Autonomic Nervous System Part I PDF

## Physiology of autonomic nervous system Dr/Hassan Ahmed **Physiology of autonomic nervous system** The nervous system (NS) is the part of the body that controls and regulates voluntary and involuntary functions of all body organs and tissues. This control is carried out through the transmission...

## Physiology of autonomic nervous system Dr/Hassan Ahmed **Physiology of autonomic nervous system** The nervous system (NS) is the part of the body that controls and regulates voluntary and involuntary functions of all body organs and tissues. This control is carried out through the transmission of sensory signals from the organ to the central nervous system (CNS) through afferent nerves as well as motor signals from CNS to the affected organs through efferent nerves. **Classification of the nervous system:** **I- Functional classification** 1- Sensory nervous system: Carry sensory neuronal signals from organs to CNS via afferent nerve fibers 2- Motor nervous system: Carry motor neuronal signals from CNS to organs (muscles and glands) via efferent nerve fibers. It is divided into: a- Somatic motor nervous system (Voluntary): It is the motor part of the nervous system responsible for voluntary function represented by the control of the skeletal muscle. b- Autonomic nervous system (Involuntary): It is the part of the motor nervous system that is responsible for involuntary function represented by the control of muscles (cardiac and smooth) and glands (endocrine and exocrine). It is divided into the sympathetic (adrenergic fibers) and parasympathetic nervous system (cholinergic fibers). **II- Anatomical classification** 1- Central Nervous system: includes the brain and spinal cord. 2- Peripheral nervous system (PNS): 12 cranial (sensory, motor, and mixed) and 31 pairs of spinal nerves (all of them are mixed) (8 cervicals, 12 thoracics, 5 lumbar, 5 sacral, and 1 coccygeal). **Transverse section of the spinal cord:** 1- Central canal: contains cerebrospinal fluid. 2- Grey matter: Anterior, lateral and posterior horns. Lateral horn found only in sympathetic nerves (12 thoracic and 3 lumber). Contains only soma of the neurons. 3- White matter: Anterior, lateral and posterior horns. Contains nerve fibers so, it appears white due to the presence of myelin sheath. 4- Spinal nerves (2 roots): Posterior root (afferent sensory fibers). Anterior root (efferent motor fibers). **The structural and functional unit of the nervous system (Neuron)** 1- Soma 2- Dendrites 3- Axon: Schwan cells produce myelin sheath and regeneration of axon. Axons are divided into: a. Myelinated axons: (Thick myelinated (A) 100 m/second such as somatic efferent fibers and thin myelinated (B) 10 m/second such as preganglionic fibers) b- Unmyelinated axons (C- 1m/second) such as postganglionic fibers. contain Schwan cells for regeneration. NB: Myelin sheath synthesized by Schwan cells in PNS and by oligodendrocytes in CNS. **Reflex arc** It is the response of the tissue to external or internal stimulus to keep the body homeostasis it either somatic reflex for skeletal muscles or autonomic reflex for internal organs like heart, lung, and viscera. The pathway of reflex action is called reflex arc, which is composed of: 1- Receptors: receive the stimulus as thermoreceptors in skin or stretch receptors in the lung. 2- Afferent nerve: carry sensory signal from organ to CNS. 3- Center: analyzes and processes the sensory signals and sends a suitable response to the organ. The center in somatic reflex is anterior horn cells in the spinal cord or cranial nuclei while the autonomic center is lateral horn cells in the spinal cord or cranial nuclei. 4- Efferent nerve: Carry signals from the center to the effector organ. It either: Somatic efferent: is a single neuron supply skeletal muscle. Autonomic efferent: two neurons in between a collection of cell bodies of these neurons and this collection is called autonomic ganglia. The first neuron called preganglionic fiber and originated from lateral horn cells of the spinal cord or cranial nuclei in the brain and the second neuron called postganglionic fibers originated from autonomic ganglia and terminate in the effector organ. **Pathway of reflex arc** **Differences between somatic and autonomic nervous systems** | | Somatic NS | Autonomic NS | |-----------|--------------|---------------| | Origin | AHCS | LHCS | | Fibers | One efferent fiber (Type A) | Two efferent fibers (pre and postganglionic, B and C respectively) | | | | It is either sympathetic or parasympathetic | | Function | Nerve operator (voluntary) for skeletal m. | Nerve regulator (involuntary for 2 muscles (cardiac and smooth) and 2 glands (endocrine and exocrine) | | Neurotransmitter | Only acetylcholine | Acetylcholine and catecholamines | | Velocity of transmission | Very rapid so, it is thick myelinated and originated from AHCS | Less velocity so, it is thin myelinated in preganglionic and unmyelinated in postganglionic and originated from LHCS | **Origin of ANS:** 1- Cranial origin: originates from cranial nerves in brain stem: 3rd cranial nerve (Oculomotor) from the midbrain. 