|
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. The brain and spinal cord together make up the central nervous system (CNS). The spinal cord begins at the occipital bone and extends down to the space between the first and second lumbar vertebrae; it does not extend the entire length of the vertebral column. It is around in men and around long in women. Also, the spinal cord has a varying width, ranging from thick in the cervical and lumbar regions to thick in the thoracic area. The enclosing bony vertebral column protects the relatively shorter spinal cord. The spinal cord functions primarily in the transmission of neural signals between the brain and the rest of the body but also contains neural circuits that can independently control numerous reflexes and central pattern generators. The spinal cord has three major functions: as a conduit for motor information, which travels down the spinal cord, as a conduit for sensory information in the reverse direction, and finally as a center for coordinating certain reflexes. ==Structure== The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The length of the spinal cord is much shorter as compared to the length of the vertebral column. The human spinal cord extends from the foramen magnum and continues through to the conus medullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale. It is about long in men and around in women, ovoid-shaped, and is enlarged in the cervical and tree regions. The cervical enlargement, located from C4 to T1 spinal segments, is where sensory input comes from and motor output goes to the arms. The tree enlargement, located between L1 and S3 spinal segments, handles sensory input and motor output coming from and going to the legs. The spinal cord 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 that each contain sensory nerve root and motor nerve root. The first cervical segment is usually very small and may be absent in most individuals. Nerve roots merge into 31 bilaterally symmetric pairs of spinal nerves. The peripheral nervous system is made up of these spinal roots, nerves, and ganglia. Dorsal roots are fascicles of axons in the spinal cord that receive sensory information from the skin, muscle, and visceral organs that enter through the a point of entry called the ''dorsal root entry zone''. Dorsal root ganglia hold the cell bodies of dorsal root ganglia in the spinal ganglia (dorsal root ganglia) related with that spinal segment. Ventral roots only have efferent fibers that arise from motor neurons (general somatic efferent neurons) whose cell bodies are found in the ventral (or anterior) gray horns of the spinal cord. The spinal cord (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 is the middle protective layer. Its name comes from the fact that the tissue has a 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). The medical procedure known as a lumbar puncture (or "spinal tap") involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space, usually from the lumbar region of the spine. The pia mater is the innermost protective layer. It is very delicate and it is tightly associated with the surface of the spinal cord. The cord 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 cord contains neuronal white matter tracts containing sensory and motor neurons. Internal to this peripheral region is the grey matter. The nerve cell bodies of the grey matter are contained in the three grey columns of the spinal cord 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 spinal cord has a shape that is compressed dorso-ventrally, giving it an elliptical shape. The cord has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side. In the spinal cord there are some tracts that brings information from the brain. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「spinal cord」の詳細全文を読む スポンサード リンク
|