The Spinal Cord

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					The Spinal Cord

   Chapter 13
                 Functions
• The spinal cord with its 31 pairs of spinal
  nerves serves two important functions.
• It is the connecting link between the brain
  and most of the body.
• It is involved in spinal reflex actions, both
  somatic and visceral.
    Basic Anatomy of the Spinal
               Cord
• The spinal cord extends caudally from the brain for about
  45 cm and has a width of ~14 mm. Its upper end is
  continuous with the brain (medulla oblongata). The cord
  is slightly thicker than a pencil.
• There are 31 pairs of spinal nerves:8 cervical, 12 thoracic,
  5 lumbar, 5 sacral, and coccygeal. The roots of the
  lumbar and sacral are called cauda equina.
• Surrounding and protecting the spinal cord is the
  vertebral column.
• The spinal cord is slightly flattened dorsally and
  ventrally, with two enlargements-cervical and
  lumbosacral from which the spinal nerves emerge
  that innervate the upper and lower limbs.
   Basic Anatomy of the Spinal
              Cord
• The cervical enlargement supplies nerves
  to the pectoral girdle and upper limbs.
• The lumbar enlargement supplies nerves
  to the pelvis and lower limbs.
• Inferior to the lumbar enlargement, the
  spinal cord becomes tapered and conical-
  conus medullaris.
• Filum terminale-slender strand of fibrous
  tissue that extends from conus medullaris.
                  Spinal Nerves
• There are 8 cervical nerves(C), 12 thoracic(T), 5 lumbar
  (L), 5 sacral (S), and 1 coccygeal (Co).
• Each pair of spinal nerves passes through a pair of
  intervertebral foramina located between two successive
  vertebrae. Each spinal nerve caudal to the first thoracic
  vertebra takes its name from the vertebra immediately
  preceding it.
• The nerves are then distributed to a specific pair of
  segments of the body.
• The spinal cord and the roots of its nerves are protected by
  the vertebral column, its ligaments, spinal meninges and
  cerebrospinal fluid.
             Spinal Meninges
• The outer layer is called dura mater. This is a
  tough, fibrous memebrane that merges with the
  filum terminale.
• The middle layer, the arachnoid, runs caudally to
  the S2 vertebral level. This is delicate and
  transparent.
• The innermost is called, pia mater. It is highly
  vascular and tightly attached to the spinal cord and
  its roots.
• Meningitis-bacterial or viral infection.
             Spinal Meninges
• Between the dura mater and periosteum of the
  vertebrae is the epidural space that contains many
  blood vessels and fat.
• Anesthetics can be injected here below the L3
  vertebral level, from which it ascends to act upon
  sensory neurons to help dull pain. This procedure
  is called caudal block.(epidural block)
• Space between dura mater and archnoid-subdural
  space-no CSF.
• Space between arachnoid and pia mater-
  subarchnoid space-CSF, blood vessels, spinal
  roots.
          Cerebrospinal Fluid
• This is a clear watery ultra filtrate solution
  primarily derived from blood.
• The basic mechanism involves an active transport
  system and passive diffusion into the four
  ventricles.
• The CSF provides a cushion that protects the
  delicate tissues of the spinal cord.
• It is also involved in the exchange of nutrients
  between the blood and neurons of the brain and
  spinal cord.
             Internal Structure
• If the spinal cord is cut in X.S., a tiny central canal
  is observed, which contains CSF.
• There is a dark portion of H-shaped or butterfly
  shaped “gray matter”, surrounded by a larger area
  of “white matter”.
• The spinal cord is divided into more or less
  symmetrical halves by a deep groove called the
  anterior(ventral) median fissure and a median
  septum called posterior (dorsal) median sulcus.
• Extending from the spinal cord are the ventral and
  dorsal roots of the spinal nerves.
