Chp. 16: AUTONOMIC N.S. (In Review: Peripheral N. S.) Peripheral nerves contain both motor and sensory neurons Among the motor neurons, some of these are somatic and innervate skeletal muscles while some are autonomic and innervate smooth muscle, cardiac muscle, and glands Sensory neurons are not subdivided into somatic and autonomic since there is overlap in function; e.g., pain receptors can stimulate both somatic and autonomic reflexes 16-1 Somatic and Autonomic Nervous Systems Somatic Skeletal muscle Conscious and unconscious movement Skeletal muscle contracts One synapse Acetylcholine Receptor molecules: nicotinic Autonomic Smooth and cardiac muscle and glands Unconscious regulation Target tissues stimulated or inhibited Two synapses Acetylcholine by preganglionic neurons and ACh or norepinephrine by postganglionic neurons Receptor molecules: varies with synapse and neurotransmitter 16-2 1
Distribution of Autonomic Nerve Fibers 16-3 Autonomic Nervous System Homeostasis Divided into sympathetic and parasympathetic divisions as well as the enteric nervous system Sympathetic and parasympathetic divisions often supply the same organs but differ in a number of features The Enteric Nervous System: Nerve plexuses within the wall of the digestive tract. Contributions from: sensory neurons between digestive tract and CNS, ANS motor neurons between the CNS and the digestive tract, and enteric neurons confined within the plexuses Functions Stimulate/inhibit smooth muscle contraction Stimulate/inhibit gland secretions Detect changes in content of lumen 16-4 2
Sympathetic (Thoracolumbar) Division Preganglionic cell bodies in lateral horns of spinal cord T1-L2: thoracolumbar Preganglionic axons pass through ventral roots to white rami communicantes to the retroperitoneal sympathetic chain ganglia. 16-5 Routes of Sympathetic Axons 1. Spinal nerves: preganglionic axons synapse (at the same or different level) with postganglionic neurons within the sympathetic chain. These postganglion neurons exit the ganglia through the gray rami communicantes and re-enter spinal nerves. 2. Sympathetic nerves: preganglionic axons synapse (at the same or different level) with postganglionic neurons, which exit the ganglia through sympathetic nerves 16-6 3
Routes of Sympathetic Axons 3. Splanchnic nerves: preganglionic axons pass through the chain ganglia without synapsing to form splanchnic (visceral) nerves. Preganglionic axons then synapse with postganglionic neurons in collateral ganglia. Postganglionic neurons then send fibers to target organs (viscera). 4. Innervation to adrenal gland: preganglionic axons synapse with the cells of the adrenal medulla. Embryologically, adrenal medulla is derived from same cells as postganglionic ANS cells. Medullary cells secrete epinephrine and norepinephrine; act as hormones promoting physical activity. 16-7 Distribution of ANS Fibers: Sympathetic (emergency mode) Sympathetic axons reach organs through Spinal nerves: innervate sweat glands, smooth muscle of blood vessels to skeletal muscle and skin, and arrector pili Head and neck nerve plexuses: innervate sweat glands, smooth muscle of blood vessels to skeletal muscle and skin, and arrector pili Thoracic nerve plexuses: cardiac and pulmonary; heart and lungs Abdominopelvic nerve plexuses: celiac, superior mesenteric, inferior mesenteric, hypogastric plexuses. Organs of abdominopelvic cavity NOTE: sweat glands, blood vessels, heart & lungs, muscles 16-8 4
Responses to Exercise (Fight or Flight Response) Increased heart rate and force of contraction Blood vessel dilation in skeletal and cardiac muscles Dilation of air passageways Energy sources availability increased Glycogen to glucose Fat cells break down triglycerides Muscles generate heat, body temperature increases Sweat gland activity increases Decrease in nonessential organ activities 16-9 Parasympathetic (Craniosacral) Division Preganglionic cell bodies in nuclei of brainstem or lateral parts of spinal cord gray matter from S2-S4 Preganglionic axons from brain pass to terminal ganglia through cranial nerves III, VII, IX and X Preganglionic axons from sacral region pass through pelvic splanchnic nerves to terminal ganglia Terminal ganglia located near organ innervated or embedded in wall of organ 16-10 5
Distribution of ANS Fibers: Parasympathetic (Peaceful-Housekeeping) Parasympathetic axons reach organs through Cranial nerves Oculomotor (III) through ciliary ganglion. Ciliary muscles,iris Facial (VII) through pterygopalatine ganglion and submandibular ganglion. Lacrimal glands, mucosal glands of nasal cavity and palate, some salivary glands Glossopharyngeal (IX) through otic ganglion. Parotid salivary Vagus (X) and thoracic nerve plexuses. Heart, lungs, esophagus through esophageal plexus Abdominopelvic nerve plexuses. Parts of vagus nerve supply stomach and other viscera Pelvic splanchnic nerves and nerve plexuses.(from S2-S4) Colon, urinary bladder, reproductive organs 16-11 Innervation of Organs by ANS 16-12 6
Physiology of ANS Neurotransmitters: primary substances produced by neurons of ANS Acetylcholine released by cholinergic neurons Norepinephrine released by adrenergic neurons Certain cells have receptors that combine with neurotransmitters causing a response in the cell Cholinergic: bind acetylcholine. Have two different forms: nicotinic and muscarinic Nicotinic: all receptors on postganglionic neurons, all skeletal muscles, adrenal glands Muscarinic: all receptors on parasympathetic effectors, receptors of some sweat glands Adrenergic receptors bind norepinephrine/epinephrine Alpha and beta receptors.these are further subdivided into categories. α 1 and β 1 usually have opposite affects than α 2 and β 2 16-13 Location of ANS Receptors 16-14 7
Regulation of ANS Autonomic reflexes control most of activity of visceral organs, glands, and blood vessels. Autonomic reflex activity influenced by hypothalamus and higher brain centers, but it is the hypothalamus that has overall control of the ANS. Sympathetic and parasympathetic divisions influence activities of enteric nervous system through autonomic reflexes. These involve the CNS.. BUT, the enteric nervous system can function independently of CNS through local reflexes (involuntary response) (E.g., when wall of digestive tract is stretched, sensory neurons send information to enteric plexus and then motor responses sent to smooth muscle of gut wall and the muscle contracts.) 16-15 Autonomic Reflexes a) Parasympathetic reflex via vagus lowers heart rate. b) Sympathetic reflex via cardiac accelerator nerves (sympathetic) cause heart rate to increase. 16-16 8
Influence of Brain on Autonomic Functions 16-17 Functional Generalizations of ANS Dual innervation to most organs with sympathetic and parasympathetic having the opposite effects. Either division alone or both working together can coordinate activities of different structures. Sympathetic prepares body for physical activity or flight-or-fight response. But also important at rest. (Blood vessel walls receive only sympathetic stimulation, so at rest, sympathetic is responsible for maintenance of blood pressure.) In general, parasympathetic more important for resting conditions: SLUDD= salivation, lacrimation, urination, digestion, defecation 16-18 9