Anatomy of the thorax


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Nerves of the thorax, the heart & pericardium- Finish

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Types of Nerves:

Somatic – innervates

and skeletal muscle

fibres supply skeletal muscle only

Sensory nerves supply sensation from the skin (dominant sensory area), muscles, bones, the parietal pleura + the parietal pericardium.

The main somatic nerves of the thorax are: the intercostal nerves (11 pairs) and the

nerves (1 pair)

Autonomic (also known as

) – innervates organs, viscera, smooth muscle, glands

Motor fibres supple smooth muscle, cardiac muscle and many exocrine glands (e.g. muscle of the heart, blood vessels, bronchi, bronchial + sweat glands)

Sensory fibres supply sensation of various kinds to the

The autonomic nerves can then be sub-divided:


- Motor nerves to smooth muscle, the cardiac pacemaker + many exocrine glands - Pain sensation to the viscera - NOT limited to the viscera – also supply somatic

muscle (particularly blood vessel walls) and the sweat glands - Chest wall obtains sympathetic supply mainly via thoracic spinal nerves

Thoracic viscera mainly supplied from thoracic spinal nerves T3-T6


- Motor to smooth muscle, the cardiac pacemaker + many exocrine glands - Sensory monitoring feedback from the visceral organs (known as

- Distribution limited to the viscera - Entire supply to the thoracic viscera comes from the brainstem in the


Thoracic somatic nerves:

The intercostal nerves-

Arise as the main branches of the thoracic somatic spinal nerves

These nerves are often described as

nerves – each pair supplies a single body segment – i.e. each pair form a repeating unit containing a single vertebrae with its associated skeletal muscle and skin

Each segmental spinal nerve forms from roots emerging from the spinal cord: o anterior (ventral) root – all motor fibres o posterior (dorsal) root – all sensory fibres

the cell bodies of the sensory neurones all form a swelling called the

(found on the dorsal root just prior to its join with the ventral root to form the spinal nerve)

the dorsal root ganglion and the junction of the roots lie within the

– a lateral gap between the pedicles of adjoining vertebrae

There is no corresponding motor neurone ganglion, as the cell bodies are within the spinal cord

Each spinal nerve then divides into two unequal-sized

The posterior ramus is

and supplies motor fibres to the column of muscle posterior to the transverse spinal processes (often called the erector spinae complex), and sensory fibres to the skin overlying these muscles

The anterior ramus is larger, and runs anteriorly between the muscle layers, supplying muscle right round to the anterior midline.

Many different spinal nerves may combine to form

suppying specialised areas (e.g. cervical, brachial, lumbosacral)

Dermatome – an area of

which is supplied by a single spinal nerve/spinal cord level

Myotome – part of a

which is supplied by a single spinal nerve/spinal cord level

Thoracic segments:




T10- Navel


The phrenic nerves:

 Derived from the anterior rami of spinal nerves

Somatic nerves – no autonomic function or visceral distribution

Motor fibres supply the skeletal muscle of the

– the most important inspiratory muscle therefore damage to the motor tracts of the spinal cord at/above the C4 segment disconnects the inspiratory muscles from the respiratory centre in the brainstem (likely to cause death from asphyxia)

Sensory fibres supply the central diaphragm, its pleural covering, mediastinal plerua + pericardium, as well as the peritoneum on the inferior surface of the central diaphragm

Spinal nerves C2-C5 produce several nerves other than the phrenic, though these are of much less importance. Such areas of shared distribution of several pairs of spinal nerves are called plexi – this one is called the


The phrenic nerves run down the neck on the muscles arising from the cervical transverse processes to enter the thorax.

They then pass on either side of the mediastinum down to the diaphragm


phrenic nerve follows the course of the great veins, running successively on the right brachiocephalic vein, the superior vena cava, and the fibrous pericardium covering the sinus venous of the right atrium and inferior vena cava

The left phrenic nerve crosses the arch of the

then rubs across the fibrous pericardium overlying the left ventricle

Diaphragmatic pain – the brain doesn’t have a map of the viscera, so pain from the diaphragm is interpreted as coming from the part of the body surface supplied by C3-C5 – the top of the shoulder and the base of the neck – this is known as

Thoracic autonomic nerves:

In the autonomic nervous system, there are two tiers of motor neurones:

Pre-ganglionic neurones – cell bodies in the spinal cord/brain with axons that fun to swellings on the nerves called

In these ganglia, the pre-ganglionic axons synapse with the much more numerous post-ganglionic neurones

Post-ganglionic neurones – axons run to target (cardiac muscle, smooth muscle, glands)

This two-stage arrangement greatly reduces the number of cell bodies needing space within the CNS, though it reduces the precision of successful targeting of specific nerves

Autonomic nerves are divided into parasympathetic and sympathetic divisions; each of which has different origins and distributions, and are often (NOT always) opposite in motor effects

Sympathetic nerves:

All the sympathetic ganglionic neurones of the body live in the spinal cord between

but the axons have to reach virtually all parts of the body (except the CNS itself)

Sympathetic motor fibres mainly travel with the

motor fibres. However their autonomic ganglia (containing the synpases between pre + post) need to be accommodated somewhere.

The fibres come out in the spinal nerve like somatic motor fibres, but then briefly leave the nerve in a slender bundle to form a

ganglion on the side of the vertebrae (thus each spinal nerve pair will -> ganglion on either side of the vertebra)

At the paravertebral ganglion, the pre-synaptic fibres synapse with the post-synaptic fibres, and then the slender bundle of post-synaptic fibres return to the spinal nerve to be distributed.

These slender bundle of nerves which join the sympathetic ganglia to the spinal nerves are called

The pre-ganglionic fibres are wrapped in myelin so form a

ramus commincans

The post-ganglionic fibres are unmyelinated so form a

ramus communicans