Brain and cognitive sciences: Touch

Brain and Cognitive Sciences: Touch
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Abstract
The sensory modality of touch plays an extremely important role in the human way of living and has a wide variety of functions. Many different neurons combine to form a pathway that transmits information received from the touch receptors to the central nervous system. Though all sensory modalities function in cohesion in the human body, the modality of touch can become heightened and work as a substitute in situations where any other sensory modality is impaired. Scientists working in the field of haptics are trying to use this capability to help visually impaired people and develop devices that can replicate the human touch pathways and work in situations where a human being is unable to function.
Keywords: Touch, sensory modalities, haptics.
“Touch has a memory.” These four words so poignantly penned down by John Keats describe the basic essence of what touch means in the context of human life. Studies show that the touch modality probably evolved as a means of protection against dangerous stimuli and helped in increasing the chances of survival of a species or organism in a hostile environment. It has been seen that the human embryo begins to perceive tactile stimulation as early as the eighth week of gestation.
This modality serves various functions; in fact we rely on touch for almost all of our daily activities beginning from social interaction, protecting our body from harm, sexual behavior, child learning and manipulation of objects.

It is quite impossible to imagine our daily existence without touch. Research has also shown that the communication between an infant and the person who takes care of the infant relies heavily on touch, this sensation can convey the emotions of the caregiver to the infant (Hertenstein, 2002).
This part of the essay will try to explain the entire touch pathway via which information travels from the skin that has the distinction of being the largest sensory organ in the human body and finally reaches the brain.
Before entering into the complex web of sensory pathways let’s first understand the different types of sensory receptors present on our skin. The diagram given below is an illustration of different types of sensory fibers presents on the skin.

