BIO 225 ch13 part 2 notes

Peripheral Nervous System Overview

  • Central Nervous System (CNS): Comprises the brain and spinal cord.

  • Peripheral Nervous System (PNS): Everything outside of the CNS, categorized into four main divisions.
      - Components:
        - Sensory Receptors: Detect external stimuli and send signals to the CNS.
        - Transmission Lines: Include nerves transmitting sensory information to the CNS (afferent) and those sending motor commands away from the CNS (efferent).
        - Motor Endings: Structures that innervate muscles, particularly skeletal muscles at neuromuscular junctions.
        - Spinal Reflexes: A summary of the reflex activities involving spinal cord processing.

Sensory Receptors and the Transmission Lines

  • Receptors classified by:
      - Structure
      - Type of Stimuli: Respond to environmental changes.

  • Types of Nerves:
      - Afferent Nerves (Sensory): Carry information towards the CNS from sensory receptors.
      - Efferent Nerves (Motor): Carry information away from the CNS to effectors such as muscles and glands.

Neuron Classification

  • Central Nervous System (CNS):
      - Bundles of Axons: Called tracts.
      - Cell Bodies: Called nuclei.

  • Peripheral Nervous System (PNS):
      - Bundles of Axons: Called nerves.
      - Cell Bodies: Called ganglia.

Cranial and Spinal Nerves

  • Cranial Nerves:
      - 12 pairs that arise from the brain.

  • Spinal Nerves: 31 pairs arise from the spinal cord, with an emphasis on spinal nerves in lecture; overview of cranial nerves in lab.

Structural Components of Nerves

  • Similar terminology to muscles:
      - Endoneurium: Surrounds each individual axon.
      - Perineurium: Surrounds a fascicle (bundle of axons).
      - Epineurium: Surrounds the entire nerve.

Types of Nerves

  • Mixed Nerves: Most nerves are mixed, carrying both sensory and motor information.
      - Sensory Nerves: Carry sensory information towards the CNS.
      - Motor Nerves: Carry impulses from the CNS.

Sensory Neuron Classification

  • Dorsal Aspect: Deals with sensory information (afferent signals).

  • Ventral Aspect: Deals with motor information (efferent signals).

  • Types of Sensory Neurons:
      - Unipolar: Typical for sensory neurons; includes a peripheral branch (to receptors) and a central branch (to CNS).
      - Dorsal Root Ganglion: Clusters of sensory cell bodies adjacent to the spinal cord.

Nerve Fiber Regeneration

  • CNS vs PNS Regeneration:
      - CNS:
        - Myelination: Oligodendrocytes produce growth-inhibiting proteins, preventing regeneration.
      - PNS:
        - Myelination: Schwann cells do not inhibit growth and support regeneration.
        - Regeneration Steps:
          1. Wallerian Degeneration: Distal segment of the axon degenerates after injury.
          2. Debris Cleanup: Macrophages clear away debris from the injury area.
          3. Growth Cone Formation: Schwann cells form a regeneration tube with growth cones.
          4. Remyelination: Regenerated axon is remyelinated by Schwann cells.

Spinal Nerve Structure and Function

  • Spinal Nerve Formation: Arises from rootlets merging to form roots, then spinal nerves.

  • Dorsal Rootlets: Carry sensory information; Ventral Rootlets: Carry motor information.

  • Branching of Spinal Nerves:
      - Dorsal Ramus: Innervates back muscles.
      - Ventral Ramus: Innervates anterior and lateral trunk as well as limbs.
      - Meningeal Ramus: Returns to innervate the meninges.

Nerve Plexuses

  • Cervical Plexus:
      - Formed by ventral rami of C1 to C4; important nerve:
        - Phrenic Nerve: Innervates the diaphragm (C3, C4, C5).

  • Brachial Plexus:
      - Formed by ventral rami of C5 to T1; major nerves include:
        - Axillary Nerve: Innervates shoulder muscles.
        - Musculocutaneous Nerve: Innervates anterior arm.
        - Median Nerve: Innervates forearm flexors.
        - Radial Nerve: Innervates posterior arm and forearm extensors.
        - Ulnar Nerve: Innervates intrinsic hand muscles.

  • Lumbosacral Plexus:
      - Lumbar Plexus: L1 to L4; includes:
        - Femoral Nerve: Innervates quadriceps.
        - Obturator Nerve: Innervates medial thigh muscles.
      - Sacral Plexus: L4 to S4; major nerve:
        - Sciatic Nerve: Largest nerve, branches into tibial and common fibular nerves.

Innervation of the Skin and Joints

  • Dermatome: Maps the skin areas innervated by different spinal nerves.

  • Hilton's Law: States that the nerve supplying a muscle also innervates the joint over which the muscle acts.
      - Example: Axillary nerve innervates the deltoid and the shoulder joint; femoral nerve innervates quadriceps and the knee joint.

Summary and Preparation

  • Focus on memorization of nerve types, the plexuses, and their corresponding structures.

  • Review charts of nerves specific to plexuses for clinical relevance, especially for exam preparation.

  • Keep mind the relationships between nerves innervating muscles and the joints they influence for potential questions.