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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