Part of human<unk> . 1 – central canal; 2 – posterior median sulcus; 3 – gray matter; 4 – white matter; 5 – dorsal root + dorsal root ganglion; 6 – ventral root; 7 – fascicles; 8 – anterior<unk>  artery; 9 – arachnoid mater; 10 – dura mater Diagram of the<unk>  showing segments The<unk>  is the main pathway for information connecting the brain and peripheral nervous system. Much shorter than its protecting<unk>  column, the human<unk>  originates in the brainstem, passes through the foramen magnum, and continues through to the conus medullaris near the second lumbar vertebra before terminating in a fibrous extension known as the filum terminale. It is about long in men and about in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, stretching from the C5 to T1 vertebrae, is where sensory input comes from and motor output goes to the arms and trunk. The lumbar enlargement, located between L1 and S3, handles sensory input and motor output coming from and going to the legs. The<unk>  is continuous with the caudal portion of the medulla, running from the base of the skull to the body of the first lumbar vertebra. It does not run the full length of the vertebral column in adults. It is made of 31 segments from which branch one pair of sensory nerve roots and one pair of motor nerve roots. The nerve roots then merge into bilaterally symmetrical pairs of<unk>  nerves. The peripheral nervous system is made up of these<unk>  roots, nerves, and ganglia. The dorsal roots are afferent fascicles, receiving sensory information from the skin, muscles, and visceral organs to be relayed to the brain. The roots terminate in dorsal root ganglia, which are composed of the cell bodies of the corresponding neurons. Ventral roots consist of efferent fibers that arise from motor neurons whose cell bodies are found in the ventral (or anterior) gray horns of the<unk> . The<unk>  (and brain) are protected by three layers of tissue or membranes called meninges, that surround the canal. The dura mater is the outermost layer, and it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space. The epidural space is filled with adipose tissue, and it contains a network of blood vessels. The arachnoid mater, the middle protective layer, is named for its open, spiderweb-like appearance. The space between the arachnoid and the underlying pia mater is called the subarachnoid space. The subarachnoid space contains cerebrospinal fluid (CSF), which can be sampled with a lumbar puncture, or<unk>  tap" procedure. The delicate pia mater, the innermost protective layer, is tightly associated with the surface of the<unk> . The<unk>  is stabilized within the dura mater by the connecting denticulate ligaments, which extend from the enveloping pia mater laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral vertebra. In cross-section, the peripheral region of the<unk>  contains neuronal white matter tracts containing sensory and motor axons. Internal to this peripheral region is the grey matter, which contains the nerve cell bodies arranged in the three grey columns that give the region its butterfly-shape. This central region surrounds the central canal, which is an extension of the fourth ventricle and contains cerebrospinal fluid. The<unk>  is elliptical in cross section, being compressed dorsolaterally. Two prominent grooves, or sulci, run along its length. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side. Segments 150px The human<unk>  is divided into segments where pairs of<unk>  nerves (mixed; sensory and motor) form. Six to eight motor nerve rootlets branch out of right and left ventralateral sulci in a very orderly manner. Nerve rootlets combine to form nerve roots. Likewise, sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots. The ventral (motor) and dorsal (sensory) roots combine to form<unk>  nerves (mixed; motor and sensory), one on each side of the<unk> .<unk>  nerves, with the exception of C1 and C2, form inside the intervertebral foramen (IVF). These rootlets form the demarcation between the central and peripheral nervous systems.A model of segments of the human spine and<unk> , nerve roots can be seen extending laterally from the (not visible)<unk> .. Generally, the<unk>  segments does not corresponds to the bony vertebrae levels. As the<unk>  terminates at L1-L2 level, other segments of the<unk>  would locate above the corresponding bony vertebrae body. For example, T11<unk>  segment is located above the T11 bony vertebrae, sacral<unk>  segment is located above the L1 vertebrae body. The grey column, (as three regions of grey columns) in the center of the<unk> , is shaped like a butterfly and consists of cell bodies of interneurons, motor neurons, neuroglia cells and unmyelinated axons. The anterior and posterior grey column present as projections of the grey matter and are also known as the horns of the<unk> . Together, the grey columns and the gray commissure form the "grey H." The white matter is located outside of the grey matter and consists almost totally of myelinated motor and sensory axons. "Columns" of white matter carry information either up or down the<unk> . The<unk>  proper terminates in a region called the conus medullaris, while the pia mater continues as an extension called the filum terminale, which anchors the<unk>  to the coccyx. The cauda equina ("horse's tail") is a collection of nerves inferior to the conus medullaris that continue to travel through the vertebral column to the coccyx. The cauda equina forms because the<unk>  stops growing in length at about age four, even though the vertebral column continues to lengthen until adulthood. This results in sacral<unk>  nerves originating in the upper lumbar region. For that reason, the<unk>  occupies only two-thirds of the vertebral canal. The inferior part of the vertebral canal is filled with cerebrospinal fluid (CSF) and the space is called the lumbar cistern. Within the Central Nervous System (CNS), nerve cell bodies are generally organized into functional clusters, called nuclei. Axons within the CNS are grouped into tracts. There are 31<unk>  nerve segments in a human<unk> : * 8 cervical segments forming 8 pairs of cervical nerves (C1<unk>  nerves exit the<unk>  column between the foramen magnum and the C1 vertebra; C2 nerves exit between the posterior arch of the C1 vertebra and the lamina of C2; C3–C8<unk>  nerves pass through the IVF above their corresponding cervical vertebrae, with the exception of the C8 pair which exit between the C7 and T1 vertebrae) * 12 thoracic segments forming 12 pairs of thoracic nerves * 5 lumbar segments forming 5 pairs of lumbar nerves * 5 sacral segments forming 5 pairs of sacral nerves * 1 coccygeal segment<unk>  segments in some common species Species Cervical Thoracic Lumbar Sacral Caudal/Coccygeal Total Dog 8 13 7 3 5 36 Cat 8 13 7 3 5 36 Cow 8 13 6 5 5 37 Horse 8 18 6 5 5 42 Pig 8 15/14 6/7 4 5 38 Human 8 12 5 5 1 31 Mouse 8 13 6 4 3 35 In the fetus, vertebral segments correspond with<unk>  segments. However, because the vertebral column grows longer than the<unk> ,<unk>  segments do not correspond to vertebral segments in the adult, particularly in the lower<unk> . For example, lumbar and sacral<unk>  segments are found between vertebral levels T9 and L2, and the<unk>  ends around the L1/L2 vertebral level, forming a structure known as the conus medullaris. Although the<unk>  cell bodies end around the L1/L2 vertebral level, the<unk>  nerves for each segment exit at the level of the corresponding vertebra. For the nerves of the lower<unk> , this means that they exit the vertebral column much lower (more caudally) than their roots. As these nerves travel from their respective roots to their point of exit from the vertebral column, the nerves of the lower<unk>  segments form a bundle called the cauda equina. There are two regions where the<unk>  enlarges: * Cervical enlargement – corresponds roughly to the brachial plexus nerves, which innervate the upper limb. It includes<unk>  segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1). * Lumbar enlargement – corresponds to the lumbosacral plexus nerves, which innervate the lower limb. It comprises the<unk>  segments from L2 to S3 and is found about the vertebral levels of T9 to T12. Development<unk>  seen in a midsection of a five-week-old embryo<unk>  seen in
Spinal cord
Brainstem
Retina
Forebrain
Hindbrain
Brain
Central nervous system
Sensory cortex