Section of spinal cord showing spinal nerves

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. It is around 45 cm long in men and around 43 cm long in women and has diameter of a human finger.. The length of the spinal cord is much shorter than the length of the bony spinal column. In fact, the spinal cord extends down to only the last of the thoracic vertebrae. Enclosed within, and protected by, the bony vertebral column, the spinal cord functions primarily in the transmission of neural signals between the brain and the rest of the body, but also contains neural circuits that can independently control numerous reflexes.
The spinal cord is enclosed within, and protected by, the bony vertebral column. and  three layers of meninges. The spinal cord is surrounded by a clear fluid called Cerebral Spinal Fluid (CSF), that acts as a cushion to protect the delicate nerve tissues against damage from banging against the inside of the vertebrae.

The peripheral region of the cord contains neuronal 'white matter' tracts containing sensory and motor neurons. Internal to this peripheral region is the 'gray matter', butterfly shaped central region made up of nerve cell bodies. The middle of the spinal cord is a hollow canal called the central canal which is also filled with cerebro spinal fluid.

Spinal nerves
Spinal cord and spinal nervesThe human spinal cord is divided into 31 different segments. At every segment, two spinal nerves(which are mixed nerves, made of a sensory and a motor root) form, one on left side and one or right side.  Each of this mixed spinal nerve are formed by the a sensory rootlet emerging fromt the dorsal (front) side, and a motor rootlet emerging from ventral (back) side of the spinal cord.

These spinal nerves  pass out through a hole between two adjacent vertebrae called the Foramen and carry the information from the spinal cord to the rest of the body, and from the body back up to the brain

The spinal nerves are grouped into 4 groups depending upon the level of vertebra they pass out.

8 pairs of cervical nerves C1-C8 : C1 spinal nerve exit spinal column between head and C1 vertebra; C2-7 exit through foramen above the corresponding vertebra, C8 exit through foramen between C7 and T1 vertebra. Please note that there are 8 pairs of cervical nerves buy only 7 cervical vertebrae.

12 pairs of thoracic nerves T1-T12 : Each pair exit through the foramen below the corresponding vertebra. Eg, T1 nerve exit below T1 vertebra.

5 pairs of lumbar nerves L1-L5 : Each pair exit through the foramen below the corresponding vertebra L1-L5)

5 pairs of sacral nerves S1-S5 : Each pair exit through the foramen below the corresponding vertebra S1-S5

1 pair of coccygeal nerves tat exit spinal column through the sacral hiatus.

As the length of the spinal cord only comes till the lumbar vertebra L1-L2, the segments of the cord where the spinal nerves originate do not relate exactly to the level of the vertebral bodies, it will be at a higher level.
For example, the L5 cord segment, where the L5 nerve orginate, is situated at the L1 vertebra and the nerve moves down and exit the vertebral coloum below L5 vertebra.
This means that damage to the bone at a particular level, say L5 vertebra, does not necessarily mean damage to the spinal cord at the same spinal nerve level. See the image on the right.

Segmental Spinal Cord Level and Function