Autonomic Nervous System

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Autonomic Nervous System
Human Neurobiology 217 2004 Avinash Bharadwaj
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Introduction The autonomic nervous system (ANS)
controls involuntary visceral functions.
Anatomically it is not a distinct entity. Parts
of the ANS are located in both central and
peripheral nervous system. Visceral structures in
the body strictly include the internal organs
which have involuntary muscle and glands and also
smooth muscle in blood vessels and the glands of
the skin. Like somatic structures, visceral
structures also have sensory and motor
innervation. Integration of sensory and motor
activity is a vital part of visceral functioning
as it is for somatic structures. Despite this,
many resources define the ANS as the motor
(efferent) component of visceral innervation.
Efferent visceral innervation does have special
organisational (anatomically) features, but so
does the sensory component. In our discussion we
shall discuss the efferent component first,
followed by some aspects of visceral
afferents. The innervation of the digestive tract
has another dimension small local circuits.
Most authorities now consider the eneteric
nervous system as a distinct functional division
of the nervous system.
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The Term Viscera Viscera is the plural of
viscus, meaning an internal organ. Strictly
speaking this definition would include organs in
the thoracic and abdominopelvic cavities. The
tissue components that require motor innervation
in thse organs are smooth (and cardiac) muscle
and glands. As mentioned earlier, smooth muscle
is present in blood vessels especially arteries
all over the body, there are glands in the skin
and the digestive and respiratory systems have
organs in the head and neck which all require
autonomic nerve supply.
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Efferent Visceral Innervation The single major
functional difference between motor control of
skeletal muscle and visceral structures is that
efferent nerves can stimulate or suppress
visceral function that is, these nerves can be
excitatory or inhibitory. Based on the anatomical
organisation and their neurotransmitters the
efferent nerves to visceral structures are
grouped into two categories or divisions
sympathetic and parasympathetic. On any given
structure, the two divisions have opposite
actions. However, it would be hasty to conclude
that they have similar actions on all viscera. To
give examples sympathetic nerves cause the
heart to beat faster and more powerfully while
parasympathetic nerves slow the heart down. On
the other hand, sympathetic nerves relax the
smooth muscle in the bronchi and bronchioles of
the lungs. Most of the glands under autonomic
control are stimulated by parasympathetic nerves,
sweat glands in the skin are stimulated by
sympathetic nerves. At best we can say that the
normal function of any given organ depends on a
balance between the two divisions. At the same
time it is worth remembering that one or the
other division does not demonstrably supply some
structures.
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ANS Common Organisational Features In case of
innervation of skeletal muscle we see that the
axon of a neuron body in the brainstem or the
spinal cord goes all the way to reach the target
muscle fibre. In case of the ANS, there are two
neurons between the CNS and the target organ. The
body of the first neuron is in the brainstem or
certain parts of the spinal cord. Its axon
terminates before it reaches its target and has a
synapse with a second neuron body in a ganglion
outside the CNS. The axon of the second neuron
reaches the target structure. The first axon is
called a preganglionic fibre and is thinly
myelinated. The second axon is a postganglionic
fibre and is non-myelinated. This general
principle is illustrated in the diagram in the
next slide.
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Sympathetic Division The preganglionic fibres of
the sympathetic division are axons of neuron
bodies in the lateral (intermediate) horn of grey
matter of the thoracic and upper lumbar spinal
cord. For this reason the sympathetic division is
also called thoracolumbar outflow. Sympathetic
ganglia form a chain close to the vertebral
column. Preganglionic fibres travel with the
ventral roots of spinal nerves and branch off the
spinal nerves to join the sympathetic chain.
These short branches are white rami communicantes
(singular ramus communicans, communicating
branches white, because the fibres are
myelinated!) From the ganglia, postganglionic
fibres join the spinal nerves as grey rami
communicantes (grey, because the fibres are not
myelinated). Via the spinal nerves, they are
distributed to the blood vessels and skin
(glands) of the region supplied by the spinal
nerve. Some preganglionic fibres travel to the
cervical and lower lumbar and sacral parts of the
chain. Through ganglia in these regions, grey
rami are given to these spinal nerves. Other
preganglionic fibres pass through the chain
without synapse to form nerves to thoracic and
abdominal viscera. They synapse with ganglion
cells located close to the viscera.
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Sympathetic Division In this diagram the
sympathetic division is shown in red. C, T, L and
S are parts of the spinal cord. Note the parts
of the sympathetic chain in the neck and the
lumbosacral regions. Between the spinal cord and
the chain, notice grey rami which join the spinal
nerves (spinal nerves are not shown). Also notice
how some preganglionic fibres pass through the
chain without synapsing, form nerves to viscera
and synapse in ganglia close to the viscera.
