47. Anatomy | Autonomics of the Thorax

What are the two main divisions of the peripheral nervous system?

Somatic and Visceral innervation.

What structures are controlled by somatic innervation?

Skeletal muscle of the body wall (striated voluntary muscle).

What structures are controlled by visceral innervation?

Cardiac muscle, smooth muscle, and glands (organs in cavities and body wall structures such as sweat glands, sebaceous glands, arrector pili, vasculature).

How many neurons are involved in a somatic motor pathway?

One neuron: cell body in CNS → axon → synapse directly on target.

How many neurons are involved in a visceral motor pathway?

Two neurons: Preganglionic (cell body in CNS) → synapse in autonomic ganglion → Postganglionic neuron to target.

Where are somatic motor neuron cell bodies located?

In the ventral horn (anterior horn) of the spinal cord.

Where are somatic sensory neuron cell bodies located?

In the dorsal root ganglion.

Where are visceral preganglionic motor neuron cell bodies located?

In the lateral horn T1-L2 (thoracolumbar) or brainstem and sacral regions (S3-S4).

Where are visceral postganglionic motor neuron cell bodies located?

In autonomic ganglia outside the CNS (paravertebral, prevertebral, or in/near target organ).

Where are visceral sensory neuron cell bodies located?

In the dorsal root ganglion, just like somatic sensory neurons.

How does somatic sensory information present clinically?

Sharp, well localized sensations.

How does visceral sensory information present clinically?

Dull, poorly localized pain caused by ischemia, distension, cramping, or inflammation; not from touch, temperature, or cutting stimuli.

What is the nickname for sympathetic nervous system function?

Fight or Flight (energy expenditure).

What is the nickname for parasympathetic nervous system function?

Rest and Digest (energy conservation).

Where are sympathetic preganglionic neuron cell bodies located?

In the lateral horn of spinal cord levels T1-L2 (thoracolumbar outflow).

Where are parasympathetic preganglionic neuron cell bodies located?

In the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord S3-S4 (craniosacral outflow).

Are sympathetic preganglionic axons short or long?

Short.

Are parasympathetic preganglionic axons short or long?

Long.

Are sympathetic postganglionic axons short or long?

Long.

Are parasympathetic postganglionic axons short or long?

Short.

What neurotransmitter do sympathetic postganglionic neurons release?

Norepinephrine/epinephrine (adrenaline).

What neurotransmitter do parasympathetic postganglionic neurons release?

Acetylcholine.

Where are sympathetic ganglia located?

Paravertebral ganglia (sympathetic chain) or prevertebral/subdiaphragmatic ganglia.

Where are parasympathetic ganglia located?

Near or within the target organ wall (e.g., cardiac plexus, pulmonary plexus).

Does sympathetic innervation reach the body wall?

Yes - sweat glands, vasculature, and arrector pili muscles.

Does parasympathetic innervation reach the body wall?

No - parasympathetics never enter the body wall.

What is the sympathetic effect on Cardiac muscle?

Increases heart rate and contractility

What is the parasympathetic effect on Cardiac muscle?

Decreases heart rate and contractility

What is the sympathetic effect on Blood vessels?

Constriction (and dilation depending on receptor subtype)

What is the parasympathetic effect on Blood vessels?

No effect or mild dilation

What is the sympathetic effect on Bronchi?

Relaxation (bronchodilation)

What is the parasympathetic effect on Bronchi?

Contraction (bronchoconstriction)

What is the sympathetic effect on Sweat glands?

Stimulates secretion

What is the parasympathetic effect on Sweat glands?

No effect

What is the sympathetic effect on Digestive tract?

Decreases motility and secretion

What is the parasympathetic effect on Digestive tract?

Increases motility and secretion

What is the sympathetic effect on Pupils?

Dilation (mydriasis)

What is the parasympathetic effect on Pupils?

Constriction (miosis)

List the stepwise pathway of sympathetic innervation to the body wall.

1. Preganglionic cell bodies in lateral horn T1-L2;2. Ventral root → spinal nerve → ventral ramus;3. White ramus communicans → paravertebral ganglion;4. Synapse at level or ascend/descend chain;5. Postganglionic axon exits via gray ramus → dorsal/ventral ramus → targets.

How many pairs of white rami communicantes are there, and what do they carry?

14 pairs (T1-L2 only); carry preganglionic sympathetics and visceral sensory fibers.

How many pairs of gray rami communicantes are there, and what do they carry?

31 pairs (C1-Co1); carry postganglionic sympathetics.

What is the stellate ganglion?

Fusion of the inferior cervical ganglion and the T1 paravertebral ganglion.

