Organisation of nerves in the thorax

Question 1

What are the functional divisions of the CNS

Answer 1

Somatic (from Greek for body)
Skin and skeletal muscles
Autonomic or visceral (from Greek for guts)
Organs and parts of organs such as smooth muscle and glands

Question 2

Describe somatic spinal nerves

Answer 2

Motor to skeletal muscle only
Skeletal muscle cannot function without them
Sensory to body wall but not to viscera
Segmental nerves may combine to form plexi supplying specialised areas (cervical, brachial, lumbosacral)
Voluntary

Question 3

Where do somatic sensory nerves enter the spinal cord

Answer 3

Somatic sensory nerves enter the posterior horn via the posterior root

Question 4

Where do somatic sensory nerves leave the spinal cord

Answer 4

Somatic motor nerves leave the ventral horn of the spinal cord through the ventral root.

Question 5

What happens after the nerves travel through their roots

Answer 5

They join to form a spinal nerve (mixed sensory and motor) seen after spinal ganglion.
Branch into anterior (front of body) and posterior (back of body) rami to innervate structures at the front and back of the body respectively.

Question 6

What is meant by a dermatome

Answer 6

Dermatome

An area of skin which is supplied by a single spinal nerve on one side or from a single spinal cord

Question 7

What is the clinical importance of dermatomes

Answer 7

It can be used to localise lesions to a specific spinal nerve or to a specific level in the spinal cord

Question 8

What is meant by a myotome

Answer 8

Myotome

Part of a skeletal muscle supplied by a single spinal nerve on one side or from a single spinal cord level

Question 9

What is the clinical importance of myotomes

Answer 9

Testing movements at successive joints can help in localising lesions to specific nerves or to specific spinal cord levels:
muscles that move the shoulder are innervated mainly by spinal nerves from C5-C6.
Muscles that move the elbow C6-C7
Muscles in the hand- C8 and T1

Question 10

Why are myotomes more difficult to test than dermatomes

Answer 10

Because each skeletal muscle in the body is usually innervated by nerves derived from more than one spinal cord level.

Question 11

List some key dermatomal landmarks

Answer 11

T1- medial arm
T3- 3rd, 4th intercostal space
T4- nipple line, 4th, 5th intercostal space
T6- Xiphoid process
T10- navel (belly button)
T12- pubis

Question 12

Summarise the intercostal nerves

Answer 12

11 pairs (+ 1 subcostal)
Mixed (= motor + sensory)
Spinal or segmental nerves
(anterior primary rami)
Supply the intercostal spaces
Lateral cutaneous branch - anterior and posterior
Anterior cutaneous branch - medial and lateral

Question 13

Summarise the phrenic nerves

Answer 13

Derived from anterior rami of spinal nerves C3 – C5
Somatic nerves – no autonomic function or visceral distribution
Motor fibres supply skeletal muscle of the diaphragm
“C3,4 and 5 keep the diaphragm alive”
Sensory fibres supply central diaphragm, its pleural covering, mediastinal pleura and pericardium
Also supply peritoneum on inferior surface of central diaphragm

Question 14

Describe the autonomic nerves

Answer 14

Motor to cardiac muscle, smooth muscle and glands
Sensory to visceral organs
Divided into parasympathetic and sympathetic divisions
Different origins and distributions
Often but not always opposite in motor actions

Question 15

Describe sympathetic outflow from the spinal cord

Answer 15

Emerge from posterior root of spinal cord and enter paravertebral sympathetic trunk (ganglia along its length) and extends from the base of the skull to the scarum where the two trunks converge anteriorly to the coccyx to form the ganglion impar.
Each trunk is attached to the anterior rami of spinal nerves and becomes the route by which sympathetics are distributed to the periphery and viscera

Question 16

Summarise sympathetic motor pathways

Answer 16

All autonomic motor pathways involve preganglionic and postganglionic neurones
Pathways to body wall synapse in ganglia of sympathetic trunk
Pathways to viscera synapse in unpaired ganglia
Trunks take fibres up or down

Question 17

Describe the basic pathway of sympathetic motor neurones

Answer 17

Preganglionic fibres leave T1-L2 part of spinal cord in intermediolateral horn.
These fibres enter the spinal nerves, pass through the anterior rami and into the sympathetic trunks
Anterior rami of T1-L2 are connected to the sympathetic trunk or a ganglion by a white ramus communicans (myelinated).
Sympathetic trunks or ganglia are connected to the anterior rami by a gray ramus communicans
Preganglionic sympathetic fibres can then take a number of different pathways.

Question 18

Describe peripheral (thoracic wall, sweat glands, smooth muscle, blood vessels).

Answer 18

Preganglionic sympathetic motor fibres may synapse with post ganglionic motor neurones in ganglia associated with the sympathetic trunk, after which postganglionic fibres via the gray ramus communicans enter the same anterior ramus and are distributed with posterior and anterior branches of that spinal nerve.
The preganglionic fibres may also ascend or descend to other vertebral levels where they synapse in ganglia associated with spinal nerves that may or may not have sympathetic motor input directly from the spinal cord.

