Sympathetic System

Introduction

The sympathetic system is a thoracolumbar network that produces widespread visceral responses through divergent pathways and adrenal catecholamine release. Its fibers extend across multiple segments, while alpha and beta receptors show selective sensitivity to noradrenaline and adrenaline, respectively.

• The sympathetic system promotes energy expenditure during stress, physical activity, and emotional arousal. It enhances cardiovascular, respiratory, and metabolic functions to meet increased demands.
• Persistent overactivation may cause tissue depletion and weight loss.
• Reduced activity can result in lethargy and increased fat accumulation.

Functional Anatomy

Sympathetic Neurons

Preganglionic Neurons

• The functional anatomy of sympathetic neurons is organized to produce widespread and coordinated responses.
• Preganglionic neurons originate from the intermediolateral region of thoracic and upper lumbar spinal segments, approximately T1 to L3. Hence, the sympathetic division is termed the thoracolumbar outflow of the autonomic nervous system. These neurons exit the spinal cord through ventral roots and enter spinal nerves.
• They then pass into white communicating branches to reach paravertebral sympathetic ganglia.
• The sympathetic chain forms a longitudinal series of ganglia on either side of the vertebral column.
• Preganglionic fibers may synapse with postganglionic neurons at the same spinal level.
• Some fibers ascend to cervical ganglia or descend to lumbar and sacral ganglia before synapsing.
• Cervical ganglia include superior, middle, and inferior groups that supply head and neck structures.
• Additional lumbar and sacral ganglia extend sympathetic influence to lower body regions.
• Certain preganglionic fibers pass through the chain without synapsing and reach prevertebral ganglia near abdominal organs.
• This arrangement allows a single preganglionic neuron to influence multiple postganglionic neurons, producing divergence.
• As a result, sympathetic activation can affect several organs simultaneously. This structural organization supports rapid, coordinated responses during stress and increased physiological demand.

Postganglionic Neurons

• Postganglionic neurons of the sympathetic system arise from paravertebral ganglia and project to target tissues.
• Fibers supplying sweat glands, piloerector muscles, and cutaneous vessels reenter spinal nerves through gray communicating branches. These fibers distribute to peripheral structures along somatic nerves.
• Neurons supplying the head, heart, and lungs originate from cervical and upper thoracic ganglia. They reach organs either as distinct nerves or along perivascular plexuses accompanying arteries.

Sympathetic Ganglia

Sympathetic ganglia are classified into paravertebral, prevertebral, and terminal groups, forming the peripheral relay system of the sympathetic division.

Paravertebral Ganglia

Paravertebral ganglia form two longitudinal chains on either side of the vertebral column, with approximately 22 to 23 ganglia in each chain.

Cervical Ganglia

• In the cervical region, three ganglia are present: superior, middle, and inferior.
• The superior cervical ganglion supplies structures of the head through perivascular plexuses, including dilation of the pupil and eyelid elevation. It also contributes to innervation of lacrimal and salivary glands.
• The middle and inferior cervical ganglia supply thoracic organs such as the heart, lungs, trachea, and esophagus.
• The inferior cervical ganglion may fuse with the first thoracic ganglion to form the stellate ganglion.

Thoracic Ganglia

• The thoracic ganglia, numbering about twelve, mainly supply thoracic viscera and contribute fibers to head, neck, and upper abdominal organs.

• Fibers from upper thoracic segments extend to cervical regions, while lower segments supply abdominal structures.

Lumbar and Sacral Ganglia

• The lumbar and sacral ganglia extend the sympathetic chain into the lower body. These ganglia supply lower limbs, pelvic organs, and lower abdominal viscera.
• This segmental and interconnected arrangement enables widespread and coordinated sympathetic responses across multiple organ systems.

Prevertebral Ganglia

• Prevertebral ganglia are collateral ganglia located anterior to the vertebral column near major abdominal arteries. They include the celiac, superior mesenteric, and inferior mesenteric ganglia, positioned near their respective arterial origins from the aorta.
• Preganglionic fibers from thoracic segments, approximately T5 to T12, synapse in the celiac ganglion to supply upper abdominal organs such as the stomach, liver, pancreas, and spleen.
• Fibers from T10 to T12 reach the superior mesenteric ganglion and innervate most of the small intestine and part of the large intestine.
• Fibers from L1 to L3 synapse in the inferior mesenteric ganglion, supplying the distal colon, rectum, urinary bladder, and reproductive organs.

