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Chapter 11 Efferent Division: Autonomic and Somatic Motor Control Part 2 On Friday, go back to part one and go over slide 50 Some definitions Endocrine gland – A ductless gland or a single cell which secretes a hormone Exocrine gland – Releases secretions through a duct Figure 11-10a The adrenal medulla secretes epinephrine into the blood Adrenal gland Kidney (a) Copyright © 2010 Pearson Education, Inc. The Adrenal Gland Adrenal cortex – Endocrine gland – Epidermal origin – Secretes steroid hormones Adrenal medulla – The interior or core of the adrenal gland – Neurosecretory structure – Developed from same embryological tissue as the sympathetic neurons Figure 11-10b The adrenal medulla secretes epinephrine into the blood Adrenal cortex is a true endocrine gland. Adrenal medulla is a modified sympathetic ganglion. (b) Copyright © 2010 Pearson Education, Inc. Adrenal Medulla is a modified sympathetic ganglion Preganglionic sympathetic neurons project from the spinal cord to the medulla, where they synapse The postganglionic neurons lack axons The axon-less cell bodies (Chromaffin cells), secrete the neurohormone, epinephrine directly into the blood In response to alarm signals from the CNS, the adrenal medulla releases large amounts of epinephrine as part of the “Fight-or-Flight” response Figure 11-10c The adrenal medulla secretes epinephrine into the blood ACh Spinal cord Preganglionic sympathetic neuron The chromaffin cell is a modified postganglionic sympathetic neuron. (c) Adrenal medulla Copyright © 2010 Pearson Education, Inc. Epinephrine is a neurohormone that enters the blood. Blood vessel To target tissues Parasympathetic Pathways Parasympathetic neurons release Acetylcholine at their targets Parasympathetic neuroeffector junctions have muscarinic cholinergic receptors Muscarinic receptors are all G protein-coupled receptors. Receptor activation initiates second messenger pathways, some of these open K+ or Ca++ channels Tissue response varies with receptor subtype (5 subtypes) Autonomic Agonists and Antagonists Agonists: – Molecules which combine with a receptor and mimic a response Antagonists: – One substance opposes the action of another Autonomic Agonists and Antagonists Direct agonist or antagonist – Combine with the target receptor – Mimic or block neurotransmitter action Indirect agonists and antagonists – These act by altering – Secretion of neurotransmitters – Re-uptake of neurotransmitters – Degradation of neurotransmitters Examples of Autonomic Agonists and Antagonists Cocaine – An indirect agonist – Blocks re-uptake of NE into adrenergic nerve terminals; extends NE's excitatory effect Anticholinesterases – Includes the organophosphate insecticides (parathion and malathion) – Cholinesterase inhibitors – Indirect agonists – Block ACh degradation – Extends the active life of each ACh molecule Examples of Autonomic Agonists and Antagonists Once alpha and beta adrenergic receptors had been discovered, drugs were designed to block specific ones: Tamsulosin (Flomax) blocks alpha-1A adrenergic receptors Beta blockers are antagonists for Beta adrenergic receptors – Used to treat high blood pressure Table 11-3 Copyright © 2010 Pearson Education, Inc. Autonomic Nervous System Summary P. 396, Summary of Sympathetic and Parasympathetic branches P. 397, Figure 11-11 Summary of Efferent Pathways (next slide) Table 11-4: Comparison of Sympathetic and Parasympathetic Branches Table 11-5: Comparison of Somatic and Autonomic Branches Figure 11-11, overview Copyright © 2010 Pearson Education, Inc. Table 11-4 Copyright © 2010 Pearson Education, Inc. Table 11-5 Copyright © 2010 Pearson Education, Inc. The Somatic Motor Division Somatic motor pathways – Control skeletal muscles – Consists of a single neuron which originates in the CNS – The target is ALWAYS a skeletal muscle – Somatic pathways are always excitatory Figure 11-11 Summary of efferent pathways SOMATIC MOTOR PATHWAY CNS Parasympathetic pathway