Question 19

Describe the ascending and descending of pre-ganglionic fibres

Answer 19

Postganglionic fibres leave fibre gray rami communicantes (associated with all spinal nerves, white only associated with spinal nerves T1 to L2).
Fibres from T1-5 tend to pass superiorly, fibres T5-L2- inferiorly
All sympathetics into the head leave T1 and ascend to superior cervical ganglion

Question 20

Describe the sympathetic motor fibres that innervate the viscera

Answer 20

Don’t synapse in paravertebral sympathetic trunk.
Synapse in unpaired ganglia (in front of aorta)- may join branches from the parasympathetic nervous system on or near the target organ, for example the cardiac or pulmonary plexi ( where post ganglion is found)

Question 21

Summarise the sympathetic trunks

Answer 21

Receive branches from spinal nerve T1 to L2
Distribute sympathetic nerves to smooth muscle and glands throughout body
Nerves to body wall synapse in ganglia of trunks
Nerves to viscera synapse in unpaired ganglia
Also bring pain fibres back to CNS from viscera
Fibres from lower T5-T12 reach abdomen in bundles called splanchnic nerves

Question 22

Summarise the parasympathetic nerves

Answer 22

Five sets of nerves contain parasympathetic fibres:
Oculomotor (III) cranial nerves
Facial (VII) cranial nerves
Glossopharyngeal (IX) cranial nerves
Vagus (X) cranial nerves
Sacral (S2 – S4) spinal nerves
The first three will be considered with the head and neck. The most important now is the vagus, supplying the viscera of the thorax & most of the abdomen

Question 23

Describe the basic pathway of sympathetic nerves to the lung and heart

Answer 23

Mainly from spinal nerves T2 – T4, passing through cervical and upper thoracic ganglia of sympathetic trunk
Many of their synapses are in micro-ganglia in the pulmonary and cardiac plexuses rather than in trunk ganglia

Question 24

Describe the pulmonary plexuses

Answer 24

Structures of the lung and the visceral pleura are supplied by visceral efferents and afferents through the anterior and posterior pulmonary plexuses
These interconnected plexuses lie anteriorly and posteriorly to the tracheal bifurcation and main bronchi- anterior is smaller

Question 25

Describe the sympathetic and parasympathetic effects of the pulmonary plexuses

Answer 25

Sympathetic nerves dilate the bronchioles

Parasympathetic (vagus) nerves constrict the bronchioles

Question 26

Describe parasympathetic innervation of the heart

Answer 26

Preganglionic parasympathetic fibres reach the heart as cardiac branches from the right and left vagus nerves. They enter the plexus and synapse within the plexus or in the walls of the atria.

Parasympathetic efferents (vagus) decrease heart rate via the pacemaker tissue and constrict coronary arteries

Question 27

Describe sympathetic innervation of the heart

Answer 27

Sympathetic efferents increase heart rate and force of contraction
Sympathetic fibres reach the cardiac plexu through cardiac nerves from the sympathetic trunk
Preganglionic sympathetic fibres form the upper four or five thoracic vertebrae enter and move up to the cervical or upper thoracic vertebrae
Synapse in micro-ganglia in the cardiac plexus.

Question 28

Describe parasympathetic afferents from the heart

Answer 28

Parasympathetic afferents (vagus) relay blood pressure and chemistry information from the  heart
Afferents associated with vagal cardiac nerves return to vagal nerve.

Question 29

Describe sympathetic afferents from the heart

Answer 29

Sympathetic afferents relay pain sensations from the heart
Return to cervical (then descend) or thoracic portions of the sympathetic trunk
Pain is often referred to cutaneous regions supplied by the same spinal cord levels

Question 30

Where is the cardiac plexus found

Answer 30

Superficial part is inferior to the aortic arch and lies between the aortic arch and pulmonary trunk
Deep part between aortic arch and tracheal bifurcation

Question 31

Describe the course of the vagal nerve

Answer 31

Cranial nerve X – arise from medulla and leave skull through jugular foramina
Descend neck posterolateral to common carotid artery
Left vagus crosses anterior to aortic arch then posterior to left lung root
Right vagus passes posterior to right lung root
Both vagi form a plexus round the oesophagus
Separate to form anterior and posterior oesophageal/gastric nerves (right posterior, left anterior)

Question 32

Describe the oesophageal plexus

Answer 32

Sympathetic afferents relay pain sensations from the oesophagus

Parasympathetic afferents (vagus) senses normal physiological information from the oesophagus

Question 33

Describe the vagus nerve and its branches

Answer 33

Branches to chest and abdomen are parasympathetic (control smooth and cardiac muscle + glands of gut and airways)
Also large sensory (enteroceptor) content from gut and lungs
Unlike the sympathetic they provide no autonomic supply to the body wall (eg arterioles & sweat glands)
Recurrent laryngeal branch of vagus nerve is not parasympathetic – runs back up neck to supply most skeletal muscles of larynx

Question 34

Describe the vagi in the posterior mediastinum

Answer 34

This posterior view of the oesophagus shows mainly the right vagus contributing to the oesophageal plexus
Remember that these nerves also acquire many sympathetic fibres
The inferior continuation of this nerve is the posterior oesophageal nerve, taking right vagal fibres through the diaphragm to the abdominal viscera
In a similar way, the left vagus provides fibres to the oesophageal plexus then continues as the anterior oesophageal nerve

Question 35

Describe the intrinsic nerves of the oesophagus

Answer 35

This plexus of ganglia and axons within the oesophageal wall coordinates its activity
This can be up- or down-regulated by the autonomic nerves
It is part of the enteric nervous system

Question 36

Why can’t the right recurrent laryngeal nerve be seen at level of the manurium

Answer 36

Winds off higher in the neck
Left vagus nerve: loops around aortic arch to form left recurrent laryngeal nerve
Right vagus nerve: loops around brachiocephalic trunk to form right recurrent laryngeal nerve