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Peripheral Nervous System
Dr. Jena Hamra
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
Central Regulation

Control centers
 Hypothalamus
 Pons
 Medulla

Control:
 Heart rate, blood pressure
 Temperature
 Water balance

Spinal reflexes
 Urination, defecation
Efferent Neurons

Two neurons in series
 Preganglionic neuron from CNS
 Autonomic ganglion outside CNS
 Postganglionic neuron to target tissue

Intrinsic neurons
 Divergence
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
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
Target Tissues of Autonomic
Nerves

Smooth muscle
 Cardiac muscle
 Many exocrine glands
 Adipose tissue
 Brown fat
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
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
Neurotransmitters

Acetylcholine
 Norepinephrine
 Release
 Action potential  Open Ca2+ channels  Ca2+
enters  Exocytosis of neurotransmitter
vesicles

More neurotransmitter released, the stronger
the response
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
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)
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)
Adrenergic Receptors

Beta
 Beta–1
 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
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
Autonomic Agonists and Antagonists

Cholinergic agonists
 Acetylcholine, nicotine, muscarine

Cholinergic antagonists
 Atropine, scopolamine, curare

Adrenergic agonists
 Norepinephrine, epinephrine
 Phenylephrine, Isoproterenol

Adrenergic antagonists
 Propanolol
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
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