Peripheral Nervous System and Autonomic Nervous System Review

Overview of the Peripheral Nervous System (PNS)

• Function of PNS: Provides essential links to and from the body’s external environment.
• Components: Includes all neural structures outside the brain:
  • Sensory Receptors: Detect environmental changes.
  • Peripheral Nerves: Carry signals between the CNS and the rest of the body.
  • Ganglia: Clusters of nerve cell bodies in the PNS.
  • Efferent Motor Endings: Neuronal terminals that activate muscles and glands.

Structural Organization of the Nervous System

• Central Nervous System (CNS): Comprises the brain and spinal cord.
• Peripheral Nervous System (PNS): Divided into two major divisions:
  • Sensory (Afferent) Division: Conveys sensory information to the CNS.
  • Motor (Efferent) Division: Transmits impulses from the CNS to effector organs:
  • Somatic Nervous System: Controls voluntary movements (skeletal muscles).
  • Autonomic Nervous System (ANS): Regulates involuntary functions (smooth and cardiac muscles, glands).
    • Contains sympathetic and parasympathetic divisions.

Sensory Receptors

• Definition: Specialized structures that respond to environmental stimuli and induce graded potentials leading to nerve impulses.
• Distinction:
  • Sensation: Awareness of stimulus.
  • Perception: Interpretation of the stimulus’ meaning.
• Classification of Receptors:
  1. Type of Stimulus:
  • Mechanoreceptors: Respond to mechanical forces (touch, pressure).
  • Thermoreceptors: Detect temperature changes.
  • Photoreceptors: Respond to light (e.g., in the retina).
  • Chemoreceptors: Respond to chemical stimuli (e.g., smell, taste).
  • Nociceptors: Sensitive to pain (e.g., extreme temperatures, pressure).
  1. Location: Cutaneous vs. internal receptors.
  2. Structural Complexity: Simple vs. complex receptors.

Pain Perception and Tolerance

• Pain Perception: Uniform perception across individuals but tolerance varies.
• Genetic Influence: Some genes affect pain tolerance and response to medications.
• Homeostatic Imbalances:
  • Hyperalgesia: Increased pain sensitivity.
  • Chronic Pain: Long-lasting discomfort affecting quality of life.
  • Phantom Limb Pain: Sensation of pain in limbs that have been removed; possibly from spinal cord changes.

Nerve Structure & Regeneration

• Structure of Nerves: Composed of bundles of myelinated and unmyelinated axons, encased in connective tissue (epineurium, perineurium, endoneurium).
• Regeneration Process:
  • Amitotic Nature of Neurons: Mature neurons cannot divide, but peripheral axons can regenerate if the cell body is intact.
  • Wallerian Degeneration: The process following axon injury, involving fragmentation and cleanup by macrophages.
  • Regeneration Tubes: Axon filaments grow through and form new myelin sheaths.

Spinal and Cranial Nerves

• Cranial Nerves: Twelve pairs, mainly mixed nerves, each associated with different functions (sensory, motor, or both).
• Spinal Nerves: 31 pairs of mixed nerves; each named according to its point of origin on the spinal cord.
  • Cervical (C1-C8): 8 pairs, distinguishable despite a lack of cervical vertebrae.
  • Thoracic (T1-T12), Lumbar (L1-L5), Sacral (S1-S5), Coccygeal (C0) pairs exist as well.

Autonomic Nervous System (ANS)

• Function: Regulates involuntary bodily functions through two branches: sympathetic and parasympathetic.
• Sympathetic vs. Parasympathetic:
  • Sympathetic: Mobilizes body for activity (fight-or-flight response). Increases heart rate, dilates pupils, etc.
  • Parasympathetic: Conserves energy; promotes maintenance activities (rest-and-digest).

Key Differences Between ANS Divisions (Table 14.1)

Reflex Arcs

• Components of Reflex Arcs:
  1. Stimulus: Initiates the reflex.
  2. Receptor: Detects stimulus.
  3. Sensory Neuron: Carries signal to integration center.
  4. Integration Center: Processes the signal, often in the spinal cord.
  5. Motor Neuron: Carries signal to the effector.
  6. Effector: Executes the reflex action (muscle or gland).

Conclusion

• Understanding the PNS and its components is crucial for grasping how the nervous system functions and how it interacts with our bodies. The distinctions between various receptors, pain perception, and nerve regeneration are key elements in neurobiology that help explain human physiology and the processes of healing and response.