U4L7 - The Reflex Arc

The Reflex Arc

Overview of Homeostasis

  • Definition:

    • Homeostasis refers to the maintenance of a "steady state" within a cell or organism.

    • Etymology:

    • Homeo = Same

    • Stasis = Unchanging

  • Importance:

    • Cells must withstand external changes by maintaining a constant internal environment.

  • Main Factors to Maintain:

    • pH

    • Temperature

    • Ion Concentration

Animal Systems of Homeostasis

  • Urinary System:

    • Responsible for water and ion balance.

  • Endocrine System:

    • Regulates hormonal functions including blood glucose, water balance, and blood pressure.

  • Nervous System:

    • Coordinates stimulus and response actions.

Understanding the Reflex Arc

Components of the Reflex Arc

  • External Cause:

    • Trigger that initiates the reflex.

  • Stimulus:

    • The specific change in the environment that is sensed.

  • Receptors:

    • Sensory cells or organs that receive the stimulus.

  • Regulator:

    • Typically consists of interneurons in the spinal cord that process the information.

  • Effector:

    • Muscles or glands that produce a response.

  • Response:

    • The action taken by the effector.

  • Effect:

    • The resulting change in the body's state or behavior.

Characteristics of the Reflex Arc

  • Nature of the Reflex:

    • The reflex arc operates as an all-or-none involuntary response to stimuli.

  • Examples of Reflex Actions:

    • Doctor taps your knee to test reflexes.

    • Withdrawal of hand upon touching a hot surface.

  • Significance:

    • The reflex action occurs without the involvement of brain pathways, allowing for extremely rapid responses (very FAST!).

Example Scenario of a Reflex Arc

  • Stimulus:

    • Touching a hot stove, resulting in a sudden increase in skin temperature of fingers.

  • Threshold for Response:

    • The reflex is triggered when temperature exceeds 42℃.

  • Involved Receptors:

    • Thermoreceptors: Respond to temperature changes.

    • Nociceptors: Sensitively detect pain.

  • Nerve Communication:

    • A message from sensory receptors is sent to a relay center in the spinal cord.

    • Can involve as few as ONE sensory neuron.

    • The relay center has interneurons, which can also be as few as one cell, then communicates with motor neurons (potentially as few as one motor neuron).

  • Outcome:

    • Skeletal muscles in the arm and hand react quickly to withdraw the hand from the hot stove, typically within 10.4 milliseconds.

Additional Feedback Mechanisms

  • Negative Feedback Theory:

    • The process demonstrates a typical example of negative feedback, where the body responds to a stimulus in a way that counteracts the change.

Role of the Cerebral Cortex

Concurrent Activity during Reflex Action

  • Cerebral Cortex:

    • Considered the thinking part of the brain and has regulatory functions.

  • Additional Messages Sent:

    • Sends messages to other effectors, including:

    • Skeletal muscles in the mouth and larynx

    • Lacrimal glands in the eye

    • Messages are transmitted via EFFERENT nerves (motor neurons).

  • Other Potential Responses:

    • Responses may include swearing or crying in reaction to the reflex action.

Further Study Resources

  • Anatomy Visualization:

    • Refer to handouts to visualize the anatomy of the nervous system related to the reflex arc.

  • Videos for Understanding:

    • Recommend watching the video "2-Minute Neuroscience: Knee-jerk Reflex" for additional visualization of the reflex arc.