                   Gray Matter
• The gray matter of the spinal cord consists of nerve cell
  bodies, dendrites and axon terminals(unmyelinated) and
  neuroglia. It is pinkish-gray color because of a rich
  network of blood vessels.
• The gray matter forms an H shape and is composed of
  three columns of neurons-posterior, anterior and lateral
  horns. The projections of gray matter toward the outer
  surface of spinal cord are called horns.
• The two that run dorsally-posterior horns which function
  in afferent input. The two that run ventrally-anterior
  horns which function in efferent somatic output. The two
  that extend laterally-lateral horns.
• The nerve fibers that form the cross of the H are known as
  gray commisure-functions in cross reflexes.
                White Matter
• The white matter gets its name because it is
  mainly composed of myelinated nerve fibers, and
  myelin has a whitish color.
• The white matter is divided into three pairs of
  columns or funiculi of myelinated fibers-anterior,
  posterior, lateral and a commisure area.
• The bundles of fibers within each funiculus are
  divided into tracts called fasciculi.
• Ascending tracts-sensory fibers carry impulse up
  the spinal cord to the brain.
• Descending tracts-motor neurons transmit
             Spinal Nerves
• A series of connective tissue layer
  surrounds each spinal nerve.
• Epineurium-outermost layer, consists of a
  dense network of collagen fibers.
• Perineurium-extend inward from th
  epineurium, dividing the nerve into a series
  of compartments.
• Endoneurium-delicate connective tissue
  fibers.
        Ventral and Dorsal Roots
• In the vicinity of the cord, each spinal nerve divides into a
  ventral (anterior, motor) root and a dorsal (posterior,
  sensory) root.
• Ventral roots contain mostly efferent nerve fibers and
  convey motor information.
• Dorsal roots contain afferent nerve fibers and convey
  sensory information.
• The axons of motor neurons whose cell bodies are located
  within the CNS in the ant. Horn emerge from the spinal
  cord to form ventral roots (motor).
• Groups of sensory neurons , whose axons make up the
  dorsal roots lie outside the cord in the dorsal root ganglia
  or spinal ganglia of the PNS.
 Peripheral distribution of Spinal
              Nerves
• A typical spinal nerve has a white ramus(this contains
  myelinated axons), and a gray ramus (unmyelinated fibers
  that innervate glands and smooth muscles in the body wall
  or limbs)
• A dorsal ramus(providing sensory and motor innervation
  to the skin and muscles of the back), and a ventral ramus
  (supplying the ventrolateral body surface, structures in the
  body wall and the limbs).
• Each pair of nerves monitors a region of the body surface
  called a dermatome.
            Nerve Plexuses
• A complex, interwoven network of nerves is
  a nerve plexus.
• The three large plexuses are the cervical
  plexus, the brachial plexus and the
  lumbosacral plexus. The latter can be
  further divided into the lumbar plexus and
  the sacral plexus.
 Functional Roles of Pathways of
              CNS
• Each pathway is composed of organized
  sequences of neurons.
• Upper motor neurons in the brain influence the
  activity of lower motor neurons in the cranial and
  spinal nerves.
• Some neurons have long axons that terminate in
  processing centers-called nucleus, ganglion, gray
  matter of spinal cord or cortex of the brain.
      General Somatic Efferent
          (Motor) System
• The brain exerts active influences on the activity
  of skeletal muscles through descending motor
  pathways that make up the upper motor neurons.
• These originate from the cell bodies in the cerebral
  cortex and brainstem.
• These act by regulating and modulating the
  activity of the lower motor neurons of the cranial
  and spinal nerves.
        Lower Motor Neurons
• These include alpha and gamma motor neurons.
• Alpha motor neurons have their cell bodied in
  their CNS. Their axons course through cranial
  and spinal nerves and terminate on the motor end
  plates of skeletal muscle fibers (extrafusal muscle
  fibers). Involved in stretch reflex.
