Anatomy of the ANS

Question 1

What are the two types of motor fibres in the peripheral nervous system?

Answer 1

Somatic and autonomic

Question 2

What does the term “lower motor neurones” refer to?

Answer 2

The somatomotor neurones - the motor supply to the voluntary striated skeletal muscles.

It is called this because in the somatic nervous system there is a single (lower motor) neuron between the central nervous system and the target structure; the nerve cell body is in the CNS (brain or spinal cord) and the axon reaches its target skeletal muscle via a peripheral nerve. Here there is a NMJ using ACh.

Question 3

What is the difference between somatic and autonomic motor neurones?

Answer 3

Somatic are single neurones running from CNS to target (muscle), release ACh and NMJs.

In autonomic there are two-neurone chains between CNS and target, pre and post-ganglionic with an intervening autonomic ganglion. Use different neurotransmitters, mainly ACh and NA (*Look at Albert ‘intro to ANS’ lecture)

Question 4

How many skeletal muscles do the somatic motor neurones innervate?

Answer 4

Approx 750

Question 5

What are some of the roles of the ANS?

Answer 5

control:

• BP
• HR/Force of contraction
• RR
• Core temp
• Blood glucose
• Plasma osmolality

Question 6

Describe the source/location of the sympathetic outflow

Answer 6

The cell bodies of all preganglionic sympathetic neurons are located in the intermediolateral column of the spinal cord grey matter, which is only present in the thoracic and upper two or three lumbar segments of the spinal cord (T1-L2/L3).

Question 7

How do axons of the preganglionic neurones leave the cord and what happens next?

Answer 7

They leave via the ventral (motor) root to join the mixed spinal nerve and almost immediately enter the sympathetic trunk/chain via myelinated (white) communicating rami.

Question 8

What is the sympathetic trunk?

Answer 8

A chain of autonomic ganglia, which lies close to the spinal cord on either side of the vertebral column.

AKA paravertebral ganglia

Question 9

Which one of four things can a a preganglionic fibre do once it has entered the sympathetic chain?

Answer 9

1. Synapse on a postganglionic neuron at its own segmental level (most common)
2. Travel up the chain to synapse in a ganglion higher up
3. Travel down the chain to synapse in a ganglion lower down
4. Pass through the chain without synapsing and travel towards the gut (splanchnic)

Question 10

When are the white and grey communicating rami used in the mixed spinal nerve?

Answer 10

White (myelinated) communicating ramus - when the preganglionic nerve axon goes from the mixed nerve to the paravertebral ganglia

Grey (unmyelinated) comunicating ramus - when the postganglionic nerve axon goes from the paravertebral ganglia to the mixed nerve

Question 11

Where are white and grey rami found and why is this?

Answer 11

White rami - between T1 - L3

Grey rami - every segmental level

Why? As sympathetic chain only receives preganglionic fibres from the thoracic and upper lumbar segments and grey rami are found at all segmental levels, because all spinal nerve roots receive postganglionic fibres from the sympathetic chain.

Question 12

What is the differentiation and pathway of the splanchnic nerves?

Answer 12

Arise from lower half of the sympathetic chain (without synapsing) to give off:
-> greater, lesser and least splanchnic nerves

The three nerves terminate in 3 places, pre-aortic ganglia that lie in front of main branches of aorta
-> coeliac, superior mesenteric and inferior mesenteric

The sympathetic axons enter the arterial walls and are distributed via the blood vessels.

Question 13

Which 2 ganglia fuse to form the stellate (star shaped) ganglion?

Answer 13

Inferior cervical and T1

Question 14

Where does the sympathetic supply to the head arise from?

Answer 14

Preganglionic fibres leave the spinal cord at T1 and ascend without synapsing to reach the superior cervical ganglion.

Question 15

Where do most of the postganglionic sympathetic fibres from the superior cervical ganglia enter?

Answer 15

Most of the postganglionic sympathetic fibres enter the wall of the internal carotid artery to form the internal carotid sympathetic plexus and are distributed to their targets by branches of this vessel.

Question 16

What structures does the internal carotid plexus innervate?

Answer 16

• dilator pupillae muscle (of the eye)
• superior tarsal muscle (the smooth muscle component of the eyelid elevator)

Question 17

What structure does the external carotid artery branches innervate?

Answer 17

Facial sweat glands

Question 18

What is Horner’s syndrome?

