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
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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