The Autonomic Nervous System Chapter 9 The Autonomic Nervous System

Neural Control of Involuntary Effectors ANS: Innervate organs not usually under voluntary control. Effectors include cardiac and smooth muscles and glands. Effectors are part of visceral organs and blood vessels.

Somatic Motor Neurons Cell bodies in CNS. Conduct APs along single axon from spinal cord to neuromuscular junction. Usually under voluntary control.

Autonomic Neurons 2 neurons in the effector pathway. 1st neuron has its cell body in gray matter of brain or spinal cord. Preganglionic neuron. Synapses with 2nd neuron within an autonomic ganglion which extends to synapse with effector organ. Postganglionic neuron.

Autonomic Neurons Preganglionic autonomic fibers originate in midbrain, hindbrain, and upper thoracic to 4th sacral levels of the spinal cord. Autonomic ganglia are located in the head, neck, and abdomen. Presynaptic neuron myelinated and postsynaptic neuron unmyelinated.

Visceral Effector Organs Involuntary effectors are somewhat independent of their innervation. Denervation hypersensitivity: Damage to autonomic nerve makes its target tissue more sensitive than normal to stimulating agents. Cardiac and many smooth muscles can contract rhythmically in absence of nerve stimulation. Maintain resting tone.

Divisions of the ANS Sympathetic Nervous System Parasympathetic Nervous System Both have preganglionic neurons that originate in CNS. Both have postganglionic neurons that originate outside of the CNS in ganglia.

Sympathetic Division Myelinated preganglionic exit spinal cord in ventral roots at T1 to L2 levels. Travel to ganglia at different levels to synapse with postganglionic neurons. Divergence: Preganglionic fibers branch to synapse with numerous postganglionic neurons. Convergence: Postganglionic neuron receives synaptic input from large # of preganglionic fibers.

Sympathetic Division Mass activation: Divergence and convergence cause the SNS to be activated as a unit. Axons of postganglionic neurons are unmyelinated to the effector organ. Preganglionic neuron is short. Post-ganglionic neuron is long.

Adrenal Glands Adrenal medulla secretes epinephrine and norepinephrine when stimulated by the SNS. Innervated by preganglionic sympathetic fibers. Stimulated by mass activation.

Parasympathetic Division Preganglionic fibers originate in midbrain, medulla, and pons; and in the 2-4 sacral levels of the spinal cord. Preganglionic fibers synapse in ganglia located next to or within organs innervated. Do not travel within spinal nerves. Do not innervate blood vessels, sweat glands,and arrector pili muscles.

Parasympathetic Division 4 of 12 pairs of cranial nerves contain preganglionic parasympathetic fibers. Preganglionic fibers are long, postganglionic fibers are short. Vagus: Innervate heart, lungs esophagus, stomach, pancreas, liver, small intestine and upper half of the large intestine.

Parasympathetic Division Preganglionic fibers from the sacral level innervate the lower half of large intestine, the rectum, urinary and reproductive systems.

Sympathetic Effects Fight or flight response. Release of norepinephrine from postganglionic fibers and epinephrine from adrenal medulla. Mass activation prepares for intense activity. Heart rate increases. Bronchioles dilate. [glucose] increases.

Parasympathetic Effects Stimulation of separate parasympathetic nerves. Release ACh. Relaxing effects: Decrease heart rate (HR). Dilate blood vessels. Increase GI activity.

Adrenergic and Cholinergic Synaptic Transmission ACh is NT for all preganglionic fibers of both sympathetic and parasympathetic nervous systems. ACh is NT released by most postganglionic parasympathetic fibers. Transmission at these synapses is termed cholinergic.

Adrenergic and Cholinergic Synaptic Transmission NT released by most postganglionic sympathetic nerve fibers is norepinephrine. Transmission at these synapses is called adrenergic. Epinephrine released by the adrenal medulla is synthesized from the same precursor as norepinephrine. Collectively called catecholamines.

Responses to Adrenergic Stimulation Has both excitatory and inhibitory effects. Responses due to different membrane receptor proteins. a1 : constricts vascular smooth muscles a2 : contraction of smooth muscle b1 : increases HR and force of contraction b2 : relaxes bronchial smooth muscles

Responses to Adrenergic Stimulation Beta receptors: Produce their effects by production of cAMP. NE binds to receptor. Alpha G complex activates adenylate cyclase, producing cAMP. cAMP activates protein kinase, opening ion channels.

Responses to Adrenergic Stimulation Alpha1 receptors: Produce their effects by the production of Ca++. Epinephrine binds to receptor. Ca++ binds to calmodulin. Calmodulin activates protein kinase, modifying enzyme action.

Responses to Cholinergic Stimulation Muscarinic receptors: Ach binds to receptor. Requires the mediation of G-proteins. Beta-gamma complex binds to chemical K+ channel, opening the channel. Functions to: Decrease HR. Decrease force of contraction of the heart. Produce bronchiole constriction. Increase GI secretions.

Responses to Cholinergic Stimulation Nicotinic receptors: ACh binds to 2 nicotinic receptor binding sites. Opens a Na+/ K+ channel.

Organs with Dual Innervation Most visceral organs receive dual innervation (innervated by both sympathetic and parasympathetic fibers). Antagonistic effects: Actions counteract each other. Heart rate. Complementary: Produce similar effects. Salivary gland secretion. Cooperative: Cooperate to produce a desired effect. Micturition.

Control by Higher Brain Centers Sensory input transmitted to brain centers that integrate information. Can modify activity of preganglionic autonomic neurons. Medulla: Most directly controls activity of autonomic system. Hypothalamus: Regulates medulla. Cerebral cortex and limbic system: Responsible for responses to emotion.