11/20/14 2:02 PM Chapter 15: The Autonomic Nervous System Autonomic Nervous System = Visceral Motor Division • Motor commands sent to viscera (internal organs) • Divided into sympathetic and parasympathetic divisions • Regulation of breathing, blood pressure, heart rate, etc. o Things outside of conscious control • Receptors control these functions: o Thermoreceptors—Body heat o Baroreceptors—Pressure § Ex: Elderly people become dizzy when they stand up quickly because their baroreceptors don’t work as quickly Visceral Reflexes • ANS is responsible for body’s visceral reflexes • Visceral reflex arc: Receptors --> Afferent Neurons --> Interneurons --> Efferent Neurons --> Effectors • Baroreflex: High blood pressure detected by arterial stretch receptors (baroreceptors) o Example of homeostatic negative feedback loop § Advantageous because body senses pressure of blood as soon as it comes off of aorta (away from heart) § If blood pressure it too high, could damage other organs ú Kidney normally first to be damaged, liver normally last to be damaged § Information sent to brain when blood pressure gets too high, two ways to decrease BP once message gets to brain: ú Blood vessels may dilate ú Decrease heart rate Autonomic Motor Pathways vs. Somatic Motor Pathways • Somatic—Myelinated fiber from somatic efferent nerve to gland • Autonomic—Two neurons from CNS to effectors o Preganglionic fiber and postganglionic fiber § Myelinated preganglionic fiber synapses on ganglion§ Fiber that leaves ganglion is an unmyelinated postganglionic fiber that will synapse on the target Functional Organization of Somatic vs. Autonomic • Conscious (somatic) vs. unconscious (autonomic) control Divisions of the ANS • Sympathetic Division: Prepares body for physical activity o “Fight or flight”—Prepares you for stressful situations o Heart rate, breathing, etc. all increase o Digestion decreases • Parasympathetic Division: Calms body functions o “Rest and digest” o Heart rate, breathing, etc. normal o Body can focus on digestion Sympathetic Division (Thoracolumbar Division): Short preganglionic and long postganglionic fibers • Short, branching preganglionic axon coming off of preganglionic neuron • Preganglionic Neurosomas: Lead to nearby sympathetic chain of ganglia o Paravertebral ganglia—right beside vertebrae o Only in thoracic and lumbar regions • White communicating ramus: Preganglionic (myelinated) fiber that is responsible for helping with communication from spinal nerve to ganglia o Information coming off of one nerve able to be distributed to other parts of body o Chain of ganglia • Gray communicating ramus: Postganglionic fiber (unmyelinated) • Nerve communicates with sympathetic chain of ganglia via WCR and leaves ganglia via GCR o Very short preganglionic fiber because fiber coming off of spinal cord immediately to chain of ganglia o Longer postganglionic fiber to reach target • When a spinal nerve synapses and sends information to ganglia, 3 possibilities:o 1. Send information up or down chain and synapse in another ganglia above/below o 2. Nerve may end (synapse with a postganglionic fiber) within ganglia itself o 3. Go through a ganglia, but not synapse on ganglia, and synapse at a later ganglion that is not prevertebral (located more distally) § Form pelvic splanchnic nerves or splanchnic nerve § Splanchnic: Nerves to organs § Pelvic Splanchnic: Nerves to organs in pelvic region • Neural Divergence: Not 1:1 ratio—may have one nerve that synapses on several postganglionic fibers (wide dispersal of information) Sympathetic Chain Ganglia The Adrenal Glands • Each is two glands with different functions: o 1. Adrenal Cortex: Outer layer that secretes steroid hormones o 2. Adrenal Medulla: Inner core that is essentially a sympathetic ganglion § Secretes catecholamines Parasympathetic Division (Craniosacral Division) • Nerves from midbrain, medulla oblongata, and pons, and sacral region S2-S4 o Oculomotor, facial, glossopharyngeal nerves o Vagus nerve (thoracic and abdominal pelvic regions) o Nerves in S2-S4 region—form pelvic splanchnic nerves § Originate in sacral region and innervate some of the lower abdominal organs and all organs in pelvic region (i.e. reproductive, bladder, etc.) Enteric Nervous System: Nervous system of the digestive system • Not connected to the CNS—Set of nerves only found in digestive system • Composed of 100 million neurons found in walls of digestive tract • Regulates esophagus, stomach, and intestinal motility and secretion of digestive enzymes and acid • Has own reflex arc that does not involve CNSClinical Corner: Megacolon: Massive dilation of bowel • Usually evident in newborns who fail to have their first bowel movement • May be caused by something getting stuck and causing blockage or inflamed material • Hirschsprung’s Disease (Aganglionic Megacolon): Born without enteric nervous system o Absence of a bowel movement indicates obstruction or hirschsprung’s disease o Always diagnosed at birth • In adults with megacolon, take out colon • In children born with hirschsprung’s, put in a bag for a short period of time and perform surgery to correct problem o Generally no problems after that Neurotransmitters and Their Receptors • How can different autonomic neurons have different effects on the same organ—constricting some vessels but dilating others? o 1. Different receptors o 2. Different neurotransmitters • Same neurotransmitter may have different effect depending on neurotransmitter • Cholinergic Fibers: Acetylcholine as neurotransmitter o All preganglionic fibers in the sympathetic and parasympathetic divisions secrete acetylcholine as their neurotransmitter*** o All postganglionic fibers in the parasympathetic division secrete acetylcholine as neurotransmitter, and some postganglionic fibers in sympathetic division secrete acetylcholine as their neurotransmitter*** o The remaining postganglionic fibers in the sympathetic division secrete norepinephrine as their neurotransmitters*** • Cholinergic Receptor: Acetylcholine as receptor, two types: o 1. Nicotinic Receptors: Always accept excitatory acetylcholine § Found on ganglion in both parasympathetic and sympathetic§ Acetylcholine released in both divisions is