The somatosensory system is part of the sensory nervous system, which is a complex system of sensory neurons and neural pathways that respond to changes on the surface or in the body. The axons (as afferent nerve fibers) of sensory neurons connect with or respond to various receptor cells. These sensory receptor cells are activated by different stimuli (such as heat and nociception) and give functions to the responding sensory neurons (such as thermoreceptors that carry information about temperature changes). Other types include mechanoreceptors, chemoreceptors, and nociceptors, which send signals along the sensory nerves to the spinal cord, where they can be processed by other sensory neurons and then passed to the brain for further processing. Sensory receptors are distributed throughout the body, including skin, epithelial tissue, muscles, bones and joints, internal organs and cardiovascular system.
All incoming touch/vibration information rises through the spinal cord via the posterior (dorsal) column-medial lemma pathway. The somatosensory pathway usually has three neurons: first-order, second-order, and third-order.
First-order neurons are a type of pseudo-unipolar neurons, and their cell bodies are always located in the dorsal root ganglia of spinal nerves. If the somatosensory pathway is located in a part of the head and neck, and the cervical nerves are not covered, the primary neurons will be the trigeminal ganglia or other sensory cranial nerve ganglia.
The cell bodies of second-order neurons are located in the spinal cord or brain stem. The ascending axon of this neuron will cross (disappear) to the opposite side in the spinal cord or brain stem.
In the case of touch and certain types of pain, the cell bodies of third-order neurons are located in the ventral posterior nucleus of the thalamus and terminate in the central posterior parietal gyrus of the primary somatosensory cortex (or S1).
Fig.1 CNS processing of sensory information.
Somatosensory defects may be caused by peripheral neuropathy involving peripheral nerves in the somatosensory system. This may manifest as numbness or paresthesias.
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