Chapter #13 Peripheral Nervous System

• Innervates


  What the nerve connects to, it will power
• PNS


  Nerves, how the brain communicates with the rest of the body. All sensory & motor info travels through, to & from the brain
• Types of movement


  • Voluntary
  • Involuntary
• Nerve


  Cordlike organ of the PNS, bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue
• Types of nerves


  • Spinal
  • Cranial
• Neuroglia (glial cells)


  Small cells that surround and wrap delicate neurons, with various functions
• Neurons (nerve cells)


  Excitable cells that transmit electrical signals
• Nerve: cordlike organ of PNS, bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue, two types: spinal or cranial, depending on where they originate
• Nerve types


  • Spinal nerves
  • Cranial nerves
• Nerve classification


  • Mixed nerves (contain both sensory and motor fibers, impulses travel both to and from CNS)
  • Sensory (afferent) nerves (impulses only toward CNS, somatic afferent, visceral afferent)
  • Motor (efferent) nerves (impulses only away from CNS, somatic efferent, visceral efferent)
• Spinal Cord Cross-sectional Anatomy

  1. Two lengthwise grooves that run length of cord partially divide it into right and left halves

  2. Ventral (anterior) median fissure

  3. Dorsal (posterior) median sulcus

  4. Gray matter is located in core (H and horns), white matter outside (faniculi)

  5. Central canal runs length of cord (filled with CSF)
• Dorsal


  Afferent, sensory
• Ventral


  Efferent, motor
• Spinal nerves: A mix of sensory & motor roots, ventral roots (bundle of motor neuron axons that exit the spinal cord), dorsal roots (sensory input to cord), dorsal root (spinal) ganglia (cell bodies of sensory neurons), spinal nerves formed by fusion of dorsal and ventral roots
• Regeneration of Nerve Fibers/ PNS axon can regenerate

  
  

  1.Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon

  2.Macrophages clean dead axon debris; Schwann cells are stimulated to divide

  3.Axon filaments grow through regeneration tube

  4.Axon regenerates, and new myelin sheath forms
• Each spinal nerve has: ventral roots (motor), dorsal roots (sensory), branch as rootlets then join to form spinal nerve, ganglia (nerve cell bodies) outside SC
• Nerve Plexuses


  All ventral rami except T2–T12 form interlacing nerve networks, found in cervical, brachial, lumbar, and sacral areas, only ventral rami form plexuses, each branch contains fibers from several different spinal nerves, each limb muscle is innervated by more than one spinal nerve, so damage to one does not cause paralysis
• Cervical Plexus C1–C4, first four ventral rami form looping cervical plexus, most branches form cutaneous nerves that innervate skin of neck, ear, back of head, and shoulders, innervate neck muscles, phrenic nerve (major motor and sensory nerve of diaphragm, major muscles for breathing, receives fibers from C3 to C5)
• Phrenic Nerve


  C3 – C5
• Irritation of the phrenic nerve causes spasms of the diaphragm, also called hiccups, if both phrenic nerves are severed, or if C3–C5 region of spinal cord is destroyed, diaphragm becomes paralyzed, respiratory arrest occurs, victim requires mechanical respirators to stay alive
• Brachial Plexus Nerves


  • Axillary nerve (to deltoid, teres major muscles; sensory input from superolateral arm)
  • Median nerve (to most anterior forearm muscles and hand muscles; sensory input from palmar side and dorsal tips of most fingers (not pinkie))
  • Musculocutaneous nerve (to anterior arm muscles (e.g., biceps brachii); sensory input from lateral forearm)
  • Radial nerve (to posterior arm muscles (e.g., triceps brachii), posterior forearm muscles; sensory input from posterior arm and forearm and dorsolateral hand)
  • Ulnar nerve (to anterior forearm muscles, most intrinsic hand muscles; sensory input from palmar & dorsal aspect of 2 medial fingers)
• Brachial Plexus C5–C8, T1
• Ulnar nerve is very vulnerable to injury, severe or chronic damage can lead to sensory loss, paralysis, and muscle atrophy, affected individuals have trouble making a fist and gripping objects, little and ring fingers become hyperextended at the knuckles and flexed at distal interphalangeal joints, causes hand to contort into a claw hand, striking the "funny bone" can make the little finger tingle
• Lumbar Plexus L1 to L4, innervates thigh, abdominal wall, and psoas muscle, femoral nerve (innervates quadriceps and skin of anterior thigh and medial surface of leg), obturator nerve (passes through obturator foramen to innervate adductor muscles)
• Sacral Plexus L4 to S4, serves the buttock, lower limb, pelvic structures, and perineum, sciatic nerve (longest and thickest nerve of body, innervates hamstring muscles, adductor magnus, and most muscles in leg and foot, composed of two nerves: tibial and common fibular)
• Sciatica, a common problem, is characterized by stabbing pain radiating over course of the sciatic nerve, injury could be caused by a fall, disc herniation, or badly placed injection into the buttock, if the nerve is transected, leg is nearly useless and cannot be flexed because hamstrings are paralyzed, foot and ankle cannot move at all, so foot drops into plantar flexion, a condition called footdrop, recovery from sciatic nerve injury is usually slow and incomplete, for lesions below knee, thigh muscles are spared, if tibial nerve is injured, paralyzed calf muscles cannot plantar flex foot, and a shuffling gait develops
• Intercostal nerves supply muscles of ribs, anterolateral thorax, and abdominal wall, give off cutaneous branches to skin along course
• Nerve Plexuses


  • Cervical Plexus (C1-C4)
  • Brachial Plexus (C5-T1)
  • Lumbar Plexus (L1-L4)
  • Sacral Plexus (L4-S4)
  • Intercostal Nerves (T2-T12)
• Dermatomes


  Body map showing relationship between nerves and skin, dermatome is area of skin innervated by cutaneous branches of single spinal nerve, all spinal nerves except C1 participate in dermatomes, extent of spinal cord injuries ascertained by affected dermatomes, most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness
• Hilton's law: Any nerve serving a muscle that produces movement at a joint also innervates that joint and skin over that joint
• Elements that innervate effectors

  • Skeletal muscle
  • Visceral muscle
  • Glands
• Innervation of Skeletal Muscle

  Takes place at neuromuscular junction, neurotransmitter acetylcholine (ACh) is released when nerve impulse reaches axon terminal
• Types of motor endings

  • Somatic motor endings
  • Autonomic motor endings
• Types of movement


  • Voluntary
  • Involuntary
• Innervation of Visceral Muscle and Glands

  Autonomic motor endings and visceral effectors are simpler than somatic junctions, branches form synapses in passing "beaded terminals" with effector cells via varicosities, visceral motor responses are slower than somatic responses
• Reflex


  An action that is performed as a response to a stimulus and without conscious thought
• Inborn (intrinsic) reflex: rapid, unlearned, unpremeditated, involuntary, predictable motor response to stimulus
• Innervation


  Activate effectors by releasing neurotransmitters
• Elements innervated


  • Skeletal muscle
  • Visceral muscle
  • Glands
• Neuromuscular junction


  Where innervation of skeletal muscle takes place