THE AUTONOMIC NERVOUS SYSTEM LECTURE 12 CH 9

Neural Control of Involuntary Effectors The autonomic nervous system helps regulate cardiac, smooth muscle and glands Impulses are conducted from the CNS to a second autonomic neuron. And then the second neuron innervates the involuntary effector.

Visceral Effector Organs The autonomic nervous system regulates all the organs. Common features of organs regulated autonomically: 1) a built-in muscle tone – they maintain a resting ‘tone’ 2) denervation hypersensitivity – they may become more sensitive to regulation when nerves are damaged 3) They may contract without autonomic input 4) The autonomic system is like an accelerator or brake

Divisions of the Autonomic Nervous System Sympathetic - thoracolumbar division – preganglionic fibers exit spinal cord at spinal nerves T1-L2 - synapse with paravertebral ganglia (sympathetic chain)

Fig. 9.2 The Sympathetic Chain of Paravertebral Ganglia

Sympathetic Neuron Pathways

Sympathetic Division Divergence – preganglionic fibers branch out to postganglionic fibers at different levels of the chain Convergence – a postganglionic neuron can receive info from many pre- ganglionic nerves. Mass activation – all postganglionic sympathetic neurons can be activated simultaneously for fight or flight

Sympathetic Collateral (prevertebral) Ganglia These are places where pre- ganglionic neurons synapse if they do not synapse in the paravertebral chain; they form splanchnic nerves E.G. celiac, superior, mesenteric, inferior mesenteric ganglia which innervate digestive, urinary, reproductive

Note: dual innervation of organs by both systems

Adrenal Glands Two functionally different glands: medulla and cortex Medulla – originates from neural crest; innervated by preganglionic sympathetic fibers which trigger the secretion of epinephrine into the blood Cortex – secretes steroid hormones Sympathoadrenal system – stimulation of adrenal medulla by mass activation of sympathetic nervous system

Question What happens to blood vessels of the digestive tract during sympathetic activation? What happens to skeletal muscles? Lungs?

Parasympathetic Division Craniosacral Division Preganglionic fibers originate in the brain (midbrain, pons, medulla) and in sacrum; they extend to terminal ganglia, which are inside the organs they stimulate. Terminal ganglia supply postganglionic fibers to synapse with effector cells.

Functions of the Autonomic Nervous System Sympathetic: Fight or flight Epinephrine/norepi released from postganglionic neurons Can be regulated as a “mass system” or finely tuned Purpose: increase ATP Parasympathetic Rest and digest Acetylcholine released from postganglionic neurons Not activated as a whole Opposite of sympathetic

Adrenergic and Cholinergic Synaptic Transmission Sympathetic and parasympathetic – preganglionic neurons release Acetylcholine Sympathetic postganglionic neurons release epi [adrenergic] Parasympathetic postganglionic neurons release Ach [cholinergic] Exception: some sympathetic fibers to skeletal muscle and sweat glands release ACh

Response to Adrenergic Stimulation Both excitatory and inhibitory effects can be produced in different tissues by the same neurotransmitter because different tissues have different receptors. α(alpha) -adrenergic receptors stimulate a rise in cytoplasmic Calcium α 1 – causes vasoconstriction by increasing Ca2+ α 2 – they are activated by norepi, but then cause a negative feedback reduction in epi levels (p. 254) clonidine (drug) - α 2 receptors in the brain  lowers sympathoadrenal system β(beta) -adrenergic receptors stimulate the production of cAMP in the target cell. β 1 – increase heart rate; β 2 – vasodilation; bronchodilation

Response to Cholinergic Stimulation The effects of ACh depend on the nature of the ACh receptor: Nicotinic – located in CNS and neuromuscular junction and in autonomic ganglia; always excitatory; always cause an inflow of Na+ Muscarinic – located in visceral organs; excitatory or inhibitory; G-protein coupled and can activate different membrane enzymes. Subtypes of muscarinic receptors exist

The effects of ACh depend on nature of the Ach receptor: Nicotinic –always excitatory; always cause an inflow of Na+ Muscarinic –excitatory or inhibitory; Subtypes exist

Other Autonomic Neurotransmitters Non-adrenergic, non-cholinergic Nitric oxide – causes vasodilation of penis, causes vasodilation of cerebral arteries (parasympathetic); sometimes called a paracrine regulator VIAGRA blocks the breakdown of cyclic GMP

Antagonistic Effects “Antagonistic” means the two systems produce opposite effects. Example: the heart – epi speeds it up/ACh slows it

Complementary and Cooperative Effects Complementary – both systems produce similar effects e.g. salivary glands – Cooperative – systems produce different effects that work together to promote a single action. e.g. erection of the penis/clitoris – arousal is parasympathetic/orgasm is sympathetic e.g. urinary bladder -

Organs Without Dual Innervation Some organs receive only sympathetic innervation: 1)Adrenal medulla 2)Arrector pili 3)Sweat glands 4)Blood vessels of skin Regulation is achieved by increases or decreases in firing rate of sympathetic fibers. E.g. During exercise, increased sympathetic activity dilates blood vessels in the muscles and stimulates sweat glands (which secrete bradykinin to stimulate dilation of surface blood vessels)

Control of the Autonomic Nervous System by Higher Brain Centers Medulla oblongata Hypothalamus Limbic system (contains the cingulate gyrus, hypothalamus, fornix, hippocampus, amygdala) – emotional control of autonomic activation (fainting, blushing, racing heartbeat) Cerebellum – control of motion sickness, nausea