The Nervous System

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Intro to Neuroanatomy

Intro to Neuroanatomy

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  • 1. The Nervous System Lectured by Bien Nillos, MD Reference: Gray’s Anatomy
  • 2. The Nervous System
    • the most complicated and highly organized of the various systems which make up the human body
    • the mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment
    • may be divided into two parts,  central  and  peripheral.
  • 3. CNS
    • The  central nervous system  consists of the  encephalon  or  brain,  contained within the cranium, and the  medulla spinalis  or  spinal cord,  lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra.
  • 4. PNS
    • The  peripheral nervous system  consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups,  cerebrospinal  and  sympathetic,  the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled. 
  • 5.
    • The  enteric nervous system , a subsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the Vagus nerve , to function independently in controlling the gastrointestinal system
  • 6. The Neuron
  • 7.
    •   The nervous tissues are composed of  nerve cells  and their various processes, together with a supporting tissue called  neuroglia,  which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as  nerve fibers.
  • 8.
    • To the naked eye a difference is obvious between certain portions of the brain and medulla spinalis: the  gray substance  and the  white substance.  
    • The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers
  • 9.  
  • 10.
    • The nerve cells vary in shape and size, and have one or more processes:
    • (1)  Unipolar cells - found in the spinal ganglia; the single process, after a short course, divides in a T-shaped manner
    • (2)  Bipolar cells- found in the spinal ganglia,  when the cells are in an embryonic condition. 
    • (3)  Multipolar cells - which are pyramidal or stellate in shape, and characterized by their large size and by the numerous processes which issue from them.
  • 11.
    • Nerve fibers are found universally in the peripheral nerves and in the white substance of the brain and medulla spinalis.
    • They are of two kinds
    • medullated  or  white fibers
    • non-medullated  or  gray fibers.
  • 12.
    • The  medullated fibers  form the white part of the brain and medulla spinalis, and also the greater part of every cranial and spinal nerve, and give to these structures their opaque, white aspect.
  • 13.
    • Most of the fibers of the sympathetic system, and some of the cerebrospinal, consist of the  gray  or  gelatinous nerve fibers  ( fibers of Remak )
  • 14.  
  • 15.
    • axis-cylinder  is the essential part of the nerve fiber, and is always present; the medullary sheath and the neurolemma are occasionally absent, expecially at the origin and termination of the nerve fiber
  • 16.
    • The  medullary sheath,  or  white matter of Schwann , is regarded as being a fatty matter in a fluid state, which insulates and protects the essential part of the nerve—the axis-cylinder. 
  • 17.
    • The medullary sheath undergoes interruptions in its continuity at regular intervals, giving to the fiber the appearance of constriction at these points: these are known as the  nodes of Ranvier
  • 18.
    • There may also be seen transverse lines beyond the nodes termed  Frommann’s lines ; the significance of these is not understood. 
    • oblique clefts may be seen in the medullary sheath, subdividing it into irregular portions, which are termed  medullary segments,  or  segments of Lantermann
  • 19.
    • The  neurolemma  or  primitive sheath  presents the appearance of a delicate, structureless membrane. Here and there beneath it, and situated in depressions in the white matter of Schwann, are nuclei surrounded by a small amount of protoplasm
  • 20. Wallerian Degeneration
    • When nerve fibers are cut across, the central ends of the fibers degenerate as far as the first node of Ranvier; but the peripheral ends degenerate simultaneously throughout their whole length. The axons break up into fragments and become surrounded by drops of fatty substance which are formed from the breaking down of the medullary sheath. The nuclei of the primitive sheath proliferate, and finally absorption of the axons and fatty substance occurs
  • 21.
    • retrograde degeneration
    • Wallerian degeneration occurs after axonal injury in both the peripheral nervous system (PNS) and central Nervous System (CNS). It occurs in the axon stump distal to a site of injury and usually begins within 24-36 hours of a lesion. Prior to degeneration distal axon stumps tend to remain electrically excitable. After injury, the axonal skeleton disintegrates and the axonal membrane breaks apart. 
  • 22. Neuroglia
    • the peculiar ground substance in which are imbedded the true nervous constituents of the brain and medulla spinalis, consists of cells and fibers.
    • Some of the cells are stellate in shape
  • 23. The Spinal Cord
    • forms the elongated, nearly cylindrical, part of the central nervous system which occupies the upper two-thirds of the vertebral canal.
