Fundamentals of Neuroscience/Pain and Touch

Type classification: this is a lesson resource.

Subject Classification: this is a neuroscience resource.

Goals

To learn how sensors of the peripheral nervous system acquire tactile information
To learn how touch signals are mapped and interpreted in the brain
To understand the distinction between pain and touch, and how the nervous system helps establish that distinction.

Schematic of several representative sensory pathways leading from the skin to brain.

Mechanoreception and the Skin

Nestled within the dermal layers of the skin lie a diverse assortment of nervous cells classified under the general term of mechanoreceptors. What unifies mechanoreceptors as a group is that they all take in certain tactile information about what the skin is in contact with- whether it be in terms of touch, vibration, pressure, pain, or temperature. Each subtype of mechanoreceptor specializes more or less in detecting one of these attributes, and may further specialize into different levels of sensitivity. Because certain mechanorectpors are concentrated in certain regions of the body, this helps produce regions of the body with different sensitivities to touch, pain, etc. Varying densities of mechanorecptors and how far they are buried within the dermis (because ones closer to the surface are easier to activate) can also play a role in this diversification.

Touch

Most mechanoreceptors related to touch are constructed in such a way that mechanical stimulus will somehow alter the receptor and thereby allow firing of an action potential. For example, the cell labeled B in the adjacent diagram will have its capsule compressed whenever a sufficient level of pressure is applied. The physical force provided by this compression can alter the permeabilites of ion channels, thus setting off the action potential. While the cell in this example is adept at detecting changes in applied pressure, other receptors incorporate different mechanisms of activation, which accounts for why different stimuli activate each.

A slice of the somatosensory cortex showing how each portion roughly corresponds to a certain body region

Once the mechanoreceptors have generated their action potentials, these signals travel across the peripheral nerves into the spinal cord. Eventually, the signal travels all the way up the spinal cord and through several intermediate processing stations to reach the primary somatosensory cortex. Here, the inputs from the sensory systems are arranged in order to roughly correspond to the body part they originated from. The feet and legs take up the inner portion, while the hands and face make up the outer. In addition, much more of the cortex is devoted to inputs from the hand in face, giving these body regions much higher sensitivities.

Pain

An fMRI image showing the wide swath of brain regions responsible for pain perception. The primary somatosensory cortex is labeled in green in the two left images.

Although there are several important similarities between pain processing and regular touch processing, the distinction between the two systems is critical. One of the most defining traits of the pain-signaling pathway is its unique class of receptor cell, known as nociceptors. Unlike standard mechanoreceptors, which simply respond to touch or pressure, nociceptors are tasked with detecting whenever damage occurs to the body.

There are two ways this can be done: through physical stimulation or through chemical stimulation. In the physical method, the exposed nerve endings of the nociceptor (cell A in the diagram) can themselves become deformed by a sufficiently strong and deep wound, and in response send a pain signal. More commonly, an injury will trigger a complex set of chemical reactions as damaged cells release certain chemicals and immune cells reacting to the damage release others. The nociceptor is stimulated by exposure to this chemical 'soup', and continue to send pain signals until the levels of these pain-generating chemicals are lowered over time as the wound heals. A very similar mechanism is at work with itching sensations.

Once the pain signal is generated, it too travels up the spinal cord to the brain (although it follows a different pathway that touch does), where the conscious perception of pain is generated. While almost all brain functions involve a wide area of brain regions, pain and its essential role in life has led it to become an almost whole-brain experience. This diffuse nature of pain perception also helps explain phenomenon such as why emotional pain can feel just as sharp as physical pain. This attribute of pain also is key for the remarkable ability for higher mental faculties to block out pain signals.