Tactile sensation: how the sense of touch works

Free nerve endings

Free nerve endings are unencapsulated terminal parts of sensory neurons located in the dermis and epidermis. They detect touch, pressure, stretch, temperature and noxious stimuli. The mechanosensitive subset includes both slowly and rapidly adapting fibers, depending on the afferent type.

Meissner corpuscle

Meissner corpuscle is an elongated, encapsulated nerve ending that is located in the dermis. These receptors are highly sensitive to changes in movement, low frequency vibrations (10-50 Hz), light touch and can detect the movement of objects on the skin’s surface. Meissner corpuscles are abundant in the fingertips, lips, and other parts of the body where the ability to differentiate spatial location of tactile sensations is highly developed.

Merkel cell-neurite complex

The Merkel cell-neurite complex consists of a non-neuronal epithelial cell (the Merkel cell) in the basal epidermal layer, in close apposition to the terminal of an Aβ sensory afferent. The Merkel cell expresses PIEZO2 and depolarizes in response to mechanical stimuli, and the afferent itself is also mechanosensitive; both contribute to the slowly adapting response of the complex. Merkel cell-neurite complexes are abundant on the fingertips, lips and external genitalia, and detect sustained pressure and low-frequency vibrations (5–15 Hz).

Hair follicle afferents (lanceolate endings)

Hair follicle afferents form lanceolate endings that wrap the base of the hair shaft and respond to hair deflection. The Aβ subtypes are rapidly adapting and signal hair movement across a range of velocities. Hairy skin also contains Aδ and C-fiber low-threshold mechanoreceptors that respond to slow, gentle stroking.

Ruffini endings

Ruffini endings are multibranched, encapsulated nerve endings located in the deeper layers of the skin and in the joints. They are slowly adapting and signal continuous tissue deformation, stretch, and joint position, contributing to the kinesthetic sense.

Pacinian corpuscle

Pacinian corpuscle is the largest of the mechanoreceptors and is located both deep in the dermis and in the subcutaneous tissues. It is stimulated by rapid local compression, mechanical changes in the tissues, and very high-frequency vibrations (250-350 Hz). The tip of the nerve fiber is encapsulated and unmyelinated, while outside of the capsule, it becomes myelinated.

The skin contains five main types of low-threshold mechanoreceptor that compare across location, adaptation and tuning. The table below summarises the differences.

Slowly adapting or tonic receptors

Tonic receptors include the Merkel cell-neurite complex and Ruffini endings. They initiate action potentials for the duration of the stimulus, ensuring the brain is constantly aware of the body's status. They require many hours or even days to completely adapt.

Rapidly adapting or phasic receptors

Phasic receptors initiate an action potential at the onset of the stimulus and again when the stimulus ceases. Such receptors include the Meissner corpuscle, the Pacinian corpuscle and hair follicle afferents. They all adapt within approximately a second and they provide the brain with information about sensory stimuli that vary in intensity.

Dorsal column-medial lemniscus pathways

The dorsal column-medial lemniscus pathway has the ability to transmit sensory stimuli such as touch with precise localization and intensity, vibratory sensations and proprioception. After the signal enters the spinal cord, the axons of the first order neurons form ascending branches of dorsal root fibers, specifically the gracile fasciculus (input from vertebral level of T6 and below) and the cuneate fasciculus (input from vertebral level of T6 and above). The axons travel upwards towards the medulla oblongata and synapse in the gracile and cuneate nucleus accordingly. Then, they cross to the opposite side of the brainstem and continue to the thalamus through the medial lemniscus, where they synapse in the ventral posterolateral nucleus (VPL). The third-order neuron then transmits the signal to the somatosensory cortex. Sensory input from the face joins the medial lemniscus from the trigeminal system and synapses in the ventral posteromedial nucleus (VPM).

Anterolateral pathways/spinothalamic tract

The anterolateral system (or spinothalamic tract) transmits pain, thermal sensations and crude (non-discriminative) touch. Itch is conveyed by a partly distinct pruriceptive pathway that also ascends in the anterolateral system. After the axon enters the spinal cord, the afferent neuron synapses in the dorsal horns of the gray matter and then crosses immediately in the opposite side. Subsequently, the axons of the second-order neuron ascend to the thalamus through the anterior and lateral white columns (anterior and lateral spinothalamic tract). The second synapse occurs in the VPL nucleus of the thalamus with the third-order neuron, whose axon projects to the somatosensory cortex.

Somatosensory area I (S1)

Somatosensory area I corresponds to Brodmann’s areas 3, 1, 2, which are crucial for processing somatosensory signals. In this area, the axons of the third-order neurons are arranged according to the location of the stimulus. There is a very precise topographic organization, meaning that every part of the body is represented by a distinct area of the primary somatosensory cortex and exclusively by the opposite hemisphere. Some parts of the body, such as the lips, fingers, and face, are represented by larger areas as the sensations in these regions are more precise.

Somatosensory area II (S2)

Somatosensory area II is an association area for sensory input, located in the parietal operculum within the upper bank of the lateral sulcus. It corresponds to Brodmann area 43 and the adjacent operculum. The function of this area is to provide more detail to the information that is processed in the S1 area. It receives sensory input from both sides of the body and has numerous connections with the primary sensory areas, the thalamus, the posterior parietal cortex, the hippocampus, the amygdala, the motor cortex and other cortical areas. This results in the involvement of memory, emotional processing, consciousness and visuospatial processing, which all modify the final perception of the sensory stimuli.