The spinal cord is also known as the “medulla spinalis” and it is part of the Central Nervous System. It has the function of integrating the Peripheral Nervous System and the brain. The spinal cord has a cylindrical, elongated shape and presents some variations in form and dimensions in some specific regions. Intumescentia corresponds to the thicker parts that give origin to the spinal nerves that supply the limbs and Conus Medullaris corresponds to the final caudal tapering of the spinal cord.
The brain and spinal cord are protected by the meninges. They are three layered membranes (from inside out): Pia Mater, Arachnoid Mater and Dura Mater.
A transverse section shows a central mass of a gray substance (neuron bodies – Perikaryon) that surround a central canal. This canal is the residue of the lumen embryonic neural tube and is filled with cerebrospinal fluid. The gray horn can be divided into three main areas (horns):
a. Dorsal Gray Horn (Alar plate): Receives afferent somatic and visceral action potentials.
b. Lateral Gray Horn: Present only at the levels of thoracic and lumbar vertebrae. This region is responsible for generating autonomic motor nerve impulses.
c. Ventral Gray Horn (Basal Plate): This area contains the Somatic and Visceral motor neurons (efferent fibers).
The Dorsal Root of the spinal cord contains the afferent nerve fibers (somatic and visceral). These neurons are unipolar neurons and the neuron bodies are grouped outside of the spinal cord in a structure called Dorsal Root Ganglion. Once afferent nerve impulses arrive at the dorsal gray horn, the stimulus is then transferred to interneurons that redirect the signals up to the brain and/or to the ventral gray horn which will produce a motor response – either somatic or visceral.
Interneurons receive this name because they are completely encapsulated within the central nervous system. They can be Intersegmental – when they transmit signals vertically in the spinal cord – or Intrasegmental – when they transmit signals horizontally in the spinal cord (reflexes).