Study Notes on the Sympathetic Nervous System

Sympathetic Nervous System

General Overview

  • Speaker: Dr. Jay Ferguson, DC

  • The sympathetic nervous system (SNS) operates alongside the parasympathetic nervous system (PNS) to control various physiological processes within the body.

Functions of the Sympathetic Nervous System

  1. Control of Dually Innervated Tissues:

    • Regulates tissue under ordinary resting conditions.

  2. Maintenance of Vasomotor Tone:

    • Helps maintain normal tone in the smooth muscles of blood vessels, significant for regulating blood pressure.

  3. Fight or Flight Response:

    • Activated in emergency situations characterized by physiological or psychological stress.

Physiological Responses to Sympathetic Stimulation

  • Following sympathetic activation, various responses are observed in the body:

    • Mydriasis: Dilation of the pupil of the eye.

    • Increased Heart Rate: Enhances both rate and force of heart contractions.

    • Dilation of Coronary Blood Vessels: Improves blood flow to the heart.

    • Elevation of Blood Glucose Levels: Provides quick energy sources for cells.

    • Bronchodilation: Air passages widen, facilitating easier breathing.

    • Increased Blood Flow to Skeletal Muscles: Blood vessels dilate to support muscular activity.

    • Cutaneous Vasoconstriction: Reduces blood flow to skin to conserve heat in emergencies.

    • Constriction of Blood Vessels in Digestive Organs: Redirects blood to skeletal muscles and vital organs.

    • Increased Breakdown of Stored Fats: Provides more energy.

    • Piloerection (pilomotor reflex): Causes hair on skin to stand on end.

    • Perspiration: Produces sweat to cool the body.

    • Ejaculation: Sympathetic stimulation influences reproductive functions.

    • Immune Effects: Impacts immune response, possibly suppressing it during stress.

Nervous System Structure

Chemical Transmitters

  • Key neurotransmitters involved at specific junctions:

    • Ach: Acetylcholine

    • NE: Norepinephrine

    • Epi: Epinephrine

Anatomy of the Nervous Systems
Preganglionic and Postganglionic Neurons

  • Sympathetic Preganglionic Neurons:

    • Originate from the lateral horn of the gray matter of the spinal cord.

    • Spinal segments: T1 - L2 (thoracolumbar division).

    • Types of ganglia:

      • Paravertebral Ganglia: Located beside the vertebral column (sympathetic chain).

      • Prevertebral Ganglia: Located in front of the vertebral column.

    • Total ganglia on each side: 22 (3 cervical, 11 thoracic, 4 lumbar, and 4 sacral).

Sympathetic Preganglionic Neurons

  • Path of sympathetic preganglionic neuron:

    1. Leaves the spinal cord via the ventral root.

    2. Enters spinal nerve and exits in white rami communicantes.

    3. Enters the paravertebral chain of ganglia.

  • Various pathways following synapse:

    1. Synapse with postganglionic neuron at the same level.

    2. Send ascending/descending axon branches to other ganglia.

    3. Pass through sympathetic chain without synapsing and travel via splanchnic nerves to prevertebral ganglia.

    • Named after blood vessels from the aorta:

      • Celiac ganglia

      • Aortic-renal ganglia

      • Superior mesenteric ganglia

      • Inferior mesenteric ganglia

Postganglionic Neurons

  • Postganglionic Neurons:

    • Contains axons of preganglionic neurons along with dendrites and cell bodies of postganglionic neurons.

    • Location types: Paravertebral or prevertebral ganglia.

  • Pathways for postganglionic axons depend on their location:

    1. In paravertebral ganglia: exit through gray rami communicantes.

    2. Some are directly involved with autonomic plexuses.

    3. In prevertebral ganglia: exit to innervate viscera.

Special Cases

  • Innervation of adrenal medulla results in the release of catecholamines (epinephrine and norepinephrine) into the bloodstream, contributing to a widespread effect.

Structural Pathways

  • Major Nerve Pathways:

    • Describe pathways like greater splanchnic nerves (T5 to T9), and lesser splanchnic nerves (T9 to T10).

    • Importance of specific sympathetic and parasympathetic pathways in bodily functions and responses.

Reflection and Discussion

  • Discussion questions:

    • What physiological effects occur during sympathetic stimulation?

    • Locations of GVE neurons controlling sympathetic vs. parasympathetic functions.

    • The role of the vagus nerve in parasympathetic innervation to thoracic and abdominal viscera.