ANS

16.1 Overview of the Autonomic Nervous System

  • The autonomic nervous system (ANS) is divided into three parts: sympathetic, parasympathetic, and enteric systems.

1. Differences Between Autonomic and Somatic Nervous Systems

  • Structure:
      - The autonomic nervous system controls involuntary functions, while the somatic nervous system controls voluntary movements.
      - ANS includes smooth muscle, cardiac muscle, and glands (visceral effectors), whereas somatic system primarily involves skeletal muscle.

  • Function:
      - ANS operates largely unconsciously.
      - Somatic system operates consciously and provides control over movements.

2. Visceral Reflex and Examples

  • Definition of Visceral Reflex:
      - ANS mediates unconscious automatic responses to stimuli.
      - Involves visceral receptors and effectors and typically results in a slower response.

  • Examples: Chilling of the body in response to temperature changes.

3. Sympathetic and Parasympathetic Division of ANS

  • General Functions:
      - Sympathetic Division: Increases alertness, heart rate, blood pressure, and reduces blood flow to skin and digestive tract, preparing the body for physical activity.
      - Parasympathetic Division: Reduces energy expenditure, generally responsible for body maintenance and homeostasis.

  • Autonomic Tone:
      - The balance maintained between sympathetic and parasympathetic activation based on the body's needs. Both divisions do not exclusively produce excitatory or inhibitory effects; they can function simultaneously for homeostasis.

4. Functional Division of ANS

  • Preganglionic and Postganglionic Fibers:
      - Preganglionic Fibers: Myelinated and originate from the CNS (brainstem or spinal cord), synapsing close to or within target organs.
      - Postganglionic Fibers: Unmyelinated, project from ganglia to effector organs and can release acetylcholine (ACh) or norepinephrine (NE).

  • Comparison with Somatic Motor System:
      - The terms preganglionic and postganglionic are not used in somatics, as the motor neurons connect directly to skeletal muscles without an intervening ganglion.

5. Action of the ANS

  • General Actions of ANS:
      - Controls glands, cardiac and smooth muscle targeted within Thoracic, Abdominopelvic cavities, and various body wall structures (e.g., blood vessels and sweat glands).

6. Neural Pathways in ANS

  • Communication in the ANS involves crossing through preganglionic and postganglionic fibers to reach target organs.

7. Anatomy of Autonomic Nervous System

A. Components and Pathways

  • Sympathetic Division:
      - Known as the thoracolumbar division because it arises from thoracic and lumbar spinal cord.
      - Pathway Characteristics: Short preganglionic fibers and long postganglionic fibers.

  • Parasympathetic Division:
      - Also called craniosacral division, originates from the brain stem and sacral region.
      - Features long preganglionic fibers (ending near target organs) and short postganglionic fibers.

B. Adrenal Glands Relationship to Sympathetic Nervous System

  • Adrenal glands (located atop kidneys) contain two parts:
      - Adrenal Cortex: Secretes steroid hormones.
      - Adrenal Medulla: Modifies postganglionic neurons with no dendrites or axons, forms part of the sympathoadrenal system for coordinated endocrine responses.

C. Enteric Plexus of Digestive Tract

  • The enteric nervous system functions individually, aptly referred to as the second brain of the body.

  • It regulates the motility of the gastrointestinal tract, monitors chemical conditions, and secretes digestive enzymes and acids.

16.3 Actions of the ANS

1. Neurotransmitters in ANS

  • Cholinergic Fibers: Nerves that secrete acetylcholine (ACh).
      - Muscarinic Receptors: Located on cardiac/smooth muscle and gland cells that respond to cholinergic innervation.
      - Nicotinic Receptors: Located at the junctions of ANS pre and postganglionic neurons and in the neuromuscular junctions stimulating excitatory effects.

  • Adrenergic Fibers: Nerves that secrete norepinephrine (NE).
      - Adrenergic Receptors:
        - Alpha Receptors: Generally exhibit excitatory effects.
        - Beta Receptors: Generally exhibit inhibitory effects.

2. Contrasting Effects of Sympathetic and Parasympathetic Divisions

  • Dual innervation: Most organs receive fibers from both divisions which often exhibit opposing (antagonistic) or complementary (cooperative) functions.
      - Antagonistic Example:
        - Dilation of pupils (sympathetic) vs. constriction of pupils (parasympathetic).
      - Cooperative Examples:
        - Parasympathetic division stimulates enzyme secretion while sympathetic stimulates mucus secretion in the salivary glands.

3. CNS Regulation of the ANS

  • The central nervous system coordinates autonomic functions through connections to the hypothalamus, midbrain, pons, and medulla.

  • Functions such as anger, fear responses, and various visceral reflexes are controlled through the spinal cord.

4. Visceral Reflexes Controlled by the Spinal Cord

  • Examples include reflex arcs related to muscle control in defecation and micturition, regulated by the spinal cord's autonomic pathways.

16.4 Development of Autonomic Nervous System

1. Ectoderm and Neural Development

  • Ectoderm forms the neural plate leading to the development of the nervous system.

  • Neurulation process leads to the formation of the neural tube, from which the CNS emerges.

2. Neurulation Process

  • The neural plate: forms during week 3 of embryonic development.

  • Formation of neural folds occurs, closing to form the neural tube, correlating to brain and spinal cord formation.

3. Brain Vesicles and Development

  • Expansion of the brain from the neural tube results in formation of three primary brain vesicles:
      - Prosencephalon: Forebrain.
      - Mesencephalon: Midbrain.
      - Rhombencephalon: Hindbrain.

  • Each primary vesicle later differentiates into secondary vesicles:
      - Telencephalon: Forms cerebral hemispheres.
      - Diencephalon: Forms thalamus, hypothalamus, and epithalamus.
      - Metencephalon: Forms pons and cerebellum.
      - Myelencephalon: Forms medulla oblongata.

4. Spinal Cord Organization

  • Neural tube structures differentiate into the alar plate (posterior section: sensory function) and basal plate (anterior section: motor function).

  • The central canal within the neural tube develops into the spinal cord's main cavity, illustrating the organization of sensory and motor pathways.