Hand Size Not Sex Determines Sense of Touch

Sometimes a difference between the sexes is not based on sex at all. Women have a finer sense of touch than men do, but a new study shows that this is simply because their fingertips tend to be smaller. Neuroscientist Daniel Goldreich of McMaster University in Hamilton, Canada, and his colleagues first became curious about the sex difference while studying differences between blind and sighted people. They found that blind people are better than those with normal vision at distinguishing fine textures but that, within each group, women are better than men. The researchers thought that the discrepancy might be the result of brain differences between men and women, but they first wanted to see if something simpler could explain it. So they tested 50 women and 50 men on a simple task: Each person touched a small, grooved surface and tried to identify the orientation of the grooves. As the grooves got closer together, it became more difficult to determine their direction. As expected, women performed better at this task than men did,but when the scientists looked at the results by finger size,

they found that the sex difference disappeared: On average, men and women with the same size fingertips perform at the same level, the team reports in the 16 December issue of The Journal of Neuroscience. (Finger size does not explain all individual variability, however; there are differences between people with the same size fingers, perhaps as a result of differences in the mechanical properties of skin or in how each person’s brain processes the information.) The researchers also came up with a potential explanation for the size effect. Cells in the finger called Merkel cells appear to transmit this type of touch information to the brain. Goldreich and his co-workers measured the number of Merkel cells in their subjects’ fingertips and found that everyone had about the same number, regardless of finger size. They suspect that this explains the effect: Merkel cells are spaced more densely in smaller hands, giving those hands the ability to distinguish finer textures.  The paper is “very solid and convincing”  regarding the effect of fingertip size on this specific task,

says neuroscientist François Tremblay of the University of Ottawa in Canada. However, other types of tactile tasks may not work the same way, he adds. For example, passively pressing the skin against a textured object–as the study participants did–involves different neuronal pathways than actively moving the fingers around an object and may be controlled differently. news from sciencenow.sciencemag.org  –  The somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch is considered one of the five traditional senses, the impression of touch is formed from several modalities; In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved.

The system reacts to diverse stimuli using different receptors: thermoreceptors, mechanoreceptors and chemoreceptors. Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex. At its simplest, the system works when a sensory neuron is triggered by a specific stimulus such as heat; this neuron passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location. The mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image. The somatosensory system is spread through all major parts of a mammal’s body (and other vertebrates). It consists both of sensory receptors and sensory (afferent) neurones in the periphery (skin, muscle and organs for example), to deeper neurones within the central nervous system. General somatosensory pathway. A somatosensory pathway will typically have three long neurons: primary, secondary and tertiary (or first, second, and third).  * The first neuron always has its cell body in the dorsal root ganglion of the spinal nerve

(if sensation is in head or neck, it will be the trigeminal nerve ganglia or the ganglia of other sensory cranial nerves). * The second neuron has its cell body either in the spinal cord or in the brainstem. This neuron’s ascending axons will cross (decussate) to the opposite side either in the spinal cord or in the brainstem. The axons of many of these neurones terminate in the thalamus (for example the ventral posterior nucleus, VPN), others terminate in the reticular system or the cerebellum.  * In the case of touch and certain types of pain, the third neuron has its cell body in the VPN of the thalamus and ends in the postcentral gyrus of the parietal lobe. In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g. by mechanoreceptors for tactile sensation and nociceptors for pain sensation. The sensory information (touch, pain, temperature etc.,) is then conveyed to the central nervous system by afferent neurones. There are a number of different types of afferent neurones which vary in their size,  structure and properties.

Generally there is a correlation between the type of sensory modality detected and the type of afferent neurone involved. So for example slow, thin unmyelinated neurones conduct pain whereas faster, thicker, myelinated neurones conduct casual touch. n the spinal cord, the somatosensory system  includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurones of the Dorsal Column Medial Lemniscal pathway and the Ventral Spinothalamic pathway. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture are projected to the cerebellum. These include the ventral and dorsal spinocerebellar tracts. Another important target for afferent somatosensory neurones which enter the spinal cord are those neurones involved with local segmental reflexes. The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe.  The postcentral gyrus is the location of the primary somatosensory area,

the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus at this location. For the primary somatosensory cortex, this is called the sensory homunculus. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Interestingly, one study showed somatosensory cortex was found to be 21% thicker in 24 migraine sufferers, on average than in 12 controls, although we do not yet know what the significance of this is. Somatosensory information involved with proprioception and posture also targets an entirely different part of the brain, the cerebellum.Touch deprivation is when someone experiences an excessive lack in the sense of touch, often during the development in infancy,affecting the wellness of a person. Touch deprivation in infants leads to many different issues later in life. It affects the behavioral, health and physiological development of a human. With only minimal research in the field of touch deprivation, there is only a short history of the research and effects. Touch, before research conducted,

was seen as only a minor impact of the development of a person. But according to an article by Robert Hatfield, Ph.D. from the University of Cincinnati, in 1945-1947, that view began to fall apart. Premature infants and sick toddlers were dying unexpectedly, so Dr. Rene Spitz, the caregiver, searched for explanation to the deaths. Not until 1958-1962, in Harry Harlow’s research, was the mystery solved (Hatfield). The children were not being provided enough touch. Through Harlow’s research with monkey infants he was able to discover the great importance of touch. Research following by John Bowlby and Mary Salter Ainsworth, confirmed the impact touch has on the attachment theory (Hatfield).  Robert Hatfield discusses the results of these experiments and stated, “Affectionate touch vs. neglect or punishing touch is a central theme of Attachment Theory and much of this work may be viewed as the human research counterpart to the Harlow studies” (Hatfield). Initiation of probably all “somatosensation” begins with activation of some sort of physical “receptor”.

These somatosensory receptors tend to lie in skin, organs or muscle. The structure of these receptors is broadly similar in all cases, consisting of either a “free nerve ending” or a nerve ending embedded in a specialised capsule. They can be activated by movement (mechanoreceptor), pressure (mechanoreceptor), chemical (chemoreceptor) and/or temperature. Another activation is by vibrations generated as a finger scans across a surface. This is the means by which we can sense fine textures in which the spatial scale is less than 200 µm. Such vibrations are around 250 Hz, which is the optimal frequency sensitivity of Pacinian corpuscles. In each case, the general principle of activation is similar; the stimulus causes depolarisation of the nerve ending and then an action potential is initiated. This action potential then (usually) travels inward towards the spinal cord. The new research area of haptic technology can provide touch sensation in virtual and real environments. This new discipline has started to provide critical insights into touch capabilities.

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