15.1

Learning Outcomes

  • Expected learning outcomes include:
      - Explain how the autonomic and somatic nervous systems differ in form and function.
      - Explain how the two divisions of the autonomic nervous system differ in general function.

15.1 General Properties of the Autonomic Nervous System (ANS)

  • Definition and Role:
      - The autonomic nervous system (ANS) is defined as a motor nervous system that controls:
        - Glands
        - Cardiac muscle
        - Smooth muscle
      - Also referred to as the visceral motor system.
      - Distinct from the somatic motor system, which controls skeletal muscles.
      - Primary target organs include:
        - Viscera of the thoracic and abdominal cavities
        - Structures of the body wall, including cutaneous blood vessels, sweat glands, and erector muscles of hair.

  • Characteristics of ANS:
      - Operates involuntarily without intent or awareness.
      - In contrast to the voluntary nature of the somatic motor system.
      - Exceptions include:
        - Some skeletal muscle responses may be involuntary (e.g., somatic reflexes).
        - Certain muscles are difficult to control (e.g., middle ear muscles).

  • Biofeedback and Autonomic Control:
      - Therapeutic biofeedback demonstrates that individuals can learn to voluntarily control autonomic functions (e.g., blood pressure, heart rate).

  • Functional Dependency:
      - Visceral effectors do not require the ANS to function.
      - ANS adjusts activity in response to the body's needs:
        - Example: The heart continues to beat even if all nerves are severed, but the ANS modulates heart rate during rest or exercise.

  • Muscle Response upon Nerve Severance:
      - Severing somatic nerves to skeletal muscles results in flaccid paralysis (muscles do not function).
      - Severing autonomic nerves affects cardiac or smooth muscles leads to exaggerated responses known as denervation hypersensitivity.

15.1 A Visceral Reflexes

  • Definition:
      - The ANS oversees visceral reflexes, which are:
        - Unconscious
        - Automatic
        - Stereotyped responses to stimulation, similar to somatic reflexes but involving visceral receptors and effectors.

  • Components of Visceral Reflex Arc:
      1. Receptors: Nerve endings that detect:
         - Stretch
         - Tissue damage
         - Blood chemistry
         - Body temperature
         - Other internal stimuli.
      2. Afferent Neurons: Carry signals from receptors to the CNS.
      3. Integrating Center: Neurons in the CNS that process information.
      4. Efferent Neurons: Carry motor signals away from the CNS to effectors.
      5. Effectors: Carry out the end response based on the stimulus.

  • Example of a Visceral Reflex:
      - Process of blood pressure regulation through a visceral reflex or baroreflex:
        - High blood pressure activates baroreceptors in the internal carotid arteries that send signals via the glossopharyngeal nerves to the brainstem.
        - The medulla integrates the information and sends signals to the heart via the vagus nerves, leading to:
          - Decreased heart rate
          - Reduced blood pressure.
        - A separate reflex accelerates the heart when blood pressure drops below normal (e.g., when transitioning from reclined to standing position).

15.1 B Divisions of the Autonomic Nervous System

  • Subdivisions of the ANS:
        - Composed of three subsystems:
          1. Sympathetic Division
          2. Parasympathetic Division
          3. Enteric Plexus

  • General Functions:
      - The two divisions differ in anatomy and function but often target the same organs, sometimes with cooperative or contrasting effects.

  • Sympathetic Division Characteristics:
      - Prepares the body for physical activity (fight or flight response).
      - Effects include:
        - Increased alertness
        - Elevated heart rate
        - Elevated blood pressure
        - Increased pulmonary airflow
        - Elevated blood glucose levels
        - Increased blood flow to cardiac and skeletal muscles.
      - Concurrently reduces blood flow to:
        - The skin
        - The digestive tract.
      - Triggers during situations of:
        - Arousal
        - Exercise
        - Competition
        - Stress
        - Danger
        - Trauma
        - Anger
        - Fear.
      - Subtle effects may go unnoticed in daily activities.

  • Parasympathetic Division Characteristics:
      - Has a calming influence on many body functions.
      - Associated with:
        - Reduced energy expenditure
        - Maintenance functions like digestion and waste elimination.
      - Referred to as the resting and digesting state.
      - Functions simultaneously with sympathetic division with an osmotic tone background rate of activity.
      - Balance between sympathetic tone and parasympathetic tone adjusts according to body needs:
        - Parasympathetic tone maintains smooth muscle tone in intestines and controls resting heart rate (70-80 beats/min).
        - Sympathetic tone keeps blood vessels partially constricted, maintaining blood pressure.
        - Loss of sympathetic tone can lead to rapid blood pressure drop, possibly resulting in shock or fainting.

  • Enteric Plexus:
        - A complex network of autonomic neurons in the digestive tract's wall.
        - Employs distinct neural pathways and neurotransmitters from the sympathetic and parasympathetic divisions.
        - Further exploration will occur in later chapters.

15.1 C Autonomic Output Pathways

  • Components of ANS:
        - Predominantly part of peripheral nervous system but includes:
          - Control nuclei in the hypothalamus and other brainstem regions.
          - Motor neurons in the spinal cord and peripheral ganglia.
          - Nerve fibers traversing cranial and spinal nerves.

  • Autonomic vs. Somatic Motor Pathways:
      - Somatic Pathway:
        - A motor neuron in the brainstem or spinal cord issues a myelinated fiber axon reaching directly to skeletal muscle.
      - Autonomic Pathway:
        - Signal traverses two neurons to reach the target organ, crossing a synapse at an autonomic ganglion.
        - First neuron (preganglionic fiber) has:
          - Cell body in either brainstem or spinal cord.
          - Axon extending to an autonomic ganglion (typically near the vertebral column or target organ).
          - Releases the neurotransmitter acetylcholine (ACh) at the synapse.
        - Second neuron (postganglionic fiber):
          - Leaves the ganglion and extends to the target organ.
          - Depending on fiber type, may secrete either ACh or norepinephrine (NE).

  • Differences in Fiber Myelination:
      - Autonomic preganglionic fibers are lightly myelinated; postganglionic fibers are unmyelinated.
      - Unlike somatic motor nerve endings, autonomic postganglionic fibers do not engage in one-to-one synapses with muscle cells but release neurotransmitters from varicosities, stimulating multiple target cells simultaneously.

Table 15.1 Comparison of Somatic and Autonomic Nervous Systems

  • Feature comparison table between somatic and autonomic nervous systems.

  • Key questions for understanding:
      1. How does the autonomic nervous system differ functionally and anatomically from the somatic motor system?
      2. How do the general effects of the sympathetic division differ from those of the parasympathetic division?