6 Skin, The Body’s Largest Organ

6.1 Human Skin on the Molecular Scale

All sensors, biological or technological, contain several elements: the sensing element

that senses the signal, the transducer that transfers the signal, and an amplification

and/or analysis/reporting element that increases the signal and/or analyzes it. Skin is

the largest and most complex organ in the body. It has a variety of sensor functions,

each with a transducer that transfers and possibly amplifies the signal. These signals

are then analyzed and identified in the brain.

The main sensors in skin are touch (mechanical forces), and heat/cold (tempera-

ture). Additionally, there are sensors for chemical stimuli. For all sensors, there are also

pain receptors that might be the same or a different set of receptors. Both are connected

to automatic actions of the body: avoidance, in the case of extreme heat or cold and

chemicals; but also, the active temperature and fluid/electrolyte regulation of the body

that skin is mostly responsible for—in the case of pressure, whatever action needed to

release the pressure/avoid the pain. All of these actions are part of the skin’s response

to the outside environment, which skin is also responsible for, since it is closest to the

outside environment.

One of the biggest other functions of skin, besides sensing and body regulation, is

protection. Skin contains the organs so that they can function under precise conditions,

called homeostasis. At the same time, skin protects from injury (including temperature

and pressure), but it also protects from bacteria and viruses that could lead to disease.

When injury does occur (not everything can be prevented), skin starts a self-healing

process to contain and then heal the injury. Skin also has additional functions such as

Vitamin D production.

All of these functions are contained in a relatively thin, two-layer membrane [1]

(Figure 6.1). The structure is rather complex, but optimized for the combination of all

the functions. This chapter will talk about the molecular structure of the senses in the

skin first, then describe briefly the body regulation system in skin, the healing process,

and the structure of skin that makes all of these functions possible.

Temperature is sensed by afferent neurons in the peripheral nervous system, i. e.,

the neurons that bring sensory information from the outside (skin) to the brain and

spinal cord. There is only limited information on the neurons that sense temperature.

It was originally assumed that the nerve endings for temperature sensing had to be dif-

ferent, but research shows that the difference mostly lies in the type of ion channel [2].

One type of ion channel that was identified and researched is the family of transient

receptor potential ion channels. First found in the drosophila fly, by now four channels

for heat and two for cold were found in humans (Figure 6.2). As the graph suggests, not

only does each receptor have a specific temperature range it reacts to, each also reacts to

specific chemicals. The chemicals are generally the ones that leave either a cooling or a

burning sensation on the skin, which makes sense since they are primarily temperature

https://doi.org/10.1515/9783110779196-006