Peripheral Nervous System

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Peripheral Nervous System
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Autonomic Nervous System

• Two divisions
• Sympathetic
• Fight or flight
• Parasympathetic
• Rest and digest
• Antagonistic control
• Dual innervation
• Exception Sweat glands, vascular smooth muscle
• Multiple receptor types
• Both tonically active

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Central Regulation

• Control centers
• Hypothalamus
• Pons
• Medulla
• Control
• Heart rate, blood pressure
• Temperature
• Water balance
• Spinal reflexes
• Urination, defecation

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Efferent Neurons

• Two neurons in series
• Preganglionic neuron from CNS
• Autonomic ganglion outside CNS
• Postganglionic neuron to target tissue
• Intrinsic neurons
• Divergence

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Anatomy of the ANS

• Sympathetic pathways
• Originate in thoracic and lumbar regions of SC
• Sympathetic ganglia run in chain alongside SC
• Short preganglionic and long postganglionic
  neurons
• Parasympathetic pathways
• Originate in brainstem or sacral region of SC
• Parasympathetic ganglia located on or near target
  organ
• Long preganglionic and short postganglionic
  neurons

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Special Parasympathetic Nerves

• Leave brainstem via cranial nerves
• Major parasympathetic tract
• Vagus nerve
• Cranial nerve X
• Carries
• Afferent sensory information
• Efferent parasympathetic to most internal organs

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Target Tissues of Autonomic Nerves

• Smooth muscle
• Cardiac muscle
• Many exocrine glands
• Adipose tissue
• Brown fat

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Autonomic Synapse

• Axon terminals have varicosities
• Swollen ends containing many vesicles
• Branched axon terminals lie across target tissue
• Target cell membrane does NOT have clusters of
  receptors
• Neurotransmitter released into interstitial fluid
• Single postganglionic neuron can affect large area

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Adrenal Medulla

• Neuroendocrine gland
• Secretes norepinephrine and epinephrine
• Modified sympathetic ganglion
• Receives preganglionic input from SC
• Postganglionic neurons lack axonal projections
• Release neurohormones into blood

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Neurotransmitters

• Acetylcholine
• Norepinephrine
• Release
• Action potential ? Open Ca2 channels ? Ca2
  enters ? Exocytosis of neurotransmitter vesicles
• More neurotransmitter released, the stronger the
  response

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Neurotransmitters

• Breakdown/Removal
• Catecholamines
• Uptake into presynaptic terminal
• Repackaged or degraded by monoamine oxidase
• Mitochondria
• Acetylcholine
• Degraded by acetylcholinesterase in synaptic
  cleft
• Choline transported back into neuron

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Location of Neurotransmitters

• All autonomic preganglionic neurons
• Acetylcholine, nicotinic cholinergic receptors
• Postganglionic sympathetic neurons
• Norepinephrine, adrenergic receptors
• Postganglionic parasympathetic neurons
• Acetylcholine, muscarinic cholinergic receptors
• Non adrenergic, non cholinergic neurons
• Substance P, Somatostatin, Vasoactive intestinal
  peptide (VIP)

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Adrenergic Receptors

• Alpha
• Binds to and responds more to norepinephrine
• Alpha 1
• Second messenger Phospholipase C and IP3
• Increase intracellular Ca2 ? muscle contraction
  or exocytosis
• Alpha-2
• Second messenger Inhibits cAMP
• Smooth muscle relaxation (GI tract)

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Adrenergic Receptors

• Beta
• Beta1
• Heart muscle and kidney cells
• Enhances contraction
• Innervated by sympathetic neurons
• Respond to both Nepi and Epi
• Beta-2
• Smooth muscle in airways, blood vessels
• Relaxation
• NOT innervated by sympathetic neurons
• More sensitive to epinephrine
• Both
• Increase cAMP

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Cholinergic Receptors

• Nicotinic
• Receptor linked to ion channel
• Muscarinic
• Coupled to G proteins
• Second messengers
• Potassium channels
• Tissue response varies with receptor subtype
• Five subtypes

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Autonomic Agonists and Antagonists

• Cholinergic agonists
• Acetylcholine, nicotine, muscarine
• Cholinergic antagonists
• Atropine, scopolamine, curare
• Adrenergic agonists
• Norepinephrine, epinephrine
• Phenylephrine, isoproterenol
• Adrenergic antagonists
• Propanolol

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Somatic Motor Division

• ONE efferent neuron
• Cell bodies in gray matter of brain or SC
• Long single axon
• Branching axons lie close to target
• Axon branches divide into boutons
• Acetylcholine only neurotransmitter
• Only ONE target tissue
• Skeletal muscle
• Excitatory effects only
• No ganglia

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Neuromuscular Junction

• Presynaptic axon terminal
• Efferent motor neuron
• Synaptic cleft
• Fibrous matrix
• Contains acetylcholinesterase
• Postsynaptic membrane
• Motor end plate
• Folds with cluster of Ach receptors
• Nicotinic receptors
• Chemically gated ion channels
• Nonspecific ion channels
• Always excitatory

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