Reflex Action and Interneurons

Reflex Action and Interneurons Notes

Definition of Reflex Action

  • Reflex Action: An automatic (involuntary) specific response of an organ caused by an adequate sensory stimulus.

Components of Reflex Action

  1. Stimulus: The trigger for the reflex action.
  2. Receptor: Detects the stimulus (e.g., sensory neurons).
  3. CNS (Central Nervous System): Spinal cord processes the signal.
  4. Effector: The organ or muscle that responds.
  5. Response: The action taken as a result of the reflex.

Types of Reflexes

Local Reflexes
  • Centers are outside the CNS (e.g. Dorsal Root Ganglia, Enteric Nervous System).
Systemic Reflexes
  • Centers are within the CNS.
Conditioned vs. Unconditioned Reflexes
  • Conditioned Reflexes: Acquired reflexes, require an intact cerebral cortex, involve training.
  • Unconditioned Reflexes: Inherent (inborn) reflexes, do not require an intact cerebral cortex, no training necessary.

Reflex Arc

  • Pathway consists of three main components:
  1. Afferents (sensory neurons)
  2. Interneurons
  3. Efferents (motor neurons)

Sensory Neurons

  • Monopolar Neurons in the Dorsal Root Ganglion (DRG).
  • Axon divides into:
    1. Peripheral branch: Terminates in sensory receptors.
    2. Central branch: Enters the spinal cord.
  • Functions:
  • Conduction of sensory signals.
  • Diverges signals to wider areas in the nervous system.

Interneurons

  • Two types: Exictatory and Inhibitory.
  • Functions:
  1. Divergence circuits: Spread signals to multiple neurons.
  2. Convergence circuits: Multiple inputs into fewer outputs, facilitating spatial summation of signals.

After-Discharge Mechanism

  • Definition: Prolonged discharge from an efferent neuron even after stimulation stops.
  • Types of Circuits:
  1. Open interneuron circuits: Afferent neuron stimulates efferents directly and through multiple interneurons.
  2. Closed interneuron circuits: Involves reverberation, can cause sustained activity until fatigue/inhibition occurs.

Inhibitory Circuits

  1. Lateral Inhibitory Circuits: Afferent neuron activates an inhibitory interneuron to inhibit adjacent neurons, crucial for focusing sensation.

  2. Recurrent Inhibitory Circuits: Efferent neuron collaterals inhibit themselves and surrounding neurons, using neurotransmitters like glycine.

Renshaw Cells

  • Inhibitory interneurons that help focus motor activity by inhibiting AHC (alpha motor neurons).

Types of Efferent Neurons in Spinal Reflexes

  1. Autonomic Motor Neurons: Mediate autonomic reflexes.
  2. Somatic Motor Neurons: Mediate somatic reflexes.

Properties of Polysynaptic Reflexes

  • All spinal reflexes are polysynaptic, except stretch reflex, due to the presence of interneurons and synapses.

Reflex Response Patterns

  • Influenced by:
  • Site of stimulus: Different areas of the limb may produce variations in reflex actions.
  • Strength of stimulation: Varies response based on intensity (mild, moderate, severe stimulus).

Total Reflex Time

  • Defined as the time from stimulus application to reflex response onset; influenced by conduction speeds along afferent and efferent pathways, neuromuscular transmission, and central processing time.

Reflex Fatigue

  • Gradual decline in response due to prolonged stimulation, leading to synaptic fatigue.

Antagonistic Muscle Inhibition

  • Reflex contraction of one muscle group often leads to inhibition of the opposing group, except in positive supporting reactions.

Significance of Reflex Actions

  • Physiological importance in protecting from harm (withdrawal reflex) and supporting weight (crossed extensor reflex).
  • Clinical importance includes testing reflex pathway integrity.

Summary of After-Discharge Mechanisms

  • Can arise from various factors including lateral inhibition, parallel circuits, and feedback loops via Renshaw cells.