Nervous System: Autonomic Nervous System Anatomy & Physiology

Nervous System: Autonomic Nervous System Anatomy & Physiology

15.1 Functional Organization

  • Somatic Nervous System (SNS)

    • Definition: Consciously perceived or controlled processes.
    • Somatic Motor Portion: Sends signals from CNS to skeletal muscles.
    • Voluntary Movements: Involve the cerebrum.
    • Reflexive Movements: Involve brainstem and spinal cord.
    • Somatic Sensory Portion: Detects signals from special senses and skin.
    • Special Senses: Vision, hearing, equilibrium, smell, taste.
    • Proprioceptors: Sensory receptors located in joints and muscles.

  • Autonomic Nervous System (ANS)

    • Also Known As: Autonomic motor or visceral motor system.
    • Processes: Regulated below conscious level.
    • Functionality: Transmits signals from CNS to heart, smooth muscle, and glands.
    • Visceral Sensory Inputs: From blood vessels, not considered part of ANS.
    • Homeostatic Mechanisms: Maintain stable internal environment.

15.1a Lower Motor Neurons of the Somatic vs. Autonomic Nervous System

  • Somatic Nervous System Lower Motor Neurons:

    • Structure: A single lower motor neuron extends from CNS to skeletal muscle fibers.
    • Location: Cell body within brainstem or spinal cord.
    • Axon: Myelinated, large diameter.
    • Neurotransmitter: Acetylcholine (ACh) released to excite muscle fiber.

  • Autonomic Nervous System Lower Motor Neurons:

    • Structure: Utilizes a chain of two motor neurons to reach effector.
    1. Preganglionic Neuron:
      • Location: Cell body in brainstem or spinal cord.
      • Axon: Myelinated, projects to autonomic ganglion, releases ACh.
    2. Ganglionic Neuron (Postganglionic):
      • Location: Cell body in autonomic ganglion.
      • Axon: Unmyelinated, projects to cardiac muscle, smooth muscle, or gland, releases ACh or norepinephrine (NE).
      • Function: Can excite or inhibit effectors based on receptor type.

15.1b CNS Control of the Autonomic Nervous System

  • Regulation Locations:
    • Hypothalamus: Crucial for emotional processes of ANS such as fight-or-flight; influenced by cortex, thalamus, limbic system.
    • Brainstem Nuclei: Mediate visceral reflexes, e.g., maintenance of blood pressure.
    • Spinal Cord: Governs some ANS reflexes, e.g., defecation and urination influenced by parasympathetic system.

15.2 Functional Differences

  • Parasympathetic Division: Known as “rest-and-digest.”

    • Functions: Conserves energy and replenishes nutrients.

  • Sympathetic Division: Known as “fight-or-flight.”

    • Functions: Activates in situations of exercise, excitement, or emergency.

15.2a Anatomical Differences

  • Parasympathetic Division:

    • Origin: Craniosacral division, preganglionic neurons from the brainstem or S2–S4 spinal cord.
    • Axon Lengths: Long preganglionic axons, short postganglionic axons.
    • Ganglia Location: Close to or within the effector organ.
    • Branching: Few branches of preganglionic axons.

  • Sympathetic Division:

    • Origin: Thoracolumbar division, preganglionic neurons in lateral horns of T1–L2.
    • Axon Lengths: Short preganglionic axons, long postganglionic axons.
    • Ganglia Location: Close to spinal cord (sympathetic trunk ganglia).
    • Branching: Many branches of preganglionic axons.

15.2b Degree of Response

  • Parasympathetic Activity: Typically produces a local response due to long preganglionic axons with few branches affecting one or few effectors.
  • Sympathetic Activity: Facilitates mass activation; short preganglionic axons with many branches affecting several effectors, e.g., exercise/fear increases several organ system functionalities.

15.3 Parasympathetic Division

  • Active State: Most active when resting or eating.
  • Ganglia Locations:
    • Terminal Ganglia: Located close to effector.
    • Intramural Ganglia: Located within wall of effector.

