Thoracic Cavity Neural Contents

The dorsal root of a spinal nerve is formed by

sensory (afferent) neurons

The ventral root of a spinal nerve is formed by

motor (efferent) neurons

The phrenic nerve is the motor nerve to

the diaphragm

The right phrenic nerve runs alongside the caudal vena cava within the

plica venae cavae

The left phrenic nerve travels within the

left mediastinal pleura

Nerve roots of origin for the phrenic nerve

C5, C6, C7

Course of the phrenic nerve

C5-C7 roots → through the thoracic inlet → on the cardiac base, ventral to the hilus of the lung

During partial pericardectomy, the incision is made

ventral to the phrenic nerve

During subtotal pericardectomy, the surgeon must

elevate the phrenic nerve from the pericardium

Intercostal nerves are formed by

the ventral branch of each thoracic spinal nerve

Structures formed by input from somatic and visceral afferent neurons (1 & 2)

the dorsal horn

Structures formed by visceral and somatic efferent neurons (3 & 4)

the intermediate and ventral horns

The dorsal root ganglion contains

cell bodies of sensory (afferent) neurons

A somatic motor neuron pathway from CNS to effector involves

one neuron only, directly to skeletal muscle

An autonomic motor neuron pathway from CNS to effector involves

two neurons in series (preganglionic and postganglionic) with a ganglion in between

The autonomic ganglion is the site where

preganglionic neuron synapses with postganglionic neuron

White rami communicantes carry

preganglionic (myelinated) sympathetic fibers

Gray rami communicantes carry

postganglionic (unmyelinated) sympathetic fibers

The lateral horn of the spinal cord gives rise to

preganglionic sympathetic neurons

Neurotransmitter released by preganglionic autonomic neurons (sympathetic and parasympathetic) at the ganglion

acetylcholine (ACh), acting on nicotinic receptors

Neurotransmitter released by postganglionic parasympathetic neurons at the target organ

acetylcholine (ACh), acting on muscarinic receptors

Neurotransmitter released by most postganglionic sympathetic neurons at the target organ

norepinephrine (NE), acting on adrenergic receptors

Exception: postganglionic sympathetic neurons to sweat glands release

acetylcholine (ACh), acting on cholinergic (muscarinic) receptors

Location of preganglionic parasympathetic neurons

cranial (brainstem) and sacral regions of the CNS

Location of preganglionic sympathetic neurons

thoracolumbar (T1-L2) segments of the spinal cord

Length of preganglionic vs postganglionic fibers in the parasympathetic division

long preganglionic, short postganglionic

Length of preganglionic vs postganglionic fibers in the sympathetic division

short preganglionic, long postganglionic

Location of parasympathetic ganglia

close to or within the wall of the target organ (terminal or intramural ganglia)

Location of sympathetic ganglia

close to the vertebral column (sympathetic trunk/paravertebral or prevertebral ganglia)

General effect of the parasympathetic division

rest-and-digest; returns the body to homeostasis

General effect of the sympathetic division

fight-or-flight response

Cranial nerves that carry parasympathetic outflow

CN III, VII, IX, X

Do parasympathetic fibers supply the body wall (skin, sweat glands, blood vessel smooth muscle)?

No — parasympathetics only supply visceral structures

Do sympathetic fibers supply the body wall?

Yes — sympathetics supply the body wall as well as viscera

The vagosympathetic trunk is a combination of

the vagus nerve and the cervical sympathetic trunk running together in the neck

The vagus nerve is Cranial Nerve

X (10)

Approximately what percent of the vagus nerve is composed of general visceral sensory (afferent) fibers?

~80%

The vagus nerve gives complete _____ supply to the thorax

parasympathetic

The vagus nerve runs ______ to the hilus of the lung

dorsal

Nerve given off by the vagus near the heart base

recurrent laryngeal nerve (RLN)

Motor components of the vagus nerve in the thorax supply

the heart, bronchial smooth musculature, and the wall of the esophagus

Sensory components of the vagus nerve in the thorax

visceral afferent fibers from the visceral organs

Each vagus nerve (right and left) divides near the heart into

dorsal and ventral branches

The dorsal branches of the right and left vagus join to form

the dorsal vagal trunk

The ventral branches of the right and left vagus join to form

the ventral vagal trunk

Where do the dorsal and ventral vagal trunks travel to reach the abdomen?

through the esophageal hiatus, alongside the esophagus

Where do the dorsal vagal branches join relative to the ventral branches?

dorsal branches join near the esophageal hiatus; ventral branches join caudal to the heart

In ruminants, enlargement of the caudal mediastinal lymph nodes can compress

the esophagus and the dorsal vagal nerve trunk

Clinical results of caudal mediastinal lymph node enlargement compressing the dorsal vagal trunk in ruminants

bloat (no eructation) and vagal indigestion

The left recurrent laryngeal nerve curves around

the aorta

The right recurrent laryngeal nerve curves around

the right subclavian artery

After curving around its vessel, the recurrent laryngeal nerve re-ascends the neck along

the trachea and esophagus (longus capitis m.)

At the level of the larynx, the recurrent laryngeal nerve terminates as

the caudal laryngeal nerve

Which intrinsic laryngeal muscle is NOT supplied by the caudal laryngeal nerve?

the cricothyroideus (supplied by the cranial laryngeal nerve instead)

Why is the left recurrent laryngeal nerve damaged more commonly than the right?

its close proximity to the aorta (pulsing artery) and to the tracheobronchial lymph nodes, which can enlarge and damage it

Result of recurrent laryngeal nerve damage in the horse

laryngeal hemiplegia, causing a respiratory noise called "roaring"

Four possible routes preganglionic sympathetic fibers may follow once they reach the sympathetic trunk

1) synapse at the paravertebral ganglion at the same level, 2) pass through and travel cranially/caudally to synapse at another trunk ganglion, 3) synapse at a prevertebral ganglion, 4) pass through without synapsing to the adrenal medulla

Where do preganglionic sympathetic fibers destined for the adrenal medulla finally synapse?

directly on neurosecretory cells within the adrenal medulla (no ganglionic synapse beforehand)

Preganglionic sympathetic neurons supplying the head are located in

the lateral horn of the first few thoracic spinal cord segments (T1-T4)

Path of sympathetic fibers to the head

T1-T4 lateral horn → cervicothoracic ganglion → ansa subclavia → middle cervical ganglion → synapse in the cranial cervical ganglion → postganglionic fibers travel with blood vessels to head targets

Where do postganglionic sympathetic neurons to the head originate?

the cranial cervical ganglion

At the caudal aspect of the thoracic cavity, the sympathetic trunks

course over the diaphragm to enter the abdomen and continue caudally

The major and minor splanchnic nerves originate from

the thoracic (sympathetic trunk) ganglia

The splanchnic nerves leave the thoracic cavity to enter

the abdominal cavity

Autonomic plexuses (e.g., cardiac and pulmonary plexus) are collections of

sympathetic postganglionic axons, parasympathetic preganglionic axons, and visceral sensory axons

Do sympathetic and parasympathetic axons synapse with each other within an autonomic plexus?

No — they run close together but do not interact or synapse