Organisation of nerves in the thorax Flashcards
What are the functional divisions of the CNS
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
Describe somatic spinal nerves
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
Where do somatic sensory nerves enter the spinal cord
Somatic sensory nerves enter the posterior horn via the posterior root
Where do somatic sensory nerves leave the spinal cord
Somatic motor nerves leave the ventral horn of the spinal cord through the ventral root.
What happens after the nerves travel through their roots
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.
What is meant by a dermatome
Dermatome
An area of skin which is supplied by a single spinal nerve on one side or from a single spinal cord
What is the clinical importance of dermatomes
It can be used to localise lesions to a specific spinal nerve or to a specific level in the spinal cord
What is meant by a myotome
Myotome
Part of a skeletal muscle supplied by a single spinal nerve on one side or from a single spinal cord level
What is the clinical importance of myotomes
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
Why are myotomes more difficult to test than dermatomes
Because each skeletal muscle in the body is usually innervated by nerves derived from more than one spinal cord level.
List some key dermatomal landmarks
T1- medial arm T3- 3rd, 4th intercostal space T4- nipple line, 4th, 5th intercostal space T6- Xiphoid process T10- navel (belly button) T12- pubis
Summarise the intercostal nerves
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
Summarise the phrenic nerves
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
Describe the autonomic nerves
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
Describe sympathetic outflow from the spinal cord
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
Summarise sympathetic motor pathways
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
Describe the basic pathway of sympathetic motor neurones
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.
Describe peripheral (thoracic wall, sweat glands, smooth muscle, blood vessels).
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.
Describe the ascending and descending of pre-ganglionic fibres
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
Describe the sympathetic motor fibres that innervate the viscera
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)
Summarise the sympathetic trunks
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
Summarise the parasympathetic nerves
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
Describe the basic pathway of sympathetic nerves to the lung and heart
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
Describe the pulmonary plexuses
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
Describe the sympathetic and parasympathetic effects of the pulmonary plexuses
Sympathetic nerves dilate the bronchioles
Parasympathetic (vagus) nerves constrict the bronchioles
Describe parasympathetic innervation of the heart
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
Describe sympathetic innervation of the heart
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.
Describe parasympathetic afferents from the heart
Parasympathetic afferents (vagus) relay blood pressure and chemistry information from the heart Afferents associated with vagal cardiac nerves return to vagal nerve.
Describe sympathetic afferents from the heart
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
Where is the cardiac plexus found
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
Describe the course of the vagal nerve
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)
Describe the oesophageal plexus
Sympathetic afferents relay pain sensations from the oesophagus
Parasympathetic afferents (vagus) senses normal physiological information from the oesophagus
Describe the vagus nerve and its branches
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
Describe the vagi in the posterior mediastinum
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
Describe the intrinsic nerves of the oesophagus
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
Why can’t the right recurrent laryngeal nerve be seen at level of the manurium
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