7th cranial nerve (Fascial) from the medulla oblongata. 9th cranial nerve (Glossopharyngeal) from the medulla oblongata. 10th cranial nerve (Vagus) from the medulla oblongata. 2- Thoraco-lumber segments of the spinal cord: originated from lateral horn cells of all thoracic and upper three lumbar segments. 3- Sacral segments of the spinal cord: from lateral horn cells of 2nd, 3rd, and 4th sacral segments. **Diagram:** * **Somatic Ns:** * Cranial Ns except sensory (128) * All spinal nerves * **Autonomic Ns:** * **Sympath:** * Thoraco Lumber * LHCS 15 * **Parasympath:** * Cranio 1973 (X, IX, VII, III * sacral 2, 3 ,4 * Corresponding 7 * 12Th 3L 4Cr 3S **Characters of parasympathetic and sympathetic nervous systems** 1- They are antagonists in action i.e. parasympathetic is anabolic (store energy) while, sympathetic is catabolic (consume energy). 2- They have reciprocal action i.e. if one of them is active the other is inhibited except in some cases such as sexual activity. 3- Parasympathetic has limited distribution (7 nerves) and acts as an isolated unit activity while sympathetic is widely distributed (15 nerves) and acts as one unit. 4- Parasympathetic ganglia away from CNS and near to effector organ so, parasympathetic NS has long preganglionic and short postganglionic fibers. On the other side, sympathetic ganglia are close to CNS and away from the effector organ, so, sympathetic NS has short preganglionic and long postganglionic fibers. 5- Parasympathetic NS is craniosacral in origin while, sympathetic is thoraco-lumber in origin. 6- Parasympathetic keep body homeostasis during the rest while sympathetic keep body homeostasis in fight and flight. 7- Neurons of parasympathetic release acetylcholine as chemical transmitters and sympathetic neurons release catecholamines. **Autonomic ganglion** **Definition:** They are a collection of postganglionic neuronal cell bodies outside the CNS. While nucleus is a collection of preganglionic neuronal cell bodies inside the CNS. **Types:** **I- Sympathetic ganglia:** 1- Lateral or paravertebral ganglia * Located on both sides of and parallel to of vertebral column forming a sympathetic chain. * They are 26 in number: 3 cervical (inferior, middle, and superior), 12 thoracics, 5 lumber, 5 sacral, and 1 coccygeal. 2- Collateral (near to lateral) or prevertebral ganglia * Unpaired and located on the mid-way between spinal cord and organ. * Occur only in the abdomen and pelvis. * Located at the origin of GIT blood vessels (celiac, superior mesenteric, and inferior mesenteric arteries) when branched from the aorta and take the same names as celiac, superior mesenteric, inferior mesenteric, inferior hypogastric ganglia. **II- Parasympathetic ganglia:** 1- Terminal ganglia * Located on the surface or embedded in the visceral organ. * They have long preganglionic and short postganglionic fibers. 2- Other ganglia: * They are for head and neck only * They are ciliary, otic, pterygopalatine, and submandibular. **III- Adrenal medulla:** * It is a modified sympathetic ganglion. During the embryonic stage, it was a fully sympathetic ganglion containing pre- and post-ganglionic fibers. * It modified and lost its postganglionic axons (tailless gland while soma converted into secretory cells secret 80% of adrenalin (ad=near) and 20% nor-adrenalin (nor=similar). **The function of autonomic ganglion** 1- They are distributing centers and points for the synaptic junction between nerve terminals of preganglionic neurons and cell bodies of postganglionic neurons. 2- Neuronal signals coming from CNS through the ganglia are not stable in frequency and these signals become stable by the ganglia according to the condition of the target organ. 