THE AUTONOMIC NERVOUS SYSTEM

INTRODUCTION

ANS in the Structural Organization of the Nervous System

COMPARISON OF THE SOMATIC AND AUTONOMIC NERVOUS SYSTEM The somatic nervous system stimulates skeletal muscles, while the ANS innervates cardiac and smooth muscle and glands In the somatic nervous system, the cell bodies of the neurons are in the spinal cord and their axons extend to the skeletal muscles they innervate The ANS consists of a two-neuron chain The neurotransmitter released by the somatic motor neurons is acetylcholine, which always has an excitatory effect; the neurotransmitters released by the ANS are epinephrine and acetylcholine, and both may have either an excitatory or an inhibitory effect There is overlap between the somatic and autonomic nervous systems, and most body responses to changing internal and external stimuli involve both skeletal muscle activity and visceral organ responses

SOMATIC VS AUTONOMIC

DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM The parasympathetic division keeps body energy use as low as possible while directing digestion and elimination activities The sympathetic division prepares the body to respond to an emergency or threatening situation (or vigorous exercise)

ANS ANATOMY

ANS SUBDIVISIONS

PARASYMPATHETIC (CRANIOSACRAL) DIVISION The preganglionic axons extend from the CNS nearly all the way to the structures to be innervated where they synapse with ganglionic neurons in the terminal ganglia The cranial outflow consists of preganglionic fibers that run in the oculomotor, facial, glossopharyngeal, and vagus cranial nerves The rest of the large intestine and the pelvic organs are served by the sacral outflow, which arises from neurons located in the lateral gray matter of spinal cord segments

PARASYMPATHETIC NERVOUS SYSTEM

SYMPATHETIC (THORACOLUMBAR) DIVISION The sympathetic division supplies the visceral organs in the internal body cavities but also all visceral structures in the somatic part of the body When synapses are made in chain ganglia, the postganglionic axons enter the ventral (or dorsal) ramus of the adjoining spinal nerves by way of communicating branches called gray rami communicates The preganglionic fibers from T5 down synapse in collateral ganglia; thus these fibers enter and leave the sympathetic chains without synapsing Some fibers of the thoracic splanchnic nerves terminate by synapsing with the hormone producing medullary cells of the adrenal cortex

SYMPATHETIC NERVOUS SYSTEM

SYMPATHETIC PATHWAYS

The visceral sensory neurons are the first link in autonomic reflexes by sending information concerning chemical changes, stretch, and irritation of the viscera

VISCERAL REFLEXES

REFERRED PAIN

ANS PHYSIOLOGY Neurotransmitters and Receptors Cholinergic receptors, such as nicotinic and muscarinic receptors, bind acetylcholine Adrenergic receptors alpha and beta bind to epinephrine Knowing the locations of the cholinergic and adrenergic receptor subtypes allows specific drugs to be prescribed to obtain desired inhibitory or stimulatory effects on target organs

ANS PHYSIOLOGY Interactions of the autonomic divisions Most visceral organs receive dual innervations by both ANS divisions, allowing for a dynamic antagonism to exist between the divisions and precise control of visceral activity The sympathetic division will increase heart and respiratory rates during a fight-or-flight situation and decrease activity of digestive and elimination organs Sympathetic tone occurs in the vascular system, and parasympathetic tone occurs in the digestive and urinary tracts The parasympathetic and sympathetic divisions may work together to achieve a common purpose For example, the parasympathetic division controls erection while the sympathetic division controls ejaculation The sympathetic division mediates reflexes that regulate body temperature, release renin from the kidneys, and promote metabolic effects The parasympathetic division exerts short-live, localized control its effectors, while the sympathetic division responds in a diffuse and interconnected way to cause a body-wide mobilization

ANS PHYSIOLOGY Control of Autonomic Functioning The brain stem appears to exert the most direct influence over autonomic functions The hypothalamus is the main integration center for the autonomic nervous system Cortical or voluntary control of the autonomic nervous system does appear to be possible

LEVELS OF ANS CONTROL

HOMEOSTATIC IMBALANCES OF THE ANS Hypertension, or high blood pressure, may result from an overactive sympathetic vasoconstrictor response due to continuous high levels of stress Raynaud’s disease is characterized by intermittent attacks causing the skin of the fingers and the toes to become pale, then cyanotic and painful Mass reflex reaction is a life-threatening condition involving uncontrolled activation of both somatic and autonomic motor neurons

DEVELOPMENTAL ASPECTS OF THE ANS Embryonic and fetal development of the autonomic nervous system ANS preganglionic neurons and somatic motor neurons derive from the embryonic neural tube ANS structures found in the PNS (ganglionic neurons, adrenal medulla, and all autonomic ganglia) derive from the neural crest Nerve growth factor is a protein secreted by target cells of the postganglionic axons On old age the efficiency of the ANS begins to decline, partly due to structural changes of some preganglionic axonal terminals