The Somatosensory System

The somatosensory is the system of nerve cells that responds to changes to the external or internal state of the body, predominately through the sense of touch, but also by the senses of body position and movement.

Spread through all major parts of the body, it consists of sensory receptors and sensory neurons in the periphery (for example, skin, muscle, and organs), along with deeper neurons within the central nervous system.

While touch is considered one of the five traditional senses, the impression of touch is actually formed from several diverse stimuli using different receptors: 

• Thermoreceptors (temperature)
• Nociceptors (pain) 
• Mechanoreceptors (pressure)

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.

Thermoreceptors

Mammals have at least two types of sensors: those that detect heat and those that detect cold. 

Upon deviation from the norm ,sensory receptors trigger an action potential that can provide feedback or lead to alterations in behavior in order to maintain homoeostasis. Two receptors that exhibit the ability to detect changes in temperature include Krause end bulbs (cold) and Ruffini endings (heat).

Nociceptors

A nociceptor is a sensory nerve cell that responds to damaging or potentially damaging stimuli by sending signals to the spinal cord and brain. Nociceptors can respond to excessive thermal, mechanical, or chemical stimulation and often result the generation of an involuntary motor respons—for example, pulling  a hand away from a hot surface.

Mechanoreceptors

Mechanoreceptors are sensory receptors that respond to pressure and vibration. Four key types of mechanoreceptor have been described based on their response to stimulation and receptive field. 

Receptors can either induce a slow response to stimulation, whereby a constant activation is initiated, or a fast response, whereby activation is only initiated at the beginning and end of stimulation. The receptive field—the region in which a receptor can sense an effect—can vary from small to large.

• The Merkel receptor is a disk-shaped receptor located near the border between the epidermis and dermis. It demonstrates a slow response and has a small receptive field; it is useful for detecting steady pressure from small objects, such as when gripping something with the hand.
• The Meissner corpuscle is a stack of flattened cells located in the dermis, near the epidermis. It demonstrates a rapid response and has a small receptive field; it is useful for detecting texture or movement of objects against the skin.
• The Ruffini cylinder is located in the dermis and has many branched fibers inside a cylindrical capsule. It demonstrates a slow response and has a large receptive field; it is good for detecting steady pressure or stretching, such as during the movement of a joint.
• The Pacinian corpuscle is a layered, onion-like capsule surrounding a nerve fiber. It is located deep in the dermis, in the subcutaneous fat. It demonstrates a fast response and has a large receptive field; it is useful for detecting large changes in the environment, such as vibrations.

Together they provide a wide range of mechanical sensitivity that enables fine motor control.