The somatosensory system in mammals collects information from receptors in the skin, muscle, and joints. The dorsal root ganglion cells and trigeminal ganglion cells that receive this information send their central processes to synapse on neurons in the spinal cord and hindbrain. These spinal cord and hindbrain neurons project in turn to specialized somatosensory nuclei in the thalamus. Finally, the thalamic nuclei project to somatosensory areas in the cerebral cortex. In the mouse, the somatosensory system is dominated by the input from facial vibrissae. A variety of different somatosensory receptors are located in the skin, muscle, and joints. These receptors are anatomically specialized for the detection of mechanical, thermal, or noxious stimuli. The axon terminals sensitive to mechanical stimuli are housed in elaborate clusters of cells or connective tissue, whereas the terminals sensitive to thermal and noxious stimuli are free nerve endings without accessory structures. The trigeminal nerve receives somatosensory input from the face. The axons carrying somatosensory information are the peripheral processes of ganglion cells in the trigeminal ganglion. The trigeminal ganglion is a swelling on the sensory root of the trigeminal nerve. The brainstem trigeminal nuclei receive the central axons of the trigeminal ganglion cells. These axons form the sensory root of the trigeminal nerve. The sensory root of the trigeminal nerve enters the hindbrain at the level of the second rhombomere (r2), where it connects with the principal trigeminal nucleus. The somatosensory pathways from the spinal cord and hindbrain project to the thalamus. The recipient thalamic nuclei project in turn to the somatosensory cortex. The largest thalamic somatosensory nucleus is the ventral posterior group.