    • Its average length in the male is about 45 cm., in the female from 42 to 43 cm., while its weight amounts to about 30 gms.
    • It extends from the level of the upper border of the atlas to that of the lower border of the first, or upper border of the second, lumbar vertebra.
    • Above, it is continuous with the brain; below, it ends in a conical extremity, the  conus medullaris,  from the apex of which a delicate filament, the  filum terminale,  descends as far as the first segment of the coccyx
  • 24.  
  • 25.
    • The Spinal Cord does not fill the part of the vertebral canal in which it lies; it is ensheathed by three protective membranes, separated from each other by two concentric spaces:
    • dura mater  
    • arachnoid
    • pia mater
  • 26. Dura Mater
    • is a strong, fibrous membrane which forms a wide, tubular sheath;
    • extends below the termination of the medulla spinalis and ends in a pointed cul-de-sac at the level of the lower border of the second sacral vertebra.
    • is separated from the wall of the vertebral canal by the  epidural cavity - which contains a quantity of loose areolar tissue and a plexus of veins;
    • between the dura mater and the subjacent arachnoid is a capillary interval, the  subdural cavity - which contains a small quantity of fluid, probably of the nature of lymph
  • 27. Arachnoid
    • a thin, transparent sheath, separated from the pia mater by a comparatively wide interval, the  subarachnoid cavity,  which is filled with cerebrospinal fluid. 
  • 28. Pia Mater
    • closely invests the spinal cord and sends delicate septa into its substance; a narrow band, the  ligamentum denticulatum,  extends along each of its lateral surfaces and is attached by a series of pointed processes to the inner surface of the dura mater.
  • 29. Spinal Nerves
    • Thirty-one pairs of spinal nerves spring from the spinal cord
    • each nerve has an anterior (ventral), and a posterior (dorsal) root, the latter being distinguished by the presence of an oval swelling, the  spinal ganglion,  which contains numerous nerve cells
  • 30.  
  • 31.
    • The pairs of spinal nerves are grouped as follows:
    • cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1
    • the spinal cord is divided into cervical, thoracic, lumbar and sacral regions, corresponding with the attachments of the different groups of nerves.
  • 32.  
  • 33.  
  • 34.  
  • 35. The Brain
    • is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system.
    • In its early embryonic condition it consists of three hollow vesicles, termed the  hind-brain  or  rhombencephalon,  the mid-brain  or  mesencephalon,  and the  fore-brain  or  prosencephalon
  • 36.
    • the wall of the hind-brain - form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle
    • The mid-brain - forms the  cerebral aqueduct  ( aqueduct of   Sylvius ), which serves as a tubular communication between the third and fourth ventricles; its walls are thickened to form the corpora quadrigemina and cerebral peduncles
    • The fore-brain undergoes great modification:
      • anterior part or  telencephalon  expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures;
      • the cavity of the posterior part or diencephalon  forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity
  • 37.  
  • 38. The Medulla Oblongata
    • extends from the lower margin of the pons to a plane passing transversely below the pyramidal decussation and above the first pair of cervical nerves; this plane corresponds with the upper border of the atlas behind, and the middle of the odontoid process of the axis in front; at this level the medulla oblongata is continuous with the spinal cord
  • 39.
    • medulla oblongata is pyramidal in shape, its broad extremity being directed upward toward the pons, while its narrow, lower end is continuous with the spinal cord
    • divided into a lower  closed part  containing the central canal, and an upper  open part  corresponding with the lower portion of the fourth ventricle
  • 40.
    •   The  anterior district  is named the  pyramid  ( pyramis medullæ oblongatæ ) and lies between the anterior median fissure and the antero-lateral sulcus.  Its  upper end is slightly constricted, and between it and the pons the fibers of the abducent nerve emerge;
  • 41.
    • The two pyramids contain the motor fibers which pass from the brain to the medulla oblongata and spinal cord: corticobulbar and corticospinal fibers
    • When these pyramidal fibers are traced downward it is found that some two-thirds or more of them leave the pyramids in successive bundles, and decussate in the anterior median fissure, forming what is termed the  pyramidal decussation
  • 42.
    • The  olive  is situated lateral to the pyramid, from which it is separated by the antero-lateral sulcus, and the fibers of the hypoglossal nerve
  • 43. Pons
    • forepart of the hind-brain is situated in front of the cerebellum. From its superior surface the cerebral peduncles emerge, one on either side of the middle line.
