sensitization

Definition

Target type: biologicalprocess

An increased in a behavioral response to a repeated stimulus. For example, a shock to the tail of the marine snail Aplysia, to which the snail responds by withdrawing its gill, will result in increased gill withdrawal the next time the skin is touched. [ISBN:0582227089]

Sensitization is a complex process in which the nervous system becomes more responsive to a stimulus after repeated exposure. This heightened responsiveness can be triggered by various stimuli, including physical, chemical, or psychological stressors. The process of sensitization can be broadly divided into two phases:

**1. Acquisition Phase:** This phase involves the initial exposure to the stimulus, leading to changes in the nervous system that underlie sensitization. This phase is characterized by:

- **Synaptic Plasticity:** Repeated exposure to the stimulus causes changes in the strength of synapses, the junctions between neurons. This can involve an increase in the release of neurotransmitters, an increase in the sensitivity of postsynaptic receptors, or both.
- **Cellular Signaling Pathways:** Various intracellular signaling pathways are activated in response to the stimulus. These pathways involve the activation of second messengers like cAMP and calcium, which ultimately lead to changes in gene expression and protein synthesis.

**2. Expression Phase:** Once the sensitization has been acquired, the nervous system becomes more responsive to the stimulus. This enhanced responsiveness can be observed in the following ways:

- **Increased Sensitivity:** The threshold for triggering a response to the stimulus is lowered.
- **Exaggerated Response:** The response to the stimulus is more intense and prolonged.
- **Generalized Response:** The sensitization can extend beyond the original stimulus and affect responses to other stimuli.

**Mechanisms of Sensitization:**

- **Classical Conditioning:** The association of a neutral stimulus with a noxious stimulus can lead to sensitization. This is evident in fear conditioning, where a neutral stimulus like a tone is paired with an aversive stimulus like a shock, leading to fear responses elicited by the tone alone.
- **Hebbian Plasticity:** This principle, "neurons that fire together wire together," states that the repeated co-activation of neurons strengthens their connections. In sensitization, repeated exposure to a stimulus leads to the co-activation of neurons, strengthening their connections and contributing to the enhanced responsiveness.
- **Neurotransmitter Systems:** Several neurotransmitter systems are involved in sensitization, including dopamine, glutamate, serotonin, and acetylcholine. These neurotransmitters play crucial roles in modulating synaptic plasticity, cellular signaling pathways, and behavioral responses.

**Examples of Sensitization:**

- **Drug Addiction:** Repeated exposure to addictive drugs like cocaine or heroin can lead to sensitization of the reward circuitry in the brain, contributing to cravings and compulsive drug seeking.
- **Chronic Pain:** Chronic pain conditions often involve sensitization of the nervous system, leading to exaggerated pain perception and heightened sensitivity to pain stimuli.
- **Post-Traumatic Stress Disorder (PTSD):** PTSD can involve sensitization to trauma-related cues, causing anxiety, fear, and avoidance behaviors.

**Overall, sensitization is a complex and highly adaptive process that plays a significant role in shaping our responses to the environment. It can be both beneficial, enhancing our ability to respond to threats, and detrimental, contributing to chronic pain, addiction, and anxiety disorders. Understanding the underlying mechanisms of sensitization is crucial for developing effective treatments for these conditions.**'
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