Anatomy and Physiology I Chapter 15 The Autonomic Nervous System and Visceral Reflexes

Autonomic Nervous System (ANS) Motor nervous system that controls glands, cardiac muscle, and smooth muscle (visceral motor) – Viscera of thoracic and abdominopelvic cavities – Blood vessels – Sweat glands – Piloerector muscles Involuntary Homeostasis

Visceral Reflexes Unconscious, automatic, stereotyped responses to stimuli Slower responses- visceral receptors and effectors Visceral reflex arc – Receptors – Afferent neurons- to CNS – Interneurons – Efferent neurons- away from CNS – Effectors

ANS divisions 2 subdivisions – Sympathetic – Parasympathetic Often innervate same organ Cooperative or contrasting effects Autonomic tone- balance b/t divisions – Both systems are active simultaneously – Shifts in accordance to body’s changing needs

Sympathetic Adapts body for physical activity Fight or flight Subtle effects

Parasympathetic Calming effect on functions Reduced energy expenditure and normal bodily maintenance

Neural Pathways Signal passes across 2 neurons to get to target organ Cross synapse – Autonomic ganglion Preganglionic neuron – Soma in CNS – Axon terminates in ganglion Postganglionic neuron – Axon extends to target cells

The Sympathetic Division Thoracic and lumbar regions Short preganglionic fibers – Myelinated – Spinal nerve to ganglion Long postganglionic fibers – Unmyelinated – Ganglion to target organ

The Parasympathetic Division Brain and sacral regions Long preganglionic fibers – Brain to ganglion – Almost to target cells Short postganglionic fibers – Ganglion to target cell

Neurotransmitters and Receptors ANS division has contrasting effects Sympathetic and Parasympathetic fibers secrete different neurotransmitters Target cells respond in different ways – Depends of type of receptor

Neurotransmitters All autonomic nerve fibers either secrete ACh or norepinepherine – ACh- secreted by preganglionic neurons of both divisions and postganglionic neurons of the parasympathetic division – NE- secreted by all sympathetic postganglionic neurons Sympathetic effects lasts longer – Ach secreted by parasympathic- quickly broke down by AChE

Dual Innervation Viscera receive nerve fibers from both sympathetic and parasympathetic divisions – Antagonist or cooperative effects Antagonist effects- oppose each other Cooperative effects- 2 divisions act on different effectors to produce unified overall effect

Control w/out Dual Innervation Dual innervation is not always necessary to produce opposite effects on an organ – Regulation of BP and blood flow – Sympathetic fibers keep vessels in partial constriction- vasomotor tone Increase firing rate- constricts vessel – Smooth muscle contracts Decrease firing rate- dilates vessel – Smooth muscle relaxes Sympathetic division can shift blood flow from organ to organ according to body’s needs