Introduction to the Autonomic Nervous System Professor John A. Peters E-mail [email protected]“The autonomic nervous system consists of nerve cells and nerve fibres, by means of which efferent impulses pass to tissues other than multi-nuclear striated muscle” [John Newport Langley in his classic text ‘The Autonomic Nervous System’ (1921)]. He was also a pioneer of the receptor theory, postulating the existence of ‘receptive substances’ as early as 1905. John Newport Langley Neuroeffector junctions between a postganglionic fibre (N) and intestinal smooth muscle cells (S) (Burnstock, 1988) N S 3 μm
23
Embed
Autonomic physiology and pharmacology 1 2017 18 jap
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
“The autonomic nervous system consists of nerve cells and nerve fibres, by means of
which efferent impulses pass to tissues other than multi-nuclear striated muscle” [John
Newport Langley in his classic text ‘The Autonomic Nervous System’ (1921)]. He was
also a pioneer of the receptor theory, postulating the existence of ‘receptive substances’
as early as 1905.
John Newport Langley
Neuroeffector junctions between a
postganglionic fibre (N) and
intestinal smooth muscle cells (S)
(Burnstock, 1988)
N
S
3 μm
Learning ObjectivesFollowing this lecture and further study students should be able to:
Appreciate that the autonomic nervous system (ANS) is essential to life due its
fundamental roles in homeostasis
Describe the anatomy of the motor ANS utilizing the terms, pre- and post-ganglionic
fibre, ganglia, paravertebral ganglia and prevertebral ganglia
Name the ‘classical’ neurotransmitters synthesised and released by pre- and post-
ganglionic fibres in the sympathetic and parasympathetic divisions of the ANS and the
receptors that they act upon understanding the meaning of the terms cholinergic,
cholinoceptor, adrenergic, adrenoceptor and non-adrenergic , non-cholinergic (NANC)
State the effect of sympathetic and parasympathetic stimulation upon selected targets
noting their frequently reciprocal, but in some instances unopposed, effects
Provide a simple description of neurochemical transmission in the sympathetic and
parasympathetic divisions of the ANS noting subtypes of cholinoceptor, adrenoceptor,
their exemplar organ distribution and physiological actions
Recommended reading:• Boron WF, Boulpaep EL (2017). ‘Medical Physiology’ (3rd. ed.). Chapter 14, pp. 334 –
347.
• Naish J, Syndercombe Court D (2014). ‘Medical Sciences’ (2nd. ed.). Chapter 4, pp.
125 - 130 and 138 - 147.
• Koeppen BM, and Stanton BA (2018). ‘Berne and Levy Physiology’ (7th. ed.). Chapter
11.
• Neal MJ (2016). ‘Medical Pharmacology at a Glance’ (8th. ed.). Chapter 7.
• Rang HP, Ritter JM, Flower RJ, Henderson G (2016). ‘Rang and Dale’s Pharmacology’
(8th. ed.). Chapter 12.
Introduction to the Autonomic Nervous System
The entire nervous system can be divided broadly into the Central Nervous
System (CNS) and Peripheral Nervous System (PNS) and their subdivisions.
The Autonomic Nervous System (ANS) has both central and peripheral
components
PNS
CNS (Brain and Spinal Cord)
Somatic afferent(i.e. sensory fibres, from
skin and skeletal muscle)
Somatic efferent(i.e. motor fibres to
skeletal muscle)
Enteric
(ENS)
Sympathetic
division
Parasympathetic
division
Afferent signals
(towards the
CNS)
Efferent signals
(away from the
CNS)
Autonomic (ANS)(motor and sensory
components)
The Overall Functions of the ANSThe motor autonomic nervous system (ANS) mediates output from the
CNS to the whole of the body, with the exception of skeletal (voluntary)
muscle. Output is modulated by external and internal sensory input, often
via reflexes involving negative feedback loops within PNS and the CNS
• contraction and relaxation of vascular and visceral smooth muscle
• the heartbeat (including rate and force)
• all exocrine and certain endocrine secretions
• aspects of metabolism (particularly in liver and skeletal muscle)
• modulation of the processes of the immune system
• is subdivided anatomically into sympathetic and parasympatheticdivisions (and, debatably, the enteric nervous system also)
regulates functions essential to human health and life that do not require conscious effort (e.g. whilst asleep) and that are largely involuntary, e.g.
