ch 16 autonomic nervous system ans 04 01 2015 The peripheral nervous system PNS is composed of sensory and motor neurons Sensory neurons carry action potentials from the periphery to the central nervous system CNS Motor neurons carry action potentials from the CNS to the periphery o Motor neurons that innervate skeletal muscle are called somatic motor neurons somatic nervous system o Motor neurons that innervate smooth muscle cardiac muscle and glands are called autonomic motor neurons autonomic nervous system ANS In the Somatic Nervous System the cell bodies of the somatic motor neurons are in CNS and their axons extend from there to skeletal muscle The ANS has two neurons in a series extending b w the CNS and the innervated organs The first neuron of the series is called the preganglionic neuron The preganglionic neuron s cell body is located in the CNS and its axon extends to an autonomic ganglion located outside the CNS The autonomic ganglion contains the cell body of the second neuron of the series which is called the postganglionic neuron Preganglionic neurons synapse with the postganglionic neurons in the autonomic ganglia The axons of postganglionic neurons extend from autonomic ganglia to effector organs where they synapse with their target tissues Sensory neurons are not classified as somatic or autonomic These neurons propagate action potentials from sensory receptors to the CNS and can provide information for reflexes mediated through the somatic nervous system or the ANS Ex Stimulation of pain receptors can initiate somatic reflexes such as the withdrawal reflex and autonomic reflexes such as an increase in heart rate LOOK TABLE 16 1 ANATOMY OF THE ANS ANS is subdivided into the sympathetic division parasympathetic division and the enteric nervous system Sympathetic thoracolumbar Division cell bodies of sympathetic preganglionic neurons are located in the lateral horns of the spinal cord gray matter b w the first thoracic T1 segment and the second lumbar L2 segment Sympathetic division can also be called thoracolumbar division Two types of sympathetic ganglia sympathetic chain ganglia and collateral ganglia Sympathetic chain ganglia are connected to each other and are so named because they form a chain paravertebral Collateral ganglia are unpaired ganglia Sympathetic axons exit the sympathetic chain ganglia by 1 Spinal nerves 2 Sympathetic nerves 3 Splanchnic nerves preganglionic axons enter sympathetic chain ganglia without synapsing and exit to form splanchnic nerves Those preganglionic axons extend to collateral ganglia where they synapse with postganglionic neurons Innervation to the adrenal gland different b c axons of the preganglionic neurons do not synapse in sympathetic chain ganglia or in collateral ganglia Instead axons pass through those ganglia and synapse with cells in the medulla of the adrenal gland A misplaced sympathetic ganglion Upon stimulation medullary cells secrete norepinephrine NE and epinephrine EPI into the blood Fibers from T5 L2 form splanchnic nerves abdomen and synapse with collateral ganglia Fibers from T1 T4 synapse form sympathetic nerves thoracic organs and synapse with the sympathetic chain ganglia Parasympathetic craniosacral division The cell bodies of parasympathetic preganglionic neurons are located either within nuclei of brainstem or lateral parts of the gray matter of the spinal cord from S2 to S4 Axons of the parasympathetic preganglionic neurons from the brain are in cranial nerves III VII IX and X and from the spinal cord in pelvic splanchnic nerves The preganglionic axons course through these nerves to terminal ganglia where they synapse with postganglionic neurons Terminal ganglia located near organ innervated or embedded in wall of organ Distribution of ANS Fibers Parasympathetic Parasympathetic axons reach organs through 1 Cranial nerves 2 Vagus nerve and thoracic nerve plexuses Heart lungs esophagus through esophageal plexus 3 Abdominopelvic nerve plexus Parts of vagus nerve supply stomach and other viscera 4 Pelvic splanchnic nerves and nerve plexus Colon urinary bladder reproductive organs Interactions of the Autonomic Divisions Most visceral organs are innervated by both sympathetic and parasympathetic fibers This results in dynamic antagonisms that precisely control visceral activity Sympathetic fibers increase heart and respiratory rates and inhibit digestion and elimination Parasympathetic fibers decreases heart and respiratory rates and allow for digestion and the discarding of wastes Enteric Nervous System Consists of nerve plexuses within the wall of the digestive tract Plexus have contributions from Sensory neurons that connect the digestive tract to the CNS ANS motor neurons that connect the CNS to the digestive tract Enteric neurons which are confined to the enteric plexuses Functions Stimulate inhibit smooth muscle contraction Stimulate inhibit gland secretions Detect changes in content of lumen Interneurons connect sensory and motor aspects of enteric Sympathetic Versus Parasympathetic Activity Sympathetic Has major influence under conditions of physical activity stress or flight or fight response Also plays a major role during rest blood vessel walls receive only sympathetic stimulation so at rest sympathetic is responsible for maintenance of blood pressure Involves E activities exercise excitement emergency and embarrassment Roles Regulates many functions not subject to parasympathetic influence o These include the activity of the adrenal medulla sweat glands kidneys and most blood vessels The sympathetic division promotes metabolic effects that are not reversed by the parasympathetic division Increases the metabolic rate of body cells Raises blood glucose levels Mobilizes fat as an energy source Parasympathetic Has a great influence during resting conditions Involves the D activities digestion defecation and diuresis It s activity is illustrated in a person who relaxes after a meal Blood pressure heart rate respiratory rates are low Gastrointestinal tract activity is high The skin is warm and the pupils are constricted Neurotransmitters All preganglionic neurons of the sympathetic and parasympathetic If a neuron secretes acetylcholine it s a cholinergic neuron If a neuron secretes norepinephrine it s an adrenergic neuron divisions and all postganglionic neurons of the parasympathetic divisions are cholinergic Almost all postganglionic neurons of the sympathetic division are adrenergic Receptors The
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