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THE SPINAL CORD AND SPINAL NERVES (DOC download)

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					               THE SPINAL CORD AND SPINAL NERVES
                                                 (05-04-11)

                                                 Functions
    1. Provides a pathway for sensory and motor impulses
    2. Reflexes—i.e., quick reactions to stimulus

                              Protective Structures of the Spinal cord
•   Vertebral column
•   Meninges
•   Denticulate ligaments
•   Cerebral spinal fluid
•   Epidural space

                                        The Vertebral Column
•   Vertebral foramina form a cavity
     – Vertebral foramina of all the vertebra form the spinal cavity
•   The vertebral canal is longer than the spinal cord

                                            The Meninges
Same layers as found in the brain
Are continuous with the cranial meninges
Layers:
   Dura mater—forms a sac that extends from the foramen magnum to the 2nd sacral vertebra.
    Arachnoid mater—has a ―spider web‖ arrangement of collagen and elastic fiber.
    Pia mater—adheres to the surface of the spinal cord and brain; contains many blood vessels that
    supply the spinal cord with oxygen and nutrients.
    Subarachnoid space
    Subdural space—contains interstitial fluid
    Epidural space—contains a cushion of adipose CT


                                      Denticulate Ligaments
Description:
      Are thickenings of the pia_ mater
      Project laterally
      Fuse with the arachnoid matter

Function: Protect the spinal cord from shock and displacement

                            External Anatomy of the Spinal Cord
•   Dimensions
        42 to 45 centimeter long, about 2 cm in diameter
•   Enlargements—result from greater number of neuron cell bodies, axons and dendrites
                                                   1
    – Cervical: from 4th cervical vertebra to 1st thoracic; has nerves to and from the upper limb

    – Lumbar: from 9th to 12th thoracic vertebra; nerves to and from lower limbs.

•   Conus medullaris—spinal cord tapers to a conical portion;




        Filum terminale

        Cauda Equina


        Spinal Nerves
            o Cervical
            o Thoracic
            o Lumbar
            o Sacral
            o Coccygeal

                                      Spinal Nerve Features
•   Roots
     – Posterior or dorsal root—contains only sensory axons

    – Anterior or ventral root—contains axons of motor neurons which conduct impulses from the
        CNS to effector organs and cells.

•   Ganglia—swelling found in each posterior root containing the cell bodies of sensory neurons.

                                                    2
                            Internal Anatomy of the Spinal Cord
•   Grooves
     – 1. Anterior median fissure: deep, wide
     – 2. Posterior median sulcus: narrower and shallower
•   Gray matter
     – Shape:
     – Consists of: primarily of the cell bodies of neurons, neuroglia, unmyelinated axons, dendrites
•   The gray commissure—forms the crossbar; permits communication between right and left sides;
    mostly unmyelinated axons.

•   Nuclei—clusters of neuronal cell bodies that form functional groups

•   Horns
     – Anterior—contain cell bodies of somatic motor neurons and motor nuclei
     – Posterior—contain somatic and autonomic sensory nuclei
     – Lateral—contain the cell bodies of autonomic motor neurons that regulate activity of smooth
       muscle, cardiac muscle and glands
     The white commissure—is anterior to the gray commissure; connects the white matter of the right
     and left sides of the spinal cord

    – Columns/funiculi
       • Anterior—both ascending and descending tracts
       • Posterior—ascending tracts or fasciculi
       • Lateral
•   Tracts
     – Ascending

    – Descending




                                                    3
                                 Branching of Spinal Nerves
•   Branches = rami
•   Rami:
     – Posterior—serves the deep muscles and skin of the dorsal surface of the trunk
     – Anterior—serves the muscles and structures of the upper and lower limbs and the skin of the
       lateral and ventral surfaces of the trunk
     – Meningeal rami reenter vertebral canal
     – Rami communicantes—are components of the autonomic nervous system

