the receptor, making it more difficult to elicit a response. This process is mediated by the activation of potassium channels, which hyperpolarize the neuron and decrease its excitability. The repeated stimulation of the receptor leads to a gradual decrease in the response, as the neurons adapt to the stimulus.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to a phenomenon known as "sensitization," which is the opposite of habituation. Sensitization is an increase in response to a stimulus that is repeated at a subthreshold intensity.
**Option B:** This option is incorrect because it refers to a phenomenon known as "summation," which is the increase in response to a stimulus that is repeated at a threshold or suprathreshold intensity.
**Option C:** This option is incorrect because it refers to a phenomenon known as "desensitization," which is similar to habituation, but is typically used to describe the decrease in response to a stimulus that is repeated at a threshold or suprathreshold intensity.
**Clinical Pearl / High-Yield Fact**
Habituation is an important aspect of sensory processing, and is thought to be mediated by the activation of potassium channels. Understanding this concept is crucial for understanding how sensory neurons adapt to repeated stimuli.
**Correct Answer: C. Desensitization.**