Neural Pathways and Sensory Systems Lecture

Overview of Neural Pathways

• Neural Pathway: Route for nerve impulses traveling within the nervous system.
• Components: Interconnected neurons for transmitting action potentials between the PNS and CNS.
• Function: Sensory neurons communicate with motor neurons via interneurons.
• Terminology:
  • Sensory Pathways: Neurons ascending toward the brain.
  • Somatic Sensory Pathways: Relay information from sensory receptors to the brain's primary sensory area.

Sensory Pathways

• Function: Transmit sensory information from PNS receptors to CNS.
• Example: Stepping on Lego triggers receptors, sends action potential to brain.
• Process:
  1. Receptor Activation: Detects stimulus (e.g., pain from stepping on Lego).
  2. Sensory Neuron Pathway: Travels up leg to spinal cord, synapses with another neuron.
  3. Decussation: Crossing over in the spinal cord.
  4. Transmission to Thalamus: Reaches thalamus, projects to primary cortex.

Sensory Modalities and Receptors

• Sensory Modality: Each type of sensation like touch, pain, vision, or hearing.
• Specialization: Each sensory neuron carries information for one modality.
• General Senses: Somatic (touch, pressure, vibration, itch, tickle) and visceral senses (internal organ conditions).
• Special Senses: Smell, taste, vision, hearing, and equilibrium.

Sensory Receptors

• Types:
  • Free Nerve Endings: Bare dendrites, e.g., pain, temperature.
  • Encapsulated Nerve Endings: Dendrites enclosed in tissue, e.g., pressure, vibration.
  • Separate Cells: Specialized cells synapsing with neurons, e.g., taste buds.
• Receptor Classification:
  • Location: Exteroceptors, interoceptors, proprioceptors.
  • Stimuli Origin: External vs. internal environment.
  • Functional: Reaction to specific stimuli.

Proprioceptors

• Muscle Spindles: Monitor muscle length, participate in stretch reflex.
• Gogi Tendon Organs: Located at tendon-muscle junction, prevent damage from excessive tension.

Major Sensory Pathways

1. Posterior Column-Medial Lemniscus Pathway:

   • Carries touch, pressure, vibration, and conscious proprioception.
   • Pathway: From limbs, trunk, neck, posterior head to primary sensory cortex.

2. Anterolateral (Spinothalamic) Pathway:

   • Transmits pain, temperature, itch, tickle.
   • Pathway: From limbs, trunk, neck, posterior head to primary sensory cortex.

3. Trigeminal Pathway:

   • Carries sensations from face, head, oral cavity.
   • Pathway: Via trigeminal nerve to sensory cortex.

Additional Pathways

• Anterior and Posterior Spinocerebellar Tracts:
  • Transmit proprioceptive info from trunk/lower limbs to cerebellum.
  • Aid in movement coordination, posture, and balance.

Conclusion

• Integration of Sensory Information: Not all sensations are perceived consciously.
• Receptor Adaptation: Over time, consistent stimuli may not be consciously detected (e.g., road noise).