ANS 14-11-2015 (2).docx

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 a...

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 the 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 or 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 a 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 stimuli to keep the body homeostasis. It is either a *[somatic reflex]* for skeletal muscles or an *[autonomic reflex]* for internal organs like the 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 the skin or stretch receptors in the lung. 2. [Afferent nerve]: carries sensory signals from organ to CNS. 3. [Center]: analyzes and processes the sensory signals and sends a suitable response to the organ. The center of 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: a ***[single neuron]*** supplies skeletal muscle. - Autonomic efferent: ***[two neurons]*** in between a collection of cell bodies of these neurons and this collection are called autonomic ganglia. The first neuron called preganglionic fiber originated from lateral horn cells of the spinal cord or cranial nuclei in the brain and the second neuron called postganglionic fibers originated from the autonomic ganglia and terminated in the effector organ. **[Differences between somatic and autonomic nervous systems]** +-----------------------+-----------------------+-----------------------+ | | Somatic NS | Autonomic NS | +=======================+=======================+=======================+ | Origin | **AHCs** | **LHCs** | +-----------------------+-----------------------+-----------------------+ | Fibers | - ***[One]{.underli | - ***[Two]{.underli | | | ne}*** | ne}*** | | | efferent fiber | efferent fibers | | | (Type A) | (pre and | | | | postganglionic, B | | | | and C | | | | respectively) | | | | | | | | - It is either | | | | sympathetic or | | | | parasympathetic | +-----------------------+-----------------------+-----------------------+ | Function | ***[Nerve | ***[Nerve | | | operator] | regulator]{.underline | | | *** | }*** | | | (voluntary) for | (involuntary for | | | skeletal m. | ***[two]* | | | | ** | | | | muscles (cardiac and | | | | smooth) and | | | | ***[two]* | | | | ** | | | | glands (endocrine and | | | | exocrine) | +-----------------------+-----------------------+-----------------------+ | Neurotransmitter | Only acetylcholine | Acetylcholine and | | | | catecholamines | +-----------------------+-----------------------+-----------------------+ | Velocity of | Very rapid so, it is | With less velocity, | | transmission | thick myelinated and | it is thin-myelinated | | | originated from AHCs | in preganglionic and | | | | unmyelinated in | | | | postganglionic and | | | | originated from LHCs | +-----------------------+-----------------------+-----------------------+  **[Origin of ANS]:** 1. Cranial origin: originates from cranial nerves in the brain stem: - 3^rd^ 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 2^nd^, 3^rd^, and 4^th^ sacral segments.  **[Characters of parasympathetic and sympathetic nervous systems]** 1. They are antagonists in action i.e., parasympathetic is anabolic (store energy) while sympathetic is catabolic (consuming 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 are 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 keeps the body homeostasis during the rest while sympathetic keeps the 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 the 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 the 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 mid-way between the spinal cord and organ. - It occurs 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:]** 3. [Terminal ganglia] - Located on the surface or embedded in the visceral organ. - They have long preganglionic and short postganglionic fibers. 4. [Other ganglia:] - They are for the 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 the 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 depends on its function in a specific condition. c. E system depends on its function as it induces exercise, excitement, and emergency.  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 the piloerector 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. - *[Vasodilation]* 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 nutrient 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 the stomach, small intestine, and proximal part of the large intestine. - Stimulates glycogenolysis ending with hyperglycemia. - Vasodilatation of blood vessels of the liver. - Contraction of the spleen pumps 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: C[ollateral ganglia; inferior mesenteric ganglia.] **[Function]** - Relaxation of smooth muscle fibers in the wall of the urinary bladder and contraction of the internal urethral sphincter lead to urine retention. - Relaxation of smooth muscle fibers in the wall of the large intestine and rectum with contraction of the internal anal sphincter leads to feces retention. - Vasoconstriction of blood vessels that supply the penis and clitoris leads to their shrinkage. - *[Ejaculation]* due to contraction of the vas deferens, seminal vesicle, and an ejaculatory duct leading to. - Stimulate anti-peristaltic movement of the uterus during sexual intercourse helping the sperm to reach the fertilization site. **[Parasympathetic nervous system]** The parasympathetic nervous system also has different synonyms: a. Cranio-sacral depending on its origin b. Rest and Digest system depending on its function in a specific condition. c. D system depends on its function as it induces digestion, defecation, and diuresis. d. Secreto-motor system  I. **[Cranial division:]** 1. **[Oculomotor nerve (3^rd^ cranial nerve)]** *[Origin:]* Edinger-Westphal nucleus in the midbrain. *[Relay: terminal ganglia;]* [ciliary.] **[Function]** - Constriction of the eye pupil (meiosis) due to contraction of the pupillary constricting muscle. - Contraction of the ciliary muscle increases eye lens thickness leading to the accommodation of the lens for near vision. 