Answer 18

Loss of sympathetic innervation to one side of the face

Question 19

What are some key characteristics of Horner’s syndrome?

Answer 19

• (i) a small, unreactive pupil (miosis),
• (ii) slight drooping of the eyelid (partial ptosis)
• (iii) loss of sweating on one side of the face (facial anhidrosis*)

*not always there, depdending on where pathway was interrupted

Question 20

What are the possible different pathologies at various anatomical sites for Horner’s syndrome?

Answer 20

• the lateral medulla (e.g. due to a stroke or brain tumour) interrupting the sympathetic control pathway from hypothalamus to T1
• the spinal cord (e.g. traumatic transection, above T1)
• the nerve root or brachial plexus (e.g. trauma, infiltrating tumour)
• the sympathetic trunk as it passes close to the apex of the lung (e.g. by a malignant tumour or so-called Pancoast tumour)
• the wall of the internal carotid artery (e.g. by trauma, aneurysm, arterial dissection)

Question 21

What is important about the adrenal medulla/gland in terms of sympathetic outflow?

Answer 21

The adrenal medulla itself is a modified sympathetic ganglion, consisting of postganglionic sympathetic neurons that have lost their axons.

The adrenal medullary cells are innervated by preganglionic sympathetic fibres, but rather than releasing noradrenaline via discrete synapses at distant targets, the modified neurosecretory cells discharge adrenaline directly into the bloodstream where it acts as a hormone rather than a neurotransmitter.

Question 22

Which post-ganglionic sympathetic fibres use ACh instead of NA?

Answer 22

Those that innervate the skin of the limbs and trunk ;

supply sweat glands, piloarrector muscles and smooth muscle within dermal blood vessels.

Termed ‘sympathetic-cholinergic’

Question 23

Describe the source/location of the parasympathetic outflow

Answer 23

Craniosacral

Question 24

Which 4 cranial nerves carry parasympathetic fibres?

(Hint: Faries Occupy Glimmering Valleys)

Answer 24

• occulomotor (CN III) – from the Edinger-Westphal nucleus, in the midbrain
• facial (CN VII) – from the superior salivatory nucleus, in the pons
• glossopharygeal (CN IX) – from the inferior salivatory nucleus, in the medulla
• vagus (CN X) – from the dorsal motor nucleus of the vagus, in the medulla

Question 25

In the head, where are the parasympathetic neurone cell bodies? Where does the preganglionic axon run?

Answer 25

The parasympathetic neurone cell body is located in the cranial nerve nucleus.

The preganglionic axon travels with the associated cranial nerve before synapsing in one of four autonomic ganglia.

Question 26

What do the facial and glossopharyngeal nerves innervate?

Answer 26

salivary and lacrimal glands

Question 27

What does the vagus nerve do?

Answer 27

Called the wandering nerve as it has such a wide territory.

Fibres leave the head and neck region and contributes to a number of autonomic plexuses which lie close to the heart, lungs, stomach and intestines. These are often situated on or in the walls of the target organ and are referred to as mural or intramural ganglia.

Principal parasympathetic supply for the body.

Question 28

Describe the origin of the pelvic splanchnic nerves in parasympathetic outflow

Answer 28

The lower part of the bowel and the pelvic contents receive their parasympathetic supply from the pelvic splanchnic nerves which arise from the S2,3,4 and terminate in the hypogastric plexus.

The pelvic splanchic nerves make important contributions to sexual function (erection) and continence.

Question 29

Where are cell bodies of visceral afferents located?

Answer 29

In the dorsal root ganglia

Question 30

Discuss referred pain, using an example of cardiac pain

Answer 30

When a noxious stimulus does occur (e.g. during angina or myocardial infarction, as a result of myocardial ischaemia) this is usually perceived by the patient as somatic pain from the correpononding segmental levels. The pain is therefore felt in the arm as well as in the centre of the chest, possibly with radiation to the neck or jaw. The sensation of discomfort at a distance from the anatomical site of the diseased organ is called “referred pain” and is clearly of considerable clinical importance.

Question 31

Why might the brain misinterpret the visceral pain as somatic pain?

Answer 31

Some evidence suggests that the visceral and somatic sensory fibres might converge on the very same second-order neurons (of the spinothalamic tract) that convey somatic pain and temperature sensations.