Of all five major senses, tactile perception is the least understood. Some two dozen unique types of nerve endings pass through the skin of mammals. How and what they detect, how they code and transmit signals to the brain, and how those signals are decoded and used are still only poorly understood.
How does a three-foot long cell, such as one of the neurons that run from your big toe to the ganglial clusters near your spine, tell you nearly instantly that a mosquito is about to bite the tip of that toe, or that a slight, warm breeze just blew in through a crack in your door?
One way — that we are just coming to find out about — is through arrayed antennae, seemingly dead cellular structures (keratin proteins) that we have long been calling, regardless of cell type or location, simply, “hair.” In actuality, “hair” is a complex matrix of distinct cooperating structures that include the part under the skin — the follicle — which is an organ that contains stem cells that can be tasked to make either more follicles or dermal cells, such as regrowing skin after a wound; the shaft (see cross-section); and an outer coating of sensory cells. We have been growing these structures since we were lizards.
Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful — elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and most other cetaceans, and naked mole rats — are effectively hairless.
It seems that during the Vietnam War special forces in the War Department had sent undercover experts to comb American Indian Reservations looking for talented scouts, for tough young men trained to move stealthily through rough terrain.
They were especially looking for men with outstanding, almost supernatural, tracking abilities. Before being approached, these carefully selected men were extensively documented as experts in tracking and survival.
Bill and Hillary
Serious causalities and failures of performance led the government to contract expensive testing of these recruits, and this is what was found.
When questioned about their failure to perform as expected, the older recruits replied consistently that when they received their required military haircuts, they could no longer 'sense' the enemy, they could no longer access a 'sixth sense', their 'intuition' no longer was reliable, they couldn't 'read' subtle signs as well or access subtle extrasensory information.
So the testing institute recruited more Indian trackers, let them keep their long hair, and tested them in multiple areas. Then they would pair two men together who had received the same scores on all the tests. They would let one man in the pair keep his hair long, and gave the other man a military haircut. Then the two men retook the tests.
Regrettably there is scant evidence offered to back up these claims, so they remain in the realm of internet fable. A search of Google science comes up with a dearth of research on sensory perception and hair. The central grain of truth, however, is that hair is an extension of the nervous system. Whether it actually transmits important information to the brain remains an open question, but some studies of our mammalian relatives raise intriguing possibilities.
Batman: The Beginning
Bats are the only mammals capable of powered flight, and they perform impressive aerial maneuvers like tight turns, hovering, and rapid acceleration and deceleration. They fly in nearly complete darkness employing sonar navigation and can fly through areas of high turbulence with astonishing stability. Their wing, which is a bony webbed-hand with five fingers joined by a membrane, is covered with stiff, domed micro-hairs. When scientists began looking at the motor control and aerodynamic feedback employed by bats, using wind tunnels, they discovered just how important their hair was.
The researchers noted that bats are not unique in these attributes, saying “in mammals, tactile hairs are usually located on the head in the form of sinus hairs (e.g., whiskers) … Sinus hairs on other body parts have been described only for the manatee. The authors suggested that the sparsely distributed hairs on the manatee’s body could form a lateral line system analogous to that of fish and amphibians, where it serves to detect water currents surrounding the animal’s body.”
The bat was somewhat unique among mammals in that it grew its hairs on its “fingertips” whereas in primates, rodents and other non-hoofed mammals, fingers and toes are hairless.
Consider your ear
The bones of the middle ear are connected to the inner ear, or cochlea where the auditory nerve is stimulated. As can be seen in the drawing, the cochlea is a tightly coiled portion of the ear, like a snail’s shell. It is liquid-filled, but projecting up into it are very tiny hair cells that may be stimulated by activity of the liquid within which they are set and the basilar membrane on which they are mounted. Most physiologists have assumed that, since hair is “dead” keratin, it is the bending of these hair cells that fires the auditory nerve. Maybe, maybe not. The effect is to translate a sound vibration transmitted through air or fluids into an electro-chemical impulse headed for your brain. As the number and rate of auditory nerve cells firing increase, perceived loudness increases. Different cells fire to reflect difference in frequency; the combination of cells firing determines the perceived timbre.
|Electromagnet imaging of brainwaves|
|Bob Marley's dreads|
So, the big question is, what do we sense with our hair? Can we detect others nearby in darkness, or beyond our range of view? Can we sense more than we think we can? Should we be cutting it off, or should we be letting it grow?
Susanne Sterbing-D’Angelo, Mohit Chadha, Chen Chiu, Ben Falk, Wei Xian, Janna Barcelo, John M. Zook, and Cynthia F. Moss, Bat wing sensors support flight control, PNAS Early Edition (2011) www.pnas.org/cgi/doi/10.1073/pnas.1018740108
Antonio Damasio, The quest to understand consciousness, TED 2011, Long Beach, CA (Feb 2011)