Section 3 Medical Literature 第三部分 医学文献
Part 1.
Nerve tissue consists of two principal types of cells，neurons and supporting cells.The neuron，or nerve cell，is the functional unit of the nervous system it consists of a cell body,containing the nucleus,and several processes of varying length.Nerve cells are specialized to receive stimuli from other cells and to conduct electrical impulses to other parts of the system via their processes.They are arranged as an integrated communications network，with several neurons in a chainlike fashion typically involved in sending impulses from one part of the system to another.Specialized contacts between neurons that provide for transmission of information from one neuron to the next are called synapses.
Supporting cells are nonconducting cells that are in intimate apposition to the neurons.In the CNS they are called ganglia or，simply，glia.In the PNS they are called Schwann cells1) and satellite cells.Schwann cells surround the processes nerve cells and isolate them from adjacent cells and extracellular matrix.Within ganglia of the PNS the supporting cells are called satellite cells.They surround the nerve cell bodies,the part of the cell that contains the nucleus,and are analogous to the Schwann cells.The supporting cells of the ganglia in the wall of the alimentary canal are called enteric glial cells2) despite the fact that they are apart of the PNS.They are morphologically and functionally similar to glial cells in CNS.
Supporting cells provide
·Physical support(protection)for delicate neuronal processes
·Electrical insulation for nerve cell bodies and processes
·Metabolic exchange pathways between the vascular system and the neurons of the nervous system
In addition to neurons and supporting cells,an extensive vasculature3) is present in both the CNS and the PNS.The blood vessels are separated from the nerve tissue by the basal laminae and variable amounts of connective tissue,depending on vessel size.The boundary between blood vessels and nerve tissue in the CNS excludes many substances that normally leave blood vessels to enter other tissues.This selective restriction of blood-borne substances in the CNS is called the blood-brain barrier.
注释：
1）schwann cells 雪旺细胞
2）enteric glial cells 肠样神经胶质细胞
3）vasculature 脉管系统，血管系统
Part 2.
The spinal cord connects the brain to the peripheral nervous system(PNS).It is located within the vertebral canal，which provides structural protection，and is held in place by spinal roots,the denticulate ligaments1) and strands of pia mater2).The adult spinal cord is only about 18 inches(46cm)long and extends from the foramen magnum3) to the level of the L2 vertebra.This is not so at birth，when it extends much lower(to the L3 vertebral level)，but because the vertebral column grows faster than the cord，it leaves the cord positioned progressively higher up in the spinal canal,this means that different spinal cord levels can be related to specific vertebral levels.The spinal cord tapers off at its caudal end to form the conus medullaris4)，and beyond this point the spinal canal is filled only with spinal roots descending caudally to find their intervertebral foramen.In the sacral region these fan out，resembling a horse’s tail，and this is termed the cauda equine5).(https://www.daowen.com)
The spinal cord and spinal roots are covered by three layers of meninges—the dura,arachnoid and pia mater—and further protection is provided by the presence of cerebrospinal fluid(CSF)，which surrounds the cord in the subarachnoid space.The pia is thin and is difficult to identify as a discrete membrane,except as the denticulate ligaments along the sides of the spinal cord.The denticulate ligaments span the space between the surface of the cord and the dura and have an important protective mechanical function in relation to the cord.The lowest level at which they are observed is the L1 lumbar root level.The filum terminale6) is an extension of the pia mater that is attached to the coccygeal segments，whose function is to suspend the cord in the CSF(like the denticulate ligaments).The arachnoid and dura mater extend beyond the L2 level to the level of the S2 vertebra.Thus，if a sample of CSF is needed，a lumbar puncture needle can be inserted below the level of the L2 vertebra without fear of damaging the spinal cord.
The size of the spinal cord is not uniform along its length.At the cervical and lumbar levels,the spinal cord enlarges to accommodate the increased sensorimotor connections involved with the limbs，via the cervical and lumbar enlargements.The cervical spinal cord is also largest because it carries tract fibres from lower body levels that are ascending to higher levels.Similarly，the size of the vertebral canal changes at different vertebral levels,being greater at the cervical and lumbar levels than at the thoracic levels.The thoracic region is small because the input is only from the ribcage area.As the sacral bones are fused，the canal is reduced to tiny foramina that contain the nerve fibres of the cauda equine.However，this has consequences，for if there is narrowing of the canal.
注释：
1）denticulate ligaments 齿状韧带
2）pia mater 软脑膜
3）foramen magnum 枕骨大孔
4）conus medullaris 圆锥，脊髓圆锥
5）cauda equine 马尾
6）filum terminale 终丝
Part 3.
Cerebrovascular disease include some of the most common and devastating disorders:ischemic stroke，hemorrhagic stoke，and cerebrovascular anomalies such as intracranial aneurysms1) and arteriovenous malformations.The incidence of cerebrovascular diseases increases with age，and the number of strokes is projected to increase as the elderly population grow，with a doubling in stroke deaths in the United States by 2030.Most cerebrovascular diseases are manifest by the abrupt onset of a focal neurologic deficit，as if the patient was “struck by the hand of God”.A stroke，or cerebrovascular accident，is defined by this abrupt onset of a neurologic deficit that is attributable to a focal vascular cause.Thus，the definition of stroke is clinical，and laboratory the diagnosis.The clinical manifestations of stroke are highly variable because of the complex anatomy of the brain and its vasculature.Cerebral ischemia is caused by a reduction in blood flow that lasts longer than several seconds.Neurologic symptoms are manifest within seconds because neurons lack glycogen2),so energy failure is rapid.When blood flow is quickly，restored，brain tissue can recover fully and the patient’s symptoms are only transient:this is called a transient ischemic attack(TIA).Typically the neurologic signs and symptoms of a TIA last for 5 to 15min but，by definition，must last＜24h.if the cessation3) of flow lasts for more than a few minutes，infarction or death of brain tissue results.Stroke has occurred if the neurologic signs and symptoms last for ＞24h.A generalized reduction in cerebral blood flow due to systemic hypotension usually produces syncope.If low cerebral blood flow persists for a longer duration，then infarction in the border zones between the major cerebral artery distributions may develop.In more severe instances，global hypoxia-ischemia causes widespread brain injury；the constellation of cognitive sequelae that ensue is called hypoxia-ischemia encephalopathy.Focal ischemia or infarction,on the other hand,is usually caused by thrombosis of the cerebral vessels themselves or by emboli4) from a proximal arterial source or the heart.Cerebral hemorrhage products neurologic symptoms by producing a mass effect on neural structures or form the toxic effects of blood itself.
注释：
1）aneurysms 动脉瘤
2）glycogen 糖原
3）cessation 停止；休止
4）emboli（pl）栓子