Peripheral Nervous System

The Brain's Link to the World!

• The PNS includes all structures outside the brain & spinal cord

• Sensory receptors, peripheral nerves & ganglia, and efferent motor endings

• Sensory receptors respond to environmental stimuli. Actual sensation and perception do not occur in the receptors &endash; they occur in the brain.
• Receptors can be classified by location
  • Exteroceptors are superficial and detect outside stimuli
  • Interoceptors are deep and detect internal stimuli
  • Proprioceptors are found typically in muscle and advise our brain of our movements
• Receptors can be classified by stimulus detected
  • Mechanoreceptors detect pressure (also called baroceptors)
  • Thermoreceptors detect temperature changes
  • Photoreceptors detect light
  • Chemoreceptors detect chemicals in solution
  • Nociceptors detect pain
  • Note that overstimulation can cause most any type of receptor to function as a nociceptor
• Receptors can also be classified by structural complexity
• Sensory membrane potentials transduce the energy of a stimulus into electrical energy. A threshold stimulus to a receptor causes a special excitatory event called the receptor potential
• Adaptation occurs when a receptor is subjected to an unchanging stimulus

• The Cranial Nerves : know `em and love `em

• Twelve pairs
• The first two pair attach to the forebrain and the rest arise from the stem
• Except for the accessory and the vagus, all of the cranial nerves "serve" the head & neck regions
• Memorize them any way you know how!
  • I. Olfactory
  • II. Optic
  • III. Oculomotor
  • IV. Trochlear
  • V. Trigeminal
  • VI. Abducens
  • VII. Facial
  • VIII. Vestibulocochlear
  • IX. Glossopharyngeal
  • X. Vagus
  • XI. Accessory
  • XII. Hypoglossal
• the cranial nerves are mostly mixed nerves, and they are composed differently than the spinal nerves

• The Spinal Nerves : less annoying to remember

• 31 pairs, all mixed nerves, that supply all parts of the body except the head and parts of the neck
• named according to where they exit the spinal cord. 8 pairs of cervical nerves (C₁-C₈), 12 pairs of thoracic (T₁-T₁₂), 5 pairs of lumbar (L₁-L₅), 5 pairs of sacral (S₁-S₅), and 1 pair of coccygeal (C₀).
• The first 7 pairs of cervical nerves leave superior to the corresponding vertebrae. C₈ leaves inferior to vertebra C₇. All other spinal nerves leave inferior to the corresponding vertebra.
• Each spinal nerve is a combination of the dorsal and ventral roots of the spinal cord. The nerve exits the vertebral column and immediately divides again into a large ventral ramus and a small dorsal ramus. A tiny meningeal branch also branches off and re-enters and innervates the cord.
  • Dorsal rami supply the posterior body trunk. Ventral rami supply the rest of the trunk and the limbs.
• The spinal nerve rami, especially the dorsal rami, follow a simple, segmental pattern. Each dorsal ramus innervates the narrow strip of muscles and skin where it emerges from the spinal cord
• The ventral rami of T₁-T₁₂ go deep to each rib and work as the intercostal nerves. T₁₂ is the subcostal nerve and T₁ is more a part of the brachial plexus (see below). The intercostal nerves serve the intercostal muscles
• All other ventral rami form nerve networks referred to as plexuses. The ventral rami branch and join lateral to the spinal column to form these networks, which primarily serve the limbs.
  • Cervical plexus : under the sternocleidomastoid, it is formed by the ventral rami of C₁-C₅. The phrenic nerve is one of the motor branches of the cervical plexus &endash; it supplies motor and sensory fibers to the diaphragm. The phrenic nerve, which receives most of its input from C₃-C₅, can be irritated to cause hiccups.
  • Brachial plexus : partly under the neck and partly in the axilla. It can be felt just superior to the clavicle at the lateral border of the SCM. It is composed of the ventral rami from C₅-C₈ and most of T₁. Occasionally, there are fibers from C₄ and/or T₂. The main nerves arising from this plexus include the ulnar, radial, axillary, pectoral and three scapular nevers. The median nerve is also part of this plexus.
  • Lumbar plexus : arises from L₁-L₄. The largest nerve from this plexus is the femoral nerve. Most herniated disks affect the functioning of this plexus.
  • Sacral plexus : L₄-S₄, it is immediately inferior to the lumbar plexus. About half of its branches serve the buttock and lower limb; the rest innervate pelvic structures and the perineum. The major branch is the sciatic nerve, the thickest and longest nerve in the body. It branches into the tibial and common peroneal nerves at the knee. The gluteal nerves are also sacral branches.

• Innervation of the Skin

• The area of skin innervated by the cutaneous branches of a single spinal nerve is called a dermatome. C₁ is the only spinal nerve that has no dermatomes.
• Hilton's Law : any nerve serving a muscle producing movement at a joint also innervates the joint itself and the skin over that joint.

This page hosted by

Get your own Free Home Page