AUTONOMIC PATHWAYS Sympathetic pathways CNS CNS Adrenal sympathetic pathway CNS ACh Adrenal cortex Nicotinic receptor Adrenal medulla Ganglia E ACh Ganglion Nicotinic receptor α receptor NE ACh Muscarinic receptor Autonomic effectors: • Smooth and cardiac muscles • Some endocrine and exocrine glands • Some adipose tissue Blood vessel β 1 receptor E ACh Nicotinic receptor Skeletal muscle Copyright © 2010 Pearson Education, Inc. β2 receptor KEY ACh = acetylcholine E = epinephrine NE = norepinephrine The Somatic Motor Division Somatic motor pathways – Cell bodies of somatic motor neurons are located in either • Ventral horn of spinal cord • or • The brain The myelinated axons of these pathways may be a meter or more long – Examples: those that innervate the muscles of the hand and the foot Somatic motor neurons branch close to their targets Each branch divides into a cluster of enlarged axon terminals that lie on the surface of the skeletal muscle fiber This arrangement allows a single motor neuron to control many muscle fibers at one time Neuromuscular Junction (NMJ): – The synapse of a somatic motor neuron + a muscle fiber Figure 11-12, part 1 Copyright © 2010 Pearson Education, Inc. Neuromuscular Junction (NMJ) 3 main components to the NMJ 1. Motor neuron's presynaptic axon terminal 2. Synaptic Cleft 3. Postsynaptic membrane of the skeletal muscle fiber Neuromuscular Junction (NMJ) Also at the NMJ are Schwann cell extensions which cover the axon terminals Once thought that this was just additional insulation to speed up the conduction of the AP Now we know that that the Schwann cells secrete a variety of chemical signal molecules These chemicals play a vital role in the formation and maintenance of the NMJ Neuromuscular Junction (NMJ): Postsynaptic side Motor End Plate – The muscle cell membrane is modified into a series of folds that look like shallow gutters – Along the upper edge of each gutter, there is an active zone filled with nicotinic ACh receptor channels (nAChR) Figure 11-12, part 2 Copyright © 2010 Pearson Education, Inc. Figure 11-12, part 3 Copyright © 2010 Pearson Education, Inc. Neuromuscular Junction (NMJ): Postsynaptic side Motor End Plate (continued) – Synaptic cleft is filled with a fibrous (collagen matrix) • Holds the motor end plate and axon terminal in alignment • Matrix also contains acetylcholinesterase which rapidly deactivates ACh by degrading it into acetyl and choline Neuromuscular Junction (NMJ): Receptors An AP arriving at the axon terminal will cause voltagegated CA++ channels to open Calcium ion diffuses into the cell and triggers the release of ACh which then diffuses across the synaptic cleft On the other side of the synaptic cleft, ACh binds with a nicotinic receptor channel (nAChR) on the skeletal muscle membrane Figure 11-13a Copyright © 2010 Pearson Education, Inc. The nAChR channels of skeletal muscle are similar to but not identical to the nicotinic ACh receptors found on neurons Their differences are highlighted by the snake toxin alpha-bungarotoxin: – This toxin binds to nicotinic skeletal muscle receptors, but not to similar receptors in the autonomic ganglia Both muscle and neuronal receptor proteins have five subunits encircling a central pore, but the SUBUNITS in each receptor type are DIFFERENT Nicotinic cholinergic receptors are chemically gated ion channels They have 2 binding sites for ACh When ACh binds, the gate opens and allows monovalent cations to flow through In skeletal muscle, the monovalent cation is Na+ Figure 11-13b Events at the neuromuscular junction Open channel Closed channel ACh Na+ K+ (b) Copyright © 2010 Pearson Education, Inc. K+ Na+ Na+ entry into the muscle fiber depolarizes it, triggering an AP which causes contraction of the skeletal muscle cell Acetylcholine acting on the skeletal muscle motor end plate is always excitatory and always causes muscle contraction There is no antagonistic innervation to relax the muscle Relaxation occurs when somatic motor neurons are inhibited by the CNS