Sensory receptors play a crucial role in maintaining homeostasis – the body's state of steady internal conditions. These receptors constantly monitor and respond to changes in the external and internal environment. For instance, thermoreceptors in the skin and hypothalamus detect temperature changes, triggering responses like sweating or shivering to regulate body temperature. Chemoreceptors in the carotid arteries and aorta monitor blood oxygen and carbon dioxide levels, helping to maintain respiratory and metabolic balance. Mechanoreceptors in the bladder wall signal the need to void, thus aiding in the excretion of waste and balance of body fluids. Similarly, receptors in the muscles and joints provide proprioceptive feedback, essential for coordinating movement and maintaining posture. By providing the nervous system with continuous feedback about the internal and external environment, sensory receptors enable the body to respond appropriately to changes, thus maintaining homeostasis.

Nociceptors are specialised sensory receptors that detect signals indicating potential or actual tissue damage, commonly perceived as pain. They are found throughout the body, particularly in the skin, muscles, joints, and some internal organs. Nociceptors respond to various harmful stimuli, including extreme temperatures, mechanical damage (like cuts or pressure), and chemical irritants. When activated, nociceptors generate nerve impulses that travel to the spinal cord and then to the brain. These impulses are processed in several brain regions, including the thalamus and cerebral cortex, leading to the perception of pain. The function of nociceptors is not merely to signal pain but also to initiate protective reflexes and behaviours, such as withdrawal from a harmful stimulus. They play a crucial role in injury prevention and in alerting the body to potential or existing damage. Nociceptors can also become sensitised after an injury, leading to heightened sensitivity, which is part of the body's mechanism to ensure protection and healing of the injured area.

Receptor specificity can indeed change over time, a phenomenon largely attributed to the plasticity of the nervous system. This plasticity allows sensory systems to adapt to changing environmental conditions or to compensate for damage. For example, in the olfactory system, exposure to certain odours over time can lead to a change in receptor sensitivity or even to the expression of different receptor proteins, altering olfactory perception. Similarly, in the visual system, prolonged exposure to specific light conditions can lead to adjustments in the sensitivity of rods and cones. On a molecular level, changes in receptor specificity can occur due to gene regulation mechanisms, alterations in receptor protein structure, or changes in the surrounding membrane or cellular environment affecting receptor function. These changes are part of an organism’s ability to adapt to its environment and maintain homeostasis. However, it's important to note that while plasticity allows for some level of adaptation, the fundamental specificity of receptors (e.g., a photoreceptor responding to light) remains largely constant.

Thermoreceptors in humans are specialised sensory receptors that detect temperature changes. These receptors are categorised into two types: those sensitive to heat (warm receptors) and those responsive to cold (cold receptors). Warm receptors are generally activated at temperatures above body temperature (around 37°C), with their response increasing up to temperatures of about 45°C, beyond which pain receptors take over to signal the risk of burns. Cold receptors, on the other hand, are activated at temperatures slightly below normal body temperature, with their maximum response at around 20°C to 25°C. The sensitivity range of these receptors is designed to alert the body to potentially harmful temperature changes. Both types of thermoreceptors employ a mechanism of action potential generation, where a change in temperature alters the permeability of receptor cell membranes to ions, thereby generating an electrical impulse. This impulse travels along the sensory neurons to the brain, which interprets the signal as a sensation of warmth or cold. The differential activation thresholds and response patterns of these receptors allow for precise temperature discrimination.

Sensory receptors interact with the nervous system through a process known as sensory transduction, where they convert physical or chemical stimuli into electrical signals (nerve impulses). These impulses are then transmitted to the central nervous system (CNS) via afferent neurons. Upon reaching the CNS, the signals are processed, integrated, and interpreted in various brain regions to produce a perception of the stimulus. This information is then often relayed to the motor neurons of the peripheral nervous system, leading to a coordinated response. For example, when mechanoreceptors in the hand detect pressure (stimulus), they generate nerve impulses that travel to the brain. The brain processes this information and may send signals back through motor neurons, instructing the hand muscles to grip or release an object. This intricate communication between sensory receptors and the nervous system allows for rapid and coordinated responses to environmental changes, critical for survival and interaction with the world.