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Parasympathetic
Nervous System (PaNS)
The PARASYMPATHETIC
NERVOUS SYSTEM (PaNS) Although as already mentioned the SyNS is most active
in times of stress it is in fact in operation continuously.
Neurones of the SyNS
emerge from the spinal cord via the ventral roots. Efferent neurones of the
SyNS pass through the branch or each spinal nerve known as the white ramus.
Together they are known as the white ramus communicans. They are white in
appearance because they contain myelinated axons. In fact all the preganglionic
neurones in the SyNS are myelinated. They are called preganglionic neurones
because they come between the CNS and a chain of ganglia which run up either
side of the spinal cord known as the paravertebral sympathetic gangilonic
chain.
Each of these paired
chains is a series of 22 ganglia located along the length of the vertebral
column. Most preganglionic neurones end within the ganglia and synapse with
postganglionic efferent neurones. (Postganglionic meaning "after the ganglia"
).
Some of the
postganglionic neurones leave the ganglia via the paravertebral sympathetic
chain from which they branch off to form various sympathetic nerves serving
mainly blood vessels in the head, neck and thorax.
Some of the
postganglionic neurones re-enter the spinal nerves via the grey rami
communicans. These appear grey because postganglionic neurones are
non-myelinated
The above diagram
shows the complexity of the reflex system of the sympathetic nervous system.
Note that for simplicity only the left paravertebral ganglionic chain is
depicted. In reality this is mirrored on the right side of the spinal
cord.
These neurones travel
with other neurones in the spinal nerves but eventually branch off to form the
visceral nerves which serve the smooth muscle and sweat glands.
Some preganglionic
neurones do not end in the paravertebral sympathetic ganglia but pass through
to further ganglia known as the collateral ganglia which are mainly located in
the abdomen close to the aorta and it's major branches.
These bundles of
collateral ganglia form clumps often called a plexus ( e.g. solar plexus [now
more correctly referred to as the coeliac ganglion] ).
They are usually named
after the blood vessel with which they are closely associated for example the
coeliac ganglion surrounds the coeliac artery.
These neurones then
synapse with non-myelinated postganglionic neurones in the collateral ganglia.
The post ganglionic neurones then branch off to form sympathetic nerves which
serve the smooth muscles of the abdomen and the pelvic viscera and the
endocrine glands found in that area.
Collectively these
nerves are often known as the splanchnic nerves. In addition these nerves have
many association neurones ( sometimes 30 or more ) this means that the effects
of the SyNS are extremely widespread rather than specific to one organ or one
part of a muscle sheet.
Click on the picture
above to view a larger version It can be seen from the above that the SyNS is
by no means simple in it's structure. The need for this complexity is due to
the fact that the effector organs that the system serves are widespread
throughout the body even in the most peripheral areas of the skin (such as
blood vessels in your big toe).
The fact that the SyNS
has it's synapses in ganglia outside the CNS has allowed extensive research
into the neurotransmitter mechanisms of the system and has led to the
development of drugs which can either mimic or block the effects of these
chemical messengers.
Preganglionic neurones
in the SyNS utilise the chemical acetylcholine as a neurotransmitter and are
therefore known as cholinergic neurones. Cholinergic means basically to work by
choline. The transmission at these ganglionic synapses is said to be nicotinic
because they respond to nicotine in a similar way to their response to
acetylcholine.
Most of the post
ganglionic neurones utilise the organic chemical noradrenaline ( norepinephrine
) as their neurotransmitter and are therefore said to be adrenergic. Adrenergic
meaning to work by adrenaline or it's derivatives. The noradrenaline is
released at the effector synapse (i.e. where the neurone ends in the smooth
muscle, cardiac muscle or endocrine gland etc.).
Noradrenaline and
adrenaline stimulate two types of' adrenergic receptors namely alpha and beta
receptors. Adrenaline stimulates both alpha and beta receptors almost equally
whereas noradrenaline acts more pronouncedly on the alpha receptors.
Stimulation of the two different types can produce different results. For
example stimulation of` the alpha receptors on capillaries causes
vasoconstriction whereas stimulation of the beta receptors causes
vasodilation.
Stimulation of either
type of receptor however will cause the intestinal walls to relax.
