Transcript File

EFFECTOR CELLS INNERVATED
BY POST-GANGLIONIC
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.
• When the action potential traffic in the
parasympathetic pathway is increased, the
acetylcholine released from post-ganglionic
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.
• The eye is thus accommodated for near vision.
• Accomodation 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.
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
• 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.
• 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 mesh-work.
• 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.
• 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.
• 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 glaucoma-acute
closed-angle glaucoma and chronic open-angle
glaucoma.
• 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 intraocular pressure
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 ).
• The sphincter pupillae receives only parasympathetic ( acetylcholinergic ) innervation
and acetylcholine released from the
postganglionic neurones causes contraction
of the muscle fibres.
• The pupil thus constricts ( miosis ).
• The dilator muscle receives only a
sympathetic innervation and nor-adrenaline
released from post-ganglionic neurones
causes contraction of the muscle fibres.
• The pupil thus dilates ( mydriasis ).
• 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.
• 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.
• 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.
• 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 inta-ocular 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.
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.
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 ).
• Parasympathetic neurones also innervate the
atrio-ventricular (AV ) node.
• This is located on the right side of the interatrial
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.
• 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.
• 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.
• 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 ( noradrenergic ) tone may dominate the heart,
irrespective of the age of the subject.
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).
• 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
exersice 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.
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 ), whilst noradrenaline release from
sympathetic neurones causes relaxation
(inhibits propulsive activity ).
• Under normal circumstances the
propulsive smooth muscle of the gut is
dominated by parasympathetic
(acetylcholinergic ) tone.
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.
• Noradrenaline release augments the
actions of other factors that promote
the secretion of renin into afferent
arteriole.
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 ).
• 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.
• 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.
• 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.
• 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.
• 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.
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.
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 ).
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.
• 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 maintanance
of BP.
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.
• The post-ganglionic neurones in this pathway,
although anatomically sympathetic, release
acetylcholine as their transmitter, which causes
vasodilation 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.
• 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 ].
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.
• In furry animals the pilomotor muscles play
an important role in thermoregulation.
• In man their role is vestigial ( gooseflesh ).
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.
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.
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.
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
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.
 H/W: Further reading 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