Figure 1: Sensory receptors present on the skin
Free nerve endings: – These nerve endings have a large variety of functions, the nerve endings that are sensitive to touch and are specifically tailored to perceive touches are known as mechanoreceptors. The ones sensitive for pain have been grouped as nociceptors and those sensitive for temperature have been classified in a separate group called thermoreceptors.
Pacinian corpuscles: – These receptors are sensitive to stimulus related to vibration and pressure sensations.
Merkel’s discs: – These receptors play an important role in localization of touch.
Meissner’s discs: – They also play a very important role in the localization of touch and tactile discrimination. They have a high sensitivity to touch stimuli.
When the receptors described above are stimulated, then the information is carried to the brain via different pathways. The various neurons involved in the touch pathway are described in the next section of the paper: –
First order neuron: -The cell body in dorsal root ganglion of the spinal nerve forms the first order neuron of the touch pathway.
Researchers have postulated that this neuron has a shape that resembles the letter ‘T’. The axon divides in two branches one of which goes into the dorsal root of the spinal cord and the other goes to the part of the body that receives innervations from this particular nerve.
Second order neuron: – The cell body located in either the spinal cord or brainstem constitutes the second-order neuron in this pathway. The ascending axons of the second order neuron cross to the opposite side or decussate. The axons then travel to the thalamus, reticular system or the cerebellum.
Third order neuron: – The cell body of the third order neuron is located in the ventral posterolateral nucleus of the thalamus (Abraira, 2013).
Finally, the axons of the third order neurons are connected to the somatosensory cortex, or it can be said that they are connected specifically to the post central gyrus of the parietal lobe. In the brain, the primary somatic sensory cortex is designated as S1, which can be divided into four different regions – Brodmann areas (3a, 3b, 1, and 2).
The representation of different parts of the body on the cortex differs according to the importance of that area, for example, hands have a larger area designated for them in the cortex whereas back has been assigned a relatively smaller area.
It must also be kept in mind that fine touch usually travels via posterior column – medial lemniscus pathway whereas the spinothalamic pathway helps in the conduction of crude touch. In case of the medial lemniscus pathway, the first order neurons have to travel through the entire length of the spinal cord and are then able to synapse with the second order neurons in the medulla. They are then divided in two different groups namely the fasciculus gracilis that terminates in nucleus gracilis and carries sensations from structures below the thorax whereas the nucleus cuneatus carries sensations from above the mid thoracic level and finally ends in the nucleus cuneatus. This pathway also carries sensations of pressure and vibration along with the fine touch. On the other hand, the anterior spinothalamic tract carries the sensation of crude touch, pressure, itching, and tickling. In this tract, the first order neurons can form synapses with second order neurons after traveling for eight or ten spinal segments.
Another point to be noted is that the entire pathway also consists of a large number of synapses in the thalamus or reticular formation before it finally ends in the cortex.
Figure 2: Diagrammatic representation of motor and sensory cortex of the human brain.
Figure 3: Diagrammatic representation of the touch pathway.
The mystery of the touch mechanism has confounded scientists worldwide, and efforts to understand how this sensation works in the human body has inspired scientists to perform path breaking research in this field. Some discoveries related to this field have been awarded the prestigious Nobel Prize. In 1906, Camillo Golgi and Santiago Raman Cajal were awarded the Nobel Prize in Physiology or Medicine for their pioneering work in developing methods that helped in studying the intricate anatomy of the central nervous system. They developed methods to color and stain the different parts of the nervous system. Although, both differed in their perception of the structure of the nervous system, Cajal’s theory of perceiving the brain and spinal cord with many individual functional units later named as neurons played an extremely important role in studying the pathways via which information was relayed to different cells and between neurons (URL 1).
In 1944, Joseph Erlanger and Herbert Gasser were awarded the Nobel Prize in Physiology or Medicine for their path-breaking research on discovering the highly differentiated function of single nerve fibers. These scientists used the cathode ray oscilloscope and employed an amplification device to amplify even the smallest impulse sensed by a nerve fiber to a magnitude million times greater than the original stimulus. This amplification made the activity of the nerve fiber so prominent that it could be visualized as characteristic waves on a screen. Erlanger and Gasser stimulated different nerves with stimuli of similar types and intensities, but they found that the different nerves produced a different pattern of waves even though the stimulus presented to them was the same. It was also seen that the speed of conduction in a nerve fiber was dependent on the thickness of that particular fiber. So, it was postulated that different sensations are carried by different nerves at different speeds depending on the need for them to be processed in the brain. The sensation of touch is carried by nerve fibers with a large diameter and impulses are transmitted at a fast speed since any touch stimulus has to be quickly processed by the brain to determine whether it is harmful or not. On the other hand, the nerve fibers that carry pain sensations especially chronic pain stimulus are fibers that are thin and the impulses are transmitted slowly through them (URL 1)
The sensory modality of touch is less glamorous than other sensory modalities of sight with its vivid perception of colors and images, hearing with its appreciation of music and noise, touch with its ability to gain pleasure from new and different foods and smell. Touch almost seems an afterthought added by creation, but an in-depth analysis shows that the sensation of touch not only has a protective role in the human body, it also has other important functions. Skin acts as a barrier to all the foreign objects, noxious stimuli that the environment constantly bombards us. Without the sensation of touch, it would be impossible to register the presence of any stimuli. Evolution has thus assigned it a large area of the brain. Studies have also shown that touch has an important role in learning and memory. This sensory modality probably travels the longest distance to reach the brain from its sensory receptors as compared to other sensations.
Scientists have spent endless hours trying to simulate the touch pathway in the human brain and use this modality for various purposes. Various new technologies have been invented and utilized in the field of electronics, mobile telephones, video games, medicine, and robotics. This technology has the potential to help visually impaired people by helping them to visualize images and patterns via using touch.
Many breakthrough inventions have been developed in the effort of replicating the touch pathways in the human body. Just like computer graphics have opened up new dimensions for the sensory modality of vision the technology related to haptics has opened up a new chapter in the study of touch and its pathways.
One such invention is the shadow hand that was developed by Richard Greenhill and his team while working on the Shadow project financed by the Shadow Robot Company. The shadow hand is actually a robotic hand that is based on the same concept of transmission of information by sensory receptors as in the human body and utilizes sensations of touch, pressure and position to reproduce the strength, finesse and range of movements associated with the human hand. This device has been used by the Carnegie Melon University, Bielefeld University, and NASA. The device is built of twenty-four different joints and has an astonishing twenty degrees of freedom; the hand is slightly wider than the human hand especially in the forearm region. The hand is designed in a way that it resembles the human hand with four fingers and one thumb. The joints present in the fingers connect the so-called middle, distal and proximal phalanx, and there is one main joint that forms the link between the finger and the metacarpal. There is a presence of two joints on the wrist which can perform movements like flexion, extension, adduction and abduction. Sensors are fitted in every joint, and they provide information just like the touch pathways. Pressure sensors are present in the muscle hand, and force sensors are fitted in the motor hand (Reichel, 2004).
In a nutshell, it can be said that the sensation of touch is the basis of the life as we know it. The various receptors and the entire touch pathway have all developed in a manner that they can provide the necessary sensory inputs at a rapid rate. Research in the field of touch has led to the development of many revolutionary ideas, and further research holds great promise.
References
Hertenstein, M. J. (2002). Touch: Its communicative functions in infancy. Human
Development.
Abraira, V. E., & Ginty, D. D. (2013). The sensory neurons of touch. Neuron.
Marco Reichel. (2004). Transformation of Shadow Dextrous Hand and Shadow Finger Test Unit from Prototype to Product for Intelligent Manipulation and Grasping.
The Shadow Robot Company, Intelligent Manipulation and Grasping, International Conference. Retrieved from: www.nobelprize.org . All nobel prizes in physiology.