1.3

• Nervous system


  Divided into central nervous system and peripheral nervous system

  
• Motor (efferent) section


  Divided into autonomic nervous system (controls involuntary activities) and somatic nervous system (controls conscious activities)

  
• Nervous system


  Sends signals through neurons or ganglion cells

  
• Components of the nervous system

  • Central nervous system
  • Peripheral nervous system

  
• Figure 3.1: Components of the nervous system

  
• Neurons


  Use neurotransmitters to cross synapses
• Somatic nervous system


  Nerve cell body originates in central nervous system

  
• Autonomic nervous system


  Interruption between central nervous system and innervation site

  
• Neurons and ganglia


  • Preganglionic neurons (lead from vertebrae to synapse)
  • Postganglionic neurons (lead from synapse to organ or gland)
  • Multiple synapses occur in ganglia (drugs can have effect here)

  
• Figure 3.2: (A) Nerve of the somatic nervous system, (B) Synapses and ganglia of the autonomic nervous system

  
• Autonomic nervous system


  Controls generally involuntary physiologic functions

  
• Autonomic nervous system


  • Two main divisions: parasympathetic and sympathetic
  • Synapses occur in autonomic ganglia
  • Sympathetic most located near spinal cord, parasympathetic most located near effector site

  
• Figure 3.3: Origin and distribution of the autonomic nervous system

  
• Preganglionic and postganglionic neurons

  • Sympathetic division: preganglionic neurons short, postganglionic neurons longer
  • Parasympathetic division: preganglionic neurons very long, postganglionic neurons short

  
• Parasympathetic and sympathetic branches

  • Parasympathetic branches through cranial and sacral segments
  • Sympathetic branches through thoracic and lumbar segments
  • Multiple synapses occur in ganglia (drugs can have effect here)
• Neurotransmitters


  Stored in nerve terminal, released with nerve impulse, diffuse across cleft, interact with receptors, either lead to production of electrical signals or cause physiologic changes in neuroeffector organ

  
• Main neurotransmitters of the autonomic nervous system

  • Acetylcholine (ACH)
  • Norepinephrine (NE)
• Acetylcholine (ACH)


  Ganglionic synapse and neuroeffector site in parasympathetic system, ganglionic synapse in sympathetic system
• Norepinephrine (NE)


  Neuroeffector site in sympathetic system
• Cholinergic agents


  Affect ganglion, parasympathetic neuroeffector site, or muscular junction by stimulating ACH release
• Anticholinergic agents


  Block ACH receptor or prevent ACH release
• Adrenergic agents


  Stimulate receptor to release NE (adrenaline)
• Antiadrenergic agents


  Block NE receptor from having effect
• Parasympathetic and sympathetic divisions


  Innervate most organs (dual innervation), usually have opposite effects
• Sympathetic nervous system


  Generally dominant system when under stress, fight or flight response, prolonged state can lead to death
• Adrenergic drugs


  Mimic sympathetic action, stimulate NE release
• Sympathetic nerve ending


  • Neurotransmitter is NE, synthesized in nerve ending, packaged in vesicles, released when impulse reaches nerve ending, combines with type of receptor present (α1-, α2-, β1-, or β2-receptors)
• Figure 3.5: Sympathetic nerve ending
• Methods of terminating physiologic effects of NE


  Transport NE back into nerve terminal, repackaged into vesicles, diffuse NE away from site, ultimately metabolized by monoamine oxidase and catechol-O-methyl-transferase
• Sites of action of adrenergic drugs


  • Presynaptic site in nerve ending (create false transmitters, block reuptake of NE, deplete NE from storage vesicles, increase NE release from nerve endings, block NE release from nerve terminals)
  • Postjunctional site where receptors are located (stimulate α-receptors, β-receptors, or both, block α-receptors or β-receptors)
• Mechanisms of adrenergic drugs

  Alpha-adrenergic drugs (stimulate α-receptor sites), beta-adrenergic drugs (stimulate β-receptor sites), neuronal activators (increase NE levels in nerve ending), alpha-blockers (block α-receptors), beta-blockers (block β-receptors), neuronal blockers (inhibit NE levels in nerve ending)
• Alpha-adrenergic drugs


  • Stimulate α-receptor sites, include NE, epinephrine, and phenylephrine, vasoconstriction most dominant clinical effect, often administered intravenously, commonly referred to as vasopressors
• Alpha-blocking drugs


  • Block α-receptor sites, create generalized, nonselective vasodilation, considered second-line antihypertensives, some specifically target bladder, prostate gland
• Beta-adrenergic drugs


  • Mainly stimulate β1- and β2-receptors, used to treat bradycardia by increasing heart rate, heart failure by stimulating cardiac contractions
• Beta-blocking drugs


  • Commonly used in clinical practice, many available, some affect β1- and β2-receptors, some relatively selective for β1-receptors, block effects of NE and epinephrine, decrease heart rate, force of heart contractions
• Adrenergic neuronal activators and blockers

  • Decrease adrenergic nerve function, affect all sites where NE released, tend to influence α- and β-receptors, agents include reserpine, methyldopa, and clonidine
• Methyldopa (Aldomet)


  Causes activation of receptor site by metabolite, stimulates centrally acting α2-receptors
• Clonidine (Catapres)


  Reduces sympathetic tone, acts on cardiovascular center in brain stem
• Parasympathetic division


  Dominant when body at rest, controls and stimulates digestion, waste elimination, genitourinary system, cardiovascular system generally decreased
• Parasympathetic nerve ending


  • ACH synthesized in nerve ending, packaged in vesicles, nerve impulse stimulates combination with receptor, acetylcholinesterase (ACHE) enzyme metabolizes excess ACH