In the mammalian central nervous system, the somatosensory system is crucial to senses such as pain, touch, temperature, and spatial orientation (a sensation termed proprioception). The system consists of a network of neurons that work together to sense and then to process this information. The network of neurons will be described in three different divisions called first-order neurons, second-order neurons, and third-order neurons.
To begin with the sensing portion of the system, it involves a somatic (body) receptor that is within the skin, bones, muscles, joints, eyes, and ears. These specialized receptors will pick up sensory information and begin a sensory impulse. This sensory receptor is on a neuron that is considered the first-order neuron or primary afferent neuron for sensory information. This neuron comes from the periphery and travels the afferent pathway (toward the central nervous system). For most of the sensory neurons, their cell body is located in the dorsal root ganglion (a group of cell bodies outside the central nervous system) just lateral of the spinal cord. For sensation to the head, neck, and face, these nuclei (a group of cell bodies inside the central nervous system) are found in the brainstem and are named for the cranial nerve that carries the sensory and/or motor information to the central nervous system.
The secondary neuron is an interneuron that receives information from the first-order neuron’s synapse, where neuronal information is transferred from one neuron to another. Some of these second-order neurons may send their axons to cross over to the other side of the spinal cord or brainstem before carrying the information to the thalamus, a deep relay structure within the brain. All second-order neurons will synapse in the thalamus, except for the sense of smell.
Lastly, the second-order neuron synapses on the third-order neuron, which is located in the thalamus. Here the sensory information is transferred and then carried to the correct sensory area of the cerebrum, such as the primary visual cortex for vision and the somatosensory cortex for pain, temperature, touch, and proprioception.
In the brain the somatosensory system involves the thalamus, reticular formation, and postcentral gyrus, which is found posterior to the central sulcus in the parietal lobe of the cerebral cortex. Along the surface of the postcentral gyrus, the body is mapped from the midline to the temporal lobe. This means that a specific body region is located in a specific region of the postcentral gyrus. This map is called a homunculus, meaning “little man.” The purpose of the thalamus is to act as the relay center from the spinal cord or brainstem to the homunculus. This is the same as the reticular formation, but the reticular formation acts as a relay for the spinal cord or brainstem to the thalamus. The homunculus is the main processing center of the somatosensory system. It processes the sensory impulses received from the thalamus. If a reaction is necessary, such as moving away from a heat source, this sensory information in the cerebral cortex will be transferred to the motor cortex to respond.
The somatosensory system has specific ascending pathways that are dependent on the information carried and how they ascend. One of the pathways is the anterolateral tract, which contains the (1) lateral spinothalamic tract, (2) anterior spinothalamic tract, and (3) spinoreticulothalamic tract. The lateral spinothalamic tract carries pain and temperature information while the anterior spinothalamic pathway carries crude touch sensory information. Lastly, the spinoreticulothalamic tract carries persistent aching or dull pain signals. In addition, it differs from all the ascending pathways because it synapses into the reticular formation in the brainstem rather than going straight to the thalamus.
Another ascending pathway is the posterior column or lemniscus pathway, which contains two main tracts in the spinal cord. The fasciculus gracilis tract carries highly localized information that involves fine touch, proprioception, vibration, and pressure from the inferior half of the body (legs and trunk), while the fasciculus cuneatus tract sends those same impulses but from the arms and upper body to the central nervous system.
Since these pathways involve multiple relay stations, they can be vulnerable to injury. If there were damage to an area anywhere along these pathways, it could lead to the loss of sensation. However, these paths act as a type of anastomosis of the nerves and provide backup routes for pain and temperature impulses, as these sensations are important for survival. For example, if the spinal cord is damaged on one side, the person would still be able to feel pain and possibly temperature. However, if the spinal cord is damaged completely through, the person might not be able to sense anything.
See also: Afferent Tracts; Discriminative Touch; Homunculus; Nociception; Sensory Receptors; Somatosensory Cortex; Thalamus; Thermal Sense; Touch
Dougherty, Patrick, & Chieyeko Tsuchitani. (1997). Somatosensory systems. In Neuroscience Online, an electronic textbook for the neurosciences (Chap. 2). Retrieved from http://neuroscience.uth.tmc.edu/s2/chapter02.html
Kandel, Eric R., James H. Schwartz, Thomas M. Jessell, Steven A. Siegelbaum, & A. J. Hudspeth (Eds.). (2012). Principles of neural science (5th ed.). New York, NY: McGraw-Hill.
Purves, Dale, et al. (Eds.). (2004). Neuroscience (3rd ed.). Sunderland, MA: Sinauer Associates.