Physiology of the Peripheral Nervous System PDF

of the Peripheral Nervous System Physiology PNS) drugs, it is necessary wastes). In addition, the system helps control vision by constricting th h al nervous system (...

of the Peripheral Nervous System Physiology PNS) drugs, it is necessary wastes). In addition, the system helps control vision by constricting th h al nervous system ( 1 pupil and conserves energy by reducing cardiac work. To understand penp er Th ose of this chapter is to he p you to first understand the p~S. e purp Therapeutic agents that alter parasympathetic nervous system functio develop that unde_rsta:d1:1 of this book concerns pharmacology-and are used primarily for their effects on the GI tract, bladder, and e}, Because the ultIIDatdig. i's limited to those aspects of PNS Occasionally, these drugs are also used for effects on the heart ar. the scuss1on not Physio1ogy- dir ct bearing on the ability to understand drugs. lungs. physiology that have a e A variety of poisons act by mimicking or blocking effects of par" sympathetic stimulation. Among these are insecticides, nerve gases, an, DIVISIONS OF THE NERVOUS SYSTEM toxic compounds found in certain mushrooms and plants. The nervous system has two main divisions, the central nervous syst~m (CNS) and the peripheral nervous system. The PNS has ~o maJor Functions of the Sympathetic Nervous System subdivisions: (1) the somatic motor system, and (2) the autonomic nervous The sympathetic nervous system has three main functions: system. The autonomic nervous system is further su~divided into the Regulating the cardiovascular system parasympathetic nervous system and the sympathetic nervo~s system. Regulating body temperature The somatic motor system controls voluntary movement ot muscles. Implementing the acute stress response (commonly called a "fight The two subdivisions of the autonomic nervous system regulate many or-flight" reaction) involuntary processes. The sympathetic nervous system exerts multiple influences on th1 The autonomic nervous system is the principal focus of this heart and blood vessels. Stimulation of sympathetic nerves to the hear chapter. The somatic motor system is also considered, but discussion increases cardiac output. Stimulation of sympathetic nerves to arteriole is brie£ and veins causes vasoconstriction. Release of epinephrine from th1 adren;:il medulla u~suk.; in vasoconstriction in most vascular beds anc OVERVIEW OF AUTONOMIC NERVOUS vasodifo.tin:·i ;.r:. c: :,~·\:m others. By influencing the heart and blood vessels the sym}i~tfr,':'.~:-; i'..'.~rvous system can achieve three homeostati1 SYSTEM FUNCTIONS objecti:1:::.~ '. The autonomic nervous system has three principal functions: (1) regula- !vfainte.n,uice of blood flow to the brain tion of the heart, (2) regulation of secretory glands (salivary, gastric, Redistribution of blood flow during exercise 'sweat, and bronchial glands), and (3) regulation of smooth muscles Compensation for loss of blood, primarily by causing vasoconstrictior (muscles of the bronchi, blood vessels, urogenital system, and gastro-~ The sympathetic nervous system helps regulate body temperatu~1 intestinal'-lGI1 tract). These regulatory activities are shared between ·n three ways: (1) By regulating blood flow to the skin, sympathet11 the sympathetic and parasympathetic divisions of the autonomic nervous rves can increase or decrease heat loss. By dilating surface vesse~ system. ~ iympathetic nerves increase blood flow to the skin and thereb:.. accelerate heat loss. Conversely, constricting cutaneous vessel Functions of the Parasympathetic Nervous System c Strves heat. (2) Sympathetic nerves to sweat glands promote th The parasympathetic nervo~tem performs seven regulatory functions se eiion of sweat, thereby helping the body cool. (3) By inducin1 that have particular releva:1ce~ drugs. Specifically, stimulation of piloerection (erection of hair), sympathetic nerves can promote hea appropriate parasympathetic nerves causes the following: conservation. Slowing of heart wte When we are faced with an acute stress-inducing