Skin sensitivity to touch exists on a spectrum, representing how the nervous system interprets mechanical input from the environment. Normal interpretation allows for the perception of texture, temperature, and pressure without discomfort. Heightened sensitivity occurs when the nervous system misinterprets signals, causing a gentle touch to be perceived as painful (allodynia), or a mildly painful stimulus to be felt as intensely painful (hyperalgesia). This pathological hypersensitivity is driven by specific alterations in the nervous system, requiring an understanding of how normal touch detection becomes disrupted.
The Biological Basis of Normal Touch Perception
The skin is equipped with specialized sensory nerve endings called mechanoreceptors, which translate mechanical pressure into electrical signals the brain can understand. These receptors respond to different qualities of touch, such as light pressure, vibration, and skin stretch. For instance, Meissner’s corpuscles detect fine, light touch, while Merkel cells help with sustained pressure and texture discrimination. The signal travels via fast-conducting Aβ-fibers from the skin, through the spinal cord, and to the brain for conscious perception, ensuring normal touch is registered quickly and accurately.
How Peripheral Nerve Damage Causes Hypersensitivity
Pathological hypersensitivity often begins with changes in the peripheral nervous system. Conditions causing nerve injury, such as metabolic imbalances like diabetes or viral infections, can lead to peripheral neuropathy, where nerve fibers become damaged or inflamed. This damage makes the nerve endings hyper-excitable, a process termed peripheral sensitization. During sensitization, low-threshold mechanoreceptors that signal gentle touch can cross-activate high-threshold nociceptors, causing the central nervous system to interpret a non-painful stimulus as pain (allodynia).
Central Nervous System Amplification of Touch Signals
Beyond the peripheral changes, a distinct mechanism for chronic hypersensitivity occurs within the central nervous system, known as central sensitization. This involves an increase in the responsiveness of neurons within the spinal cord and brain to both normal and sub-threshold sensory input. Persistent, intense input from the damaged peripheral nerves can trigger a cascade of changes in the spinal cord, essentially turning up the volume on all incoming signals. One mechanism involved is called “wind-up,” where repetitive, low-frequency input from C-fibers leads to a progressive increase in the excitability of spinal cord neurons. This sustained release of neurotransmitters, particularly glutamate, activates N-methyl-D-aspartate (NMDA) receptors, which are involved in generating spinal hypersensitivity.
The result is a state of long-term potentiation, a persistent elevation of neuronal activity that lowers the pain threshold and expands the receptive fields of the spinal cord neurons. This central change means the nervous system is now in a persistent state of high reactivity, confusing normal touch signals with pain even after the initial peripheral injury may have healed. The brain’s pain matrix processes the information and misinterprets the gentle touch signals as a threat, maintaining the chronic hypersensitivity. The central nervous system has effectively learned to be hypersensitive, leading to pain that is disproportionate in amplitude and duration to the original stimulus.
Medical Conditions Associated With Heightened Skin Sensitivity
Heightened skin sensitivity, presenting as allodynia or hyperalgesia, is a common symptom across several medical conditions that disrupt normal nervous system function. Peripheral Neuropathy, often caused by diabetes, results in damage to small nerve fibers that leads to peripheral and central sensitization. Post-herpetic neuralgia, a complication of shingles, causes chronic pain and allodynia in the affected area.
Fibromyalgia is a chronic disorder where central sensitization is a major underlying mechanism, causing widespread pain and sensitivity to touch. Complex Regional Pain Syndrome (CRPS) involves both peripheral nerve issues and significant central sensitization. Migraine headaches are also frequently associated with cutaneous allodynia.