Simple circuits

Reflexes

• Definition: Simple patterns of behavior in response to stimuli.

• Example: Pupil response to light and pain (dilation and constriction).

Types of Reflexes

1. Two Neuron Reflex Arc

   • Components:

     • Afferent neuron (sensory)

     • Efferent neuron (motor)

   • Example: Knee Jerk Reflex

     • Mechanism:

       • Stretching of the extensor via patellar tendon tap activates muscle spindle fibers.

       • Sends action potential along unipolar neuron to spinal cord.

       • Collateral branch relays info to the brain and to Renshaw cells (inhibitory neurons).

       • Result: Extensor contracts, flexor relaxes (due to Renshaw cell inhibition).

2. Three Neuron Reflex Arc

   • Components:

     • Afferent neuron

     • Interneuron

     • Efferent neuron

   • Example: Withdrawal Reflex (e.g., pulling arm back from a hot stove)

     • Mechanism:

       • Pain detected at fingertips activates unipolar neuron.

       • Signal splits at the spinal cord:

       • One pathway to the brain for interpretation.

       • Another pathway to the interneuron, activating the lower motor neuron for limb movement.

Neural Circuit Concepts

• Convergence: Multiple inputs into a single output

  • Example: Multiple photoreceptors connect to fewer bipolar cells.

• Divergence: One input spreads out to multiple outputs

  • Example: Ganglion cells connecting to multiple cortical cells.

Facilitation and Occlusion

• Facilitation:

  • More output than expected when signals combine (greater than the sum of parts).

  • Example:

    • Neuron A activates neuron 1.

    • Neuron B can activate neuron 3 due to connections.

    • When stimulated together, neurons 1, 2, and 3 activate.

• Occlusion:

  • Less output than expected when signals combine (less than the sum of parts).

  • Example:

    • Neuron A activates both neurons 1 and 2.

    • Neuron B activates neurons 2 and 3.

    • When stimulated together, only neurons 2 and 3 activate (totaling three instead of four).

Application in Sensory Systems

• Facilitation in Low Stimulus Intensity:

  • Enhances detection probability (evolutionarily beneficial).

• Occlusion at High Stimulus Intensity:

  • Protective mechanism to prevent overstimulation and damage to sensory cells.

• Linear Relationship: Between low and high stimulations under various psychophysical laws explaining physiological and psychological experiences.