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EFFECTOR CELLS
INNERVATED BY POSTGANGLIONIC
AUTONOMIC NEURONES:
PHYSIOLOGICAL EFFECTS CAUSED
BY NERVOUS ACTIVITY
Structures Related To The Eye
1. Ciliary muscles
 The ciliary muscle may be regarded as
a ring of smooth muscle. The lens is
suspended at the centre of the ring by
ligaments.
 The ciliary muscle receives only
parasympathetic innervation.
2
Structures Related To The Eye
1. Ciliary muscles
…
 When the action potential traffic in the
parasympathetic pathway is increased,
the acetylcholine released from postganglionic neurones evokes ciliary
muscle contraction.
 Tension in suspensory ligaments is
reduced and, acting under its own
internal pressure, the lens adopts a
more spherical shape.
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Structures Related To The Eye…
1. Ciliary muscles
…
 The eye is thus accommodated for near
vision.
 Accommodation can be altered
voluntarily.
 Normally, however, the ciliary muscle is
automatically regulated to keep the
most distinct image of the object of
fixation imposed on the retina.
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Structures Related To The Eye…
2. Production and Drainage of Aqueous
Humour
 Aqueous humour is produced mainly by
the activity of epithelial cells that covers
the processes of the ciliary body.
 These epithelial cells secrete Na+ into that
part of the posterior chamber of the eye
between the iris and the lens
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Structures Related To The Eye…
2. Production and Drainage of Aqueous
Humour…
 Cl- and HCO3 ( rapid generation
catalysed by carbonic anhydrase )
follows the movement of Na+ to
maintain electrical neutrality and water
follows it to maintain isotonicity.
 The secretion of aqueous humour is
modulated by adrenoceptors on the
ciliary epithelium.
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Structures Related To The Eye…
2.Production and Drainage of Aqueous
Humour…
 In addition, the ciliary body is highly vascular
and ultra-filtration is a source of some of the
aqueous humour.
 AQUEOUS HUMOUR flows forward through
the pupil into the anterior chamber.
 It then flows into the filtration angle between
the base of the iris and the inner surface of
the cornea and enters the trabecular meshwork.
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Structures Related To The Eye…
2.Production and Drainage of Aqueous Humour…
 It finally enters the canals of Schlemm
which empty into the episcleral veins.
 Intra-ocular pressure is maintained by a
balance between the production of
aqueous humour in the ciliary body and its
drainage via the canals of Schlemm.
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Structures Related To The Eye…
2.Production and Drainage of Aqueous
Humour…
 When drainage is reduced, either by
blockage of narrow filtration angle [ which
occurs rather readily, e,g. by a relaxed iris ] or
by impaired movement through the
trabecular mesh-work in people with an open
angle, a rise in intraocular pressure ensures.
 The increased hydrostatic pressure of the
aqueous humour in the anterior chamber of
the eye is directly transmitted to the vitreous
humour.
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Structures Related To The Eye…
2.Production and Drainage of Aqueous
Humour…
 The increased hydrostatic pressure there
compresses retinal blood vessels and, if this
is severe or prolonged enough, may cause
ischaemic damage to retinal cells (loss of
visual fields and cupping of the optic disc;
glaucoma) and, if untreated, blindness.
 There are two major forms of glaucomaacute closed-angle glaucoma and chronic
open-angle glaucoma.
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Structures Related To The Eye…
2.Production and Drainage of Aqueous Humour…
 Activity of the ciliary muscle aids pumping
of aqueous humour from the canals of
Schlemm into the veins.
 Interference with ciliary muscle control
may thus not only paralyse accomodation
(cycloplegia) but may also predispose to
an increased intra-ocular pressure
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Structures Related To The Eye…
3. The Iris
 The iris contains cells that give the eye
its characteristic colour and render the
iris opaque.
 It contains 2 layers of smooth muscle
-the sphincter pupillae ( fibres
arranged concentrically around the pupil
) and
-the dilator pupillae ( fibres arranged
radially ).