• Gamma neurons also have cell bodied within the
  CNS. Their axons pass through cranial and spinal
  nerves to innervate the intrafusal muscle fibers
  inside the neuromuscular spindles. Involved in the
  gamma motor neuron reflex.
          Lower Motor Neurons
• These are the only neurons that innervate the skeletal
  muscle fibers, they function as the final common pathway,
  the final link between the CNS and skeletal muscles.
• Axons are located both in the cranial and spinal nerves.
• Those in cranial nerves innervate the skeletal muscles
  associated with the movements of the eyes, tongue,
  chewing, swallowing, vocalizing.
• These are influenced by two sources: sensory receptors
  that are integrated into reflexes and upper motor neurons
  from the brain that form the “voluntary descending
  pathways”.
        Upper Motor Neurons
• This is entirely located in the CNS.
            Sensory Pathways
• Some of these have sequences that are made of
  three neurons.
• They may be called first, second and third order
  neurons.
• A first-order neuron extends from sensory receptor
  to CNS.
• A second-order neuron extends from the spinal
  cord or brainstem to nucleus in the thalamus.
• A third-order neuron extends from the thalamus to
  a sensory area of the cerebral cortex.
• A critical feature of many pathways is that they
  cross over or decussate. By knowing where a
  pathway crosses over, a physician can use this
  information to help locate the site of an injury in
  the CNS.
• Example is touch-pressure pathway that
  decussates in the medulla oblongata.
                     Tracts
• Many tracts are named after their nuclei of origin,
  their termination as well as their location in the
  spinal cord (eg. Lateral spinothalamic tract).
• Anterolateral System: this consists of the lateral
  spinothalamic tract, spinoreticulothalamic tract
  and anterior spinothalamic tract. This involves the
  general sensatins of pain, temperature and light
  touch.
• Posterior-column medial lemniscus pathway.
                 Reflexes
• A reflex is a predictable involuntary
  response to a stimulus.
• A reflex involving the skeletal muscles is
  called a somatic reflex.
• A reflex involving responses of smooth
  muscle, cardiac muscle, or a gland is a
  visceral reflex.
      Classification of Reflexes
• Reflexes are classified according to :
• 1) their development : innate and acquired
• 2) site of information processing: cranial and
  spinal reflexes.
• 3)nature of resulting motor response : somatic and
  visceral reflexes.
• 4) the complexity of the neural circuit :
  monosynaptic and polysynaptic reflexes.
             Spinal Reflexes
• A reflex always starts with a sensory neuron and
  ends with a motor neuron.
• In a monosynaptic (one synapse, two neuron)
  reflex arc, the sensory and motor neurons synapse
  directly.
• More often, however, one or more interneurons
  synapse with the sensory and motor neurons in a
  polysynaptc reflex arc.
• Most reflex actions never travel any higher than
  spinal cord. Some maybe to brain stem.
          Types of Reflexes
• Stretch (Myotatic Reflex)-monosynaptic
  reflex arc. Maintains erect posture. Eg.
  Knee-jerk or patellar reflex. Ipsilateral-
  response and stimulus on same side.
• Gamma Motor Neuron Reflex Arc-this
  acts to smooth out the movements of muscle
  contractions or to sustain the contraction of
  a muscle.
          Types of Reflexes
• Plantar Reflex: this clinically tests the
  integrity of the spinal cord from L4 to S2.
  It is tested by drawing a blunt instrument
  down the lateral aspect of the sole(plantar
  surface) of the foot. A normal response is a
  curling or downward flexion of the foot.
• Withdrawal reflex arc: involves sensory
  receptors, afferent neurons, interneurons,
  alpha motor neurons, skeletal muscles.
       Some Diagnostic Reflexes
•   Abdominal reflex
•   Achilles Reflex
•   Biceps Reflex
•   Babinski’s reflex
•   Hoffmann’s reflex
•   Patellar Reflex
•   Plantar reflex
•   Triceps reflex

				
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