List the stepwise pathway of sympathetic innervation to thoracic organs.

1. Preganglionic cell bodies in T1-T5 lateral horn;2. Ventral root → spinal nerve → ventral ramus → white ramus → sympathetic chain;3. Synapse in paravertebral ganglia (T1-T5 or ascend to cervical levels);4. Postganglionic exits as thoracic or cervical sympathetic organ nerves → organ plexuses (cardiac, pulmonary, esophageal).

List the stepwise pathway of parasympathetic innervation to thoracic organs.

1. Preganglionic cell bodies in brainstem;2. Axons travel with CN X (Vagus nerve);3. Preganglionic fibers enter organ plexuses (cardiac, pulmonary, esophageal, bronchial);4. Postganglionic cell bodies in organ wall (short axons).

What is the quality of visceral sensation when it reaches consciousness?

Uncomfortable, dull, poorly localized.

What stimuli can activate visceral sensation?

Ischemia, distension, cramping, or inflammation.

What stimuli do NOT activate visceral sensation?

Touch, temperature, or cutting.

What is the general pathway of visceral sensory fibers back to the spinal cord?

Organ → sympathetic pathway backward → dorsal root ganglion (cell body) → dorsal root → dorsal horn.

Where are visceral sensory cell bodies located?

Dorsal root ganglion.

Where do visceral sensory neurons synapse?

In the dorsal horn of the spinal cord.

What is the exception to the general visceral sensory pathway?

Pelvic organs follow a parasympathetic-associated pathway.

Trace the pathway of visceral sensation from a sweat gland in the neck.

Peripheral process → ventral ramus → gray ramus → sympathetic chain → white ramus → spinal nerve → dorsal root ganglion → dorsal root → dorsal horn.

Trace the pathway of visceral sensation from thoracic organs (e.g., heart, lungs, esophagus).

Organ → sympathetic organ nerves → sympathetic chain (C1-T5) → descend to T1-T5 → white ramus → spinal nerve → dorsal root ganglion → dorsal root → dorsal horn.

Define referred pain.

A true visceral sensation misinterpreted as a somatic sensation.

Why does referred pain occur at the spinal cord level?

The dorsal horn intermixes somatic and viscerosomatic neurons; there are no purely visceral laminae.

What dermatome is involved in early appendicitis referred pain?

T10 dermatome → periumbilical pain.

What dermatomes are involved in myocardial infarction referred pain?

T1-T5 dermatomes → chest pain, left arm pain, jaw, neck, or back pain.

List classic male heart attack symptoms.

Crushing substernal chest pain, radiation to left arm, sweating, shortness of breath.

List common atypical female heart attack symptoms.

Neck, jaw, back, epigastric pain, nausea, indigestion, fainting, fatigue, upper abdominal discomfort, dizziness.

Why are women more likely to experience worse outcomes from heart attacks?

Women are 50% more likely to receive wrong initial diagnosis, delay seeking help, and are less likely to receive aspirin, statins, beta blockers, angiography, or cardiac rehab.

How many preventable female deaths were estimated in the UK between 2002-2013 due to heart attack mismanagement?

Over 8,000 deaths.

Why might diagnostic thresholds like troponin miss female heart attacks?

Women are underrepresented in cardiovascular clinical trials, so thresholds are set based on male values.

What is affected by a ventral root lesion at T10?

Loss of somatic motor to epaxial and hypaxial muscles; loss of preganglionic sympathetic fibers to sweat glands at umbilicus dermatome.

What is affected by a disc herniation compressing the L4 dorsal root?

Loss of somatic sensory at L4 (not visceral, since visceral sensory fibers only enter T1-L2).

Sympathetic nervous system is also called?

Thoracolumbar system (T1-L2).

Parasympathetic nervous system is also called?

Craniosacral system (brainstem + S3-S4).

Fight or Flight is associated with?

Sympathetic nervous system.

Rest and Digest is associated with?

Parasympathetic nervous system.

Which neurotransmitter is released by postganglionic sympathetics?

Norepinephrine (NE) or Epinephrine.

Which neurotransmitter is released by postganglionic parasympathetics?

Acetylcholine (ACh).

White rami communicantes carry?

Preganglionic sympathetics + visceral sensory (T1-L2 only).

Gray rami communicantes carry?

Postganglionic sympathetics (C1-Co1, all spinal levels).

Stellate ganglion is formed by fusion of?

Inferior cervical ganglion + T1 paravertebral ganglion.

Sweat gland innervation is provided by?

Sympathetic nervous system (cholinergic).

Appendicitis referred pain is felt at?