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Structures Related To The Eye…
3. The Iris
…
 The sphincter pupillae receives only
para-sympathetic (acetylcholinergic)
innervation and acetylcholine
released from the postganglionic
neurones causes contraction of the
muscle fibres.
 The pupil thus constricts ( miosis ).
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Structures Related To The Eye…
3. The Iris
…
 The dilator muscle receives only a
sympathetic innervation and noradrenaline released from post-ganglionic
neurones causes contraction of the
muscle fibres.
 The pupil thus dilates ( mydriasis ).
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Structures Related To The Eye…
3. The Iris
…
 Changes in the activity of the
parasympathetic pathway supplying the
sphincter pupillae are responsible for
the pupil diameter changes associated
with light reflex.
 An increase in the intensity of light
falling on the retina induces a reflex
increase in parasympathetic discharge
to the sphincter pupillae.
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Structures Related To The Eye…
3. The Iris
…
 The pupil constricts and reduces the
amount of light entering the eye.
 Parasympathetic discharge to the
sphincter pupillae is also increased
when viewing a near object.
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Structures Related To The Eye…
3. The Iris
…
 The pupillary constriction results in
utilization of only the central portion of the
lens.
 Relaxation of the sphincter pupillae
causes mydriasis, which can lead to
photophobia and also restriction of the
filtration angle.
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Structures Related To The Eye…
3. The Iris
…
 In a patient with an already narrow angle,
this may rarely cause impaired drainage of
aqueous humuor into the canals of
Schlemm and result in an increase in intraocular pressure (closed angle glaucoma)
 The dilator pupillae plays little part in the
light reflex.
 Sympathetic discharge in response to fright
or other emotional states may evoke
mydriasis.
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Structures Related To The Eye…
4. The Eyelids
 The eyelids are largely controlled by skeletal
muscle but also contain some smooth muscle,
which receives only sympathetic innervation.
 The release of nor-adrenaline from the postganglionic neurones evokes contraction of the
smooth muscle and the eyelid retract ( that is,
the palpebral fissure widens ).
 Paralysis of either the skeletal muscle or the
smooth muscle of the eyelids allows the upper
eyelid to droop (ptosis)- the palpebral fissure
narrows.
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The Heart
 The heart receives both parasympathetic and
sympathetic innervations.
 Parasympathetic neurones in the vagus nerve
innervate the sino-atrial (SA) node [the cardiac
pacemaker].
 The release of acetylcholine from
parasympathetic nerve terminals reduces the
discharge rate of the node and the heart rate
decreases ( bradycardia or negative
chronotropic effect ).
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The Heart…
 Parasympathetic neurones also innervate
the atrio-ventricular (AV ) node.
 This is located on the right side of the
inter-atrial septum and gives rise to a
bundle of specialized conducting cells
(Purkinje fibres), which carry the cardiac
excitation wave across the AV septum and
distribute the excitation wave to the
ventricles.
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The Heart…
 The release of acetylcholine from
parasympathetic neurone terminals
depresses conducting through the AV
node.
 The ventricular myocardium (which
performs most of the cardiac pumping
work ) does not receive a
parasympathetic innervation.
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The Heart…
 Sympathetic neurones innervate all regions of the
heart.
 The release of nor-adrenaline from these
neurones augments the discharge rate of the
sinoatrial (SA) node and the heart rate increases
(tachycardia or positive chronotropic effect ).
 It also increases conduction through the AV node
and its associated Purkinje fibres and increases
the force of contraction (positive inotropic effect) of
the ventricular myocardium.
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The Heart…
 In a healthy young adult person heart
rate is normally dominated by vagal
(acetylcholinergic) tone when the
subject is at rest.
 With increasing age, vagal tone
becomes less dominant.
 During heavy exercise, sympathetic
(nor-adrenergic) tone may dominate
the heart, irrespective of the age of the
subject.
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Respiratory Smooth Muscle:
 The smooth muscle tone of the respiratory
tract receives both parasympathetic and
(sparse) sympathetic innervation.
 Acetylcholine release from
parasympathetic neurone terminals
evokes contraction of respiratory smooth
muscle (bronchoconstriction), while
noradrenaline release from sympathetic
neurones evokes relaxation (
bronchodilatation).
