organization of motor systems
DESCRIPTION
Organization of Motor Systems. The motor systems encompass 2 divisions of the PNS . Somatic (‘voluntary’) – controls skeletal muscle Autonomic (‘involuntary’) – controls visceral effectors: Smooth muscle surrounding blood vessels Cardiac muscle GI tract smooth muscle - PowerPoint PPT PresentationTRANSCRIPT
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Organization of Motor Systems
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The motor systems encompass 2 divisions of the PNS
• Somatic (‘voluntary’) – controls skeletal muscle
• Autonomic (‘involuntary’) – controls visceral effectors:– Smooth muscle surrounding blood vessels– Cardiac muscle– GI tract smooth muscle– Exocrine glands (salivary, sweat, gastric secretion, etc)– metabolic effects in variety of tissues (SNS)
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Before we start, just a little about receptors
• Receptors divided into two classes:– Ionotropic – receptor is an ion channel that opens a conductive
pathway when bound with transmitter – effect may be depolarization, hyperpolarization or stabilization.
– Metabotropic – transmitter-receptor binding initiates a chemical 2nd message within the target cell – this typically activates ion channels and transduces an electrical effect – but it doesn’t always have to do so to be effective.
• Drugs that act on receptors may be– agonists (acting like or facilitating the action of the native
transmitter)– antagonists (opposing the action of the native transmitter)
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Are there excitatory and inhibitory transmitters and receptors?
• Although we sometimes speak as if this were so,• In most cases, the outcome of applying a
particular transmitter to a tissue depends entirely on what happens in the target tissue after the receptor has been activated, and not on any intrinsic property of the transmitter or receptor
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Comparison of Autonomic and Somatic Motor Systems
• The next slide compares the basic organization, transmitter identities, and receptor identities in the two branches of the autonomic system with the somatic system.
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Comparison of ANS and SNS
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Some Important things to know about the motor systems slide
• In all systems, at the 1st synapse outside the CNS, the transmitter is always Ach and the receptors nicotinic.
• Nicotinic receptors are always excitatory; muscarinic and adrenergic receptors may be excitatory or inhibitory depending on the effect of the 2nd messengers coupled to the receptor in each tissue.
• Single innervation of effectors in the somatic system vs (typically) dual innervation in the autonomic system.
• Sympathetic ganglia are generally remote from the target organ whereas parasympathetic ones are generally in or on the target.
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The nature of effectors in the autonomic system
• Target tissues may include smooth muscle, ducted glands, and, in the case of the sympathetic branch, the metabolic activity of a number of tissues, including the liver, adipose tissue, skeletal muscle, and the brain itself.
• Generally, autonomic inputs do not have a off/on effect on their targets. Instead, they modulate tissue activities that are already ongoing.
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Smooth muscle as an effector of the autonomic system
• Characteristics of smooth muscle:– Cells small– Surrounds hollow internal organs and blood
vessels– Contractile machinery diffuse (no sarcomeres)– Electrical activation is dependent on external
Ca++
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Control features of smooth muscle • Two basic types:• 1. Single unit
– Cells coupled by gap junctions– Spontaneous activity due to pacemakers– ANS does not need to make a synapse on each cell– ANS modulates spontaneous activity
• 2. Multi-unit– Few gap junctions; little or no spontaneous activity– ANS must make synapses on each cell– Contractile activity reflects the balance of S versus PS inputs
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There are two routes to activate smooth muscle
Electromechanical:Chemical message > depolarization > Ca++ entry >
contractile activity
Pharmacomechanical: Chemical message > intracellular 2nd message >
Ca++ release from internal stores > contractile activity
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Anatomical summary of the ANS
Basic Anatomical Layout of the Autonomic Nervous System
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The Sympathetic Branch
• Cell bodies of preganglionic neurons are in thoracic and upper lumbar cord
• Preganglionic axons pass out through T1-L2 ventral roots as B and C fibers – small to very small.
