Autonomic Nervous System Lecture Notes

Nervous System, Cont. Chapter 14 Autonomic Nervous System Objectives Explain differences between somatic & autonomic nervous systems Explain differences between autonomic divisions Contrast and describe the anatomy of sympathetic and parasympathetic nervous systems Describe the...

Nervous System, Cont. Chapter 14 Autonomic Nervous System Objectives Explain differences between somatic & autonomic nervous systems Explain differences between autonomic divisions Contrast and describe the anatomy of sympathetic and parasympathetic nervous systems Describe the enteric nervous system Name and describe neurotransmitters released at different synapses of the ANS Explain mechanistically how the ANS controls target organs Autonomic Nervous System (ANS) Involuntary Motor (visceral) Modulate activity (adjust) Glands, cardiac, smooth muscle Regulates homeostasis Heart rate Blood pressure Respiration rate Digestion Pupil diameter Metabolism Visceral Reflexes Similar to somatic reflexes 1. Receptor 2. Afferent sensory neurons 3. Integration center (interneurons) 4. Efferent motor neurons 5. Effector Slower responses Divisions Sympathetic Alertness, heart rate, blood pressure, etc. Exercise, competition, stress, anger, fear Flight-or-flight reaction Parasympathetic Reduced energy usage Body maintenance (digestion, waste removal) Resting and digesting state Neither division has universally excitatory nor inhibitory effects Divisions Autonomic tone Normal background rate (both active) Parasympathetic E.g., Smooth muscles in intestines & slow heart rate Maintains muscle tone and heartrate ~70-80 bpm. Sympathetic E.g., Blood vessels partially constricted Components CNS Control nuclei - hypothallus and other brainstem regions Motor neurons – spinal cord Peripheral nervous system Ganglion Two fibers to target organ Preganglionic fiber Myelinated Soma in CNS Releases ACh Postganglionic fiber Unmyelinated Extends to effector Ach or NE Components Varicosities Terminal portion NT released along a significant length of the axon “looser”  easier diffusion NT covers a large surface area of the Autonomic effector tissue Nerve Terminal Varicosity Sympathetic Division (SNS) Thoracic and lumbar regions T1 to L2 Fibers extend to every level of the body Ganglion chains (paravertebral) Longitudinal ganglia Cervical to coccygeal levels Input Short preganglionic fibers Myelinated via White Rami Output long postganglionic fibers Unmyelinated via Gray Rami  White rami Grey rami Leaving Sympathetic Ganglia 1. Spinal nerve route Gray ramus  returns to spinal nerve  target organ 2. Sympathetic nerve route Sympathetic nerves  carotid or cardiac plexus  target 3. Splanchnic nerve route No ganglia synapse Continue as splanchnic nerves  collateral Collateral Ganglia Splanchnic nerve route 1. Celiac ganglia 2. Superior mesenteric ganglia 3. Inferior mesenteric ganglia Located at points where arteries with the same names branch off aorta Postganglionic fibers accompany these arteries and their branches to the target Adrenal Gland Medulla Essentially a sympathetic ganglion “Sympathoadrenal system” Has postganglionic neurons neuroendocrine Stimulated by preganglionic sympathetic fibers Parasympathetic Division (PSNS) Fibers from brain & sacral region Ganglia near target organs Long preganglionic fibers Divergence of neurons closer to organ Fewer postganglionic fiber More specific stimulation control Enteric Nervous System Digestive tract ~500 million neurons (Young, 2012) Communication & coordination Smooth muscle & glands Independent from CNS Does not arise from CNS Has its own ganglia & reflex arcs Regulation by PSNS & SNS 1. Movement Esophagus Stomach Intestines Peristaltic reflex *Local stimulation Secretomotor and vasodilator reflexes are mediated by neurons located in the submucosal plexus intrinsic primary afferent neurons Ascending (IPANs) excitatory Descending reflex inhibitory reflex Hirschsprung Disease Hereditary - Absence of enteric plexus No innervation in sigmoid colon and rectum Constricts permanently and will not allow passage of feces Feces becomes impacted above constriction Shrunken rectum & Megacolon – massive colon dilation w/ chronic constipation  Hirschsprung Disease Adult Zonal Hirschsprung's Disease: A Diagno stic Challenge Grossly dilated transverse colon and caecum at the time of emergency exploration. Neurotransmitters & Receptors Divisions & Contrasting effects Between & within SNS & PSNS How? 1. Different neurotransmitter (NTs) Acetylcholine Norepinephrine 2. Different receptor types Two classes for each NT Acetylcholine (ACh) Preganglionic in both ANS divisions Postganglionic parasympathetic Few sympathetic (blood vessels, some glands) 1. Muscarine receptors Second messenger systems Cardiac muscle, smooth muscle, glands Diff. subclasses E.g., excites intestinal muscle Few E.g., inhibits cardiac muscle 2. Nicotinic receptors Ligand-gated channels Ganglia synapses, Adrenal medulla, Skeletal muscle Norepinephrine (NE) Sympathetic postganglionic 1. α-adrenergic Second messenger system Different subclasses Usually excitatory effects (e.g., labor contractions) Ca2+ messengers Can be inhibitory (e.g., lowers intestinal motility) preventing cAMP production 2. β-adrenergic Usually inhibitory (e.g., dilates bronchioles, relaxes) Different subclasses Can be excitatory (e.g., excites cardiac Dual Innervation Nerve fibers from both divisions Antagonistic Same effector cells - e.g., heart muscle cells Different cells – e.g., eye muscles Cooperative Different effectors  unified response E.g., saliva production Single Innervation Nerve fibers from one division Can also produce opposite effects when needed E.g., Blood routes during emergency/stress 1. Sympathetic fibers decrease firing rate 2. Smooth muscles relax (blood vessels) 3. Blood pressure pushes on walls  dilation Can dilate vessels leading to vital organs (brain, heart, Next: Stimulation of receptors and sensory neurons