Nervous System PDF

Chapter 5 | Nervous system Part 2- Autonomic nervous system  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Introduction ANS is the part of the nervous system that is responsible for control of the involuntary actions...

Chapter 5 | Nervous system Part 2- Autonomic nervous system  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Introduction ANS is the part of the nervous system that is responsible for control of the involuntary actions (cardiac m., smooth m. or glands). It occupies both PNS as well as CNS. The autonomic nerve pathway extending from the CNS to an effector organ consists of two-neurons (except to the adrenal medulla). 1st neuron located in the CNS (Cr. n. nucleus or LHC), Its axon, named preganglionic fiber, synapses with the cell body of the second neuron 2nd neuron lies within autonomic ganglion, its axon, named postganglionic fiber, innervates the effector organ ANS divisions Anatomically (according to origin): Cranial: arise from Cr. n. nuclei of III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus) Thoraco-lumber: arise from LHC of all thoracic & upper lumber segments. Sacral: arise from LHC of sacral 2,3,4 segments. Physiologically (according to function): Sympathetic system (Thoraco-lumbar) Parasympathetic system (Cranio-sacral) 58  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Autonomic ganglia Definition: They are collection of nerve cells outside the CNS where the preganglionic fibers relay with the postganglionic fibers Types: according to their sites, it may be 1. Lateral (paravertebral, symp. chain): Bilaterally along the vertebral column (3 Cervical, 10:12 Thoracic, 4 Lumbar, 4 Sacral, and 1 Coccygeal) For sympathetic relay only 2. Collateral (prevertebral or preaortic): anterior to aorta and vertebral column (Celiac, mesenteric, renal, and Hypogastric ganglia) for sympathetic and some parasympathetic relay 3. Terminal: in the wall of organs, for relay of parasympathetic fibers only Properties One way: Impulses travel in one direction from the preganglionic fibers to the postganglionic fibers and not in the reverse direction. One relay: autonomic nerve relay only once in the autonomic ganglia, it may pass through another autonomic ganglion but without relay One chemical transmitter (acetylcholine) Delay: due to chemical transmission in the synapse. Convergence/Divergence (Distributing centers) 59  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Sympathetic and parasympathetic systems General features: Generally, most organs are supplied by both sympathetic and parasympathetic (double innervation) that exert either reciprocal, antagonistic, complementary, or same actions. Some organs are supplied by one system only (Single innervations). Mode of Autonomic Action 1. Reciprocal action 2. Antagonistic action 3. Complementary action 4. The same action 5. Single innervations 1. Reciprocal action: When one system is stimulated the other is inhibited in the same effector organ e.g. heart is double innervated by sympathetic (stimulates all cardiac properties) & parasympathetic (inhibits all cardiac properties). 2. Antagonistic action: Sympathetic and parasympathetic exert opposite actions on two different effector tissues in the same effector organ e.g. iris has parasympathetic supply to constrictor muscle (constricts pupil), & sympathetic supply to the dilator muscle (dilates pupil) 60  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system 3. Complementary action: The sympathetic and parasympathetic actions complete each other e.g. during Grief (sadness): sympathetic system increases catecholamines that give energy for performance (increased breathing and heart rate) and parasympathetic produces crying. Also, during Coitus: The parasympathetic system causes erection and seminal secretions while the sympathetic system produces ejaculation. 4. The same action: The sympathetic and parasympathetic systems produce the same action in some organs e.g. in salivary glands both increase salivary secretion; the parasympathetic produces profuse watery secretion poor in enzymes while the sympathetic produces scanty secretion rich in enzymes 5. Single innervation: In some organs there is only a single autonomic supply either sympathetic or parasympathetic e.g. Skin, skeletal blood vessels, mammalian ventricle and Suprarenal medulla supplied only by sympathetic. While ciliary muscle, gastric and pancreatic glands are supplied only by parasympathetic. 