Nervous System and Sensory Pathways

• Ganglion and Neuron Types

  • Posterior Ganglion: Site where neuron axons converge before entering the spinal cord. Axons may ascend or cross over.
  • Neurons:
  • Proprioceptive Neurons: Large, myelinated neurons that sense body position (muscle spindles and tendons).
  • Haptic Neurons: Large, myelinated neurons from mechanoreceptors (sensations of touch and pressure).

• Deafferentation:

  • Defined as the process of undoing afferent pathways; causes numbness.
  • Affects the spinal cord and peripheral system, leading to loss of sensory information from nerves.

• Spinal Cord Pathways:

  • Bell and Magendie's Law: Afferent neurons (input) enter through the dorsal root (posterior root ganglion) and efferent neurons exit via the ventral root.
  • Neon neuron crossings: Nociceptive neurons cross over immediately in the spinal cord, while proprioceptive and haptic neurons ascend on the same side before crossing in the brain stem.

• Myelination in Neurons:

  • Myelination is beneficial for faster conduction of afferent signals (e.g., proprioceptive and haptic neurons have extensive myelination).

• Nociceptive Neurons:

  • Small, unmyelinated fibers related to pain perception.
  • Ascend contralaterally after crossing at the spinal cord, whereas proprioceptive and haptic fibers ascend ipsilaterally before crossing at the brainstem.

• Reflex Pathways:

  • Monosynaptic Reflex: Involves one synapse (e.g., the patellar tendon reflex) where afferent neurons connect directly to efferent neurons for quick response.
  • Multisynaptic Reflexes: More complex, involving additional interneurons.

• Interneurons and Pain Perception:

  • Free nerve endings involved in nociception; pain pathways can alter the perception of pain based on inhibition of signaling via interneurons (inhibiting painful responses).

• Pain Mechanism and Management:

  • Pain perception can be suppressed by applying pressure, which stimulates large myelinated fibers that inhibit nociceptive pathways.
  • Gating Theory of Pain: Explains how increased activity in non-pain fibers can diminish pain signals to the brain.

• Vestibular System and Balance:

  • Hearing vs. Balance: Two distinct portions of the inner ear; semicircular canals respond to angular motion and movement, while otolithic organs respond to gravity and linear movement.
  • Fluid in the inner ear contributes to equilibrium response through hair cells that detect head position adjustments.

• Meniere's Disease:

  • Disorder of the inner ear impacting balance and causing vertigo.

• Somatosensory Cortex:

  • Involved in processing sensory information; different sections respond to different types of sensory input (e.g., fast and slow adapting receptors, proprioception).

This comprehensive understanding of the neural pathways, types of receptors, pain perception, and related anatomy will be essential for the upcoming exam.