Spinal cord, a slightly flat cylinder in front and back, with varying thickness throughout, located atSpinal canalInside, the upper end is connected with the medulla oblongata at the foramen magnum, and the lower end is tapered into a cone, called conus medullaris. The tip of the cone continues into a filament, called filum terminale, which goes down through the sacral canal to the back of the second caudal vertebra. The adult spinal cord is about 42~45cm in length.
There are two enlargement of the spinal cord, the upper one is called cervical enlargement, which is located from the third segment of the cervical spinal cord to the second segment of the thoracic spinal cord, and the thickest at the sixth segment of the cervical spinal cord;The lower one is called lumbar enlargement, which starts from the ninth segment of the thoracic spinal cord to the conus medullaris, and is the thickest at the 12th thoracic vertebra.The formation of these two swellings is related to the appearance of limbs, which is caused by the increase of neurons in the spinal cord.
There are six parallel longitudinal grooves on the surface of the spinal cord. The deeper groove in the front median is called the anterior (ventral) median fissure. There is the anterior (ventral) lateral groove in the front lateral side, through which the anterior roots come out;There is a shallow groove in the posterior median, called the posterior (dorsal) median groove, and there is a posterior (dorsal) lateral groove in the posterior lateral side, from which the posterior root fibers enter the spinal cord.Between the posterior median sulcus and the posterior lateral sulcus, there is a posterior median sulcus.The anterior and posterior root fibers converge at the intervertebral foramen to formspinal nerve。Before confluence, a swelling is formed at the back root, calledSpinal ganglia, containing pseudomonopolar sensory neurons.The whole spinal cord sends out 31 pairs of spinal nerves. The part of the spinal cord corresponding to each pair of spinal nerves is called the spinal cord segment. There are 31 segments in total, including 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments and 1 caudal segment.
During the occurrence and development of the spinal cord and spine, due to the imbalance of their growth speed (the growth speed of the spinal cord is slower than that of the spine), the lower end of the adult spinal cord only reaches the lower edge of the first lumbar spine. Therefore, the spinal nerve roots of the lumbar, sacral, and caudal regions gather around the terminal filament and form a bundle that drops vertically, forming the cauda equina.Since there is no spinal cord below the 1st lumbar vertebra, the puncture is usually performed between the 3rd and 4th lumbar vertebrae in clinical practice.
Spinal cord inSpinal canalIts upper end is flat with the foramen magnum and connected to the medulla oblongata of the brain stem, its lower end is flat with the lower edge of the 1st lumbar vertebra, and it is attached to the back of the tailbone with terminal wire downward.The surface of the spinal cord is covered with three layers of membranes, from the outside to the inside, they are dura mater, spinal arachnoid membrane and pia mater.There are cavities between each layer of capsule and between the dura mater and the spinal canal periosteum, which form the epidural space, subdural space and subarachnoid space from the outside to the inside.[1]
(1) Spinal capsuleFrom outside to inside, they are dura mater, spinal arachnoid membrane and pia mater.[1]
Spinal dura mater: It is composed of dense connective tissue, thick and tough, forming a long tubular dura sac.The upper part is attached to the edge of the foramen magnum, which is connected with the dura mater, and forms a blind end at the height of the second sacral vertebra downward, and is attached to the tailbone by the terminal filament.There are spinal cord, cauda equina and 31 pairs of spinal nerve roots in the dural sac. Each pair of spinal nerve roots is wrapped by the dural sac to form an adventitia, which is closely connected with the connective tissue around the intervertebral foramen for fixation.
Spinal arachnoid mater: thin and translucent, connected with the arachnoid membrane of the brain upward, and leveled downward at the height of the second sacral vertebra to form a blind end.This membrane sends out many connective tissue trabeculae to connect with the soft spinal membrane.