3- Some ganglia as cervical ganglia in cats act as centers for the reflexes. **Sympathetic nervous system** The sympathetic nervous system also has different synonyms: * a. Thoracolumbar depending on its origin * b. The fight and flight system depend on its function in a specific condition. * c. E system depends on its function as it induces exercise, excitement, and emergency. **Sympathetic Division of ANS** **I- Head and neck region** * **Origin:** LHCs of T1 and T2. * **Relay:** Cervical ganglia * **Function:** 1- Eye: * Dilatation of eye pupil (Mydriasis) due to contraction of the radial muscle. * Elevation of the upper eyelid and widening of the palpebral fissure. * Exophthalmos (protrusion of eyeball) due to contraction of Muller's muscle. * Vasoconstriction of conjunctival blood vessels. * Decrease tear secretion. * Adaptation for far vision by dilatation of ciliary m. and flattening of the lens. 2- Salivary glands: * Stimulates trophic secretion of saliva (viscous, low water content, rich in organic substances, and small amount) due to vasoconstriction of blood vessels supply salivary glands. * Evacuation of acini due to contraction of myoepithelial cells. 3- Skin: * Stimulates sweat gland secretion. * Vasoconstriction to cutaneous blood vessels. * Erection of the hair due to contraction of piloerrector muscle. 4- Cerebral circulation: * Stimulates vasoconstriction to meningeal blood vessels. **II-Thoracic viscera** * **Origin:** LHCs of T1-T4. * **Relay:** Cervical and thoracic ganglia. * **Function:** 1- Heart: * Increase all properties of cardiac muscle; increase heart rate, the force of contraction, and conduction velocity. * Increase arterial blood pressure. * Vasodilatation to coronary blood vessels. 2- Lungs: * Bronchodilatation due to relaxation of smooth muscle fibers. * Vasoconstriction of pulmonary blood vessels * Decrease secretion of the lung such as surfactant and mucous. **III- Limbs and trunk** * **Forelimbs:** * **Origin:** LHCs of T5-T9. * **Relay:** Cervical and thoracic ganglia. * **Hind limbs:** * **Origin:** LHCs of T10 and L2. * **Relay:** Sacral and lumbar ganglia. * **Trunk (thoracic and abdominal parts)** * **Origin:** LHCs of all thoracic and lumbar segments. * **Relay:** preganglionic fibers relay on corresponding sympathetic ganglia. * **Function** * Enhances muscle contraction. * Vasodilatation of skeletal muscle blood vessels. * Delay muscle fatigue during exercise. * Early recovery after muscle fatigue by increasing oxygen and nutrients supply through blood. **IV- Abdominal viscera** * **Origin:** LHCs of lower 6 thoracic segments of (T6-T12) which form the greater splanchnic nerve. * **Relay:** Collateral ganglia; celiac and superior mesenteric. * **Function:** * Relaxation wall of all GIT inhibiting movement of the stomach, small intestine, proximal part of the large intestine, and gall bladder. * Contraction of sphincters. * Reduce GIT secretion. * Vasoconstriction to blood vessels in stomach, small intestine, proximal part of the large intestine. * Stimulates glycogenolysis ending with hyperglycemia. * Vasodilatation of blood vessels of the liver. * Contraction of spleen pumping more RBCs in the circulation (increase PCV). * Stimulate adrenal gland secretion of adrenaline, nor-adrenaline, and renin which elevates arterial blood pressure. **V- Function on pelvic viscera** * **Origin:** LHCs of T12, L1-L3 which for two lesser splanchnic nerves that join each other, forming presacral nerve. * **Relay:** Collateral ganglia; inferior mesenteric ganglia. * **Function** * Relaxation of smooth muscle fibers in the wall of urinary bladder and contraction of internal urethral sphincter leading to urine retention. * Relaxation of smooth muscle fibers in the wall of large intestine and rectum with contraction of internal anal sphincter leading to feces retention. * Vasoconstriction of blood vessels that supply the penis and clitoris leads to their shrinkage. * Ejaculation due to contraction of vas deferens, seminal vesicle, and an ejaculatory duct. * Stimulate anti-peristaltic movement of the uterus during sexual intercourse helping the sperm to reach the fertilization site. **Diagram** * **Greater splanchnic nerve** * **Lesser splanchnic nerve** * **Celiac ganglion** * **Liver and gallbladder** * **Stomach** * **Spleen** * **Adrenal medulla** * **Kidney** * **Small intestine** * **Large intestine** * **Rectum** * **Genitalia (uterus, vagina, and penis) and urinary bladder** * **Sacral splanchnic nerves** * **Superior mesenteric ganglion** * **Inferior mesenteric ganglion** * **Lumbar splanchnic nerves** * **Preganglionic** * **Postganglionic** **Sympathetic supply to abdominal and pelvic viscera**