The Autonomic Nervous System

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• The Autonomic Nervous System

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Autonomic Nervous System (ANS)

• Motor neurons that
• Innervate smooth cardiac muscle glands
• Subconscious control

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ANS differs from the SNS

• Effectors
• SNS skeletal muscle
• ANS non-skeletal muscle gland cells
• Efferent pathways
• SNS single PNS neuron
• ANS 2 PNS neurons
• Target organ responses
• SNS contraction of muscle
• ANS contraction or relaxation, excretion
• Neurotransmitters used
• SNS acetylcholine
• ANS acetylcholine, norepinephrine epinephrine

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Distinctions of Efferent Pathways

• SNS motor neurons
• Single neuron extends from CNS to effector
• Heavily myelinated axons
• ANS motor neurons
• Two-neuron PNS chain
• Preganglionic neuron postganglionic neuron
• Lightly myelinated preganglionic axon from CNS to
  ganglion
• Unmyelinated postganglionic axon extends to
  effector

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Neurotransmitter Differences

• SNS neurons release acetylcholine (ACh), which
  has an excitatory effect
• In the ANS
• Preganglionic fibers release ACh
• Postganglionic fibers
• release norepinephrine or ACh
• effect is stimulatory or inhibitory
• effect depends on neurotransmitter receptor in
  cells of effector tissue

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Comparison of Somatic Autonomic Systems
Figure 14.2
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Anatomy of ANS

• Short preganglionic
• Long postganglionic
• Ganglia close to spinal cord

• Long preganglionic
• Short postganglionic
• Ganglia on/in target organ

Figure 14.3
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Divisions of the ANS

• Sympathetic (SANS)
• mobilizes the body during stressfull situations
• Parasympathetic (PANS)
• stimulates maintenance activities conserves
  body energy
• The two divisions counterbalance each others
  activity
• SANS signals usually override PANS

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Examples of ANS Effects

• PANS
• Lowers BP, heart respiratory rates
• Increases gastrointestinal tract activity
• Superficial arterioles open (smooth muscle
  relaxed)
• Pupils are constricted
• SANS
• Blood flow to organs/skin reduced, flow to
  muscles increased
• Heart respiratory rates increased
• Iris contracts - Pupils dilate

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Parasympathetic Division Outflow
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Parasympathetic Division Outflow

• Longer preganglionic axons
• Ganglion near/on target organ
• Short postganglionic axons
• Vagus nerve (CN X) innervates all visceral organs

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Sympathetic Outflow

• Sympathetic neurons in lateral horns of spinal
  cord segments T1 through L2
• T1-T4 preganglionic fibers pass through the white
  rami communicantes synapse in sympathetic chain
  ganglia
• T5-L2 preganglionic fibers pass through the gray
  rami communicantes chain ganglia to form
  splanchnic nerves synapse in collateral ganglia
  around abdominal aorta
• Postganglionic fibers innervate the numerous
  organs of the body

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Sympathetic Outflow

• Sympathetic neurons in lateral horns of spinal
  cord segments T1- L2
• T1-T4 preganglionic fibers synapse in sympathetic
  chain ganglia
• T5-L2 preganglionic fibers form splanchnic nerves
  synapse in collateral ganglia on abdominal
  aorta

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Sympathetic Trunks Pathways
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Pathways to the Head

• T1-T4 preganglionic axons synapse in the superior
  cervical ganglion
• Serve skin blood vessels of the head
• Stimulate dilator muscles of the iris
• Inhibit nasal salivary gland secretions

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Pathways to the Thorax

• T1-T6 preganglionic axons synapse in cervical
  chain ganglia
• Postganglionic axons from middle inferior
  cervical ganglia enter spinal nerves C4-C8 to
  innervate the heart, thyroid skin of neck
• Other T1-T6 preganglionic axons synapse in
  nearest chain ganglia to directly serve the
  heart, aorta, lungs, esophagus

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Pathways with Synapses in Collateral Ganglia

• T5-L2 preganglionic axons exit sympathetic chain
  ganglia form splanchnic nerves
• Splanchnic nerves form aortic plexus numerous
  ganglia
• Postganglionic axons from abdominal ganglia
  innervate viscera

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Pathways with Synapses in the Adrenal Medulla

• Axons of the thoracic splanchnic nerve go
  directly to the adrenal medulla
• Upon stimulation, medullary cells secrete
  norepinephrine epinephrine into the blood

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Visceral Reflexes

• Visceral reflexes have the same elements as
  somatic reflexes
• Afferent fibers are found in spinal autonomic
  nerves

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ANS Neurotransmitters

• PANS
• Acetylcholine (ACh) released by pre-
  postganglionic axons
• SANS
• ACh released by preganglionic axons
• ACh or norepinephrine (NE) released by
  postganglionic axons
• Cholinergic fibers ACh-releasing axons
• Adrenergic fibers NE-releasing postganglionic
  SANS axons
• Excitatory or inhibitory effects depend upon the
  receptor type

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Cholinergic Receptors

• Bind ACh
• Nicotinic receptors
• Muscarinic receptors
• Named distinguished by interaction w/ agonists
• Nicotine
• Muscarine
• Agonist stimulates effect
• Antagonist blocks effect

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Cholinergic Receptors

• Nicotinic Receptors
• Locations
• Skeletal muscle motor end plates, CNS neurons
• SANS PANS ganglionic neurons
• Adrenal medulla cells
• Ion channels
• ACh always stimulatory
• Muscarinic Receptors
• Locations
• Cells stimulated by postganglionic PANS fibers,
  CNS
• ACh inhibition or excitation depends on receptor
  subtype
• subtypes M1, M2, M3

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Adrenergic Receptors

• Receptors that bind to norepinephrin
  epinephrine
• In cells innervated by SANS postganglionic axons
• Alpha
• subclasses - ?1, ?2,
• NE is stimulatory
• Beta
• Subclasses - ?1, ?2 , ?3
• NE is generally inhibitory
• Exception NE binding to ? receptors of the
  heart is stimulatory

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Drugs that Influence the ANS
Table 14.4.1
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Levels of ANS Control
Figure 14.9
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Interactions of the Autonomic Divisions

• Most visceral organs innervated by both
  sympathetic parasympathetic fibers
• results in dynamic antagonisms that precisely
  control visceral activity
• Sympathetic fibers increase heart respiratory
  rates, inhibit digestion elimination
• Parasympathetic fibers decrease heart
  respiratory rates, allow for digestion the
  discarding of wastes