The spinal cord (medulla) and the brain together make up the central nervous system (CNS). During the embryological development it arises from the neural tube.
Anatomy, structure and tracts
The spinal cord is 40 to 45 cm long, weighs 30 grams and runs beginning from the foramen magnum all the way down to about the first or second lumbar vertebra securely enclosed in the vertebral column. Here it tapers into the so called conus medullaris. The spinal cord is surrounded by meninges: The innermost layer is the pia mater which closely adheres to the surface of the spinal cord. It is followed by the subarachnoid space filled with cerebrospinal fluid (CSF), the arachnoid mater and the dura mater. Between the dura mater and the vertebral canal is the epidural (or peridural) space.
The spinal cord may be subdivided into a cervical, thoracic, lumbar, sacral and coccygeal region. Two spindle-shaped intumescences are located between segments C4-T1 (cervical intumescence) and L1-S3 (lumbosacral intumescence). These areas are enlarged due to the increased number of neurons contributing to the innervation of the limbs.
A deep sulcus (anterior median fissure) runs on the ventral side of the spinal cord. Its dorsal counterpart is the rather flat posterior median sulcus. Laterally on both sides of the anterior median fissure the anterolateral sulci mark the exits of the anterior roots. The posterior roots enter the spinal cord through the posterolateral sulci on the dorsal side. The spinal ganglia which contain the perikarya (cell bodies) of the sensory neurons are located within the posterior roots. In each segment the anterior and posterior roots unite and form a spinal nerve which then leaves the spinal cord through its respective intervertebral foramen. Altogether 31 to 33 spinal nerve pairs emerge from the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 to 3 coccygeal). The spinal nerves are only about 1 cm long before they branch further into the various peripheral nerves.
During the embryological development the vertebral column outgrows the spinal cord. Therefore the caudal nerve roots need to travel a longer distance until they reach their respective vertebral foramina compared to the cranial ones. These nerve roots descending below the spinal cord are referred to as cauda equine (“horse’s tail”).
The spinal cord consists of the gray matter and the white matter. The butterfly-shaped gray matter lies centrally and contains the perikarya of the neurons. It is surrounded by the white matter which mainly contains nerve fibers. As the axons are myelinated the white matter appears brighter than the gray matter.
The central canal containing cerebrospinal fluid runs through the center of the spinal cord surrounded by gray matter. The ventral “butterfly wing” represents the anterior horn with the motor neurons. The afferent nerve fibers of the posterior roots enter the dorsal “butterfly wing” known as the posterior horn. Between the anterior and posterior horn lies the intermediate column from where sympathetic neurons originate from the lateral horn (only in segments C8-L3).
The white matter comprises ascending and descending tracts of the spinal cord. Thus the spinal cord plays an essential role in the mediation of information between the brain and the periphery. Nonetheless exchange of information also occurs within the different spinal cord segments. In both halves of the spinal cord the white matter is grossly divided into three columns: anterior, lateral and posterior funiculi. The dorsal column contains both the gracile and cuneate fasciculi which are responsible for the epicritic sensibility. The ventral and lateral column comprise the anterior and lateral spinothalamic tracts for the protopathic sensibility and the anterior and lateral corticospinal tracts (pyramidal tract) to the motor neurons which control the motor functions. Additionally the anterior and posterior spinocerebellar tracts run through the lateral column to the cerebellum and mediate proprioceptive information.