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Peripheral Nervous System Dr. Jena Hamra Autonomic Nervous System Two divisions Sympathetic “Fight or flight” Parasympathetic “Rest and digest” Antagonistic control Dual innervation Exception: Sweat glands, vascular smooth muscle Multiple receptor types Both tonically active Central Regulation Control centers Hypothalamus Pons Medulla Control: Heart rate, blood pressure Temperature Water balance Spinal reflexes Urination, defecation Efferent Neurons Two neurons in series Preganglionic neuron from CNS Autonomic ganglion outside CNS Postganglionic neuron to target tissue Intrinsic neurons Divergence Anatomy of the ANS Sympathetic pathways Originate in thoracic and lumbar regions of SC Sympathetic ganglia run in chain alongside SC Short preganglionic and long postganglionic neurons Parasympathetic pathways Originate in brainstem or sacral region of SC Parasympathetic ganglia located on or near target organ Long preganglionic and short postganglionic neurons Special Parasympathetic Nerves Leave brainstem via cranial nerves Major parasympathetic tract Vagus nerve Cranial nerve X Carries: Afferent sensory information Efferent parasympathetic to most internal organs Target Tissues of Autonomic Nerves Smooth muscle Cardiac muscle Many exocrine glands Adipose tissue Brown fat Autonomic Synapse Axon terminals have “varicosities” Swollen ends containing many vesicles Branched axon terminals lie across target tissue Target cell membrane does NOT have clusters of receptors Neurotransmitter released into interstitial fluid Single postganglionic neuron can affect large area Adrenal Medulla Neuroendocrine gland Secretes norepinephrine and epinephrine Modified sympathetic ganglion Receives preganglionic input from SC Postganglionic neurons lack axonal projections Release neurohormones into blood Neurotransmitters Acetylcholine Norepinephrine Release Action potential Open Ca2+ channels Ca2+ enters Exocytosis of neurotransmitter vesicles More neurotransmitter released, the stronger the response Neurotransmitters Breakdown/Removal Catecholamines Uptake into presynaptic terminal Repackaged or degraded by monoamine oxidase • Mitochondria Acetylcholine Degraded by acetylcholinesterase in synaptic cleft Choline transported back into neuron Location of Neurotransmitters All autonomic preganglionic neurons Acetylcholine, nicotinic cholinergic receptors Postganglionic sympathetic neurons Norepinephrine, adrenergic receptors Postganglionic parasympathetic neurons Acetylcholine, muscarinic cholinergic receptors Non adrenergic, non cholinergic neurons Substance P, Somatostatin, Vasoactive intestinal peptide (VIP) Adrenergic Receptors Alpha Binds to and responds more to norepinephrine Alpha –1 Second messenger – Phospholipase C and IP3 Increase intracellular Ca2+ muscle contraction or exocytosis Alpha-2 Second messenger – Inhibits cAMP Smooth muscle relaxation (GI tract) Adrenergic Receptors Beta Beta–1 Heart muscle and kidney cells • Enhances contraction Innervated by sympathetic neurons Respond to both Nepi and Epi Beta-2 Smooth muscle in airways, blood vessels • Relaxation NOT innervated by sympathetic neurons More sensitive to epinephrine Both Increase cAMP Cholinergic Receptors Nicotinic Receptor linked to ion channel Muscarinic Coupled to G proteins Second messengers Potassium channels Tissue response varies with receptor subtype Five subtypes Autonomic Agonists and Antagonists Cholinergic agonists Acetylcholine, nicotine, muscarine Cholinergic antagonists Atropine, scopolamine, curare Adrenergic agonists Norepinephrine, epinephrine Phenylephrine, Isoproterenol Adrenergic antagonists Propanolol Somatic Motor Division ONE efferent neuron Cell bodies in gray matter of brain or SC Long single axon Branching axons lie close to target Axon branches divide into boutons Acetylcholine only neurotransmitter Only ONE target tissue Skeletal muscle Excitatory effects only No ganglia Neuromuscular Junction Presynaptic axon terminal Efferent motor neuron Synaptic cleft Fibrous matrix Contains acetylcholinesterase Postsynaptic membrane Motor end plate Folds with cluster of Ach receptors Nicotinic receptors Chemically gated ion channels Nonspecific ion channels Always excitatory