Nerves of the Thorax Flashcards
What is the main branch of the vagus nerve?
The recurrent laryngeal nerve
Where are the phrenic nerves positioned?
They travel down either side of the pericardium
They are anterior to the root of the lung
What is the main division of the nervous system?
- central (brain and spinal cord)
2. peripheral
How is the peripheral nervous system divided?
- somatic
2. autonomic
What does the somatic nervous system control?
It is voluntary control
It controls external actions of skin and muscles
It has a sensory and a motor component
What does the autonomic nervous system control?
It is involuntary control
It controls the internal activities of organs and glands
How is the autonomic system divided?
- sympathetic (arousing)
2. parasympathetic (calming)
What type of inputs are interpreted by the CNS?
Both sensory and motor inputs
What is the peripheral nervous system involved with?
Getting information from the peripheries and viscera to the central processing unit (CNS)
What types of fibres are found in both the sympathetic and parasympathetic nervous systems?
They both contain both sensory and motor fibres
Sensory fibres lead back to the CNS, and motor fibres lead to the viscera
What is meant by afferent and efferent fibres?
Sensory fibres are afferent fibres
Motor fibres are efferent fibres
What type of input does the vagus nerve provide to the heart?
Parasympathetic input
What type of fibres are within the vagus nerve?
Parasympathetic and somatic fibres
Why does the vagus nerve contain somatic fibres?
It gives off a branch - the recurrent laryngeal nerve
The larynx is under somatic control
How is the spinal cord divided into segments?
What is significant about each segment (vertebral level)?
Spinal cord is divided into 31 segments
Each segment (vertebral level) gives rise to a pair of spinal nerves
What types of nerves are spinal nerves predominantly?
What are the thoracic spinal nerves?
They are predominantly somatic nerves
The thoracic spinal nerves are the intercostal nerves
How is the spinal cord divided according to the regions supplied by the spinal nerves?
- 8 pairs of cervical spinal nerves
- 12 pairs of thoracic spinal nerves
- 5 pairs of lumbar spinal nerves
- 5 pairs of sacral spinal nerves
- 1 pair of coccygeal spinal nerves
What is the central part of the spinal cord comprised of?
Grey matter which contains cell bodies
What is the external part of the spinal cord comprised of?
White matter which contains the axons of neurones
It is white due to the presence of the myelin sheath
Where will sensory (afferent) information arrive to the spinal cord?
How does it enter into the spinal cord?
It arrives at the dorsal aspect of the spinal cord
Dorsal rootlets carry sensory information back from the relevant area and enter into the spinal cord
Where will motor (efferent) information leave the spinal cord and how does it do so?
It leaves the spinal cord at the ventral aspect to supply the relevant area
Information leaves via ventral rootlets
How are the motor and sensory rootlets related?
The motor rootlets and sensory rootlets will join together to form a spinal nerve
why is a spinal nerve described as a ‘mixed nerve’?
It contains both sensory and motor information in the same sheath
Where are the cell bodies of sensory nerves found?
In the dorsal root ganglion
This is just outside of the CNS
What will the spinal nerves branch to form?
An anterior (ventral) and a posterior (dorsal) ramus
What structures are supplied by the anterior and the posterior rami?
Anterior (ventral) ramus supplies the body wall and front of the body
Posterior (dorsal) ramus supplies back musculature and skin
Why is the autonomic nervous system described as a 2-neurone pathway?
There is a preganglionic neurone in the CNS
This synapses on to a postganglionic neurone in the PNS that leads to the target tissue/organ
Where is the preganglionic neurone’s cell body found?
In the brain stem or spinal cord (CNS)
This gives rise to a preganglionic axon
Where will the preganglionic axon synapse onto the postganglionic cell body?
The postganglionic cell body is in an autonomic ganglion
This is located outside of the CNS, in the PNS
What is significant about the length of the fibres in the parasympathetic system?
Very long preganglionic fibres
Short postganglionic fibres
What is significant about the length of the fibres in the sympathetic system?
Why?
Short preganglionic fibres
Long postganglionic fibres
The preganglionic fibre only has to travel a short distance to the sympathetic chain before it synapses
Where does the vagus nerve originate?
Medulla of the brainstem
How close will the vagus nerve get to the organ before it synapses?
Why?
It gets very close to the organ before it synapses
There is a long preganglionic fibre and short postganglionic fibre
What is the origin of the parasympathetic system?
What does this mean?
It has a cranio-sacral origin
The cranial outflow is the vagus nerve
There is some contribution from the sacral region (S2-S4)
Where are the only regions that the parasympathetic nervous system originates from?
Medulla of brainstem
Sacral region of spinal cord
What do the autonomic ganglia form?
What do these contain?
The autonomic ganglia form plexi
There are both pre- and post-ganglionic fibres within a plexus
Where are the only regions where sympathetic preganglionic fibres will leave the spinal cord?