  • 44. Cerebellum
    • constitutes the largest part of the hindbrain. It lies behind the pons and medulla oblongata; between its central portion and these structures is the cavity of the fourth ventricle. It rests on the inferior occipital fossæ, while above it is the tentorium cerebelli, a fold of dura mater which separates it from the tentorial surface of the cerebrum
  • 45.
    • The cerebellum consists of three parts, a median and two lateral, which are continuous with each other, and are substantially the same in structure. The median portion is constricted, and is called the  vermis,  from its annulated appearance which it owes to the transverse ridges and furrows upon it; the lateral expanded portions are named the hemispheres.
  • 46. Fourth Ventricle
    • cavity of the hind-brain, is situated in front of the cerebellum and behind the pons and upper half of the medulla oblongata
    • It presents four  angles,  and possesses a  roof  or dorsal wall, a  floor  or ventral wall, and  lateral boundaries
  • 47.  
  • 48.  
  • 49. Cerebral Peduncles
    • two cylindrical masses situated at the base of the brain, and largely hidden by the temporal lobes of the cerebrum
    • The depressed area between the crura is termed the  interpeduncular fossa,  and consists of a layer of grayish substance, the  posterior perforated substance,  which is pierced by small apertures for the transmission of bloodvessels
  • 50. Thalamus
    • two large ovoid masses, situated one on either side of the third ventricle and reaching for some distance behind that cavity. Each measures about 4 cm. in length, and presents two extremities, an anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral.
  • 51. Optic Chiasm
    • is a flattened, somewhat quadrilateral band of fibers, situated at the junction of the floor and anterior wall of the third ventricle. Most of its fibers have their origins in the retina, and reach the chiasma through the optic nerves, which are continuous with its antero-lateral angles. In the chiasma, they undergo a partial decussation
  • 52.  
  • 53. Optic Tracts
    • continued backward and lateralward from the postero-lateral angles of the optic chiasma
  • 54. Third Ventricle
    • is a median cleft between the two thalami. Behind, it communicates with the fourth ventricle through the cerebral aqueduct, and in front with the lateral ventricles through the interventricular foramen. Somewhat triangular in shape, with the apex directed backward, it has a  roof,  a floor,  an  anterior  and a  posterior boundary  and a pair of  lateral walls
  • 55. Cerebral Hemispheres
    • constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences. 
  • 56. Longitudinal Cerebral Fissure
    • contains a sickle-shaped process of dura mater, the  falx cerebri.  In front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the  corpus callosum
  • 57.  
  • 58.  
  • 59. Lateral Cerebral Fissure
    • fissure of Sylvius
    • is a well-marked cleft on the inferior and lateral surfaces of the hemisphere, and consists of a short stem which divides into three rami.
  • 60. Central Sulcus
    • fissure of Rolando
    • is situated about the middle of the lateral surface of the hemisphere, and begins in or near the longitudinal cerebral fissure, a little behind its mid-point. It runs sinuously downward and forward, and ends a little above the posterior ramus of the lateral fissure, and about 2.5 cm. behind the anterior ascending ramus of the same fissure
  • 61.  
  • 62. Calcarine Fissure
    • on the medial surface of the hemisphere. It begins near the occipital pole in two converging rami, and runs forward to a point a little below the splenium of the corpus callosum, where it is joined at an acute angle by the medial part of the parietoöccipital fissure
  • 63. Cingulate Fissure
    • on the medial surface of the hemisphere; it begins below the anterior end of the corpus callosum and runs upward and forward nearly parallel to the rostrum of this body and, curving in front of the genu, is continued backward above the corpus callosum, and finally ascends to the supero-medial border of the hemisphere a short distance behind the upper end of the central sulcus
  • 64. The Lobes of the Hemisphere
    • the  frontal,  the  parietal, the  temporal,  the  occipital,  the  limbic,  and the  insula.
  • 65.  
  • 66. Lateral Ventricles
    • are irregular cavities situated in the lower and medial parts of the cerebral hemispheres, one on either side of the middle line. They are separated from each other by a median vertical partition, the  septum pellucidum,  but communicate with the third ventricle and indirectly with each other through the  interventricular foramen
  • 67.  
  • 68. The Cranial Nerves
  • 69.  
  • 70.  
  • 71.  
  • 72.  
  • 73. END OF LECTURE “ The brain is like a muscle. When it is in use we feel very good. Understanding is joyous” - Carl Sagan