14.4 Spinal Nerves

• Spinal nerves associated with adjacent vertebrae. Three layers of connective tissue surround peripheral nerves: outer epineurium, central perineurium, inner endoneurium
• 31 pairs of spinal nerves 

  • 8 cervical 
  • 12 thoracic 
  • 5 lumbar 
  • 5 sacral 
  • 1 coccygeal
• Three layers of connective tissue surround each peripheral nerve: an outer epineurium, a central perineurium, and an inner endoneurium
• Arrangement of layers comparable to connective tissue layers in skeletal muscles
• Epineurium a tough fibrous sheath forming outermost layer of peripheral nerve
• Epineurium has dense irregular connective tissue mainly of collagen fibers and fibrocytes
• Epineurium is continuous with dura mater of spinal cord
• Perineurium composed of collagenous fibers, elastic fibers, fibrocytes
• Perineurium divides nerve into compartments with bundles of axons
• Perineurium, a single bundle of axons is known as fascicle or fasciculus
• Perineurium protects and isolates peripheral nerves from circulation
• Endoneurium surrounding each individual axon is composed of loose irregular connective tissue with collagen and elastic connective tissue fibers and fibrocyte
• Endoneurium surrounding each individual axon is composed of loose irregular connective tissue with collagen and elastic connective tissue fibers and fibrocyte
• Peripheral Distribution of Spinal Nerves. Dorsal and ventral roots of spinal nerve pass through intervertebral foramen, unite to form spinal nerve
• Peripheral Distribution of Spinal Nerves. Spinal nerves have four branches: 

  • White ramus communicans 
  • Gray ramus communicans 
  • Dorsal ramus 
  • Ventral ramus 
• Peripheral Distribution of Spinal Nerves. Rami communicantes carry visceral motor fibers to and from a nearby autonomic ganglion associated with sympathetic division of autonomic nervous system
• Peripheral Distribution of Spinal Nerves. Preganglionic axons myelinated, branch carrying fibers to ganglion is white ramus communicans
• Peripheral Distribution of Spinal Nerves. Innervate gland/smooth muscles from 2nd branch, gray ramus communicantes, rejoins spinal nerve
• Peripheral Distribution of Spinal Nerves. Dorsal (posterior) ramus of each spinal nerve receives sensory innervation from, sends motor innervation to, skeletal muscles of back 
• Peripheral Distribution of Spinal Nerves. Ventral (anterior) ramus supplies ventrolateral body surface, structures in body wall, and limbs
• Peripheral Distribution of Spinal Nerves. Each pair of spinal nerves supplies a specific region of skin, area known as dermatome
• Describe, in order from outermost to innermost, the three connective tissue layers surrounding each peripheral nerve.

  The outermost layer is called the epineurium. It surrounds the entire nerve. The middle layer, or perineurium, divides the nerve into a series of compartments that contain bundles of axons. A single bundle is called a fascicle. The endoneurium is the innermost layer, and it surrounds individual axons.
• Distinguish between a white ramus and a gray ramus.

  The white and gray rami connect the spinal nerve to a nearby autonomic ganglion. The white ramus carries preganglionic axons that are myelinated from the nerve to the ganglion. The gray ramus carries postganglionic unmyelinated axons from the ganglion back to the spinal nerve.