situation, th Increased gastric secretion sympathetic nervous system orchestrates the fight-or-flight responSl Emptying of the bladder which consists of the following: Emptying of the bowel Increasing heart rate and blood pressure Focusing the eye for near vision Shunting blood away from the skin and viscera and into skelet, Constricting the pupil.j muscles Contracting bronchial smooth muscle Dilating the bronchi to improve oxygenation Just how the parasympathetic nervous system elicits these responses Dilating the pupils (perhaps to enhance visual acuity) is discussed later in the section "Functions of Cholinergic Receptor Mobilizing stored energy, thereby providing glucose for the br~ Subtypes." and fatty acids for muscles From the previous discussion we can see that the parasympathetic Many therapeutic agents produce their effects by altering functiol nervous system is concerned primarily with what might be called the under sympathetic control. These drugs are used primarily for effe -< > "Preganglio U1 Contraction of the ra dia ~ ard 5)1t1pathetie postgangltOl'IIC the.. cl,~ c r s~ganghaar-d IOS causes mydrias,s -, tt-e adrenal med11lla nenes ard retease of {lnaeased pupil size) epineJllrine from the adrenal Arterioles Constriction medulla S\in NICO\mict, Neurt)rMCUlar jl,letion Contraction of skeletal mustle Viscera Munrinic All parasympathetlC Mucous membranes target organs Veins Contraction of the tihary muscle Constriction Eye Sex organs, male Ejaculation focuses the lens for near vision Prostatic capsule Contraction of the ins sphmcter Contraction Bladder muscle causes m1os1s 02 Conu:a_cnon of trigone and sphT\. Presynaptic nerve (decreased pupil diameter) Inhibition of transmitter release terminals" Decreased rate ll, Heart Increased rate Constriction of bronthi Promotion of secretions Increased force of contraction Bladder C,ontraction of detrusor increases Increased atrioventricular condlA bladder pressure velocity Relaxation of trigone and Kidney Release of renin sphincter allows urine to leave ~l Arterioles Dilation the bladder Heart Coordinated contraction of Lung detrusor and relaxation of Skeletal muscle trigone and sphincter causes Bronchi Dilation voiding of the bladder Uterus Relaxation Gastroinlestinal tract Salivation Liver Glycogenolysis Increased gastric secretions Skeletal muscle Enhanced contraction. glycogenct, Dopamine Kidney Increased intestinal tone and Dilation of kidney vasculature motility Defecation Sweat glands" Generalized sweating Suor gMS Erection ( l) increased glandular secretions (from pulm Blood vesself Vasodilation onary, gastric, intesL~L and sweat glands), (2) cont racti on of smoo th muscle in the brod. Atthough sweating is due primarily to stimu and GI tract , (3) slow ing of the hear t rate, lation of muscarinic ( 4) contraction of tJ:; receptors by acetylcholine, the nerves that sphincter muscle of the iris, resulting in mios supply acetylcholine to is (reduction in pupil'.ir, sweat glands belong to the sympathetic diam eter) , (5) cont racti on of the ciliary nervous system rather than muscle of the eye, causin; the parasympathetic nervous system. the lens to focus for near visio n, (6) dilat tchohnergic receptors on blood vessels are ion of bloo d vessels, m not associated with the (7) void ing of the urina ry blad der (by caus nervous system. ing contraction of th, detru sor muscle [whic h form s the blad der wall] and relaxation~ the trigo ne and sphin cter muscles [whic h bloc k the bladder neJ when cont racte d]). Functions of Cho line rgic Receptor Sub Muscarinic cholinergic receptors on bloo types d vessels require addition! Table 12.2 shows the phar maco logic ally relev com men t. Thes e receptors are not assoc iated with the nervous systen ant responses to activation in any way. That is, no autonomic nerves term of the three majo r subt ypes of chol inerg inate at vascular muscarin1 ic recep tors: nicotinicN, nico - receptors. It is not at all clear as to how, tinicM, and musc arini c. or even if, these receptors ar activated physiologically. However, regar Responses