Terminal Ganglia

• Terminal ganglia are located within or very close to target organs.
• Examples include the adrenal medulla and ganglia associated with the heart, pancreas, and urinary bladder. These arrangements ensure precise autonomic control of abdominal and pelvic viscera.

Adrenal Medulla

• The adrenal medulla is a specialized neuroendocrine structure forming the inner part of the adrenal gland. It functions as a modified sympathetic ganglion, receiving direct preganglionic sympathetic innervation from lower thoracic segments.
• Preganglionic fibers synapse on chromaffin cells, which are modified postganglionic neurons.
• These cells release epinephrine and norepinephrine into the bloodstream instead of transmitting impulses through axons. This mechanism produces widespread systemic effects during stress.

Other Terminal Ganglia

• Other terminal ganglia are present in organs such as the heart, pancreas, and urinary bladder.
• Specialized cardiac cells contribute to autonomic regulation and support early heart development.

Table 31.1: Segmental distribution of sympathetic fibers.

Spinal Segments	Major Sympathetic Targets
T1–T2	Eye muscles, head and neck vessels, sweat glands
T3–T4	Heart, airways, and esophagus
T5–T9	Upper limb vessels and sweat glands
T6–T12	Gastrointestinal organs, liver, spleen, adrenal medulla
T10–L2	Lower limb vessels and sweat glands
L1–L3	Pelvic organs including bladder and reproductive structures

Neurotransmitters

• All preganglionic fibers in the autonomic nervous system release acetylcholine and are cholinergic.
• Most sympathetic postganglionic fibers are adrenergic and release noradrenaline or adrenaline. Exceptions include fibers supplying sweat glands and certain skeletal muscle vessels, which are cholinergic.
• Some visceral vessels may also receive limited cholinergic sympathetic innervation.

Sympathetic Responses

• Sympathetic responses include basal activity at rest and widespread activation during stress.

Basal Sympathetic Activity

• Basal sympathetic tone represents continuous low-level discharge that maintains organ function.
• Variations in this tone regulate heart rate, vascular resistance, and blood pressure during routine activities such as posture changes.
• Sympathetic output can be adjusted rapidly to meet physiological demands, such as increasing cardiac output during exercise.
• Enhanced stimulation elevates heart rate, myocardial contractility, and vascular tone.

Widespread Sympathetic Response

• Widespread sympathetic response occurs during stress, producing coordinated changes across multiple organs. This response prepares the body for increased activity and environmental challenges. It is mediated by divergence of neural pathways, where one preganglionic neuron influences many postganglionic neurons.
• Additional amplification occurs through activation of the adrenal medulla, releasing catecholamines into circulation. These mechanisms ensure rapid, integrated, and sustained systemic effects.

Sympathetic Divergence

• Sympathetic divergence refers to the extensive branching of preganglionic neurons, allowing one neuron to synapse with many postganglionic neurons.
• The ratio of postganglionic to preganglionic neurons is approximately 100:1, enabling wide distribution of signals.
• Preganglionic fibers branch within the paravertebral chain and connect with neurons at multiple spinal levels.
• This arrangement produces coordinated activation of several organs simultaneously. It forms the structural basis for widespread sympathetic responses during stress.

Role of Adrenal Medulla

• The adrenal medulla complements neural divergence by providing a hormonal mechanism for systemic effects. It functions as a modified sympathetic ganglion composed of chromaffin cells. These cells secrete epinephrine and norepinephrine, typically in a ratio close to 8:1.
• Hormones are released directly into the bloodstream following stimulation by preganglionic fibers.
• Circulating catecholamines amplify and prolong sympathetic effects across multiple tissues.
• Epinephrine plays a dominant role due to its higher secretion and wider distribution through circulation.
• Norepinephrine released from nerve terminals acts locally, whereas circulating epinephrine reaches distant organs.
• Epinephrine activates both alpha and beta receptors effectively, producing diverse physiological effects.
• Together, neural divergence and hormonal release ensure rapid, extensive, and sustained sympathetic activation.

Effects Of Sympathetic Stimulation

• Sympathetic stimulation produces its effects through catecholamines released from nerve endings and the adrenal medulla.

