Ch.14-Anatomy

• Nerve plexuses


  Complex interwoven network of nerves
• Formation of nerve plexuses

  1. Small skeletal muscles fuse with their neighbors to form larger muscles with compound origins

  2. Ventral rami converge and branch to form these compound nerves
• Major nerve plexuses


  • Cervical plexus
  • Brachial plexus
  • Lumbar plexus
  • Sacral plexus
• Cervical plexus


  • Consists of cutaneous & muscular branches in the ventral rami of spinal nerves C1-C4 & some nerve fibers from C5
  • Lies deep to the sternocleidomastoid muscle & anterior to the middle scalene, levator scapulae muscles
  • Phrenic nerve is the major nerve of this plexus, provides the entire nerve supply to the diaphragm
• Brachial plexus


  • Larger and more complex
  • Innervates the pectoral girdle & upper limb
  • Formed by the ventral rami of spinal nerves C5-T1
  • Ventral rami converge to form the superior, middle, inferior trunks
  • Branches of the trunks divide into anterior & posterior divisions
  • All posterior divisions unite to form the posterior cord
  • Anterior divisions of the superior & middle trunks unite to form the lateral cord
  • Medial cord is formed by a continuation of the anterior division of the inferior trunk
  • Lateral cord forms the musculocutaneous nerve exclusively & together with the medial cord contributes to the median nerve
  • Ulnar nerve is the other major nerve of the medial cord
  • Posterior cord gives rise to the axillary & radial nerves
• Lumbar plexus


  • Arises from the lumbar & sacral segments of the spinal cord
  • Ventral rami of these nerves supply the pelvis & lower limb
  • Often referred to as the lumbosacral plexus
  • Major nerves are the genitofemoral, lateral femoral cutaneous, femoral nerve
• Sacral plexus


  • Contributes to the sacral plexus with ventral rami of L4-S5
  • Plexus contains ventral rami from spinal nerves L4-S5
  • Major nerves are the sciatic nerve, pudendal nerve
  • As the sciatic nerve approaches the popliteal fossa it divides into the common fibular & tibial nerves
• Reflex


  Immediate involuntary motor response to a specific stimulus
• Neural reflex


  1. Arrival of stimulus & activation of receptor

  2. Relay of information to the CNS

  3. Information processing

  4. Activation of a motor neuron

  5. Response of a peripheral effector
• Classification of reflexes


  • Based on location of information processing (spinal, cranial reflexes)
  • Based on nature of motor response (somatic, visceral/autonomic reflexes)
  • Based on complexity of neural circuit (monosynaptic, polysynaptic reflexes)
• Monosynaptic reflex


  Sensory neuron synapses directly on a motor neuron
• Polysynaptic reflex


  Longer delay between stimulus & response, proportional to number of synapses involved
• Spinal reflexes


  • Range from simple monosynaptic to complex polysynaptic
  • Stretch reflex is a simple monosynaptic reflex that regulates skeletal muscle length & tone
  • Patellar reflex is a stretch reflex of the quadriceps muscles
  • Postural reflexes maintain normal upright posture with firm muscle tone & sensitive stretch receptors
• Higher centers


  Can modulate or build on reflexive motor patterns
• Motor control involves a series of interacting levels, from low-level monosynaptic reflexes to high-level brain centers