The ANS:
training allows a degree of conscious influence over the execution of some
ANS reflexes (e.g., micturition, defecation via voluntary control of skeletal muscle
external sphincters of the urethra and anus commanded by somatic efferents).
Uniquely, accommodation in the eye (focus of the lens, for near vision) can be
voluntarily controlled, despite it being an autonomic function
Sympathetic and parasympathetic divisions of the ANS often work
simultaneously in a reciprocal and complementary manner maintaining
homeostasis
Basic Organisation of the Motor ANS (1)
Inside
CNS
Autonomic
ganglion
Preganglionic
neurone
Postganglionic
neurone
Eff
ec
tor
ce
lls
Chemical synaptic
transmission in the
ganglia
e.g
., c
ard
iac’
sm
oo
th m
uscle
, o
r secre
tory
ce
lls
Outside CNS
Chemical transmission
at the neuroeffector
junction
The motor (efferent) component comprises two neurones in series:
preganglionic and postganglionic fibres
Parasympathetic ANS
• regulates many functions, some
of which are restorative and
energy conserving ‘rest and
digest’
Sympathetic ANS
• orchestrates the stress response and
energy consumption associated with
‘fight or flight’ reactions, but also has
very important ongoing activity
‘Fight or flight’ and ‘rest and digest’, although memorable, are simplistic
descriptions of the extremes of sympathetic and parasympathetic activity
Basic Organisation of the Motor ANS (2)The transmitter of the preganglionic neurones, sympathetic and parasympathetic,
is always acetylcholine (ACh) acting via excitatory nicotinic cholinoceptors, but
the classical transmitters of the postganglionic neurones are different [i.e.
noradrenaline (NA), aka norepinephrine (NE)] and ACh, respectively
Th
ora
colu
mb
ar o
utf
low
fro
m s
pin
al c
ord
Preganglionic neurone (cholinergic,
synthesises and releases ACh as
transmitter)
Postganglionic neurone (usually
adrenergic, synthesises and
releases NA as transmitter)
Sympathetic divisionacetylcholine (ACh) usually noradrenaline (NA)
Eff
ecto
r cells
(ac
tio
n v
ia
ad
ren
oc
ep
tors
)
Cra
nio
sacr
al o
utf
low
fro
m b
rain
stem
an
d s
pin
al
cord
Preganglionic
neurone (cholinergic)
Postganglionic
neurone (cholinergic)
Parasympathetic division Acetylcholine
(ACh)
Eff
ecto
r cells
(ac
tio
n v
iam
us
ca
rin
ic
ch
oli
no
ce
po
rs)
Basic Organisation of the Motor ANS (3) Sympathetic preganglionic neurones synapse with postganglionic neurones in
either (i) paravertebral ganglia, or (ii) prevertebral ganglia (see next slide), both
of which are close to the spinal cord. Their axons (fibres) are typically short
Sympathetic postganglionic neurones innervate effector cells in organs distant
to the sympathetic ganglia. Their axons (fibres) are generally long
Parasympathetic preganglionic neurones synapse with postganglionic
neurones in terminal ganglia that are distant to the CNS and often located in
the walls of the target organ. Their axons (fibres) are thus long.