2. **[The facial nerve (7^th^ cranial nerve)]** *[Origin]*: Superior salivatory nucleus in the pons. *[Relay]*: Chorda tympani rely on [submandibular ganglion] (for submandibular, sublingual salivary glands and anterior two-thirds of the tongue) while, greater superficial petrosal nerve relay in [sphenopalatine ganglion (for] lacrimal, nasal glans, and mucosa of the pharynx). **[Function]** - Secretary and vasodilator to submandibular and sublingual salivary, lacrimal, and nasal glands (saliva large in volume, watery, and poor in organic matter). - Secretary and vasodilator to the mucosa of the soft palate, nasopharynx, and pharynx. - Vasodilator to blood vessels of anterior two-thirds of the tongue. 3. **[The glossopharyngeal nerve (9^th^ cranial nerve)]** *[Origin]*: Inferior salivatory nucleus in the medulla oblongata. *[Relay]*: otic ganglion. **[Function]** - Secretary and vasodilator to parotid salivary gland (profuse secretion rich in enzymes). - Vasodilator to blood vessels of the posterior third of the tongue. 4. **[Vagus nerve (10^th^ cranial nerve)]** *[Origin]*: Vagal nucleus in the medulla oblongata. *[Relay]*: Terminal ganglia. **[Function]** - Decreasing heart rate *[without effect on the force of contraction due to Vagus doesn't supply ventricles.]* - Coronary vasoconstriction. - Bronchoconstriction. - Increase bronchial secretion as mucous. - Pulmonary vasodilation. - Motor (contraction) of the esophagus, stomach, small intestine, and proximal part of the large intestine. - Secretory to gastric, intestinal, pancreatic, and bile juices. - Stimulate insulin secretion. - Contraction wall of gall bladder and relaxation of the sphincter of Oddi leading to the evacuation of the gall bladder. II. **[Sacral division:]** *[Origin]*: S2, S3, and S4 then join to form the pelvic nerve. *[Relay]*: Terminal ganglia.  **[Function]** - Micturition (urination) due to contraction of urinary bladder wall and relaxation of the internal urethral sphincter. - Defecation is due to contraction of the wall of the colon and rectum and relaxation of the internal anal sphincter. - Erection of penis and clitoris due to vasodilatation of their blood vessels. - Secretary of seminal vesicle and prostate gland.  **[Centers controlling autonomic nervous system function]** Regulation and control of autonomic activity depend on the reflex, simple reflexes are controlled by lower centers such as the spinal cord while more complicated reflexes are controlled by higher centers such as the cerebral cortex and hypothalamus. 1. **[Lower centers (spinal cord and brain stem):]** a. Lateral horn cells of the spinal cord. b. Medulla oblongata and pons: contain cardiac, respiratory, and vasomotor centers. c. The midbrain contains centers controlling the pupillary reflex of the eye. 2. **[Higher centers (control the activity of lower centers):]** a. Cerebral cortex: controls autonomic activity directly or indirectly through lower centers. b. Hypothalamus: is the main higher center controlling autonomic activity through *[posterior nuclei]* which control *[sympathetic function]* and the *[anterior nucleus]* which controls *[parasympathetic function]*. **[Chemical transmitters (Neurotransmitters)]** They are chemical substances synthesized in the synaptic knob and stored in synaptic vesicles found in the nerve terminal. These neurotransmitters are released at neural activity into the synaptic cleft (space between two neurons or between neuron and organ) or autonomic ganglia. The main function of these substances conveys neural signals from neuron to neuron or from neuron to effector organs. There are a hundred types of neurotransmitters, but the most important types are Acetylcholine and Catecholamines. 1. **[Acetylcholine (Ach)]** Fibers release acetylcholine called cholinergic fibers which are: - All preganglionic sympathetic and parasympathetic fibers. - All postganglionic parasympathetic fibers. - Postganglionic sympathetic fibers to sweat gland and blood vessels of skeletal muscle. - Preganglionic fibers to the adrenal medulla. - Neuromuscular junction. **[Synthesis of acetylcholine]** - The site of ACh synthesis is synaptic knobs. - Synthesis of Ach requires a reaction between choline and acetate. - Choline is formed in neurons or reuptake after the breakdown of ACh. - Acetate is activated by combination with reduced enzyme A to give active acetate or Acetyl-Co A. - Acetyl-Co A combined with choline in the presence of *[choline acetyltransferase]* to give ACh. - Enzymes used in Ach synthesis are found in the cell body, they are transported to the synaptic knob where the ACh synthesis site is. - Ach synthesis needs energy derived from ATP within mitochondria that migrate from the cell body to the synaptic knob. - Finally, ACh is stored in synaptic vesicles. **[Release of acetylcholine]** - The initiation of neural activity (action potential) stimulates the permeability of the nerve terminal membrane to Ca^2+^ which diffuses