I was working in the woods a while today behind my house and got cut up pretty good. There were lots of thorns bushes and stuff like that. Anyway, I came inside to wash my hands and all the cuts burned a little. Now, I understand that the central nervous system would relay the message of pain through the nerve cells and up to the brain were the brain tells me it hurts. So, the question: How does the brain distinguish where the pain began? If it is a relay of messages, how does the brain know where it all started?

The full answer is hugely big and would be very long. So, to keep it short (believe it or not):

 

We have what is known as a 'pain matrix'. This is kind of like a circuit in the brain that receives signals that ascend to the brain via the spinothalamic tract (which carries most pain information to the brain). The pain matrix consists of two divisions; the medial pain matrix and the lateral pain matrix.

 

Projections through the reticular formation and the medial (intralamina) thalamus to the limbic system constitute the medial division of the pain matrix and are associated with the aversive, cognitive-evaluative and emotional-motivational components of pain.

 

Projections through the lateral (ventrobasal) thalamus to the secondary somatosensory cortex (SII) constitute the lateral division of the pain matrix, and account for the sensory-discriminative component of pain. This division allows us to localize pain in time and space (and intensity).

 

Spinal nerves serve particular areas of the body. In the skin, these areas are known as dermatomes, so, for example, the arms are innervated mainly by spinal nerves projecting from the 5th, 6th and 9th thoracic vertebrae (T5, T6 & T9 dermatomes).

 

The somatosensory cortex is mapped out in such a way that topographically, it represents our whole body (this is sometimes knows as the sensory homunculus). I.e. each dermatome maps to a corresponding area of the somatosensory cortex. So, signals from any dermatome are sent to very specific areas of the somatosensory cortex that correspond exactly to the area of the body the signals are coming from. That's how the brain knows where the pain is coming from.

Great. That was exactly what I was looking for. Thanks.