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• Peripheral Nervous System


  Consists of the Autonomic Nervous System and the Somatic Nervous System

  
• Autonomic Nervous System


  • Operates without conscious instruction
  • Coordinates functions of visceral organs (cardiovascular, urinary, respiratory, digestive, reproductive, cardiac & smooth muscle & glands)
  • Signal can be excitatory or inhibitory
  • Has 2 axons to effector
  • Both are efferent motor

  
• Somatic Nervous System


  • Operates under conscious control
  • Seldom affects long-term survival
  • Excitatory with skeletal muscle
  • Cell bodies in CNS
  • Single axon to effector

  
• Visceral Motor Neurons of Autonomic Nervous System


  1. Preganglionic neurons - cell bodies in brain stem and spinal cord, axons leave CNS and synapse on ganglionic neurons

  2. Autonomic Ganglia - contains cell bodies of 2nd neuron (postganglionic neurons), postganglionic fibers extend to peripheral

  
• Sympathetic originates in lateral horn (visceral motor), out ventral root along preganglionic fibers that are myelinated - called White Ramus, go to sympathetic ganglion
• Post-ganglion fibers - if directly to muscles or glands then become sympathetic nerves, if innervating body wall or limbs, the fibers go back and join spinal nerve, unmyelinated so called the Gray Ramus
• Rami Communicantes


  Name for both white and gray rami
• Sympathetic Division


  • Kicks in only during exertion, stress, or emergency
  • Readies body for crisis - Fight or Flight
• Parasympathetic Division


  • Controls during resting conditions
  • Stimulates visceral activity
  • Conserves energy and promotes sedentary activities
• Sympathetic Responses


  • Heightened mental alertness
  • Increased metabolic rate
  • Reduced digestive and urinary functions
  • Energy reserves activated
  • Increased respiratory rate and respiratory passageways dilate
  • Increased heart rate and blood pressure
  • Sweat glands activated
• Preganglionic neurons of Sympathetic Division located between segments T1 and L2 of spinal cord, ganglionic neurons in ganglia near vertebral column, cell bodies of preganglionic neurons in lateral gray horns, axons enter ventral roots of segments

  
• Sympathetic Chain Ganglia are on both sides of vertebral column, innervate via postganglionic fibers, control visceral effectors in body wall, thoracic cavity, head, and limbs
• Collateral Ganglia - Splanchnic nerves (preganglionic fibers fused together) synapse in collateral ganglia, control visceral effectors in abdominopelvic cavity by reducing blood flow to non-important organs and releasing energy stores
• Adrenal Medullae - at center of each adrenal gland, modified sympathetic ganglion, when stimulated release neurotransmitters epinephrine (80%) and norepinephrine (20%) into bloodstream to act as hormones

  
• Epinephrine


  Also called adrenaline, 75-80% of secretory output by adrenal medulla
• Norepinephrine


  Released at most peripheral sympathetic synapses, 25-30% of medulla secretions, major neurotransmitter
• Acetylcholine


  Released at all preganglionic sympathetic synapses, some postganglionic varicosities, called cholinergic, excitatory
• Alpha Receptors


  • Activates enzymes on inside of cell membrane, NE stimulation has isolated local effect (stronger than beta), E stimulation has general effect
• Beta Receptors


  • NE stimulation has weak effect, E stimulation has general effect, both use G proteins and secondary messengers
• Alpha-1 Receptors


  More common, release intracellular calcium ions, have excitatory effect
• Alpha-2 Receptors


  Lower cAMP levels, have inhibitory effect, help coordinate sympathetic and parasympathetic activities
• Beta-1 Receptors


  • Increase metabolic activity, increase heart rate and contractile force
• Beta-2 Receptors


  • Cause inhibition, trigger relaxation of smooth muscles along respiratory tract
• Beta-3 Receptors


  • In adipose tissue, lead to lipolysis and release of fatty acids
• Acetylcholine is released preganglionic in sympathetic division, and also by parasympathetic division to innervate sweat glands and cause vasodilation, both NE and ACh needed to regulate visceral functions

  
• Nitroxidergic synapses release nitric oxide as neurotransmitter, innervate smooth muscles in walls of blood vessels in skeletal muscles and brain, produce vasodilation and increased blood flow
• Parasympathetic preganglionic fibers originate in brain stem and sacral segments of spinal cord, synapse in ganglia close to or within target organs

  
• Parasympathetic Effects


  • Constriction of pupils
  • Secretion by digestive glands
  • Secretion of hormones affecting cell nutrient absorption
  • Changes in blood flow and glandular activity associated with sexual arousal
  • Increases smooth muscle activity along digestive tract
  • Defecation
  • Contraction of urinary bladder
  • Constriction of respiratory passageways
  • Reduction in heart rate and force of contraction
• Vagus nerve provides 75% of all parasympathetic outflow, innervates structures in neck, thoracic and abdominopelvic cavity, and distal portion of large intestine
• Nicotinic Receptors


  At neuromuscular junctions of somatic nervous system, exposure to ACh causes excitation
• Muscarinic Receptors


  At cholinergic neuromuscular or neuroglandular junctions (parasympathetic) & some sympathetic, G protein coupled, effects are longer lasting, can be excitatory or inhibitory
• Parasympathetic neurons are all cholinergic, ganglionic neurons have nicotinic receptors, muscarinic receptors at neuromuscular or neuroglandular junctions produce either excitation or inhibition

  
• Sympathetic division has widespread impact reaching organs and tissues throughout body, parasympathetic division innervates only specific visceral structures
• Most vital organs receive instructions from both sympathetic and parasympathetic divisions, the two divisions commonly have opposing effects
• Parasympathetic postganglionic fibers accompany cranial nerves to peripheral destinations, sympathetic innervation reaches same structures by traveling directly from superior cervical ganglia of sympathetic chain

  
• Enteric Nervous System


  Third division of Autonomic Nervous System, extensive network in digestive tract walls, complex visceral reflexes coordinated locally, roughly 100 million neurons