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The spinal cord is part of the central nervous system that functions as a channel for outgoing (efferent) neural motor transmission, and incoming (afferent) sensory information. It also serves as the site of neural reflex circuits and central pattern generators. It is shaped as a long tubular structure beginning at the occipital bone and extending down to the space between the first and second lumbar vertebrae (in humans).
The spinal cord is encased in protective layers of spinal meninges, outer dura mater, middle arachnoid mater and the innermost pia mater. The spinal cord is also protected by a layer of cerebrospinal fluid in the subarachnoid space. The entire spinal cord and its protective layers are encased in the bony vertebral column.
The spinal cord is built in a basic three-zone pattern which is maintained during spinal cord (and brain) development: the grey matter layer where cell bodies (soma) reside is called the mantle zone, the white matter (myelinated axons) forms the marginal zone and the ventricular zone (later called the ependymal layer), which is the main site of proliferation, and forms the lining of the central canal of the spinal cord. Gradually, the mantle layer becomes a butterfly-shaped structure (see cross-section view).
The spinal cord is derived is a multistage process, which can be divided into four major stages: neural plate, neural fold, neural tube and finally the spinal cord, where neural differentiation occurs. Ectoderm cells differentiate into neuroectoderm, which condenses to form the neural plate, which then undergoes a process called neurulation to form the neural tube. Neural differentiation follows, giving rise to the distinct neuronal populations of the spinal cord, as will be described.
As the neural tube matures, the lateral walls thicken and form a longitudinal groove called the sulcus limitans, which extends the length of the spinal cord and divides the spinal cord into dorsal and ventral portions. The dorsal portions are called the alar columns (or plates), the location of the sensory somatic and visceral input, and the ventral portions are called the basal columns (or plates), location of the motor function output. While dorsal patterning is induced by signals coming from the surrounding epidermis, ventral patterning is induced by signals from the adjacent notochord.
Neuroblasts in the dorsal half of the spinal cord form commissural and association neurons, while ventrally located neuroblasts form somatic and visceral motor neurons and ventral interneurons. Circuits involved in proprioception and motor control are largely confined to the ventral spinal cord. Dorsal root ganglion neurons differentiate from neural crest progenitors. As the dorsal and ventral column cells proliferate, the lumen of the neural tube narrows to form the small central canal of the spinal cord. The grey matter bulges, forming white funiculi, and septi. Following the closure of the caudal neuropore and formation of the brain ventricles that contain the choroid plexus tissue, the central canal of the caudal spinal cord fills with cerebrospinal fluid (CSF).