Lecture Notes on Reflexes

Overview

• Reflexes are simple control systems.
• Defined as involuntary and rapid responses to stimuli.
• Utilize a feed-forward control system without feedback.

Feed-forward Control System

• Initial stimulus triggers a response without affecting the state variable (e.g., furnace and temperature analogy).
• Efferent output is independent of afferent input.
• No modulation after initiation.

Somatic Reflex Pathways

• End with skeletal muscles; example: musculoskeletal reflexes.
• Example: Patellar Tendon Reflex
  • Hammer hits patellar ligament causing rapid elongation of quadriceps muscle.
  • Speed matters: Quick stimulus triggers reflex.
  • Muscle spindle organ encodes muscle length.
  • Control flow chart:
    • Afferent pathway (green): Sensory side.
    • Efferent pathway (red): Quadriceps muscle activated by motor neuron.
  • Integration center in spinal cord (not brain).
  • Monosynaptic reflex (single synapse between afferent and efferent pathways).
  • Reciprocal Inhibition: Hamstring muscle is inhibited to allow quadriceps contraction.

Autonomic Reflex Pathways

• Include both parasympathetic and sympathetic pathways.
• Example: Urination Reflex
  • Bladder fills, stretch receptors detect tension.
  • Afferent pathway: Stretch receptors in bladder walls.
  • Efferent pathway: Parasympathetic control of smooth muscle.
  • Integration through an interneuron.
  • Polysynaptic reflex (two synapses before efferent recruitment).
    • Integration center involves bladder tension and threshold activation.
    • Sphincter control by skeletal muscle (motor neuron regulation).
  • Reflex overridden by brain commands during toilet training.

Modulation of Reflexes

• Reflexes can be overridden by descending commands from the brain.
• Interneurons facilitate modulation by providing additional synapse points.

Key Concepts

• Feed-forward systems: Initiated by stimulus without modulation.
• Muscle spindle organs: Important in encoding muscle state.
• Monosynaptic vs. Polysynaptic Reflexes: Depend on the number of synapses.
• Reciprocal inhibition: Ensures coordinated muscle action.
• Reflex modulation: Possible through learning and central nervous system (CNS) involvement.

Examples in Life

• Reflexes play a role in maintaining equilibrium while moving.
• They respond to startling stimuli and maintain functions like urination.

This lecture covers the basic understanding of how reflexes work, their pathways, and how they can be modulated by the brain.