Sympathetic fibres only leave the spinal cord in segmental spinal nerves T1 - L2
What happens after the sympathetic fibre leaves the spinal cord in segmental spinal nerves?
They run to the sympathetic chains, where they synapse
From the sympathetic chain, they are distributed to the body
Why is the sympathetic chain longer than the input it is getting from the nervous system?
Fibres can enter the chain and travel up or down
How does motor information leave the spinal cord?
It leaves the spinal cord and joins onto the spinal nerve
Why can the motor information not remain in the spinal nerve?
It needs to travel to the viscera
The spinal nerve is travelling to the body wall
what happens to the motor information once it has entered a spinal nerve?
It jumps off the spinal nerve and enters the sympathetic chain
It does this by travelling through a white ramus communicans
What is a white ramus communicans?
It is a fibre that is white as it contains a myelin sheath
It is a ramus as it is a branch
What happens after the motor information has entered the sympathetic chain?
It synapses in the sympathetic chain and then the postganglionic fibre leads to the organ requiring sympathetic input
How will the sympathetic motor preganglionic fibre synapse onto the postganglionic fibre?
Through the sympathetic ganglia
This is a collection of cell bodies that is outside of the CNS
How does the sympathetic chain act as an ‘elevator’?
Once the efferent information has entered the sympathetic chain via the white ramus communicans, it can synapse and move either up or down the sympathetic chain
What happens after the efferent nerve has synapsed at the sympathetic ganglion?
It jumps back onto the spinal nerve to travel to the effector organs
It does this via a grey ramus communicans
What is the nerve that returns to the spinal nerve from the sympathetic chain?
Grey ramus communicans
It is grey as the nerves have no myelin sheaths
What is a plexus formed from?
Fibres from the sympathetic and parasympathetic nervous systems forming a braid-like structure
where will a plexus form?
It will form around an organ
Small fibres then go on to supply the relevant organ
Where must parasympathetic information pass before it reaches the target organ?
Parasympathetic input has information coming from the vagus nerve, and passing towards relevant structures
The nerves must pass through the relevant plexus before they reach the structure
What plexus is found just behind the right atrium?
What fibres are found within it?
Cardiac plexus
Fibres from the vagus nerve synapse here
There are also postganglionic fibres from the sympathetic chains
What is the difference in fibres from the sympathetic and parasympathetic nervous systems found within a plexus?
The plexus contains:
- preganglionic fibres from the parasympathetic NS (vagus)
- postganglionic fibres from the sympathetic NS (sympathetic chain)
What is the origin of the intercostal nerves?
Thoracic segments T1-T11
Subcostal nerve comes from T12
What are the components of the intercostal nerves?
- autonomic - sympathetic
- somatic - motor
- somatic - sensory
What structures do the intercostal nerves supply?
- intercostal muscles
- costal and cervical pleura
- skin adjacent to the intercostal space
Where does the vagus nerve originate?
Medulla of brainstem
What are the components of the vagus nerve?
- autonomic - parasympathetic
- somatic - motor
- somatic - sensory
What does the vagus nerve supply?
Parasympathetic output to the thoracic viscera
Where does the phrenic nerve originate?
Cervical segments C3 - C5
What are the components of the phrenic nerve?
- somatic - motor
2. somatic - sensory
what does the phrenic nerve supply?
- diaphragm
- diaphragmatic and mediastinal pleura
- pericardium
Where does the recurrent laryngeal nerve originate?
It is a branch of the vagus nerve
What are the components of the recurrent laryngeal nerve?
- somatic - motor
2. somatic - sensory
What does the recurrent laryngeal nerve supply?
- muscles of the larynx
2. intrinsic sensory from membranes below vocal folds
what type of information is carried in the sensory fibres of the parasympathetic nervous system?
Sensory information relating to the state of the viscera
This is involved in homeostasis or the organs
Which pathway is used by pain fibres and noxious stimuli?
Sympathetic pathway
How do spinal nerves usually receive sensory information?
They receive sensory information from the body wall and carry it to the spinal cord
This comes from particular regions (spinal levels)
Where does information from the heart return to?
Spinal level T1 - T4
what happens to spinal nerves if there is a noxious stimulus (e.g. in heart attack)?
There is an increased amount of sensory impulses travelling along the spinal nerves to the CNS
How does referred pain result from noxious stimuli?
The CNS is not used to receiving a large quantity of information from an organ at a particular spinal level
This leads to pain at this spinal level
Why does referred pain occur in a heart attack?
Where does it occur?
Due to noxious stimuli, the CNS is receiving a large amount of information from T1 - T4
It assumes that, as it does not usually receive much information from the heart, the increased amount of impulses are coming from the body wall
What is a dermatome?
An area of skin supplied by a particular segmental spinal nerve
Why does referred pain occur?
The CNS interprets sensory information from the relevant spinal level
It gets ‘confused’ and thinks the increased impulses are coming from the body wall rather than the relevant organ
