Medical brain notes 31

  The Nervous System - AnOverall View Development and Subdivision The nervous system serves information processing. In the most primitive forms of organization  (A) , this function is assumed by the  sensory cells  ( A  –  C1 ) themselves. These cells are excited by stimuli coming from the environment; the excitation is conducted to a  muscle cell  ( A  –  C2 ) through a cellular projection, or  process . The simplest response to environmental stimuli is achieved in this way. (In humans, sensory cells that still have processes of their own are only found in the olfactory epithelium.) In more differentiated organisms ( B ), an ad-ditional cell is interposed between the sensory cell and the muscle cell – the nerve cell, or  neuron  ( BC3 ) which takes on the transmission of messages. This cell can transmit the excitation to several muscle cells or to additional nerve cells, thus form-ing a  neural network  ( C ). A diffuse network of this type also runs through the human body and innerva

Functional Circuits The nervous system, the remaining organism, and the environment are function-ally linked with each other. Stimuli from the environment ( exteroceptive stimuli ) ( E9 ) are conducted by sensory cells ( E10 ) via  sensory (afferent) nerves  ( E11 ) to the CNS( E12 ). In response, there is a command from the CNS via  motor (efferent) nerves  ( E13 ) to the muscles ( E14 ). For control and regula-tion of the muscular response ( E15 ), there is  internal feedback  from sensory cells in themuscles via sensory nerves ( E16 ) to the CNS. This afferent tract does not transmit environmental stimuli but stimuli from within the body ( proprioceptive stimuli ). We therefore distinguish between  exterocep-tive  and  proprioceptive sensitivities . However, the organism does not only re-spond to the environment; it also influences it spontaneously. In this case, too, there is a corresponding functional circuit: the action ( F17 ) started by the brain via efferent nerves ( F13 ) is

  Position of the Nervous System in the Body The  central nervous system  (CNS) is divided into the brain,  encephalon  ( A1 ), and the spinal cord (SC),  medulla spinalis  ( A2 ). The brain in the cranial cavity is surrounded by a bony capsule; the spinal cord in the vertebral canal is enclosed by the bony vertebral column. Both are covered by meninges that enclose a cavity filled with a fluid, the  cerebrospinal fluid . Thus, the CNS is protected from all sides by bony walls and the cushioning effect of a fluid ( fluid cush-ion ). The  peripheral nervous system  (PNS) in-cludes the  cranial nerves , which emerge through holes ( foramina ) in the base of the skull, and the  spinal nerves , which emerge through spaces between the vertebrae ( in-tervertebral foramina ) ( A3 ). The peripheralnerves extend to muscles and skin areas. They form  nerve plexuses  before entering the limbs: the  brachial plexus  ( A4 ) and the  lumbosacral plexus  ( A5 ) in which the fibers ofthe spinal nerves

  Development of the Brain The closure of the neural groove into the neural tube begins at the level of the upper cervical cord. From here, further closure runs in the oral direction up to the rostral end of the brain (oral neuropore, later the terminal lamina) and in the caudal direction up to the end of the spinal cord. Further developmental events in the CNS proceed in the same directions. Thus, the brain’s divi-sions do not mature simultaneously but at intervals (heterochronous maturation). The neural tube in the head region expands into several vesicles (p. 171, A). The rostral vesicle  is  the  future  forebrain,  prosen- cephalon (yellow and red); the caudal ves- icles are the future brain stem, encephalic trunk (blue). Two curvatures of the neural tube appear  at this time: the  cephalic flexure (A1) and the cervical flexure (A2). Although the brain stem still shows a uni- form structure at this early stage, the future divisions can already be identified: medulla oblongata(elon

  Anatomy of the Brain Overview The individual subdivisions of the brain contain cavities or ventricles of various shapes and widths. The  primary cavity of theneural tube and cerebral vesicle  becomesmuch narrower as the walls thicken. In the spinal cord of lower vertebrates, it survives as the central canal . In the human spinal cord, it becomes completely occluded ( ob-literated ). In a cross section, only a few cellsof the former lining of the spinal cord mark the site of the early central canal ( A1 ). In the brain, the cavity survives and forms the ventricular system  (p. 280) which is filledwith a clear fluid, the  cerebrospinal fluid . The  fourth ventricle  ( AD2 ) is located in the segment of the medulla oblongata and the pons. After a narrowing of the cavity in the midbrain, the  third ventricle  ( CD3 ) lies in the diencephalon. A passage on both sides of its lateral walls, the  interventricular foramen  ( foramen of Monro ) ( C  –  E4 ), opens into the lateral ventricles 