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Respiratory Smooth Muscle:
 In a healthy young subject the bronchial
airways are almost maximally dilated, even
when the subject is at rest.
 The activation of sympathetic pathways
during exercise does not therefore evoke
much more bronchodilatation.
 The parasympathetic pathway to respiratory
smooth muscle is reflexly activated in
response to inhalation of irritant substances
or particles.
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Gastro-intestinal Smooth Muscle:
 The propulsive smooth muscle of gut
receives both parasympathetic and
sympathetic innervation.
 The release of acetylcholine from
parasympathetic neurones causes smooth
muscle contraction {stimulates propulsive
activity (increase motility)}, whilst
noradrenaline release from sympathetic
neurones causes relaxation (inhibits
propulsive activity ).
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Gastro-intestinal Smooth Muscle:…
 Under normal circumstances the
propulsive smooth muscle of the gut
is dominated by parasympathetic
(acetylcholinergic ) tone.
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The Genitourinary System:
1. The Juxtraglomerular Apparatus of the
Kidney:
 The juxtraglomerular apparatus
comprises groups of granulated
endocrine gland cells that surround
afferent arterioles close to the point of
their entry into the renal glomerulus.
 These cells are innervated by
postganglionic sympathetic
neurones carried in the renal nerves.
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The Genitourinary System:
1. The Juxtraglomerular Apparatus of the Kidney:
 Noradrenaline release augments the
actions of other factors that promote
the secretion of renin into afferent
arteriole.
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary Bladder:
 The urinary bladder comprises a capsule of
smooth muscle whose function is the storage
and periodic evacuation of urine.
 The smooth muscle of the bladder comprises:
 the detrusor ( the greater part of the
capsule ) and
 the trigone ( that part bounded by the
ureteric orifices and the bladder neck ).
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary Bladder:
 An external sphincter of skeletal muscle
surrounds the bladder neck.
 The detrusor receives parasympathetic
innervation only.
 Bladder distension is the normal stimulus for
micturition (passage of urine), which is normally
started at will.
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary Bladder:
 The release of acetylcholine from
parasympathetic neurone terminals causes
contraction of the detrusor and closure of
the ureteric orifices.
 The bladder neck is shortened and widened
as it is pulled upwards.
 This causes a reduction in the resistance of
the urethra and allows the passage of urine.
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary Bladder:
 The activity of skeletal muscle is involved to
a variable degree in voluntary micturition.
 The first event may be relaxation of the
external sphincter round the bladder neck,
accompanied by contraction of the
diaphragm and abdominal muscles.
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary
Bladder:…
 As intra-abdominal pressure increases,
urine may start to flow before detrusor
activity reaches its peak.
 However, continence and voluntary
micturition are possible in the absence
of skeletal muscle activity.
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The Genitourinary System:…
2. Smooth Muscle Of The Urinary Bladder:
 The trigone and bladder receive only
sympathetic innervation but the role
of this sympathetic innervation in
continence and micturition is negligible.
 In males the release of noradrenaline
from sympathetic nerve terminals during
ejaculation causes a contraction of the
trigone and bladder neck that prevents
the reflux of seminal fluid into the
bladder.
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The Genitourinary System:…
3.Seminal Vesicles and Vas Deferens
 The seminal vesicles and vas deferens
receives only sympathetic innervation.
 Nor-adrenaline release evokes contraction of
the smooth muscle of these organs and
hence ejaculation of spermatozoa into the
prostatic urethra.
 Ejection of seminal fluid from urethra
(emission ) is dependent on the clonic
contraction of skeletal muscle.
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The Genitourinary System:…
4.Arterioles of External Genital Organs.
 The arterioles of the erectile tissue of
the external genital organs receive only
parasympathetic innervation.
 The release of acetylcholine from the
parasympathetic neurone terminals
causes relaxation of the vascular
muscle, with resultant engorgement of
the organ with blood ( aided by reduced
drainage due to venous compression ).
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Vascular Smooth Muscle
 The smooth muscle of blood vessels is
arranged circularly around the lumen.
