Spinal cord and nerve roots

The spinal cord contains longitudinally oriented spinal tracts (white matter) surrounding central areas (grey matter) where most spinal neuronal cell bodies are located. 

Axons from spinal sensory neurons enter and axons from motor neurons leave the spinal cord via segmental nerves or roots.

Each root receives sensory information from skin areas called dermatomes and innervates a group of muscles called a myotome.

Bony vertebrae

Cervical Spine: seven veterbrae (C1-C7), eight nerve roots (C1-C8).
Cervical roots exit above the level of their respective vertebrae.
C1 exits above C1 vertebra (C1 has no testable sensory nerve root)
C8 exits above T1 vertebra

Thoracic spine: twelve vertebrae (T1-T12), twelve nerve roots (T1-T12).
Lumbar spine: five vertebrae (L1-L5), five nerve roots (L1-L5).
Thoracic and lumbar roots exit below the level of their respective vertebrae.

Sacrum has five distinct nerve roots (S1-S5) that exit via the sacral foramina. 

Conus medullaris and cauda equina

The distal end of the spinal cord is located at L1-L2 vertebral level and is called the conus medullaris.

The cauda equina is a cluster of paired (right and left) lumbosacral nerve roots that originate in the region of the conus medullaris and travel down through the thecal sac and exit via the intervertebral foramen below their respective vertebral levels.

Dermatomes

Motor nerve roots

C5 –Elbow flexors (biceps, brachialis)
C6 –Wrist extensors (extensor carpi radialis longus and brevis)
C7 –Elbow extensors (triceps)
C8 –Finger flexors (flexor digitorum profundus) to the middle finger
T1 –Small finger abductors (abductor digiti minimi)
L2 –Hip flexors (iliopsoas)
L3 –Knee extensors (quadriceps)
L4 –Ankle dorsiflexors (tibialis anterior)
L5 –Long toe extensors (extensor hallucis longus)
S1 –Ankle plantar flexors (gastrocnemius, soleus)