17.1 Comparison of the Somatic and Autonomic Nervous System

• • Peripheral nervous system has two subdivisions: the somatic nervous system and autonomic nervous system. Somatic nervous system associated with voluntary movements of skeletal muscle and involves afferent sensory and efferent motor nerves. Autonomic nervous system associated with involuntary activities of smooth muscles, glands, visceral structures and involves afferent sensory and efferent motor nerves.
• Autonomic nervous system innervates visceral effectors with somatic nervous system, whose lower motor neurons innervate skeletal muscles 
• Like somatic nervous system, autonomic nervous system has afferent and efferent neurons
• Like somatic nervous system, afferent sensory info of autonomic nervous system processed in central nervous system, efferent impulses sent to effector organs
• However, in autonomic nervous system, afferent pathways originate in visceral receptors, efferent pathways connect to visceral effector organs (smooth muscle or glands)
• In autonomic nervous system axon of visceral motor neuron within central nervous system innervates second neuron located in peripheral ganglion
• Second neuron innervates peripheral effector
• Visceral motor neurons in CNS send short, myelinated axons called preganglionic fibers, to synapse on group of neurons located within ganglion located outside CNS
• Axons leaving ganglia are relatively long and unmyelinated, called postganglionic fibers 
• Postganglionic fibers...

  • Carry impulses away from ganglion 
  • Innervate peripheral tissues and organs, cardiac/smooth muscle, adipose tissue, glands
• Sympathetic and Parasympathetic Subdivisions of the Autonomic Nervous System. Two divisions of autonomic nervous system have opposing effects
• Sympathetic and Parasympathetic Subdivisions of the Autonomic Nervous System. Sympathetic division causes excitation, parasympathetic division caused inhibition
• Sympathetic and Parasympathetic Subdivisions of the Autonomic Nervous System. Sympathetic and parasympathetic not always opposite because… 

  • (1) Divisions work independently with structures innervated by only one division 
  • (2) Divisions work together, each controlling one stage of process
• Sympathetic and Parasympathetic Subdivisions of the Autonomic Nervous System. Parasympathetic division most active during excretion, stress, emergency
• Sympathetic and Parasympathetic Subdivisions of the Autonomic Nervous System. Autonomic nervous system also includes third division, enteric nervous system (ENS). Extensive network of neurons within walls of digestive tract 
• Describe preganglionic fibers and neurons and postganglionic fibers and neurons.

  Preganglionic fibers are short with myelinated axons; preganglionic neurons have their cell bodies in the central nervous system, and their axons project to ganglia in the peripheral nervous system. Postganglionic fibers are long and unmyelinated; postganglionic neurons have their cell bodies in ganglia in the peripheral nervous system, and their axons innervate effector cells.
• Name the neurotransmitter released by most postganglionic sympathetic fibers.

  Norepinephrine is the neurotransmitter released by most postganglionic sympathetic fibers.
• Where do the preganglionic fibers of the parasympathetic division of the autonomic nervous system originate?

  Preganglionic fibers of parasympathetic division of autonomic nervous system originate in the brainstem or the sacral spinal cord.