 Most arterioles and veins receive
sympathetic innervation only.
 The release of noradrenaline from the
sympathetic neurone terminals causes
contraction of vascular smooth
muscle and hence vasoconstriction.
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Vascular Smooth Muscle…
 The brain stem vasomotor centre
governs the tonic discharge of
sympathetic neurones innervating blood
vessels and the resultant vascular
muscle tone is one of the factors
responsible for maintenance of BP.
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Arterioles of Skeletal Muscle:
 The arterioles of skeletal muscle receive
a noradrenergic, sympathetic
innervation controlled by the vasomotor
centre, as described for other vascular
muscle.
 In addition, they receive a second
sympathetic innervation.
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Arterioles of Skeletal Muscle:…
 The post-ganglionic neurones in this
pathway, although anatomically sympathetic,
release acetylcholine as their transmitter,
which causes vasodilatation of skeletal
muscle arterioles.
 The receptor sites for the acetylcholine are
located not on the vascular smooth muscle
cells but on the endothelial cells which line
the vessel lumen.
 Activation of the receptors for acetylcholine
induces the production of nitric oxide.
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Arterioles of Skeletal Muscle:…
 This gaseous local hormone diffuses to
the vascular smooth cells and evokes
their relaxation and hence dilatation of
the arteriole.
 This sympathetic vasodilator pathway is
activated in response to emotional
shock
[ and so produces fainting ]
or in response to exercise [ anticipated
or current ].
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Effectors in the Skin:
1.Pilomotor Muscles:
 Pilomotor muscles are responsible for
the attitude of the hair shaft.
 They receive only a sympathetic
innervation.
 Noradrenaline release from the
sympathetic neurone terminals evokes
muscle contraction and the hair shaft
erects.
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Effectors in the Skin:
1.Pilomotor Muscles:
 In furry animals the pilomotor muscles
play an important role in
thermoregulation.
 In man their role is vestigial
(gooseflesh).
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Effectors in the Skin:…
2.Eccrine Sweat Glands
 The eccrine glands receive only a
sympathetic innervation.
 The post-ganglionic neurones of this
pathway, although anatomically
sympathetic, release acetylcholine as their
transmitter and thereby evoke sweat
secretion.
 The eccrine sweat glands play an important
role in thermoregulation by removing excess
body heat as the latent heat of vaporization
of sweat.
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Effectors In The Skin:…
3.Apocrine Sweat Glands.
 Are located mainly in the skin of the
palms of the hands and axillae.
 They produce the nervous sweating
associated with circulating adrenaline.
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Other Exocrine Glands:
 The lacrimal glands, salivary glands,
glands of respiratory tract, gastric oxyntic
glands and digestive glands of the
gastrointestinal tract receive
parasympathetic innervation.
 The release of acetylcholine from
parasympathetic neurone terminals in
each case stimulates glandular
secretion.
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The Pharmacology of Acetylcholinergic Axons
and their Terminals:
 Revision on the following:
 The anatomy of somatic motor neurones and
anatomy of parasympathetic nerves
 The effects of stimulating parasympathetic
nerves
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The Pharmacology of Acetylcholinergic Axons
and their Terminals:
 Acetylcholinergic neurones synthesize, store
and release acetyl choline as their
transmitter. They include:
 All preganglionic autonomic neurones
(parasympathetic and sympathetic ).
 All post-ganglionic parasympathetic
neurones.
 A few post-ganglionic sympathetic neurones.
 All somatic ( lower ) motor neurones.
 Some neurones lying entirely within the CNS.
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H/W: Read on the:
 Pharmacology of Acetylcholinergic axons and
their terminals
 Pharmacology of the acetylcholine receptors
of skeletal muscle
 Pharmacology of the acetylcholine receptors
of ganglia
 Pharmacology of the acetylcholine receptors
of smooth muscle, cardiac muscle and
exocrine glands
 Acetyl-cholinestrases and their inhibitors
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Read:
 The Main Effects of the ANS are summarized
in page 142 of Rang & Dales Pharmaology7th Edition
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