intracellularly. - Ca^2+^ stimulates synaptic vesicles to move and fuse with the neural membrane to release their content from ACh into synapse by exocytosis. **[Inactivation of acetylcholine]** After Ach is released, binds with postsynaptic receptors, and produces its physiological effect, ACh must be destroyed to avoid escaping to the blood and activate other parasympathetic fibers which are followed by death. ACh inactivated by: - Reuptake by presynaptic nerve ending. - Break down by *[Acetylcholinesterase]* enzymes into choline and acetic acid - have two types: - True ACh esterase is found in the synaptic cleft. - Pseudo ACh esterase is found in plasma to destroy any ACh that escapes to the blood. - Choline is reuptake by presynaptic nerve ending for the synthesis of new ACh. **[Acetylcholine receptors]** a. Nicotinic receptors: Although it is activated by ACh, they are sensitive to nicotine. They are found in the *[autonomic ganglia]* (on postganglionic cell bodies) and *[neuromuscular junction]* (on the cell membrane of skeletal muscles). b. Muscarinic receptors: They can be activated by muscarine (toxic material extracted from algae) found in all *[effector organs]* stimulated by postganglionic parasympathetic nerves, postganglionic sympathetic nerves [ *sweat glands,*] and *[blood vessels of skeletal muscles]*. **[Parasympathomimetic]**: They are drugs that have the same action of parasympathetic directly by stimulation of nicotinic and muscarinic receptors as nicotinic drugs (a small dose of nicotine) and muscarinic drugs such as pilocarpine. In addition, indirect mechanism by inhibiting the anti-cholinesterase enzyme. Anti-cholinesterases are: a. Irreversible anti-cholinesterase inhibits ACh destruction (war gas poisoning and insecticides). b. Reversible anti-cholinesterase which is used for medical purposes in case of a weak parasympathetic system (Eserine and prostigmine). **[Parasympatholytic]**: Inhibits action of the parasympathetic system such as muscarinic blocker as atropine and nicotinic blocker (*[curare is neuromuscular junction]* blocker, *[nicotine in large dose]*s is ganglionic blocker). 2. **[Catecholamines (Adrenaline and nor-adrenaline)]** Fibers release catecholamines called adrenergic fibers, which are all postganglionic sympathetic fibers except those that innervate sweat glands and blood vessels of skeletal muscles. **[Synthesis Catecholamines]** - The site of synthesis is synaptic knobs. - In the liver, the amino acid phenylalanine is converted into tyrosine in the presence of phenylalanine hydroxylase enzyme (+OH). - In the axoplasm of adrenergic fibers, tyrosine is converted into DOPA in the presence of tyrosine hydroxylase enzyme (+OH). - DOPA is converted into dopamine in the presence of the DOPA decarboxylase enzyme (CO2). - In synaptic vesicles, dopamine is converted into nor-adrenaline in the presence of the dopamine hydroxylase enzyme (+OH). - Finally, part of nor-adrenaline is converted into adrenaline in the presence of N-methyltransferase (NMT) enzymes (+Ch3). **[Release of Catecholamines]** - The initiation of neural activity (action potential) stimulates the permeability of the nerve terminal membrane to Ca ^2+^ which diffuses intracellularly. - Ca ^2+^ stimulates synaptic vesicles to move and fuse with the neural membrane to release their content from catecholamines into synapses by exocytosis. **[Inactivation of catecholamines]** a. 50-80% reuptake by adrenergic nerve ending by active transport. b. Diffusion away from the nerve ends in surrounded body fluids then to blood. c. Destruction by enzymes: - Catechol-ortho-methyl-transferase (COMT) found in the brain, liver, and kidney - Monoamine oxidase (MAO) is found in the brain, adrenergic nerves, liver, and kidney **[Catecholamines receptors]** Adrenergic receptors are found on the surface of the effector organ (postsynaptic receptors) and present in the membrane of the postganglionic nerve ending (presynaptic receptors). The adrenergic receptors are: +-----------------------+-----------------------+-----------------------+ | | Alpha (α) | Beta (β) | +=======================+=======================+=======================+ | Effect | Mostly excitatory | Mostly inhibitory | +-----------------------+-----------------------+-----------------------+ | Example | VC | VD | | | | | | | Pupil constriction | Bronchodilatation | | | | (relax alveolus SMF) | | | Contract all | | | | sphincters | Relax the wall of GIT | | | | and UB | +-----------------------+-----------------------+-----------------------+ | Exception | Intestinal relaxation | Heart, hormones, | | | | metabolism | | | | (glycogenolysis), | | | | skeletal muscles | +-----------------------+-----------------------+-----------------------+ | Types | α1 and α2 | β1 and β2 | +-----------------------+-----------------------+-----------------------+ | Mechanism of action | α1: Ca^+2^ | β1 and β2: ATP to | | | | cAMP | | | α2: ATP to cAMP | | +-----------------------+-----------------------+-----------------------+ | | Nor-adrenaline | Adrenaline stimulates | | | stimulate α\>β | α and β equally | +-----------------------+-----------------------+-----------------------+ **[Sympathomimetics]**: They are drugs, which produce the same effect of the sympathetic nervous system such as synthetic adrenaline and nor-adrenaline or isoprenaline (β stimulant). **[Sympatholytics]**: They are drugs, which antagonize catecholamine action such as reserpine, phentolamine (α blocker), and propranolol (β blocker).