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Parasympathetic Division Preganglionic fibres are
axons of neuron bodies in the brainstem and
sacral spinal cord (craniosacral
outflow). Parasympathetic ganglia lie very close
to or within the organs. Preganglionic
parasympathetic fibres travel with cranial nerves
III, VII, IX and X and spinal nerves S1, S2 and
S3. Postganglionic fibres join a sensory nerve
in the head and neck to supply the target organs.
There are four major ganglia in the head and
neck. Sacral parasympathetic fibres join
plexuses in the lower abdomen and pelvis and
through ganglion cells close to the target
organs, are supplied to the lower abdominal and
pelvic organs.
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Parasympathetic Division Parasympathetic system
is shown in blue. Note that the vagus nerve does
not contribute parasympathetic fibres to the
head/neck ganglia it supplies structures lower
down the neck, thorax and the abdomen. (The
ganglion cells are located close to or within the
organs).
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Higher Control The neurons in the brainstem and
spinal cord which give rise to the preganglionic
fibres are in turn controlled by higher
centres. The hypothalamus plays a major role in
the higher control of ANS. In general the
anterior/preoptic nuclei influence
parasympathetic activity and the posterior/later
ones, sympathetic activity. ANS is also
instrumental in bringing about visceral responses
to smell, emotions and even cortical functions
such as thought and vision. This is largely done
by the extensive connections of the hypothalamus
with the cerebral cortex, olfactory areas and the
limbic system (see PowerPoints on Hypothalamus
and limbic system). You can look for common
examples of such phenomena from experience
tears of joy or sadness, smell, sight or even the
thought of favourite food causing salivation,
anger or fear causing powerful beating of the
heart or sweating
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Neurotransmitters All preganglionic nerve
endings use acetylcholine as the
neurotransmitter. Parasympathetic postganglionic
nerve endings use acetylcholine (they are
cholinergic). Most sympathetic nerve endings use
noradrenaline (noradrenergic). Sympathetic nerve
endings on sweat glands use acetylcholine
(cholinergic). Local circuit neurons in the
ganglia use other transmitters as well. The
actual effect of ANS stimulation on an organ
depends not so much on the neurotransmitter as on
the receptor molecules on the postsynaptic
membrane.
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Some Functional Considerations It is important
to realise that synapses in the autonomic ganglia
are not one-to-one. A preganglionic fibre
influences many ganglion cells and a ganglion
cell can be influenced by many preganglionic
fibres. Either way there is a significant degree
of divergence from preganglionic to
postganglionic fibres. In case of the sympathetic
the divergence is much greater. Effects of
sympathetic stimulation are therefore more
widespread. Noradrenaline is taken up by the
presynaptic membrane which is a slower process.
Sympathetic effects are also sustained longer. In
the parasympathetic the divergence is less, and
acetylcholine is enzymatically (rapidly)
destroyed. Parasympathetic effects are therefore
more localised and short-lived. In general we can
say that the sympathetic division launches the
body into action (fight or flight) the heart
beats faster and stronger, blood circulation
increases in skeletal muscle, excess heat is
dissipated by sweating and the pupils dilate to
make the eyes take in environmemtal information
easily. Parasympathetic activity is symbolised by
sitting down to a relaxed cup of tea with a
newspaper the heart need not exert so much,
gatsric juice is being secreted in anticipation
and the eyes are focussed on the nearby objects!
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Visceral Afferents Visceral sensations differ
from somatic sensations. Viscera are not
sensitive to the wide variety of stimuli as say,
the skin is. Only stretching, pressure or
insufficient blood supply give rise to visceral
sensations. Sensations created by stretching and
pressure within physiological limits hardly reach
conscious perception these are used by the
brainstem and other centres for mediating
reflexes which govern the normal functioning of
viscera. Such sensations are carried by nerve
fibres which generally accompany the
parasympathetic nerves. The vagus nerve, for
example, carries a vast number of such afferent
fibres. Any stimuli which generate sensations of
a pathological nature are carried by fibres that
go with the sympathetic system. Since sympathetic
fibres are segmentally arranged with spinal
nerves, such sensations are felt as if they arise
in somatic parts of the body they are
referred to the parts of the body wall
depending on the spinal segment and the location
of the organ. Pain arising in the stomach is thus
referred to the epigastric region, that arising
in the midgut is referred to the umbilical region
and from the hindgut, to the hypogastric region.
Visceral pain is not easy to categorise ( as
pain from the skin which can be described as
pricking, burning etc), nor is it precisely
localised. It is more of a vague discomfort which
can however be quite intense. End of notes