61  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Sympathetic system: The preganglionic sympathetic fibers leave the spinal cord through the ventral (anterior) roots of the thoracic and upper lumbar spinal nerves and enter the white rami communicans (white RC). Through these rami. These neurons reach the lateral and collateral autonomic ganglia. in which they synapse with the cell bodies of the postganglionic neurons. Some of the postganglionic neurons pass to the viscera, while others turn back and join the spinal somatic nerves (through the grey rami communicans) and are distributed to the autonomic effector structures in the skin and skeletal muscles (sweat glands, erector pilae muscle and blood vessels). Feature Sympathetic system Origin Relay Function (Thoraco-lumber) (lateral/collateral gang) (Dominant in fight or flight situations) Head and neck LHCs of T1,2 superior cervical Dilatation of pupils, lid retraction, exophthalmos, VC, gang hair erection, ↑sweat and salivary secretions Thoracic viscera LHCs of T1:5 3 cervical & upper Excitatory to all cardiac m. properties, coronary VD, thoracic gang bronchodilatation, VC of pulmonary blood vessels Abdominal viscera LHCs of T6:L2 celiac, superior Glycogenolysis, contraction of splenic capsule, ↓ mesenteric & renal evacuation of stomach, gall bladder and intestine, gang pregang. to adrenal medulla, VC to splanchnic bl v Pelvic viscera LHCs of L1:4 hypogastric gang Urine and stool retention, ejaculation, VC of pelvic bl v Horner’s Syndrome This syndrome is due to lesion of the cervical sympathetic nerves in the cervical ganglia and consists of miosis, ptosis, enophthalmos, anhidrosis, and vasodilatations of the cutaneous blood vessels on the same side of the lesion. Parasympathetic system: preganglionic parasympathetic fibers in both the cranial and sacral outputs end mostly at terminal ganglia from which short postganglionic neurons arise and supply the effector organs Feature Parasympathetic system Origin Relay Function (Cranio-sacral) (Terminal/collateral gang) (Dominant in rest or digest situations) Head and neck -Cr. n. III Edinger Westphal n. ciliary gang Constriction of pupils, accommodation, -Cr. n. VII S. Salivary nucleus, submandibular & ↑salivary and lacrimal secretions, VD of bl v of lacrimal nucleus sphenopalatine gang. salivary, lacrimal glands & tongue -Cr. n. IX I. Salivary nucleus otic gang Thoracic viscera dorsal motor nucleus terminal gang Inhibitory to all cardiac m. properties (atria), -Cr. n. X of vagus coronary VC, bronchoconstriction, VD of pulmonary blood vessels Abdominal viscera dorsal motor nucleus terminal gang ↑ evacuation of stomach, gall bladder, small -Cr. n. X of vagus intestine, and proximal part of large intestine ↑ gastric, pancreatic, and intestinal secretions Pelvic viscera LHCs of S2,3,4 terminal ganglia Micturation, defecation, erection and ↑seminal -Pelvic n. hypogastric gang secretion 62 CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system 63 CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system 64  CHAPTER 5 NERVOUS SYSTEM 2-autonomic nervous system Neurotransmitters Sites of release of acetylcholine and catecholamines Acetylcholine is released by cholinergic neurons which are classified into 2 types: Central: mother cells present in CNS → act on Nicotinic receptors a) All autonomic ganglia b) Adrenal medulla c) The motor-end plate Peripheral: mother cells present outside CNS → act on Muscarinic receptors effector organs supplied by the postganglionic parasympathetic fibers, or cholinergic sympathetic fibers (eccrine sweat glands) Catecholamines include both adrenaline and noradrenaline. Chemical transmitter at the postgang. symp. adrenergic fibers is noradrenaline. The secretion of the adrenal medulla is composed of 80% adrenaline and 20% noradrenaline. Adrenergic receptors include 2 types: 1) Alpha receptors (α1 & α2). 2) Beta receptors (β1 & β2). 65