Anatomy of the Peripheral Nervous System

Anatomy of the Peripheral Sensory Nervous System

• The peripheral nervous system includes spinal nerves, brachial plexus, and lumbosacral plexus.
• These plexuses supply sensory innervation to the head via the trigeminal ganglion, and to the body parts via spinal nerves.
• 30 segmental spinal nerves are present, categorized by regions (e.g., C1-C8, T1-T12, L1-L5, S1-S5).
• Dorsal horns, dorsal columns, and dorsal root ganglia (DRG) are parts of the spinal cord.
• Sensory innervation reaches the head via the trigeminal ganglion, located near the pons.

Adult DRG Neuron Subclasses

• Three major functional sub-classes of adult DRG neurons exist.
• Proprioceptive neurons: Large soma and thickly myelinated axons.
  • Located centrally in the intermediate zone and ventral horn.
  • Innervate muscle spindles and Golgi tendon organs, and give spatial awareness of limbs.
• Low threshold mechanoreceptors: Medium to large soma, moderately myelinated axons.
  • Centrally located in the deep dorsal horn, projecting to dorsal columns.
  • Innervate specialized skin structures (e.g., tactile receptors), sensing light touch, vibration, and texture.
• Temperature, itch, and pain signaling (nociceptive) neurons: Small soma, either lightly myelinated Aδ or non-myelinated C fibers.
  • Terminate in the superficial dorsal horn, targeting mainly dermal skin.
  • Detect painful stimuli, temperature, and itch.

Sensory Pathways (Pain)

• Several ascending tracts within the spinal cord carry nociceptive signals to the brain.
• Spinothalamic tract: Conveys sharp, precisely localized pain signals. Signals project to the thalamus, and then to the primary sensory cortex.
• Spinoreticular tract: Carries dull, persistent pain and arousal responses. Inputs to the nuclei of the reticular formation(responsible for arousal).
• Spinomesencephalic tract: Autonomic and emotional responses related to pain. It projects nociceptive signals to higher centers.

Additional Information

• Spinal pain reflex: Heat sensitive nociceptors transmit action potentials to the dorsal horn, then to the motor neurons in the ventral horn, causing responses to remove the limb from the heat source.
• Temperature perception: Different temperature receptors (e.g., TrpVs) are activated at varying temperatures, triggering physiological/ noxious responses.
• Tissue damage triggers chemical release: ATP, K+, H+, prostaglandins, and bradykinin are liberated, activating nociceptors, increasing nerve sensitivities.
• Pain gating in dorsal horn: Activity in low-threshold mechanoreceptors inhibits incoming nociceptive signals in the dorsal horn (reducing pain). Descending regulation from areas like the periaqueductal gray reduces nociceptive signaling to the dorsal horn.
• Nerve Growth Factor (NGF) plays a role in prolonged hyperalgesia. NGF effects both the peripheral and central areas, influencing nociceptors.
• Endogenous opioids (e.g., endorphins, enkephalins, dynorphins) are produced by the body to modulate pain signals.

This quiz explores the anatomy and function of the peripheral sensory nervous system, focusing on spinal nerves and various plexuses. It covers the subclasses of adult dorsal root ganglion neurons and their roles in sensory innervation. Test your knowledge on how these components integrate to support sensory perception.