  Evolution of the Brain In the course of evolution, the vertebrate brain developed into the organ of human intelligence. Since the ancestors are extinct, the developmental sequence can only be re-constructed by means of species that have retained a primitive brain structure. In  am-phibians and  reptiles , the telencephalon( A1 ) appears as an appendix to the large ol-factory bulb ( A2 ); mesencephalon ( A3 ) and diencephalon ( A4 ) lie free at the surface. Al-ready in primitive  mammals  (such as the hedgehog), however, the telencephalon ex-pands over the rostral parts of the brain stem; in  lemurs,  it completely overlays the diencephalon and mesencephalon. Thus, the phylogenetic development of the brain essentially consists of a progressive enlarge-ment of the telencephalon and a transfer of the highest integrative functions to this part of the brain. This is called  telencephaliza-tion . Ancient primitive structures are stillretained in the human brain and are inter-mingled with n

  The Nerve Cell The nervous tissue consists of  nerve cells  and  glial cells  which originate from the ectoderm (the latter are supporting and covering cells).  Blood vessels and meninges do not belong to the nervous tissue; they are of mesodermal origin. The nerve cell ( gan-glion cell  or  neuron ) is the functional unitof the nervous system. In its mature state, it is no longer able to divide, thus making pro-liferation and the replacement of old cells impossible. Very few nerve cells are formed after birth. A neuron consists of the cell body, the  peri-karyon  ( A1 ), the processes,  dendrites  ( A2 ),and one main process, the  axon  or  neurite  ( A  –  D3 ). The  perikaryon  is the  trophic center  of the cell, and processes that become separated from it degenerate. It contains the  cell nu-cleus  ( A4 ) with a large, chromatin-rich  nucleolus  ( A5 ) to which the  Barr body  (sexchromatin) ( A6 ) is attached in females. The  dendrites  enlarge the cell surface by branching. Th

  Methods in Neuroanatomy The availability of methods for studying the structure and function of cells, tissues, and organs is often the limiting factor in expanding our knowledge. Certain terms and interpretations can only be understood if the background of the method used is known. Therefore, the methods commonly used in neuroanatomy are presented here briefly. Nerve cells and glial cells can be demon-strated in thin histological sections by various histological techniques. The  Nisslmethod  has proven helpful because of excel-lent visualization of the  rough endoplasmicreticulum  (p. 18), which is abundant innerve cells. However, the different types of nerve cells are essentially characterized by their long processes, the dendrites and the axon, which are not stained by the Nissl method. For demonstration of as many of these processes as possible, thick sections (200 µm) are required. By using  silver im-pregnation  ( Golgi method , p. 18), individual nerve cells  with a large numbe

  Ultrastructure of the Nerve Cell Electron micrographs show the  cell nucleus  ( A  –  C1 ) to be enclosed by a  double-layeredmembrane  ( A2 ). It contains the  nuclear pores  ( BC3 ) that probably open only temporarily. The  karyoplasm  of the nucleus contains finely dispersed  chromatin granules , which consist of DNA and proteins. The  nucleolus  ( A  –  C4 ), a spongiform area of the nucleus made up of a dense granular component and a loose filamentous component, con-sists of RNA and proteins. In the  cytoplasm , the Nissl bodies appear as  rough endoplasmic reticulum  ( A  –  C5 ), a lamel-lar system of membranes that enclose flat-tened, intercommunicating cisternae ( BC6 ). Attached to the cytoplasmic side of the membranes are the protein-synthesizing  ri-bosomes  ( BC7 ). To maintain the long axon(up to 1 m long), it is essential that the cell has an extremely high rate of protein syn-thesis (structural metabolism). Ribosome-free membranes form the agranular or  smooth endopla