Effects via Adrenergic Receptors

• These actions are mediated by adrenergic receptors, classified as alpha and beta types with specific subtypes.
• In general, alpha receptors respond more to noradrenaline, whereas beta receptors are more sensitive to adrenaline.

Effects of α Receptor Stimulation

Effects of α1 Stimulation:

• Alpha one receptors are located in vascular smooth muscle, gastrointestinal and urinary sphincters, and the radial muscle of the iris.
• Their activation causes constriction of blood vessels, closure of sphincters, and dilation of the pupil. These effects are mediated through intracellular signaling involving inositol trisphosphate and increased calcium levels.

Effects of α2 Stimulation:

• Alpha two receptors are found on presynaptic nerve endings, gastrointestinal tract, platelets, and adipose tissue.
• Their activation reduces neurotransmitter release and produces inhibitory or relaxant effects. This occurs through decreased cyclic adenosine monophosphate formation.
• Together, these receptor-mediated actions regulate vascular tone, glandular activity, and metabolic responses during sympathetic activation.

Table 31.2: Effects of sympathetic and parasympathetic stimulation.

Organ/System	Sympathetic Effects	Parasympathetic Effects
Eye	Pupil dilation (α1); ciliary relaxation (β2); eyelid elevation	Pupil constriction; ciliary contraction
Glands	Reduced nasal secretion (α1); enzyme-rich saliva	Increased lacrimal, nasal, and salivary secretion
Heart	Increased heart rate, conduction, and contractility (β1)	Decreased heart rate and conduction
Blood Vessels	Predominant vasoconstriction (α1); skeletal muscle dilation (β2)	Limited direct supply; vasodilation in select regions
Lungs	Bronchodilation (β2); variable secretion	Bronchoconstriction and increased secretion
Skin	Sweat secretion; piloerection (α1)	Minimal effect
Gastrointestinal Tract	Reduced motility and secretion; sphincter contraction	Increased motility, secretion, and sphincter relaxation
Liver and Metabolism	Glycogen breakdown and glucose production	No significant effect
Pancreas	Decreased secretion (α2), variable increase (β2)	Increased secretion
Urinary System	Bladder relaxation (β2); sphincter contraction (α1)	Bladder contraction; sphincter relaxation
Reproductive Organs	Ejaculation (α1); uterine contraction or relaxation	Erection; variable uterine response
Others	Lipolysis; increased renin and melatonin secretion	Minimal effect

Effects of β Receptor Stimulation

• Beta receptor stimulation produces excitatory or relaxant effects depending on receptor subtype and tissue distribution.
• Beta one receptors are present in the sinoatrial node, atrioventricular node, and ventricular muscle. Their activation increases heart rate, conduction velocity, and contractility through elevated intracellular cyclic adenosine monophosphate.
• Beta two receptors are located in bronchial smooth muscle, vascular smooth muscle of skeletal muscle, and gastrointestinal walls. Stimulation causes relaxation, leading to bronchodilation and vasodilation. These effects are also mediated by increased cyclic adenosine monophosphate levels.
• Beta three receptors are found in adipose tissue. Their activation promotes lipolysis, mobilizing stored fat for energy utilization.

Fight-or-Flight Response

• The fight-or-flight response is a coordinated sympathetic reaction that prepares the body for acute stress or danger. It involves both neural activation and catecholamine release from the adrenal medulla, producing rapid systemic effects.
• Cardiovascular activity increases, with elevated heart rate, cardiac output, and blood pressure.
• Blood flow is redistributed toward skeletal muscles and the heart, while supply to skin and gastrointestinal organs decreases.
• Respiratory rate rises and bronchodilation occurs, improving oxygen delivery to tissues.
• Salivary secretion decreases, while mucus production increases to maintain oral lubrication.
• Metabolic activity is enhanced, with increased glycogenolysis and lipolysis, raising plasma glucose and free fatty acid levels.
• Sweat gland activity increases, producing perspiration that aids in heat loss, often accompanied by cool skin due to vasoconstriction.
• Contraction of piloerector muscles causes hair elevation, which may assist in thermoregulation and defensive appearance.
• Pupillary dilation improves visual acuity and environmental awareness.
• Central nervous system activation enhances alertness, attention, and rapid decision-making.
• Gastrointestinal motility and bladder activity are temporarily suppressed due to sphincter contraction.
• These integrated changes optimize physical performance and survival during acute stress.