                                              Plexuses
•   Formed by the anterior (ventral) rami of spinal nerves except
•   Such networks are called plexuses
     – Rami join to form trunks
     – Trunks divide into divisions
     – Divisions converge to form cords
     – Branches emerge from cords
•   Cervical
     – Formed from: the anterior rami of nerves C1 through C4
     – The phrenic nerve arises from this plexus,
•   Brachial
     – Supplies the _______________________
     – Formed by: the anterior rami of nerves C5-T1

    – Innervates the: pectoral girdle and the entire upper limb




                                                    4
•   Lumbar
     – Formed by: anterior rami of spinal nerves L1-L4

    – Femoral nerve—main nerve of the posterior division
•   Sacral Plexus
     – Formed by: anterior rami of L4-S4
     – Sciatic nerve: longest nerve in the body; composed of two nerves:
        • Fibular
        • Tibial
•   Coccygeal Plexus

                                           Intercostal nerves
•   The anterior rami of T2 to T12 do not enter into the formation of plexuses
•   They are known as intercostal or thoracic nerves
•   They go directly to the structure they supply

                                                    Reflexes
•   Definition: rapid, automatic, involuntary reactions of muscles or glands to a stimulus

•   Functions: a survival mechanism that allows quick response; does not have to wait for the brain to
    respond

                                                Reflex Arcs
•   A reflex arc is the wiring of a single reflex
•   Components of a reflex arc:
     – Stimulus
     – Sensory receptor
     – Interneuron (not always present)
     – Motor neuron to effector
     – Effector

                                               Dermatomes
• Dermatome—the area of the skin that provides sensory input to the CNS via one pair of spinal
  nerves or cranial nerve V is called a dermatome.
• All spinal nerves except C1 innervate an area of the skin
• Importance: dermatomes can indicate potential damage to one or more spinal nerves
• Also involved in referred visceral pain—pain from one organ is referred to a dermatome




                                                       5
                 Autonomic Nervous System
      Comparison of ANS and Somatic Nervous Systems
• SOMATIC
• Receptors:                                • Autonomic Nervous System
   – receptors for the special              • Main input: autonomic sensory
     senses (vision, hearing, etc)            neurons
   – receptors for somatic senses           • interoreceptors (e.g.,
     (pain, thermal, tactile and              chemoreceptors, mechano
     proprioreception)                        receptors) located in blood
                                              vessels, visceral organs, muscles,
• Sensations are consciously                  + the nervous system
  perceived                                 • Monitor the internal environment
• Output: Somatic motor neurons             • Autonomic motor neurons may
  innervate skeletal muscle                   stimulate or inhibit effectors
• Movements: voluntary

                   Comparison of Neural Pathways
• Somatic                                   • Autonomic
• Sensory neuron with cell body in          • Sensory neuron with cell body in
  posterior root ganglion                     posterior root ganglion
• Somatic motor neuron with cell            • ANS motor neuron with cell body
  body in CNS                                 in CNS (preganglionic motor
• Effector (skeletal muscle)                  neuron)
                                            • Second motor neuron with cell
                                              body in second ganglion
                                              (ganglionic motor neuron)
•                                           • Effector (smooth muscle, glands,
•                                             organs)


           Divisions of the Autonomic Nervous System
• Sympathetic and Parasympathetic Divisions
• In common:
    – Both use a preganglionic neuron and a ganglionic neuron to innervate
      muscles or glands
    – Both are involuntary
    – Both are concerned with the body’s internal environment
    – Preganglionic axons are myelinated, postganglionic are unmyelinated

Parasympathetic Division (Craniosacral Division)
• Functions
   – ―Rest and digest‖ division
   – Maintain homeostasis
• Preganglionic neurons
   – Originate: brain stem or S-2 to S-4 spinal cord regions

                                        6
   – Length of axons: longer than those of sympathetic division
   – Number of axon branches: few (<4)
• Postganglionic neurons—axons are shorter
• Location of ganglia—close to or within the wall of the effector organ

Sympathetic Division (Thoracolumbar Division)
   – Prepares the body for emergencies
   – The fight or flight division—increases metabolic activity
   – Neurons originate in the lateral horns of the T1-L2 spinal cord regions
   – Ganglia are relatively close to the vertebral column
   – Preganglionic axons tend to have many branches (> 20)




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posted:8/18/2011
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