The autonomic nervous system

Comparison Of Autonomic & Somatic Nervous Systems

2 Neuron Chain

Preganglionic Neurons 1st neuron in spinal cord: thoracic and lumbar spinal nerves 1st neuron in 4 cranial nerves in the brain stem (III, VII, IX, X) or in S2 –S4 of spinal cord Sympathetic Parasympatheitc

Postganglionic Neurons 2nd neuron in sympathetic trunk ganglion, a chain of ganglion that is found on either side lateral to spine 2nd neuron in individual ganglion closer to organ it serves both sympathetic & parasympathetic neurons lie outside CNS Sympathetic Parasympathetic

Axons of Autonomic Nerves Preganglionic: in cranial or spinal n. (from CNS  ganglion) myelinated Postganglionic: from ganglion  visceral effector unmyelinated

ANS Neurotransmitters based on the neurotransmitter they produce & release autonomic neurons are classified as either: Cholinergic release acetylcholine (ACh) Adrenergic release norepinephrine (NE) aka noradrenalin

Cholinergic Neurons & Receptors Cholinergic neurons include: all Sympathetic & Parasympathetic preganglionic neurons Sympathetic postganglionic neurons that innervate most sweat glands all Parasympathetic postganglionic neurons

Receptors that bind ACh called cholinergic receptors 2 types: Nicotinic receptors in plasma membranes & dendrites of symp. & parasymp postganglionic neurons & in NMJ Muscarinic receptors in plasma membrane of all effectors (smooth muscle, cardiac muscle, glands) Muscarine: mushroom poisonmimics actions of ACh

ACh when activates nicotinic receptors  depolarization (excitation) when activates muscarinic receptors  sometimes depolarization, sometimes hyperpolarization (inhibition) depending on the cell

NE most sympathetic postganglionic neurons are adrenergic Adrenergic receptors bind both NE & Epinephrine (Epi) 2 types receptors: Alpha receptors (α) subtypes: α1, α2 Beta receptors (β) subtypes: β1, β2, β3

αlpha & βeta Receptors α1 & β1 produce excitation when activated α2 & β2 receptors cause inhibition of effector tissues β3 found only on cells of brown adipose where activation causes thermogenesis (heat production)

αlpha & βeta Receptors cells of most effectors have either α or β receptors some visceral effectors contain both NE stimulates α more strongly than β Epi is potent stimulator of both

MAO MonoAmine Oxidase: inactivates NE in synaptic cleft group of pharmaceuticals that are MAO inhibitors so prolong effect of NE

Agonists substance that binds to & activates a receptor in the process mimicking the effect of a natural neurotransmitter or hormone example: phenylephrine is an adrenergic agonist @ α1 receptors; used in cold remedies constricts blood vessels in nasal mucosa  reduces production of mucus

Antagonists substance that blocks receptors so prevents the natural neurotransmitter or hormone from exerting its effect example: propanolol a β1 blocker, used to treat HTN  decreases heart rate & force of contraction  lowers BP side effects: hypoglycemia, mild bronchoconstriction, decreases frequency & severity of migraines

Autonomic Tone balance between sympathetic & parasympathetic activity regulated by hypothalamus if turns up sympathetic tone, turns down parasympathetic tone @ same time

Sympathetic Responses dominate during physical or emotional stress occur during “E situations” Exercise Emotions Emergency Excitement

Sympathetic Responses Fight or Flight Response pupils dilate HR, force of contraction, & BP increase airways dilate vessels to kidneys & GI tract constrict slowing down digestion & urine production vessels  muscles (skeletal & cardiac), liver, & adipose tissue  dilate hepatocytes increase glycogenolysis & adipose increase lipolysis  blood glucose increases anything nonessential slowed down

Sympathetic Stimulation effects longer lasting than parasympathetic responses (NE lasts longer in synaptic cleft than ACh) effects more widespread (more tissues activated)

Parasympathetic Responses enhance “rest & digest” activities remeber SLUDD: Salivation Lacrimation Urination Digestion Defecation