Autonomic Nervous System Physiology PDF

The physiology of the Autonomic Nervous System Centers of the autonomic reflexes & Chemical transmission at autonomic junctions 1st Medical year / Lecture (4) 2024/2025  Centers of the autonomic reflexes Reflexes...

The physiology of the Autonomic Nervous System Centers of the autonomic reflexes & Chemical transmission at autonomic junctions 1st Medical year / Lecture (4) 2024/2025  Centers of the autonomic reflexes Reflexes Centers (1) Simple reflexes (as micturition & defecation) ➔Sacral segments of the spinal cord (2) More complex reflexes (regulate respiration & ➔ Medulla ABP) (3) Reflexes of the eye (light & accommodation ➔ Midbrain reflexes) (4) Complex mechanisms maintaining the temperature ➔ Hypothalamus & chemical composition of the internal environment ➔ Hypothalamus & limbic system (5) Reflexes of emotions & behavior in the cerebral cortex Autonomic function of hypothalamus Hypothalamus, for its major role in autonomic functions, is called the HEAD ganglion of ANS. ✓ Anterior hypothalamus is the parasympathetic area → Stimulation of preoptic area decreases the heart rate and the arterial blood pressure and produces cutaneous vasodilatation. ✓ Posterior hypothalamus is the sympathetic area →Stimulation of posterior and lateral nuclei of hypothalamus increases the heart rate and the arterial blood pressure and produces cutaneous vasoconstriction.  Chemical transmission at autonomic junctions A neurotransmitter is a chemical substance that transmits nerve impulses across a synapse A Ligand is any chemical compound that binds to a specific site on a receptor. A ligand for a receptor can be an AGONIST, ANTAGONIST, or the chemical neurotransmitter itself. More than 50 chemical substances have been proved or postulated to function as synaptic transmitters. Transmission at the synaptic The 2 main chemical junctions between the pre-and transmitters are: postganglionic neurons and 1.Acetylcholine between postganglionic neurons and autonomic effectors are 2.Noradrenaline chemically mediated. On the basis of the chemical the receptors which are activated by transmitter released, the acetylcholine are called: cholinergic autonomic fibers are divided receptors and those activated by into: cholinergic fibers and noradrenaline are called: adrenergic adrenergic fibers. receptors  Cholinergic fibers Adrenergic fibers Cholinergic fibers Cholinergic fibers Acetylcholine➔ is a chemical transmitter released by cholinergic fibers. Sites of cholinergic fibers: 1. All preganglionic autonomic fibers including ✓ All preganglionic sympathetic fibers. ✓ All preganglionic parasympathetic fibers. 2. Preganglionic sympathetic fibers to the suprarenal medulla, the modified sympathetic ganglion. On stimulation by acetylcholine, it releases adrenaline and noradrenaline. 3. All postganglionic parasympathetic fibers. 4. Some postganglionic sympathetic fibers: ✓ Secretory fibers to sweat glands ✓ Vasodilator fibers to blood vessels of skeletal muscles.  Synthesis of ACh Synthesis of Acetyl Choline: Acetylcholine is synthesized in the terminal endings of cholinergic nerve fibers. The terminations of any nerve fiber have several enlargements called: synaptic knobs (= terminal knobs). Synaptic knobs are rich in mitochondria to supply the ATP required to synthesis of the chemical transmitter. Also they contain the transmitter vesicles which contain the chemical transmitter. Synthesis of ACh involves the reaction of choline with acetate. 1-Choline is synthesized in the neurons and also there is an active uptake of choline into cholinergic neurons. 2-Thie acetate is first activated by the combination of acetate group with reduced coenzyme A to give: Active acetate (= Acetyl - CoA). 3-Then: Choline acetyl Acetyl Acetyl-CoA + Choline Transferase Acetylcholine. The enzyme choline acetyltransferase is found in high concentrations in the cytoplasm of cholinergic nerve endings. Most of this synthesis occurs in the axoplasm  Storage of acetylcholine Most of the acetylcholine is transported to be stored in CLEAR VESICLES in the terminations of cholinergic fibers. Release of acetylcholine When an action potential spreads over the terminal fibers, the depolarization process increases the permeability of the fiber membrane to calcium ions, thus allowing these to diffuse in moderate numbers into the nerve terminals. They interact with the vesicles adjacent to the membrane, causing them to fuse with the membrane and to empty their contents to the exterior. Thus, acetylcholine is released by exocytosis.  Ca2+ Preganglionic fiber Postganglionic fiber Removal of acetylcholine Once the acetylcholine has been secreted by the cholinergic nerve endings, most of it, within a fraction of a second, is split into acetate ion and choline by the enzyme acetylcholinesterase (AChE). The true AChE has a great affinity for acetylcholine, and is present in high concentration in membranes of cholinergic nerve endings, in cell membrane at nerve terminal that are cholinergic, and in the synaptic area bound with the local collagen. The pseudocholinesterase (PsChE) is present in plasma and acts on acetylcholine that diffuses into the surrounding fluids or in the plasma. The choline that is produced is transported back into the terminal nerve endings where it is used again for synthesis of new acetylcholine. Noradrenaline (Norepinephrine) Synthesis of Noradrenaline It is the chemical transmitter released by adrenergic fibers, which include: Postganglionic sympathetic fibers, except: ▪ the secretory fibers to sweat glands & ▪ the vasodilator cholinergic fibers to the blood vessels of skeletal muscles.  Synthesis of Noradrenaline: It is synthesized in the terminal endings of adrenergic nerve fibers. Storage of Noradrenaline It is also secreted from the adrenal medulla with adrenaline. The noradrenaline is stored in the 1- In the liver: granulated dark vesicles and is bound to ATP and a protein called chromogranin. Phenyl alanine +OH tyrosine Hydroxylase Release of Noradrenaline: 2- In the axoplasm of adrenergic nerves: When the nerve impulse reaches the +OH Tyrosine DOPA terminations of adrenergic fibers, Hydroxylase noradrenaline is released by exocytosis as -Co2 previously discussed in the release of DOPA Dopamine Decarboxylase acetylcholine. 3- In the granulated vesicles: +OH Dopamine ẞ-hydroxylase Noradrenaline Removal of Noradrenaline: Following secretion of noradrenaline by the terminal nerve endings, it is three different ways: - removed from the secretory site, within a few seconds 1. Re-uptake into the adrenergic nerve endings themselves by an active transport process – accounting for the removal of 80% of the secreted Norepinephrine. 2. Diffusion away from the nerve endings into the surrounding body fluids and then into the blood. 3. Destruction by M.A.O. (monoamine oxidase) which is located on the outer surface of mitochondria. It is widely distributed, being particularly plentiful in adrenergic nerve fibers, brain, liver, and kidneys. C.OM.T (catechol-0-methyl-transferase) is present diffusely in all tissues, but not found in adrenergic nerve endings. Synthesis & Removal of Noradrenaline