The Peripheral and Autonomic Nervous Systems: Stress Response

The Peripheral Nervous System (PNS) and Autonomic Nervous System (ANS)

Nerves: Bridging the CNS and PNS

  • Definition: Nerves are crucial bundles of axons that facilitate communication between the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
  • Classification based on information direction:
    • Afferent Nerves (Sensory Nerves):
      • Function: Transmit information from the external environment (e.g., sensory receptors in the skin) to the brain.
      • Direction: From the PNS to the CNS.
    • Efferent Nerves (Motor Nerves):
      • Function: Transmit a response from the brain (after processing) to muscles or other cells.
      • Direction: From the CNS to the PNS.
      • Note: Despite being called "motor" nerves, their action does not always result in visible movement.

The Reflex Arc: An Example of Nerve Communication

  • Scenario: Touching a hot stove.
  • Process breakdown:
    1. Sensory Detection: Specialized sensory receptors in the skin detect the heat.
    2. Action Potential Initiation: The detection of heat initiates an action potential.
    3. Sensory Transmission: This action potential travels via sensory neurons (sensory nerve fibers) from the skin to the spinal cord.
    4. Spinal Cord Activation: Within the spinal cord, specifically in the ventral horn, the sensory neuron directly activates a motor neuron.
    5. Motor Transmission: The activated motor neuron sends a signal along its axon to the effector muscle.
    6. Effector Response: In this example, the signal reaches the biceps brachii (the effector), prompting the muscle to contract.
    7. Behavioral Outcome: The contraction causes the forearm to flex at the elbow, leading to the rapid withdrawal of the hand from the hot surface.
    • This rapid, involuntary response is an example of a reflex arc, bypassing conscious brain processing for immediate action.

The Autonomic Nervous System (ANS)

  • Role: The ANS is a critical component of the PNS and is responsible for autonomously regulating all bodily actions over which individuals have no conscious control.
  • Examples of ANS functions: Digesting food, constricting pupils, regulating heart rate, breathing, and blood pressure.
  • Key Distinction: Often referred to as the "fight or flight" or "rest and digest" system due to its two main divisions.
  • Evolutionary Advantage: The "fight or flight" response provides an evolutionary advantage, enabling organisms to react swiftly to perceived threats, as illustrated by a smaller cat successfully frightening a larger dog.

Divisions of the Autonomic Nervous System

The ANS is divided into two main parts, which generally have opposing effects:

1. The Sympathetic Nervous System ("Fight or Flight" System)
  • Trigger: Activated when the brain receives a stressful stimulus, such as perceiving danger.
  • Initial Brain Response: The response begins in the amygdala and the hypothalamus.
  • Fast Sympathetic Response (within seconds):
    • Cardiovascular: Heart rate starts to beat faster.
    • Respiratory: Lungs are able to pull in more air.
    • Circulatory: Blood pressure rises.
    • Other effects: These responses prepare the body for immediate physical action, whether to confront a threat or escape from it. Examples include a dry mouth when speaking in front of a large group, indicating a mild stress response.
  • Secondary Response (Hormonal involvement):
    • Pituitary Gland: The initial sympathetic response also triggers the pituitary gland.
    • Hormone Release: The pituitary gland releases Adrenocorticotropic Hormone (ACTH).
    • Adrenal Gland Activation: ACTH then triggers the release of cortisol and adrenaline (also known as epinephrine).
    • Prolongation of Response: Cortisol and adrenaline are stress hormones that prolong and sustain the "fight or flight" signal, maintaining the body's heightened state of arousal.
    • Future Connection: The broader impact of stress and how it affects this system will be explored later in the lesson.
2. The Parasympathetic Nervous System ("Rest and Digest" System)
  • Trigger: Activated once the danger or excitement has passed, signaling the need for the body to return to a state of calm and recuperation.
  • Initiation: The hypothalamus plays a key role in triggering the parasympathetic division.
  • Actions (Reversal of Sympathetic Effects):
    • Cardiovascular: Reduces heart rate.
    • Respiratory: Reduces breathing rate.
    • Circulatory: Lowers blood pressure.
    • Digestive: Activates and restores normal digestive functions.
    • Overall Goal: To conserve energy and facilitate bodily maintenance after a stressful event.