 Dermatomes in the anatomy of dermatomes are an important concept to understand for USMLE step one and beyond The spinal nerves are what give rise to the dermatomes Spinal nerves are the major nerve branches that stem directly from the spinal cord and exit between the vertebrae as seen here They carry both sensory innovation and motor innovation The motor signal or efferent signal is carried on the interior root Which exits from the interior horn of the spinal cord as seen here The sensory signal or efferent signal is carried on the posterior root of the spinal nerve and carries sensory information Into the posterior horn of the spinal cord as seen here So it can be carried to the brain The posterior root is what's going to be important in the discussion of dermatomes because dermatomes are part of the way That the body organizes sensory signals There are 31 total spinal nerves There's eight cervical which we label C1 through C8 There's 12 thoracic which we label T1 through T12 Five lumbar which are L1 through L5 and five sacral which is S1 through S5 And altogether that adds up to 31 total spinal nerves, and there's a right and a left spinal nerve on each side Now a dermatome is an area of skin whose sensory innovation is supplied by a particular spinal nerve The efferent signal for various sensations of the skin including light touch, pain Temperature, vibration and proprioception are organized by dermatomes You can see a map of which spinal nerves correspond to which dermatomes here on the right For example, the T10 dermatome extends in a belt-like distribution around the belly button Don't worry though. You don't have to memorize the exact distribution of every dermatome for step one We'll have a separate video laying out some of the important landmarks and dermatomes to be familiar with So you don't have to memorize all of them as you can see from this map All the dermatomes are different shapes and sizes. So memorizing them all can be quite challenging Now since there's 31 pairs of spinal nerves, you'd expect there to be 31 dermatomes However, the C1 nerve route has no corresponding dermatome So actually there's only 30 dermatomes from a total of 31 spinal nerves This video has been an introduction to spinal nerves and the relationship to dermatomes We'll be putting out a separate video on which specific dermatomes you need to know for step one in order to be successful