T10 dermatome (umbilicus).

Myocardial infarction referred pain is felt at?

T1-T5 dermatomes (chest, left arm, jaw/neck).

Which cranial nerve carries parasympathetics to thoracic organs?

Vagus nerve (CN X).

Sympathetic chain ganglia are also called?

Paravertebral ganglia.

Visceral motor system is also called?

Autonomic nervous system.

Dull, poorly localized pain indicates?

Visceral sensory input.

Sharp, well localized pain indicates?

Somatic sensory input.

Sympathetic effect on pupils?

Dilation (mydriasis).

Parasympathetic effect on pupils?

Constriction (miosis).

Classic male MI symptom mnemonic?

Chest pain radiating to left arm, diaphoresis, SOB.

Common atypical female MI symptoms mnemonic?

Jaw pain, fatigue, indigestion, nausea, dizziness.

A 67-year-old man presents with radiating pain down his right leg after lifting a heavy object. MRI reveals a lateral disc herniation at L4/L5, compressing the dorsal root of L4. Which nerve function is most affected?

A. Somatic motor to paraspinal muscles
B. Visceral sensory from abdominal viscera
C. Preganglionic sympathetics to the pelvis
D. Postganglionic sympathetics to the lower limb
E. Somatic sensory from the anterior thigh

E

• A. Somatic motor to paraspinal muscles – ❌ Wrong. Motor fibers run in the ventral root, not the dorsal root.

• B. Visceral sensory from abdominal viscera – ❌ Wrong. Visceral sensory only returns through T1–L2 dorsal roots, not L4.

• C. Preganglionic sympathetics to the pelvis – ❌ Wrong. Preganglionic sympathetics exit via ventral roots, not dorsal.

• D. Postganglionic sympathetics to the lower limb – ❌ Wrong. Postganglionic sympathetics leave via gray rami, not through the dorsal root.

• E. Somatic sensory from the anterior thigh – ✅ Correct. The dorsal root of L4 carries somatic sensory information, e.g., from the anterior thigh/knee region.

A 58-year-old man presents with substernal chest pain radiating to his left arm and jaw. This is an example of referred pain. Which mechanism best explains this phenomenon?

A. Ischemic nociceptors are more sensitive in the myocardium
B. Parasympathetic visceral afferents follow cranial nerves to the brainstem
C. Somatic and visceral afferents converge on the same dorsal horn neurons
D. Visceral sensory neurons synapse in paravertebral ganglia before reaching the cord
E. Visceral pain is transmitted through the ventral horn

C

• A. Ischemic nociceptors are more sensitive in the myocardium – ❌ Incorrect; this doesn’t explain referred pain.

• B. Parasympathetic visceral afferents follow cranial nerves to the brainstem – ❌ This is true for some organs, but does not explain cardiac referred pain.

• C. Somatic and visceral afferents converge on the same dorsal horn neurons – ✅ Correct. The CNS misinterprets visceral pain as somatic because of overlapping dorsal horn processing.

• D. Visceral sensory neurons synapse in paravertebral ganglia before reaching the cord – ❌ Wrong. There is no synapse in the paravertebral ganglia for sensory.

• E. Visceral pain is transmitted through the ventral horn – ❌ Wrong; sensory enters via dorsal horn, not ventral.

During surgery, a thoracic sympathetic chain is transected at the T2 level. Which of the following findings is most likely to be observed on the same side of the body?

A. Loss of parasympathetic innervation to the bronchi
B. Loss of postganglionic sympathetic innervation to the upper limb sweat glands
C. Loss of preganglionic sympathetic innervation to the heart
D. Loss of somatic motor innervation to intercostal muscles
E. Loss of visceral sensory afferents from the lungs

B

• A. Loss of parasympathetic innervation to the bronchi – ❌ Parasympathetic supply is via the vagus (CN X), unaffected by thoracic chain transection.

• B. Loss of postganglionic sympathetic innervation to the upper limb sweat glands – ✅ Correct. The sympathetic chain carries fibers up to cervical ganglia → upper limb/body wall. Transection at T2 disrupts these postganglionic sympathetics.

• C. Loss of preganglionic sympathetic innervation to the heart – ❌ Preganglionic cell bodies to the heart originate from T1–T5, but they exit before synapsing; transection at T2 chain would cut postganglionics, not preganglionics.

• D. Loss of somatic motor innervation to intercostal muscles – ❌ Somatic motor runs in ventral roots, not in sympathetic chain.

• E. Loss of visceral sensory afferents from the lungs – ❌ Sensory travels with sympathetic fibers but does not synapse in chain ganglia, so it is not lost by transection.