Correspondingly, the fibres of the postganglionic neurones are short
Typically, preganglionic fibres, both sympathetic and parasympathetic are
myelinated (see lecture upon the action potential) and are termed motor B-
fibres. They give a white appearance. By contrast, postganglionic fibres are
largely unmyelinated and appear grey and are termed motor C-fibres
Sympathetic preganglionic fibres branch extensively to synapse with many
postganglionic neurones located in one, or several, pre- or para-vertebral
ganglia. The effect of sympathetic stimulation may sometimes be widespread
(as in the ‘fight or flight’ reaction)
Postganglionic neurone
– usually releases NA
L2, or L3, spinal
nerve
Thoraco-
lumbar
outflow
Sympathetic chain
Paravertebral ganglia:
pre- and post-ganglionic
neurones synapse here at
segmental, or more
rostral/caudal locations
T1 spinal nerve
The Sympathetic Outflow (1)
Preganglionic neurone
– releases ACh: note the
‘anatomical logic’ of the
segment of the cord at
which the preganglionic
neurone cell bodies are
located in relation to the
location of the target
tissue/organ
Prevertebral ganglia:
pre- and post-ganglionic
neurones synapse here
Adrenal gland – note the
innervation is pre
ganglionic and the
transmitter is ACh, not NA.
The medulla of the gland
releases adrenaline (A) and
NA as hormones
Higher centres in the
brainstem regulate
sympathetic outflow
Cervical ganglia (superior, middle
and inferior)
1
2
3
1, coeliac; 2,
aorticorticorenal, 3,
superior mesenteric and 4,
inferior mesenteric
prevertebral ganglia
Modified from Moore’s
Clinically Oriented
Anatomy (2006)
4
The Sympathetic Outflow (2) – further anatomical features
Preganglionic fibre cell bodies are located in the intermediolateral (IML) cell
column (lateral horn) of the spinal cord. Those controlling a particular organ
(e.g. the heart) may be spread over several segmental levels
Preganglionic fibres exit the cord
via the ventral (anterior) roots,
follow the spinal nerves and white
rami communicantes (at levels T1
to L2/3) and then synapse with
postganglionic cell bodies in
either:
• paravertebral sympathetic
ganglia, from which the
postganglionic fibres join the
peripheral nerves, via grey
rami communicantes, to travel
to their target organs in the
skin and blood vessels
or• prevertebral sympathetic
ganglia of the abdomen via
paravertebral ganglia (without
synapsing), and onwards in
splanchnic nerves to internal
organs/vessels From Koeppen and Stanton (2018)
Postganglionic fibres (sudomotor neurones) innervating the
thermoregulatory (eccrine) sweat glands, and a few blood vessels are
cholinergic: thus the transmitter is ACh, not NA. Correspondingly, the
receptors on the effector cells are muscarinic cholinoceptors, not
adrenoceptors. However, the postganglionic fibres innervating the stress
(apocrine) sweat glands are adrenergic and activate adrenoceptors
Preganglionic fibres also innervate neurones in the pelvic plexuses
Additional to the classical transmitter, NA, postganglionic fibres store and
release others [e.g. adenosine triphosphate (ATP) and neuropeptide Y (NPY)
(see later)]
The Sympathetic Outflow (3) – additions and exceptions to the
general rules
Preganglionic cholinergic fibres
innervate the adrenal medulla,
chromaffin cells specifically,
directly via splanchnic nerves.• Chromaffin cells are modified
postganglionic neurones that
secrete, primarily adrenaline (80%),
but also NA (20%) that enter the
capillary circulation as hormones
Cranial nerves (CN) III,
VII, IX & X X
The Parasympathetic Outflow (1)
Preganglionic neurone
– releases ACh
Postganglionic neurone
– releases ACh
Parasympathetic are usually
in the target organs (discrete
ganglia in head and neck and
some plexuses in the pelvis)
IX
VII
III
Sacral spinal nerves
(S2-S4)
Modified from Moore’s
Clinically Oriented
Anatomy (2006)
Origin and CN Ganglion Postganglionic fibre target
Midbrain
CN III (oculomotor)
Ciliary Eye (pupillary constrictor and ciliary body)