Medical brain notes 24

  The Spinal Cord :  Gross Anatomy and Some features of Internal Structure         The spinal cord (or spinal medulla) is the most important content of the vertebral canal. The upper end of the spinal cord becomes continuous with the medulla oblongata at the level of the  upperborder  of the first cervical vertebra. The lower end of the spinal cord lies at the level of the lowerborder of the first lumbar vertebra. The level is, however, variable and the cord may terminate one vertebra higher or lower than this level. The level also varies with flexion or extension of the spine.             The lowest part of the spinal cord is conical and is called the  conus medullaris . The conus is continuous, below, with a fibrous cord called the  filum terminale  (Fig. 2.2).         When seen in transverse section the grey matter of the spin...

  Spinal Nerves & Spinal Segments          The spinal cord gives attachment, on either side, to a series of spinal nerves. Each spinal nerve arises by two roots,  anterior (or ventral)  and  posterior (or dorsal)  (Fig. 2.4). Each root is formed by aggregation of a number of rootlets that arise from the cord over a certain length (Fig. 2.5).  Thelength of the spinal cord giving origin to the rootlets of one spinal nerve constitutes one spinal segment .However, this definition applies only to the superficial attachment of nerve roots. The neurons associated with one spinal nerve extend well beyond the confines of a spinal segment.          The spinal cord is made up of thirty one segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal.         Note that in the cervical and coccygeal regions the number of spinal segments, and ...

  SEGMENTAL INNERVATION:       Any condition that leads to pressure on the spinal cord, or on spinal nerve roots, can give rise to symptoms in the region supplied by nerves. In such cases it is important to be able to localize the particular spinal segments, or roots, involved. For this purpose it is necessary to know which areas of skin, (and which muscles) are innervated through each segment. DERMATOMES        Areas of skin supplied by individual spinal nerves are called  dermatomes . To understand the arrangement of dermatomes it is necessary to know some facts about the development of the limbs. Development The upper and lower limbs are derived from limb buds. These are, paddle-shaped, outgrowths that arise from the side-wall of the embryo. They are at first directed forward and laterally from the body of the embryo (Fig. 2.8). Each bud has a preaxial (or cranial) border and a postaxial border (Fig. 2.9). The thumb and...

  SEGMENTAL INNERVATION OF MUSCLES        The nerve supply of muscles can also be described on the basis of spinal nerves from which the fibres are derived (or in terms of spinal segments). a.                It is rare for a muscle to be supplied only by one segment. One example is innervation of intrinsic muscles of the hand. b.                Most muscles derive innervation from two or more segments. c.                 The segments supplying muscles acting on a joint also supply the joint itself. d.                Muscles having a common action are usually supplied by the same spinal segments.          The spinal segments supplying...