Organization of the central nervous system

The human spinal cord has about 10⁸ neurons. A transverse section through the spinal cord shows a butterfly shaped central gray matter which contains neuron cell bodies, neuropil and glia. Sensory fibers enter the spinal cord via the dorsal roots to synapse largely with cells in the dorsal horns of the spinal gray matter. Motor neurons cell bodies lie in the ventral horns of the spinal gray matter and their axons exit via the ventral roots. In the human embryo at the end of the fourth week the central nervous system (CNS) is a hollow tube, the neural tube, the caudal end of which becomes the spinal cord. At its rostral end are three swellings, primary vesicles, which are the most fundamental anatomical divisions of the brain. These are the hindbrain, midbrain and forebrain. As embryogenesis proceeds, the forebrain differentiates into a caudal diencephalon and a rostral telencephalon, which in turn acquires two lateral swellings, the cerebral hemispheres.