Tactile

The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch is considered one of the five traditional senses, the impression of touch is formed from several modalities; In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved.
The system reacts to diverse stimuli using different receptors: thermoreceptors, mechanoreceptors and chemoreceptors.

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.
At its simplest, the system works when a sensory neuron is triggered by a specific stimulus such as heat; this neuron passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location.

The mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image.


Anatomy
The somatosensory system is spread through all major parts of a mammal's body (and other vertebrates). It consists both of sensory receptors and sensory (afferent) neurones in the periphery (skin, muscle and organs for example), to deeper neurones within the central nervous system.
General somatosensory pathway
A somatosensory pathway will typically have three long neurons: primary, secondary and tertiary (or first, second, and third).
The first neuron always has its cell body in the dorsal root ganglion of the spinal nerve (if sensation is in head or neck, it will be the trigeminal nerve ganglia or the ganglia of other sensory cranial nerves).
The second neuron has its cell body either in the spinal cord or in the brainstem.This neuron's ascending axons will cross (decussate) to the opposite side either in the spinal cord or in the brainstem. The axons of many of these neurones terminate in the thalamus (for example the ventral posterior nucleus, VPN), others terminate in the reticular system or the cerebellum.
In the case of touch and certain types of pain, the third neuron has its cell body in the VPN of the thalamus and ends in the postcentral gyrus of the parietal lobe.
Periphery
In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g.

The sensory information (touch, pain, temperature etc.,) is then conveyed to the central nervous system by afferent neurones. Generally there is a correlation between the type of sensory modality detected and the type of afferent neurone involved.

For example, slow, thin, unmyelinated neurones conduct pain whereas faster, thicker, myelinated neurones conduct casual touch.
Spinal cord
In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex.Other ascending pathways, particularly those involved with control of posture are projected to the cerebellum. Another important target for afferent somatosensory neurones which enter the spinal cord are those neurones involved with local segmental reflexes.
Brain
The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe.

The postcentral gyrus is the location of the primary somatosensory area, the main sensory receptive area for the sense of touch. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area.

Interestingly, one study showed somatosensory cortex was found to be 21% thicker in 24 migraine sufferers, on average than in 12 controls, although we do not yet know what the significance of this is. Somatosensory information involved with proprioception and posture also targets an entirely different part of the brain, the cerebellum.
Physiology
Initiation of "somatosensation" begins with activation of a physical "receptor".