THE AUTONOMIC NERVOUS SYSTEM PART I

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