Fingertip Sensory Nerve Endings
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The human hand contains a high density of several specialized nerve endings. These nerve endings in concert with anatomical adaptations of the hand (muscular and bone) allow for enhanced tool use.
There are several sensory nerve endings and receptors in the human hand. Rapidly-adapting nerve receptors deliver information on changes that occur at the skin, such as occurs as you move your finger over a gritty surface. Slowly-adapting receptors deliver information regarding prolonged touch, such as occurs when you are grasping a tool for an extended period of time. The receptors also differ in the size of their receptive fields. Smaller receptive fields contribute to acuity. For example, the receptive field size of sensory nerves on the human back is much larger than on the fingertip. If two needles spaced a millimeter apart are touched to a large receptive field, both points will lie within the receptive field and will be encoded as a single point. If the needles are touched to two smaller receptive fields, such that each needle point contacts a separate receptive field, the nervous system can distinguish these as two separate points. Sensory nerve endings in the fingers include Meisnner’s corpuscles (light touch, rapidly adapting) , Pacinian corpuscles (large receptive fields, rapidly adapting), free nerve endings (temperature and pain), Merkel discs (light touch discrimination of objects and texture), and Ruffini endings (slowly adapting, finger position information). Combined, these receptors allow for the sense of touch and discrimination of shapes, textures, and objects, are resonsible for the sensations of heat and pain, and give information on where the hand and fingers are in relation to the body (i.e. is my hand flat or in a fist? Am I holding a bat or a ball?). There are also sensory nerves encircling hair follicles that give information on when the hair is displaced. However, it is the sensory nerve endings on glabrous (non-hairy) skin that likely contribute to the proprioceptive and tactile control necessary for fine touch discrimination and tool use.
Observations of human specific differences in sensory nerve endings, or the cortical areas dedicated to the representation of the fingers, are pre-1965. Napier (1965) noted an expanded representation of the hand and fingers in the human sensory and motor cortex. In the comparison of many primates, including, gorilla, orangutan, and chimpanzee, it was found that only humans had complex, lobulated Meissner corpuscles. Meissner-like sensory receptors were identified in other receptors that were not as specialized. More recent studies exploring this phenomenon find similar receptor types and cortical representations between humans and primates. Human specific differences that mediate enhanced fine motor control might instead rely more heavily on muscular and bone differences rather than differences in sensory nerve receptors.
Selection for enhanced sensory nerve endings is likely insofar as those sensory nerves contribute and improve the ability to use tools, hunt, obtain meat, scavenge, protect offspring, and otherwise aid in acquiring a mate. The ability to throw and club effectively contributes to all of the above pursuits and likely involves proprioceptive and sensory feedback from sensory nerve endings. The exact mechanism that promotes greater sensory nerve density and/or integration of the sensory information to promote enhanced tool use is not clear.
Enhanced tactile sensory information in the hand likely contributes to the ability to wield tools. Insofar as sensory nerve endings contribute to the precision and the power grips, sensory nerve endings likely played an important role in the development of tool use, and will continue to contribute to the fine motor control necessary for the operation of current technology. Indeed, fine motor control is vitally important in many modern tasks from writing to complex surgical procedures. It remains to be seen whether a lack of sensory nerve fibers in the hand of non-human hominids contributes to the lack of tool use in those species. Understanding the contributions of sensory nerve endings also have application for medical conditions that affect touch and tool use, such as diabetes, fibromyalgia, and peripheral neuropathy.
Meisner-like and Pacinian corpuscle were identified in the lowland gorilla
Human bipedal instability in tree canopy environments is reduced by “light touch” fingertip support, , Nature Scientific Reports, 2017//, Volume 7, Issue 1, p.1135, (2017)
Merkel cells transduce and encode tactile stimuli to drive Aβ-afferent impulses., , Cell, 2014 Apr 24, Volume 157, Issue 3, p.664-75, (2014)
Evolution of the human hand: the role of throwing and clubbing., , J Anat, 2003 Jan, Volume 202, Issue 1, p.165-74, (2003)
Evolution of the Human Hand, , Proc Royal Inst Great Britain, Volume 40, p.544-557, (1965)
Nerve endings in the skin of the gorilla., , J Comp Neurol, 1961 Apr, Volume 116, p.145-55, (1961)