Study Notes on the Sympathetic Nervous System

Autonomic Nervous System

Overview

  • Key Focus: Sympathetic Nervous System

  • Instructor: Dinda Victor

Lecture 3: Sympathetic Nervous System

  • Introduction to the components and functions of the Sympathetic Nervous System (SNS).

Organizational Structure of the Sympathetic Nervous System

  • Sympathetic Division of the Autonomic Nervous System (ANS)

  • Innervation:

    • Postganglionic fibers innervate various target organs.

Preganglionic Neurons

  • Location: Lateral gray horns of spinal segments T1-L2.

  • Fibers:

    • Short and myelinated (Type B fibers).

    • Leave spinal cord via ventral roots and enter sympathetic ganglia via white rami communicates.

Ganglia in Sympathetic Nervous System

  • Types of Ganglia:

    • Sympathetic Chain Ganglia (paired): Located adjacent to the spinal column.

    • Collateral Ganglia (unpaired): Found further from the spinal column, innervating abdominal organs.

    • Adrenal Medullae (paired): Hormone-releasing glands in adrenal glands.

Function of Sympathetic Nervous System

  • Splanchnic Nerves: Supply visceral organs in thoracic and abdominopelvic cavities.

  • Target Organs: Receive visceral effectors including thoracic cavity, head, body wall, limbs, and organs across the body.

Characteristics of Preganglionic Fibers

  • Preganglionic sympathetic fibers:

    • Arise from the lateral horn of the spinal cord.

    • Synapse with postganglionic neurons in sympathetic ganglia.

  • Preganglionic Fiber Pathways:

    • May terminate in the same ganglia, ascend, descend, or exit without synapsing to reach distant ganglia.

Sympathetic Ganglia

  • Definition: Structures where synapsing between preganglionic and postganglionic fibers occurs.

  • Important Groups:

    • Sympathetic Trunk (Paravertebral Ganglia)

    • Prevertebral Ganglia

    • Peripheral Ganglia

Postganglionic Neurons

  • Cell body in ganglia, characterized by:

    • Short, nonmyelinated fibers (Type C fibers).

  • Postganglionic Fiber Pathways:

    • Can re-enter the spinal nerve via gray ramus communicantes to innervate sweat glands, blood vessels, and piloerector muscles.

    • Can reach cranial nerves or vascular branches, innervating visceral organs.

Distribution of Sympathetic Neurons

  • Segmental Levels:

    • T1, T2: Head and Neck

    • T3, T4: Thoracic viscera

    • T5 to T9: Upper limb

    • T10 to L2: Lower limb, and upper abdominal viscera

    • T6 to T12: Lower abdominal viscera

    • L1, L2: Thoracic and abdominal vessels

Effects of Sympathetic Stimulation

Cardiovascular Effects

  • Heart:

    • Increased heart rate (HR), stroke volume (SV), cardiac output (CO).

    • Increases in systolic blood pressure (SBP) and diastolic blood pressure (DBP).

    • Positive effects: Chronotropic (+HR), Dromotropic, Bathmotropic, Inotropic (+Effect).

  • Blood Vessels:

    • Vasoconstriction of splanchnic and cutaneous vessels.

    • Vasodilation of skeletal muscle blood vessels.

Respiratory Effects

  • Lungs:

    • Bronchodilation (widening of the air passages).

    • Increased respiratory rate (Tachypnea).

Other Organ Effects

  • Gastrointestinal Tract (GIT):

    • Relaxation of smooth muscle, constriction of sphincters.

  • Central Nervous System (CNS):

    • Increased alertness, reduction in sleep.

  • Genitourinary System:

    • Relaxation of the detrusor muscle, stimulation of sphincter contraction, ejaculation in males.

Sympathetic Responses

  • Actions in various organs:

    • Eye: Dilates pupil.

    • Lung: Dilates bronchioles.

    • Heart: Increases heart rate.

    • Blood vessels: Constricts.

    • Gastrointestinal: Relaxes.

    • Bladder: Relaxes.

    • Uterus: Relaxes.

Sympathetic Cholinergic Innervations

  • Seen in the following organs:

    • Apocrine sweat glands.

    • Eccrine sweat glands.

    • Iris.

    • Pancreas.

Further Details on Fibers

  • Preganglionic Sympathetic Fibers: Classifications include small and myelinated, small and nonmyelinated, large and myelinated, and large and unmyelinated.

  • Preganglionic fibers to the heart arise from spinal segments T1-L2.

Precapillary to Postcapillary Resistance Ratio

  • Approximately 4:1.

Sympathetic Fibers Specific to Heart

  • Include endocardial and epicardial fibers. Other types mentioned are serotonergic and glycinergic fibers.

Applied Physiology Topics

  • Pathophysiology:

    • Horner's syndrome

    • Sympathomimetic drugs.

    • Sympatholytic drugs.

    • Parasympathomimetic drugs.

    • Parasympatholytics drugs.

    • Ganglion blockers.

Conclusion

  • The sympathetic nervous system is critical for the 'fight or flight' response and regulates many involuntary bodily functions to promote survival during stressful situations.

Thank You!!!