Spinal Cord and Spinal Nerves TeacherWeb

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           Ch 12 & 13
  Spinal Cord and Spinal Nerves

                       Spinal Cord
• This part of the central nervous system
  (CNS) is enclosed in the vertebral cavity
• It s protected by:
   – Vertebrae
   – Meninges –
       • dura, arachnoid and pia
       • covered in previous lecture
   – Cerebrospinal Fluid (CSF)
• Epidural space – space between the
  vertebrae and the dural sheath (dura

                     Spinal Cord
• The spinal cord extends inferiorly
  from the medulla oblongata to the
  lumbar region
• Along its length there are two
  notably thicker regions: The
  Cervical and Lumbosacral
   – These are sites where the
     nerves serving the limbs emerge
• Conus medullaris – the end of the
  spinal cord in the lumbar region
• Cauda equina – the tail-like
  collection of spinal nerve roots
  inferior to the conus medullaris
• Spinal nerves (PNS) will be covered   3
            Spinal Cord Gray Matter
• Gray matter consists of cell bodies, unmyelinated processes,
  and neuroglia
• Gray commissure – connects masses of gray matter on the left
  and right
• The central canal runs through the gray commissure
• Posterior (dorsal) horns – interneurons (sensory)
• Anterior (ventral) horns – some interneurons but mostly somatic
  motor neurons
• Lateral horns – contain autonomic nerve fibers

Gray Matter and Spinal Roots

Cross Section of Spinal Cord

                         Reflex Arc
• The reflex arc is the simplest network of cells capable of
  receiving a stimulus and producing a response

• A reflex is an automatic response to a stimulus that occurs
  without conscious thought.
• Components of a reflex arc
   –   Sensory receptor is stimulated and transmits an action potential to
   –   Sensory neuron. (in the case of pain reception 2-in-1 cell) To-
   –   Interneurons. To-
   –   Motor neuron. To-
   –   Effector organ which responds with a reflex                      7
                      Spinal Nerves
• 31 pairs of spinal nerves supply all of the body except head
• Each nerve is named according to its nearby vertebra
   – 8 cervical (C1-C8)
   – 12 thoracic (T1-T12)
   – 5 Lumbar (L1-L5)
   – 5 Sacral (S1-S5)
   – 1 Coccygeal (C0)

                      Spinal Nerves
• Each spinal nerve is formed from two roots
   – Dorsal root – sensory roots and ganglia
   – Ventral root – motor roots
• A dorsal root and a ventral root fuse laterally (on the side) to
  form spinal nerves

               Spinal Nerves: Roots
• Each spinal nerve arises from the spinal cord via two roots
• Each root is formed from a set of rootlets that attach to the
  spinal cord
• Ventral roots arise from the ventral horn of the spinal cord and
  contain motor (efferent) fibers
• Dorsal roots arise from sensory neurons in the dorsal root
  ganglia and contain sensory (afferent) fibers

Spinal Nerves: Roots

                 Spinal Nerve Rami
• Spinal nerves are actually quite short.
• Two spinal nerve roots give rise to each spinal nerve.
   – The dorsal root carries afferent (sensory) neurons.
   – The ventral root carries efferent (motor) neurons.
   – So each spinal nerve is a mixed nerve carrying both sensory
     and motor neurons.
• Each spinal nerve, in turn, gives rise to two spinal nerve rami
  (singular: ramus).
   – Both the dorsal and ventral spinal nerve rami carry both
     sensory and motor neurons.

Spinal Nerve: Rami

   Ventral Rami Form Nerve Plexuses
• All ventral rami except T2-T12 form interlacing networks
  called plexuses (i.e. spaghetti bowls)
• Plexuses are found in the:
  –   Cervical region
  –   Brachial region
  –   Lumbar region
  –   Sacral region
  –   The last two will be dealt with as the Lumbosacral plexus
• Each resulting branch of a plexus contains fibers from
  several spinal nerves
• Each muscle receives a nerve supply from more than
  one spinal nerve
• Damage to one spinal segment cannot completely
  paralyze a muscle                                   14
                     Cervical Plexus
The cervical plexus is formed by
  ventral rami of C1-C4
Most branches are cutaneous
  nerves of the neck, ear, back
  of head, and shoulders
The most important nerve of this
  plexus is the phrenic nerve .
The phrenic nerve is the major
  motor and sensory nerve of
  the diaphragm.
As long as an injury to the spinal
  cord occurs inferior to the
  emergence of the phrenic
  nerve the ability to breathe
  independently may be spared.
                     Brachial Plexus
• Formed by C5-C8 and T1
   – In some individuals C4 and T2 may also contribute to this plexus.
• The brachial plexus gives rise to nerves supplying the arm.
• The five major branches
   of this plexus are:
   – Axillary
   – Radial
   – Musculocutaneous
   – Ulnar
   – Median

Brachial Plexus for PT and OT
              • Additional detail is provided for
                those headed to PT and OT
              • Five ventral rami (C5-T1) form
              • three trunks that separate into
                six divisions then form
              • three cords that give rise to:
              • Branches/nerves
                 – Axillary
                 – Radial
                 – Musculocutaneous
                 – Ulnar
                 – Median
                 – Smaller nerves, too.
            Lumbosacral Plexus
      (Lumbar and Sacral Plexuses)
• Arises from L1-S4 and innervates the thigh, abdominal
  wall, psoas muscle, the buttocks, lower limb, pelvic
  structures, and the perineum
• The major nerves are the
   – Femoral (Lumbar)
   – Obturator (Lumbar)
   – Sciatic (Sacral)
       • Tibial
       • Common fibular (peroneal)

Practice Questions

Which of the following is NOT a nerve
A. The thoracic rami
B. The lumbosacral plexus
C. The cervical plexus
D. The brachial plexus

Which of the following nerves does NOT
   arise from the brachial plexus?
A. The musculocutaneous nerve
B. The radial nerve
C. The axillary nerve
D. The ulnar nerve
E. The phrenic nerve

The muscle fiber or gland cell that responds
   to the efferent impulses is known as the
A. Effector
B. Motor neuron
C. Integration center
D. Sensory neuron
E. Receptor

The basic pattern of the CNS in the spinal
 cord is a central cavity surrounded by a
 gray matter core and a white matter layer
 lying outside of that.


Although the pairs of spinal nerves
  correspond to the number of vertebrae in
  the each area, there are eight pairs of
  cervical cranial nerves, but only seven
  cervical vertebrae.


Each muscle in a limb receives its nerve
 supply from one spinal nerve.


A sensory neuron transmits afferent
  impulses to the CNS.



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