2.6 ANS Flashcards
The Anatomy of the Autonomic Nervous System
Sympathetic Division
The anatomy of the sympathetic division is such that it can function better as a mass
unit.
Parasympathetic Division
The parasympathetic division, in contrast, comprises relatively independent
components
Sympathetic Division
Origins + location
Pre-ganglionic myelinated efferents from the
hypothalamus,
medulla oblongata and
spinal cord
leave the cord with the ventral nerve roots
of the first thoracic nerve down to the second, third and,
in some subjects, the fourth lumbar spinal nerves
(T1–L2–4).
These efferents pass via the white rami communicantes
to synapse in the sympathetic ganglia lying in the
paravertebral sympathetic trunk,
which is closely related throughout its length to the spinal column.
Sympathetic synapses
They synapse with post-ganglionic neurons,
usually non-myelinated, some of which pass directly to viscera.
Others pass back via the grey rami communicantes to rejoin
the spinal nerves with which they travel to their effector sites.
A number of preganglionic fibres (from T5 and below)
synapse in collateral ganglia which are close to the
viscera that they innervate.
These collateral ganglia include the coeliac ganglion
(receiving fibres from the greater and lesser splanchnic nerves)
and the superior and inferior mesenteric ganglia.
The adrenal medulla is innervated directly by pre-ganglionic fibres
via the splanchnic nerves, which pass without relay through
the coeliac ganglion.
Supply to head
Supply to viscera
The sympathetic supply to the head originates from three structures: the superior
cervical ganglion, the middle cervical ganglion and the stellate ganglion.
Distribution of the sympathetic supply to the viscera occurs via a series of sympathetic
plexuses. The main three are the cardiac, the coeliac and the hypogastric
plexuses
The segmental sympathetic supply to the head and neck is from T1 to T5, to the
upper limb from T2 to T5, to the lower limb from T10 to L2 and to the heart from
T1 to T5.
PS
The parasympathetic nervous system has a cranial and a sacral outflow.
The cranial efferents originate in the brain stem
and travel with the third (oculomotor),
seventh (facial) and
ninth (glossopharyngeal) cranial nerves.
These pass via the ciliary, sphenopalatine,
submaxillary and otic ganglia to subserve
parasympathetic function in the head.
The most important cranial efferent is the
tenth (vagus) cranial nerve,
which supplies the thoracic and abdominal viscera.
Its fibres synapse with short
post-ganglionic neurons that are on or near the effector organs.
PS outflow
The sacral outflow originates from the
second, third and fourth sacral spinal nerves
to supply the pelvic viscera.
As with the vagus nerve, the fibres synapse with short
post-ganglionic neurons that are close to the effector organs.
Autonomic Afferents
These mediate the afferent arc of autonomic reflexes and conduct visceral pain
stimuli.
The vagus has a substantial visceral afferent component,
the importance of which is well recognized by anaesthetists who
commonly have to deal with vagally
mediated bradycardia or laryngeal spasm.
Sympathetic afferent fibres are also
involved in the transmission of visceral pain impulses,
including those originating from the myocardium.
This is the rationale for using stellate ganglion block to treat
refractory angina pectoris.
Sympathetic afferents are also involved in sympathetically
maintained pain states such as the complex regional pain syndrome.
There is usually no direct communication between afferent neurons and sympathetic post-ganglionic fibres,
but following injury there is some form of sympathetic–afferent coupling
Neurotransmitters
Sympathetic:
acetylcholine is the neurotransmitter at sympathetic pre-ganglionic fibres
(at nicotinic receptors).
Noradrenaline is the neurotransmitter at most postganglionic
fibres,
apart from those to sweat glands and to some vasodilator fibres in
skeletal muscle.
Parasympathetic:
acetylcholine is the neurotransmitter throughout the parasympathetic division,
acting at nicotinic receptors in autonomic ganglia,
and at muscarinic post-ganglionic receptors thereafter.
Autonomic neuropathy
conditions such as diabetes,
chronic alcoholism, nutritional deficiency,
Guillain–Barré syndrome, Parkinson’s
disease and AIDS.
.
Rarely, it is seen as a primary condition in the Shy–Drager syndrome or familial dysautonomia
Its clinical features include disordered cardiovascular
responses and orthostatic hypotension, the absence of sinus arrhythmia and
inability to compensate during the Valsalva manoeuvre.
Autonomic neuropathy sequelae
Patients may complain of flushing, erratic temperature control with night sweats, episodic diarrhoea and nocturnal diuresis.
The normal response to hypoglycaemia is lost, as are normal diurnal rhythms.
A more recently identified form of autonomic dysfunction is the
postural orthostatic tachycardia syndrome (POTS),
treated by a high-salt diet and in
more severe cases with mineralocorticoids.
Sympathetic blocks:
Examples include lumbar sympathectomy, stellate ganglion
block, and coeliac plexus block.
Chemical or surgical sympathectomy has been used
to improve the blood supply in vasospastic or atherosclerotic disorders of the
peripheral circulation,
to control hyperhidrosis and to treat pain associated with myocardial ischaemia.
Sympathetic blocks also have a place in the management of
sympathetically maintained pain
(see under ‘Complex Regional Pain Syndrome’ in Chapter 3),
although much of the evidence shows them to be no more effective than placebo.
Vagal reflexes:
the nerve distributes widely, hence its name; the word ‘vagus’ comes
from the Latin, meaning ‘wandering’.
(Had it been derived instead from Greek, then
the nerve – improbably – would have been called the ‘plankton’).
Sources of stimulation that can lead to bradycardia and sometimes to asystolic cardiac arrest
include the dura, the zygoma, the extraocular muscles – particularly the medial
rectus – the carotid sinus, the pharynx, the glottis, the bronchial tree, the heart, the
mesentery and peritoneum, the bladder and urethra, the testis, and the rectum and
anus.
The Brewer–Luckhardt reflex describes laryngospasm that is provoked by a
distant stimulus.
Vagal reflexes can be attenuated by the use of an anticholinergic
such as atropine, but in low doses this paradoxically can stimulate the vagus before it
blocks it (the Bezold–Jarisch reflex).
Sympathetically maintained pain
In some pain syndromes, it appears that efferent noradrenergic sympathetic activity and circulating catecholamines have a role in maintaining chronic pain.
There is usually no communication between sympathetic
efferent and afferent fibres,
but following nerve injury it is apparent that modulation
of nociceptive impulses can occur not only at the site of injury but also in distal
undamaged fibres and the dorsal root ganglion itself around which sympathetic
axons may proliferate (see under ‘Chronic Regional Pain Syndrome’ in Chapter 3).