to chol inerg ic recep tor activ dless of their physiologic rel ation can be grou ped into evance, the cholinergic receptors on bloo d three majo r categ ories base d on the subt ype vessels do have phcmrwol()f of recep tor involved: significance because drug s that are able to Activ ation of nicoti11icN (neu rona l) activate these receptors cau· recep tors prom otes ganglionic vasodilation, whic h in turn causes bloo d transmission at all gang lia of the symp athet pressure to fall. ic and para symp athet ic nerv ous syste ms. In addi tion, activ ation Functions of Adrenergic Receptor Sub of nicotinicN receptors types prom otes release of epi11ephrine from the Adrenergic recep tor subty pes and their adrenal medulla. functions are shown in T3r Acti vatio n of nicotinicM (mus cle) recep tors causes conrraction of 12.3. skeletal muscle. Activation of muscnri11ic receptors, whic a, Receptors h are located on target organs of the paras ympa theti c nervous system, elicit cx1 receptors are locat ed in the eyes, bloo s an appropriate response d vessels, male sex ofl?1 from the orga n involved. Specifically, musc prost atic capsule, and blad der (trig one and arini c activ ation causes sphin cter).  S of the Peripheral Nervous ystem CHAPTER 12 Physiology amin ial muscle of the iris. In the CNS, receptors for do e are of great therapeutic sign ifi- are present on the rad (dil n of th e pupil). e p cance. The functions of thes receptors are discussed in Chapters 19 ocw__ ,ar exI receptor tors leads to mydrrasrs. l atio s. ill and 26.. of these receent p on vein s and on artenod es m many cap..ary Activanon tors are pres els pro uces vasoamstnctron. a, rece ption of ex, receptors in blood vess of males causes RECEPTOR SPECIFICITY OF TH E LJ , ActJV3 tors in the sexual apparatus bl er ITTER S ADRENERGIC TRANSM add n of CX1 rece p rs in smooth muscle of the. ActlvauoAcU.vatJ. on Of ex I recepto [)CU" " con trac tron.. more comple ,iacufat1on. ,. ) nd prostatic capsule causes The receptor specificity of adr energic trans m1'tters 1s x and sphincter a cificity of A Ch. Wh ereas AC h. can activate all thre e than the receptor.spe v, (tngone ry ad ·. cholme.rgic rece ptors ' not eve renergic transmitter Table 12.3) su b.types of.. ract with each of the (J.,z Recepto:;the PNS are located on nerve terminals (see (epmephrme, norepmeph~me, dopamine) can inte system. d by the autono.mic nervous five subtypes of adrenergic rece pto rs. a2 receptors the organs m·nervate these rec eptors ws: (I) located on nerve terminals,. f h energic transmitters is as follo and not rv_ on receptor s are fun ctw n o t ese. Rece~tor spec~ficity of adr not dop am preiunctionaL The activate all a and Preceptors, but me receptors, 5ecause.,.,d to as presynar,ntt·c or 1 icted in Tab le 12.3, epinephnne. can. p, rece ptors, but not p2 or '- tter release. As dep vate CXi, and are re,err~is to regulaIe transmi (2) no~epinephnne can acti CXi,_ e neu ron A and doparrun ·e b' d to ex2 receptors located on the sam amine can activate a 1, ,.,., dopamme receptors, and (3) dop receptor s. uen ce of this smitter cap abl f. hnne can m eph rine was released. The con seq t dopamine itself is the only tran eo norepinep the norepm.. h rece. pto.rs. d(Note tha adre nerg ic hich sswn of furt er norep1-. rs.) Receptor specificity of the from w. receptor interaction is suppre rs can help red uce act1vatmg opamme recepto phrme- nee presynaptic cx2 recepto wn in Tab le 12.4. norepine I se. He d' th transmitters is sho t acts at A , te m e e is the only transmitter tha h nn ·er eea hen too mu ch transmitter. has. accumula. h I Knowing that epi nep hrin ns of this ~ nep ease w cts resulting from act1vatJon of perrp era CX2 'd erin g the fun ctio itter reID serv e as an a1 to rem emb trans~ effe. receptors can the adrenal apuc gap. rug imal clinical significance. t epinephrine is released from yn of min ipheral receptor subtype. Recall tha n of epin ephrine is s in the CNS. In contrast to per me dul la- not from neu ron s-a nd that the functio receptors are are also present ·11 , bec aus e epin ephrine