Although we usually study the spinal cord as a series of cross sections, it is important to remember that it is in fact a column, with continuous tracts and cell columns. However, the cord can be divided into segments by the nerve roots that come off of it; although the rootlets branch off nearly continuously, they coalesce into about 31 discrete nerves along the cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerves). At each segment, rootlets appear to come out of both the dorsal and ventral halves of the spinal cord.
脊髓是聯繫腦部與周邊神經系統的重要訊息通道。組合成脊柱的骨頭稱作脊椎骨，雖然脊柱具有一些彈性，但脊柱下部的一些脊椎骨相連的。脊髓位於脊柱孔中並有31節：八節頸椎，十二節胸椎，五節腰椎，五節薦骨，一節尾骨。男性的脊髓約為45公分女性則約43公分。脊髓脊椎骨短得多。事實上脊髓經只到達胸椎而已。因此由腰椎與薦骨分出來的脊髓神經支幹，在離開脊柱之前必須進入脊椎管才得以延伸。脊椎管中所聚集的神經稱為"cauda equina" (「馬尾」的意思)。皮膚中的受體將訊息透過脊髓神經傳送至脊髓。這些神經纖維的細胞體位於背根神經節中。神經纖維由背根部進入脊髓。部份的神經纖維會與背側角(dorsal horn)中的其他神經元形成突觸，而其他的則繼續進入腦部。脊髓腹側角(ventral horn)中的細胞體將軸索經由腹側根至肌肉上以便控制動作。