Spinal pia mater: thin and soft, rich in blood vessels, closely attached to the surface of the spinal cord.At the anterior median fissure and the posterior median sulcus, there are fibrous membranes connected with the spinal cord, which are called the anterior fibrous cord of the spinal cord and the posterior fibrous septum, respectively.On both sides of the spinal cord, the pia mater thickens and protrudes outward, forming a dentate ligament in a triangular, frontal position between the anterior and posterior roots. The triangular tooth tip of its outer edge pushes against the spinal arachnoid membrane and connects with the dura mater.There are 15-22 on each side to fix the spinal cord.[1]
(2) Spinal meningeal cavityThere are three spaces between the spinal cord capsule, namely, epidural space, subdural space and subarachnoid space.[1]
Epidural space: the narrow space between the spinal canal periosteum and the dura mater, filled with fat, intravertebral venous plexus and lymphatic vessels, with spinal nerve roots and their accompanying vessels passing through, presenting negative pressure.This gap starts from the height of the foramen magnum and ends at the sacral hiatus at the lower end.Because the dura mater is attached to the edge of the foramen magnum, this cavity is not connected with the intracranial.Clinical epidural anesthesia is to inject drugs into this gap to block spinal nerve roots.
Subdural space: the potential space between the dura mater and the spinal cord arachnoid membrane, which is connected to the lymphatic space around the spinal nerve and contains a small amount of fluid.
Subarachnoid space (subarachnoid space): located between the spinal cord arachnoid and the spinal cord pia mater, the space is filled with cerebrospinal fluid, connects with the intracranial subarachnoid space through the foramen magnum upward, and descends to the height of the second sacral vertebra.This space expands at the level from the 1st lumbar vertebra to the 2nd sacral vertebra, calledTerminal pool(terminal cervical), in which there are cauda equina composed of lumbosacral nerve roots and terminal filaments extending downward from the pia mater.Since the lower end of the adult spinal cord is flat at the lower edge of the first lumbar spine, and the cauda equina is immersed in the cerebrospinal fluid of the terminal cistern, lumbar puncture between the third to fourth or fourth to fifth lumbar spine will not damage the spinal cord and cauda equina.The layers of lumbar puncture are skin, superficial fascia, deep fascia, supraspinal ligament, interspinous ligament, ligamentum flavum, dura mater, subdural space and spinal cord arachnoid membrane, and finally reach the terminal cistern.[1]
Spinal cord injury
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Spinal cord injury (SCI) often leads to limb paralysis, incontinence and sexual dysfunction below the injured segment.
Due to the different stages of injury, spinal cord injury can be divided into primary spinal cord injury and secondary spinal cord injury.Primary spinal cord injury is directly caused by immediate violence, and the magnitude of violence is closely related to the severity of spinal cord injury.Pathologically, extensive edema may occur in the spinal cord after injury. Due to the bone limitation of the spinal canal and the bondage of the dura mater and the pia mater, nerve compression and intramedullary edema may further aggravate, resulting in circulation obstruction of the epidural vein and spinal artery and vein between the spinal cord and spinal canal, resulting in spinal cord ischemia, edema, bleeding and necrosis.In addition, spinal cord edema leads to adhesion, stenosis and even obstruction of the subarachnoid space, affecting the normal physiological circulation of cerebrospinal fluid and the physiological metabolism of the spinal cord.At the molecular level, a large number of catecholamine neurotransmitters such as dopamine and norepinephrine are released and accumulated in the injured part, and free radicals are accumulated, resulting in microvascular spasm and ischemia in the spinal cord, increased release of inflammatory factors, increased vascular permeability, small vein rupture, autophagy and apoptosis of cells, leading to secondary hemorrhage and necrosis of the spinal cord.Secondary spinal cord injury refers to a series of secondary changes in the spinal cord after primary injury.Similar to the pathological injury mechanism of osteofascial compartment syndrome, spinal cord edema and/or intramedullary hematoma will lead to increased intramedullary pressure. Due to the constraints of the spinal cord, arachnoid membrane, dura mater and the limitation of the osseous spinal canal, there will be a vicious circle of ischemia edema ischemia.Some scholars named this series of symptoms "spinal meningeal syndrome".Some patients may even have ascending myelitis due to the continuous expansion of edema, which eventually leads to respiratory depression, lung infection, respiratory and circulatory failure and death.
