 In this video, I will describe the somatosensory receptors of the skin and describe the general structure of the major somatosensory pathways. The somatosensory receptors of the skin include numerous free nerve endings throughout the dermis and epidermis. These free nerve endings are involved in detecting multiple submodalities of somatosensation. There are nociceptors that detect stimuli associated with tissue damage and produce both the submodalities of pain and itch. There are thermoreceptors, which react to changes in temperature. There are two main types of thermoreceptors, cold thermoreceptors that respond to decreasing temperature, and warm thermoreceptors that respond to increasing temperature. There are more cold thermoreceptors than warm thermoreceptors, and cold thermoreceptors are also found closer to the surface, which makes us more sensitive to decreasing temperature than increasing temperature. The root hair plexus is another example of free nerve endings that are somatosensory receptors of the skin. The root hair plexus are free nerve endings surrounding hair follicles that detect movement of the hair. There are also encapsulated nerve endings found in the skin. The ruffini corpuscles, also known as bulbous corpuscles, detect stretching of the dermis. Ruffini corpuscles are also found in joint capsules and detect changes in the body position, enabling the modality of proprioception, the detection of our body posture. Meisner's corpuscles are another encapsulated nerve ending found in the dermis. Meisner's corpuscles are also known as tactile corpuscles, and the Meisner's or tactile corpuscles are found in dermal papillae in the superficial papillary layer of the dermis. There's a high concentration of these Meisner's corpuscles in the skin on the tips of the fingers. Meisner's corpuscles are very sensitive to the onset of light pressure, and they also respond to medium frequency vibrations on the surface of the skin. Another type of encapsulated nerve ending found in the dermis of the skin is the Bacinian corpuscle, also known as a laminated corpuscle. The Bacinian corpuscles are located deep in the dermis and are responsible for detecting deep pressure on the surface of the skin and high frequency vibration. While the Bacinian corpuscles are found deep in the dermis, each Bacinian corpuscle can respond to a relatively large region of the surface of the skin. So Bacinian corpuscles are found at a relatively low density in the skin, but each Bacinian corpuscle has a very large receptive field. There are also specialized receptor cells found in the epidermis of the skin known as merkle cells. The merkle cells detect continuous light pressure as well as low frequency vibrations on the surface of the skin. When the merkle cells are stimulated, they release neurotransmitter onto the dendrites of afferent neurons, which are known as the merkle discs or tactile discs, and then the afferent neuron relays information into the central nervous system. The sensory pathway for the somatosensory modalities will start with the first order neuron or primary neuron, that is the afferent neuron, that relays information through the peripheral nervous system into the central nervous system. This primary first order neuron will have its soma, its cell body, in the peripheral nervous system in a ganglia known as the dorsal root ganglion. Then the first order neuron will form a synapse with the second order neuron within the central nervous system, either in the spinal cord or in the brain stem. Then the second order neuron will cross the midline. This enables information from the left half of the body to be sent to the right half of the brain and information from the right half of the body to be sent to the left half of the brain. The second order neuron will always form a synapse in the thalamus with the third order neuron, and then the third order neuron will form a synapse with neurons in the post-central gyrus of the parietal lobe in the primary somatosensory cortex. There are two major sensory pathways for the somatosensory modality, the dorsal column system and the spinal thalamic tract. The dorsal column system relays the somatosensory information of pressure and vibration and hair movement as well as proprioceptive information, the posture of the body. This information is being relayed from the Merkel cells, Meisner corpuscles, Raffini corpuscles, and Hair root plexus sensory receptors distributed through the skin. The first order neuron in the dorsal column system enters the dorsal root into the spinal cord, and then the axon continues up the spinal cord as an ascending tract located on the dorsal side of the spinal cord. This ascending tract is known as the dorsal white column or posterior white column. The first order neuron travels all the way up the spinal cord to reach the second order neuron in a region of the brainstem known as the medulla oblongata. Then the second order neuron crosses the midline in the medulla oblongata and then projects to the third order neuron in the thalamus. In the spinal thalamic tract sensory pathway, the submodalities of nociception and temperature thermoreception are being relayed from free nerve endings that are the nociceptors and thermoreceptors distributed throughout the skin. The afferent fiber of that neuron, the first order neuron, enters the dorsal root and then forms a synapse with the second order neuron that's located in the dorsal gray horn of the spinal cord. Then the second order neuron crosses the midline and projects through an ascending tract of the spinal cord, which is known as the lateral white column. The second order neuron will then enter the brain and form a synapse with the third order neuron in the thalamus. Here we see an illustration of the somatosensory homunculus, which is a representation of the mapping of the somatosensory information in the primary somatosensory cortex of the post-central gyrus in the parietal lobe. The way this map was created was by using an electrode to stimulate neurons in the primary somatosensory cortex while a patient was awake during neurosurgery. Then the patient reported where on the surface of the body they experienced the perception of touch, moving the electrode through the post-central gyrus of the parietal lobe and recording the locations on the skin that the patient experienced the perception of a somatosensory modality, enabled the formation of this somatosensory map on the surface of the post-central gyrus of the parietal lobe. And we know that the sensory pathways form a connection directly from the receptors in the surface of the skin to specific locations where the neurons in the primary somatosensory cortex process the perception of that somatosensory modality.