35. Autonomic Nervous System Flashcards
What is the autonomic nervous
system?
The ANS is a collection of nerves and ganglia that are involved in the involuntary control of homeostasis and stress response.
Describe the structure of the autonomic nervous system.
> The ANS consists of two divisions,
the parasympathetic nervous system (PNS)
and the sympathetic nervous system (SNS).
The PNS is involved in ‘Rest and Digest’
processes
(picture Mr Parasympathetic
sitting on the toilet,
sphincters open and
reading a newspaper with eyes accommodated).
The SNS is involved in
‘Fight-or-Flight’ processes
(just imagine what you are going to feel like at the exam – dilated pupils, sweaty, tachycardic, tachypnoeaic, dry mouth and shivering!).
> The ANS receives afferent information
from chemoreceptors, baroreceptors, mechanoreceptors and from regions
within the central
nervous system.
Once processed
it relays this information through
efferent pathways to target tissues
(e.g. cardiac muscle, smooth muscle and glands).
> The efferent pathways of the
PNS and SNS consist of a
1 pre-ganglionic fibre,
2 an autonomic ganglion
and a
3 post-ganglionic fibre.
(N.B. The exception to this is the SNS innervation of the adrenal gland where there is only a single pre-ganglionic fibre that terminates on the adrenal medulla.
The adrenal medulla then acts like a
glorified ‘post-ganglionic’
fibre and releases
neurotransmitters/hormones into the bloodstream.)
Pre-ganglionic fibres (myelinated type B fibres):
> The cell bodies of the
PNS pre-ganglionic fibres lie
within the nuclei of
III, VII, IX and X cranial nerves
and also in the lateral
grey horns of the
second to fourth sacral segments.
These fibres are long and
form the craniosacral outflow tract.
> The cell bodies of the SNS pre-ganglionic fibres lie within the lateral grey horns of the first to twelfth thoracic segments and the first to third lumbar segments.
These fibres are short and
form the thoraco-lumbar outflow tract.
Autonomic ganglia:
Autonomic ganglia:
> The PNS ganglia are known
as terminal ganglia as they are located close
to or within the wall of the target tissue
> The SNS consists of two types of ganglia:
the paravertebral ganglia
(also called the sympathetic trunk)
and
the prevertebral ganglia.
The paravertebral ganglia lie on
either side of the vertebral column
from the base of the skull to the coccyx.
The prevertebral ganglia
(e.g. coeliac,
superior mesenteric and
inferior mesenteric ganglia)
lie anterior to the
vertebral column
next to the major arteries.
> White rami communicantes connect
SNS pre-ganglionic fibres to
paravertebral ganglia.
Post-ganglionic fibres (unmyelinated type C fibres):
> PNS post-ganglionic fibres are
short and
their cell bodies
lie within the
autonomic ganglia.
> SNS post-ganglionic fibres
are long and
their cell bodies lie within the
autonomic ganglia.
> Grey rami communicantes connect SNS ganglia to spinal nerves
Neurotransmitters:
> All PNS and SNS
pre-ganglion fibres are
cholinergic
(they release acetylcholine).
> Whereas all PNS
post-ganglionic fibres
are also cholinergic,
only those SNS post-ganglionic
fibres innervating
sweat glands are cholinergic.
> All other SNS post-ganglionic fibres
are adrenergic
(they release noradrenaline).
> The adrenal medulla,
which acts like a ‘glorified’
SNS post-ganglionic fibre,
releases adrenaline (80%) and noradrenaline (20%).
Receptors:
> Receptors sited within
all autonomic ganglia
and on the adrenal medulla
are nicotinic acetylcholine receptors (nAChR).
> Receptors on the
effector organs innervated by the
PNS are muscarinic acetylcholine receptors (mAChR)
while those innervated by the SNS are adrenergic receptors (either α or β).
> Muscarinic acetylcholine receptors are
also located on sweat glands that
are innervated by the SNS.
Which organs are not under dual innervation from the ANS?
Most target tissues receive
dual innervation from the ANS.
However, structures that only receive SNS innervation are the sweat glands, arrector pili muscles, adipose cells, kidneys and most blood vessels.
The lacrimal glands only receive PNS innervation.
Compare and contrast the PNS and SNS.
Table 35.1 Comparison of the PNS and SNS
Function Rest and digest Fight or flight
109
Fig. 35.1 Schematic representation of the ANS
pg 110