How the noradrenaline
is actually released into the effector is not exactly clear. No end plates
similar to those found in skeletal muscle have yet been found in the smooth
muscle, cardiac muscle or glands. It is thought that the noradrenaline just
diffuses from the axons to the effector cells.
As mentioned above
most of the postganglionic neurones are adrenergic however those which serve
the sweat glands are in fact cholinergic in their action (i.e. they use the
neurotransmitter acetylcholine) except those on the palms of the hands which
are adrenergic. This accounts for those people who get sweaty palms if they are
nervous or under stress!
The way in which the
SyNS interacts with the adrenal medulla of the adrenal glands will be explored
in more detail in the sessions on the endocrine system. For the time being
suffice to say that stimulation of this by the SyNS results in the release of
large amounts of adrenaline and noradrenaline (epinephrine and norepinephrine)
into the blood stream. These hormones are then carried throughout the body
where they reinforce the effects of the SyNS. In some ways it appears that the
adrenal medulla provides a back up system for the SyNS in case there should be
any damage to any of the sympathetic nerve pathways to any of the organs
etc.
The adrenaline and
noradrenaline released by the combine efforts of the SyNS and the adrenal
glands is eventually dissipated either by being taken back into the synaptic
nerve endings or by action of the enzyme mono-amine oxidase. Knowledge of this
has led to the development of Mono-Amine Oxidase Inhibitors (MAOI's) as
anti-depressant drugs. The overall length of time for this process to occur can
take as long as 11/2 hours.
Thus the effects of
the SyNS are both widespread and long lived.
Bronchial constriction
which occurs in asthma can be countered by stimulation of the beta receptors of
the smooth muscle of the bronchi by caffeine or theophylline. Beta-blockers are
drugs which are used to block the beta receptors of the SyNS. They have the
effect of slowing down the heart rate and are often used in treatment of
cardiac conditions. They also have use in the treatment of anxiety especially
in those people who suffer tremor and excess perspiration as a result of
continuous SyNS stimulation due to their anxiety state.
The most dramatic
effect of the SyNS is the response commonly known as the Flight or Fight
Response.
This is often now more
correctly called the Flight, Fright or Fight Response.
As can be seen from
the diagram there are sympathetic nerve links to all parts of the body via the
spinal cord. Fairly logically those at the top end of the spinal cord serve
parts of the head and neck whilst those sympathetic nerves originating in the
bottom end of the spinal cord serve the lower limbs etc.
If suddenly confronted
with a terrifying sight or a highly stressful situations ( e.g. a car coming
head on towards you or an attacker confronting you ) the whole of the SyNS
comes into play at the same time. As mentioned earlier the effects are
extremely wide spread throughout the body. The dual combination of the SyNS and
the adrenal glands means that huge amounts or adrenaline and noradrenaline are
suddenly made available within the blood stream.
The ultimate effect is
to allow the individual to run faster, fight harder, think faster than is
normal for that particular person.
The individual's basal
metabolic rate can increase by as much as 100%.
The response causes
blood to be re-routed to those organs essential for emergency reactions. Blood
flow to the skin and to most of the internal organs is reduced. This is caused
by the effect of the sympathetic nervous system contracting the walls of blood
vessels thus narrowing the diameter and therefore allowing less blood to flow
through those vessels at that time. 'This process is known as vasoconstriction.
Constriction of blood flow to the skin has the added advantage of decreasing
blood loss from superficial wounds should a fight be required,.This also
accounts for the pallor associated with extreme rage or fear. The opposite of
vasoconstriction occurs within the blood vessels of the brain, heart, lungs and
muscles. This process is known as vasodilation and results in the diameter of
the blood vessels supplying these areas to increase thus allowing greater
volumes of blood to provide oxygen and essential nutrients to these
areas.
In addition the
strength of muscular contraction increases and the airways enlarge, breathing
becomes more efficient etc.
Many of the same
effects occur when taking part in competitive sports which are after all
usually some form of highly stylised combat which hopefully helps to dispel
man's baser instincts for violence and aggression. Also to a lesser extent when
taking part in sports for fun just simply to assist the body in coping with the
higher levels of physical activity than normal. |
