General Properties of the Skin
The skin is an organ, one of the largest organs in the body in terms of area, and it has a number of important functions. To perform all these functions, the skin has several layers with different properties.
The dermis and epidermis
The outermost layer of the skin is called the epidermis (epi means “on” or “above,” and dermis means “skin”). The epidermis is actually several layers of dead cell ghosts that provide mechanical protection from the outside. Because these cells are dead and have no pain receptors, dragging your fingernail lightly along your skin removes a layer or two of these dead cells but doesn’t hurt (much). Cells at the bottom of the epidermis, where it meets the dermis, are constantly dividing, migrating outward, and dying to replace the dead layers as they wear off.
Below the epidermis is the dermis, the living layer of the skin that includes the bulk of the somatosensory receptors.
Throughout the skin are a variety of receptors for touch, temperature, and pain. Skin receptors allow us to have both passive and active senses of touch:
- Passive sense of touch: The passive touch sense occurs when something brushes our skin and we register the fact of the touch before we know what has done the touching.
- Active sense of touch: The active sense of touch is mostly done with our hands and fingertips. We can hold an egg or an apple or a pinecone in our hands and know what we’re holding without looking. These different kinds of perceptions occur when different kinds of receptors in the skin are activated and the brain processes that information and compares it to memory.
The sense of touch, or somatosensory perception, for most of the body (below the head) is relayed through the spinal cord, to the thalamus, and then to a strip in the parietal lobe just posterior to the central sulcus where a “touch” map of the body exists. This area of the cortex is called the somatosensory cortex.
The epidermis has very few receptors. The bulk of the somatosensory receptors are within the dermis.