• They may synapse in paravertebral ganglion of the same segment, or turn to go rostrally or caudally to other segments, or continue through a splanchnic nerve to a prevertebral ganglion among the viscera– Celiac ganglion – organs of upper abdomen– Superior mesenteric ganglion – intestines– Inferior mesenteric ganglion – distal colon, bladder, genitalia
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Sympathetic branch, continued.
• Postganglionic axons are unmyelinated C fibers that may pass through gray ramus to spinal nerve running to body wall, or into a sympathetic nerve to abdominal organs
• Adrenal medulla – preganglionics are in T10 and T11 – they go through the celiac ganglion to adrenal medulla where they synapse on chromaffin cells.
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The Parasympathetic Branch
• Preganglionic cell bodies are in brainstem nuclei – associated with particular cranial nerve roots, or– in S2-S4.
• Ganglionic synapses are typically in or at least close to the target organs.
• Cranial: ciliary ganglion – Cranial N. III; sphenopalatine and submaxillary ganglia – C. N. VII, Otic ganglion – C.N. IX
• Abdomen - splenic flexure of colon marks the boundary between turf of the vagus N. (C.N. X) and the sacral part of the P.S.B.
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The cholinergic synapse• Presynaptic: acetylcholine is synthesized
from glucose• Postsynaptic: receptors may be nicotinic
(ionotropic-excitatory) or muscarinic (2nd messenger); there are at least 4 subtypes of muscarinic receptors
• After release, Ach is degraded by acetylcholinesterase and choline is reabsorbed by presynaptic terminal.
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Cholinergic synapses as drug targets
• Vesicle release blocker: botulinum toxin• Nicotinic receptor blockers: curare, cobra toxin,
hexamethonium• Muscarinic receptor blockers: muscarine (toxin
from fly agaric mushroom Amantia muscarina), atropine and scopalomine (Belladonna alkaloids)
• Cholinesterase inhibitors: physostigmine, neostigmine, nerve gases, insecticides
• Choline reuptake inhibitor: hemicholinium
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The Adrenergic Synapse
• Norepinephrine synthesized by synaptic terminal:– Tyrosine > DOPA>Dopamine>NENorepinephrine is recovered by the presynaptic
cell and may be recycled or degraded (monoamine oxidase)
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Adrenergic synapse,cont.
• 4 main classes of adrenergic receptors, characterized by agonist specificity
• Alpha 1: sweat glands, blood vessels• Alpha 2: GI tract, presynaptic terminals• Beta 1: heart muscle• Beta 2: Lung airway; blood vessels in
heart, skeletal muscle
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The adrenergic synapse as a drug target
• Alpha agonists: phenylephrine, pseudephedrine, amphetamines (decongestants, stimulants)
• Alpha blockers: phentolamine, phenoxybenzamine, ergot alkaloids, yohimbine
• Beta agonist: isoproterenol• Beta blockers: propranolol and its congeners
(antihypertensives)• Monoamine oxidase inhibitors (antidepressants)
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Higher Centers of Autonomic Function
• Location: Brainstem, hypothalamus and other parts of the limbic system
• Physiological variables controlled by the limbic system and A.N.S.:– Body temperature– Blood pressure– Blood glucose– Digestive activities– Sex response– Emotions
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An example – baroreceptor reflex and blood pressure regulation
Medullary CV center (brainstem)
Medullary Respiratory Center Limbic System
Baroreceptors (carotid sinus, aortic arch)
VasculatureHeart
Vagus (inhibits)
Sympathetics (stimulate)
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Reflexive response to a drop in blood pressure versus the medullary setpoint
• Sensed by baroreceptors• Integrated in MCV center• Results of reflex activation:
– Increases outflow of action potentials in thoracic sympathetics to heart • Heart rate rises• Stroke force increases
– decreases or shuts off vagal outflow to heart • Parasympathetic tone removed – facilitates rate increase
– Increases sympathetic outflow to blood vessels• Arterioles - Peripheral resistance rises• Veins - Venous capacitance falls, so blood moves to arterial
side of circulation.