15.3a Cranial Components

  • Cranial Nerves Conveying Signals:
    • Oculomotor Nerve (CN III):
    • Preganglionic axons extend to ciliary ganglion.
    • Postganglionic axons control pupil constriction and lens focus.
    • Facial Nerve (CN VII):
    • Preganglionic axons extend to pterygopalatine and submandibular ganglia.
    • Postganglionic axons control lacrimal glands; small glands of nose and mouth; salivary glands.
    • Glossopharyngeal Nerve (CN IX):
    • Preganglionic axons extend to otic ganglion.
    • Postganglionic axons control parotid salivary gland.
    • Vagus Nerve (CN X):
    • Preganglionic axons project extensively to thoracic and abdominal ganglia.
    • Functions include slowing heart rate, constricting lung bronchioles, enhancing digestive secretions.

15.3b Pelvic Splanchnic Nerves

  • Components:
    • Preganglionic axons from cell bodies in S2–S4 project to ganglionic neurons, contributing to hypogastric plexus.
    • Postganglionic axons influence smooth muscle contraction and increase secretions in urinary and digestive systems.

15.4 Sympathetic Division

  • Function: Maintains body during exercise and emergency; complex anatomical pathways.
  • Ganglia Location: Close to CNS.

15.4a Organization and Anatomy of the Sympathetic Division

  • Preganglionic Neurons:
    • Cell bodies in lateral horns of T1–L2, axons course through anterior roots to sympathetic trunks.
  • Sympathetic Trunks: Resembles a pearl necklace with lateral and anterior ganglia.
    • Cervical Portion: Three ganglia (superior, middle, inferior) that innervate neck and thoracic organs.
    • Superior Cervical Ganglion: Innervates sweat glands, blood vessels, and eye muscles.

15.4b Sympathetic Pathways

  • Exiting Axons: Four pathways for autonomous effectors:
    1. Spinal Nerve Pathway: For skin effectors – synapse with ganglionic neuron and travel via gray ramus to the effector.
    2. Postganglionic Sympathetic Nerve Pathway: For internal thoracic organs.
    3. Splanchnic Nerve Pathway: For abdominal and pelvic viscera, synapsing in prevertebral ganglia.
    4. Adrenal Medulla Pathway: Preganglionic axons stimulate release of hormones to enhance fight-or-flight response.

15.4c Effector Stimulation by the Sympathetic Division

  • Cardiovascular System: Increases heart rate and force; vasodilation of coronary arteries.
  • Digestive System: Inhibits secretion and motility; triggers glycogenolysis in liver.
  • Respiratory System: Dilation of bronchi for airflow increase.
  • Reproductive System, Urinary System, and Other.

15.5 Autonomic Plexuses

  • Definition: Networks of nerves containing sympathetic postganglionic axons, parasympathetic preganglionic axons, and visceral sensory axons.
  • Components: Cardiac, pulmonary, esophageal, abdominal aortic, and hypogastric plexuses.

15.6 Autonomic Neurotransmitters

  • Types: Acetylcholine (ACh) and Norepinephrine (NE) act depending on receptor type.
  • Cholinergic Neurons: Include all preganglionic and parasympathetic ganglionic neurons.
  • Adrenergic Neurons: Most sympathetic ganglionic neurons.

15.7 Autonomic Tone and Dual Innervation

  • Autonomic Tone: Continuous neurotransmitter release generating tone, allowing varied control.
  • Dual Innervation: Example of antagonistic effect (heart rate regulation) and cooperative effects in reproductive organs.

15.8 Autonomic Reflexes

  • Definition: Pre-programmed responses through reflex arcs including sensory receptors, relay to CNS, integration, motor neuron, and effector response.
  • Types of Reflexes: Cardiovascular, gastrointestinal, and micturition reflexes, demonstrating involuntary autonomic control.