Divyan Bavan
Introduction
Mobile phones have provided a simple interface for providing communication, information, and entertainment. These all rely on stimuli being processed by the brain. Many systems work together to understand this interface, integrating what we see, think, and feel. The latter is controlled by the somatosensory system. This network of mechanoreceptors, nerve fibres, and specialized areas of the brain work together to process the tactile information from a mobile phone. Through understanding these components of the somatosensory pathway, it is clear how we can interact with our mobile phones daily.
Mechanoreceptors
To receive tactile information from the environment, our bodies have developed several mechanoreceptors capable of detecting these stimuli. They are found at the endings of sensory neurons, directly connected to the axon. However, they are not evenly distributed across the body; mechanoreceptors are found more densely in glabrous skin. This type of skin does not have hair follicles and includes the palms and fingertips. These areas directly interface with a mobile phone, facilitating the fine, discriminatory processing of tactile information from the device. There are four main receptors which are responsible for this: Meissner corpuscles, Pacinian corpuscles, Merkel discs, and Ruffini endings (Koeppen and Stanton, 2018). These receptors are classified as low-threshold mechanoreceptors.
| Receptor | Rapidly/Slowly Adapting | Type 1/2 | Detects (Iheanacho et al., 2023) |
|---|---|---|---|
| Meissner corpuscle | Rapidly adapting | 1 | Onset and offset of motion on fingertips |
| Pacinian corpuscle | Rapidly adapting | 2 | Vibrations |
| Merkel discs | Slowly adapting | 1 | Fine spatial detail, edges |
| Ruffini endings | Slowly adapting | 2 | Finger posture, movement |
As shown in the table above there are two differentiators which split these four mechanoreceptors: rate of adaptation and type. If a receptor fires action potentials only during the onset or offset of a stimulus, it is rapidly adapting. However, if it fires during the entire duration of the stimulus, it is said to be slowly adapting. While adaptation explains the temporal reception of a mechanoreceptor, type explains the spatial reception. Type 1 receptors are closer to the epidermis, which means their receptive field is smaller and more defined. By contrast, type 2 receptors are buried deeper within the skin, making their receptive fields larger and less defined (Koeppen and Stanton, 2018).
Our understanding of these traits came from various experiments. One of these was single-unit recording: a process which measures the output from individual afferent nerve fibres. This method was introduced by Adrian and Zotterman to study the firing patterns of different mechanoreceptors. This gave rise to our understanding of the rapidly and slowly adapting receptors (Adrian, 1926). A similar experiment was done by Johansson and Vallbo to map the different receptive fields of each mechanoreceptor. They used a combination of single-unit recording and controlled skin stimuli to measure these traits, identifying type 1 and type 2 receptors (Johannson and Vallbo, 1983).
By understanding these differences between mechanoreceptors, it is easier to understand how they respond to stimuli from mobile phones. For example, Pacinian corpuscles can detect vibrations from a phone. Similarly, Merkel discs can detect when our hand has left the screen through the phone’s edge. Meissner corpuscles enable us to understand that our hand is swiping. Finally, Ruffini endings help us orient our fingers to the phone by detecting their positions. It is through these mechanoreceptors that we can receive all this tactile information from our mobile phones.
Mechanotransduction
After a stimulus is received by a mechanoreceptor, it must relay this information to the central nervous system. This is achieved through the generation of action potentials. For an action potential to be generated in a neuron, its membrane must be depolarized through the opening of gated cation channels. This is no different in mechanoreceptors. These constituents possess channels such as Piezo2, a mechanically gated sodium channel. Piezo2 opens in response to mechanical stretch, allowing sodium ions into the receptor (Coste et al., 2010). This depolarizes the membrane. If the membrane passes the threshold potential, it will generate an action potential at the first node of Ranvier.
The sensory neurons’ axons have large diameters and are myelinated, so they are classified as Aβ fibres. This means that signals from mechanoreceptors will propagate very rapidly: around 35-75m/s. The action potentials travel along the digital nerves, which then cluster into the median and ulnar nerves in the arm. Eventually, they enter the dorsal root ganglia (DRG). This is where the cell bodies of the sensory neurons sit. The DRG for the hand and fingertips are primarily located within the C6 to C8 section of the spinal cord. As signals pass through the dorsal root, they reach the dorsal column within the spinal cord (Koeppen and Stanton, 2018).
The Dorsal Column-Medial Lemniscus (DCML) System
The DCML system is critical for the propagation of sensory information from the afferent neurons to the brain. It is an efficient system which utilizes several structures, the first of which is the dorsal column.
The dorsal column is positioned within the spinal cord to allow vertical movement of signals to the brain. As action potentials from the dorsal root ganglia reach the column, the central axon bifurcates. Some of the branches synapse directly within the dorsal horn, facilitating spinal reflexes. However, most of the central axon instead travels upward. This movement is deemed ipsilateral, as it occurs on the same side of the body as the stimulus. As the action potential propagates further along the fibre, it forms the fasciculus cutaneous, which travels up the dorsal column. Eventually these axons terminate in the brain stem, specifically in the medulla oblongata (Koeppen and Stanton, 2018; Al-Chalabi et al., 2023).
The neurons with which these fibres synapse with are termed second-order neurons, as they are the second neurons in the DCML pathway. The synapses are found within the caudal medulla, and form a region called the nucleus cutaneous. The purpose of the dorsal column neurons is to enrich the somatosensory message from the primary neurons. For example, dorsal column neurons can integrate signals from several types of sensory neurons. This facilitates the integration of these specific signals into a more general one. This is important for creating efficient perception of tactile information. As the dorsal column neurons integrate the information and fire action potentials, the axons leave the medulla and decussate. These axons are thus referred to as the internal arcuate fibres. As soon as the fibres cross the midline of the brain, however, they form the medial lemniscus. This crossing explains why the right side of the brain perceives sensation from the left side of the body and vice versa (Koeppen and Stanton, 2018).
Once the medial lemniscus reaches the thalamus, the fibres synapse with the third-order neurons in the ventral posterior lateral (VPL) nucleus (Koeppen and Stanton, 2017). This cluster maintains the somatosensory image (Sheriden et al., 2023). This is important for the proper integration in the somatosensory cortex: the main processing unit for sensory information. It is located within the parietal lobe of the cerebral cortex, just behind the central sulcus. It contains many regions, but the most important regions for somatosensation are the primary (S1) and secondary (S2) somatosensory cortex. Neurons from the VPL synapse onto S1, interacting with specific areas (Koeppen and Stanton, 2017). For cutaneous input, this is the 3b region. This region receives the sensory information, but it is S2 that stores, processes, and retains the information. It is connected to the hippocampus and amygdala, allowing it to integrate past experiences into perceiving the sensory information. In the case of the mobile phone, S1 would map the characteristics of the interaction, while S2 would process and integrate it with other information (Raju et al., 2022).
Conclusion
This pathway illustrates the intricacy behind using a mobile phone. Although the process feels instantaneous and simple, several systems are constantly at work sending signals to the brain. This includes the mechanoreceptors, DCML system, and somatosensory cortex. Stimuli must trigger action potentials in the diverse receptors, before being rapidly propagated along the defined pathway. Despite the large number of sensory neurons across the body, this pathway is constrained to just three neurons. This creates a highly efficient “motorway” for sensory information to travel through. The result of this is rapid transmission of signals to the somatosensory cortex, where they are stored or integrated into responses. This whole timeline of events takes just a few milliseconds. It shows how efficient the human nervous system is at executing complex tasks, and it is what allows us to use mobile phones so seamlessly.
References
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