From the Guidelines
Light touch sensation is transmitted to the brain through the dorsal column-medial lemniscus pathway, involving the primary somatosensory cortex (BA2) and supramarginal gyrus (BA40), as supported by recent meta-analytic evidence 1. The transmission of light touch sensation to the brain involves a complex neural pathway. When light touch stimulates specialized mechanoreceptors in the skin, such as Meissner's corpuscles and Merkel's discs, they convert this mechanical stimulus into electrical signals. These signals are then carried by large, myelinated A-beta sensory fibers through the peripheral nerves to the dorsal root ganglia, where the cell bodies of these neurons are located. From there, the signals ascend through the dorsal columns of the spinal cord without crossing to the opposite side. The signals eventually synapse with second-order neurons in the medulla oblongata, which then cross to the opposite side and form the medial lemniscus, ascending through the brainstem to the thalamus. In the thalamus, these fibers synapse with third-order neurons that project to the primary somatosensory cortex, where the sensation is consciously perceived, with recent studies highlighting the involvement of the primary somatosensory cortex (BA2) and supramarginal gyrus (BA40) in tactile processing 1. Key aspects of this pathway include:
- The role of specialized mechanoreceptors in the skin
- The involvement of large, myelinated A-beta sensory fibers
- The ascent of signals through the dorsal columns of the spinal cord
- The synapse with second-order neurons in the medulla oblongata and subsequent crossing to the opposite side
- The projection to the primary somatosensory cortex, where the sensation is consciously perceived, with specific involvement of areas such as BA2 and BA40, as supported by recent meta-analytic evidence 1.
From the Research
Mechanisms of Light Touch Sensation
The transmission of light touch sensation to the brain involves a complex process that starts with the activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs) 2, 3. These LTMRs are responsible for detecting innocuous touch and are found in various regions of the skin. The sensory information from LTMRs is then transmitted to the spinal cord and eventually to the brain, where it is processed and perceived as touch.
Role of Mechanosensory Neurons
Mechanosensory neurons, including LTMRs, play a crucial role in the transmission of light touch sensation 3. These neurons have unique tuning properties that allow them to respond to different types of mechanical stimuli. The activation of mechanosensory neurons leads to the generation of electrical signals that propagate to the central nervous system (CNS) via the axons of LTMRs.
Importance of PIEZO2
PIEZO2 has emerged as the principal mechanosensitive channel involved in light touch of the skin 3, 4. This channel is necessary for the transformation of mechanical energy into electrical signals in mechanoreceptors. Studies have shown that PIEZO2 is expressed in human digital skin, particularly in Merkel cells and Meissner's corpuscles, which are involved in fine and discriminative touch 4.
Innervation of Mechanoreceptors
The innervation of mechanoreceptors, such as Merkel cells and Meissner's corpuscles, is critical for the transmission of light touch sensation 5, 6. Studies have shown that the transplantation of embryonic sensory neurons can improve sensory nerve fiber number and innervation of Merkel cells and Meissner's corpuscles in peripheral nerves 5. Additionally, the absence of Meissner corpuscles in mice lacking functional TrkB suggests that TrkB-expressing sensory neurons are involved in the development and maintenance of rapidly adapting cutaneous mechanoreceptors 6.
Key Findings
- LTMRs are responsible for detecting innocuous touch and transmitting sensory information to the spinal cord and brain 2, 3.
- Mechanosensory neurons, including LTMRs, have unique tuning properties that allow them to respond to different types of mechanical stimuli 3.
- PIEZO2 is the principal mechanosensitive channel involved in light touch of the skin 3, 4.
- The innervation of mechanoreceptors, such as Merkel cells and Meissner's corpuscles, is critical for the transmission of light touch sensation 5, 6.