Chapter 14 - The Autonomic Nervous System (ANS)

Two Motor Divisions

• ________________- prepares body for fight or flight situations
  
• _______________ - prepares body for resting and digesting activities

Autonomic Nervous System Description

• The sympathetic mobilizes the body during extreme situations
• The parasympathetic performs maintenance activities and conserves body energy
• The two divisions counterbalance each other's activity
• The ANS consists of motor neurons that:
  • Innervate _________and _________ muscle and __________
  • Make adjustments to ensure optimal support for body activities
  • Operate via ____________ control
  • Have ________ as most of their effectors

Roles of Sympathetic & Parasympathetic Divisions

• Parasympathetic
  • Concerned with keeping body energy use low
  • Involves the D activities - digestion, defecation, and diuresis
  • Its activity is illustrated in a person who relaxes after a meal
    • Blood pressure, heart rate, and respiratory rates are low
    • Gastrointestinal tract activity is high
    • The skin is warm and the pupils are constricted
• Sympathetic
  • The sympathetic division is the "fight-or-flight" system
  • Involves E activities - exercise, excitement, emergency, & embarrassment
  • Promotes adjustments during exercise - blood flow to organs is reduced, flow to muscles is increased
  • Its activity is illustrated by a person who is threatened
    • Heart rate increases, and breathing is rapid and deep
    • The skin is cold and sweaty, and the pupils dilate

The ANS differs from the SNS in the following three areas

• Effectors
  • SNS are skeletal muscles
  • ANS are cardiac muscle, smooth muscle, and glands
• Efferent pathways
  • Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector
  • Axons of the ANS are a two-neuron chain
    • The preganglionic (first) neuron has a lightly myelinated axon
    • The ganglionic (second) neuron extends to an effector organ
• Target organ responses and neurotransmitters
  • All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect
  • In the ANS:
    • Preganglionic fibers release ACh
    • Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory
    • ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Referred Pain

• Pain stimuli arising from the viscera are perceived as somatic in origin
• This may be due to the fact that visceral pain afferents travel along the same __________ as _______ pain fibers

Parasympathetic (Craniosacral) Division Anatomy

• The preganglionic axons extend from the CNS nearly all the way to the structures to be innervated where they synapse with ganglionic neurons in the terminal ganglia.
• The cranial outflow consists of preganglionic fibers that run in the __________,_______, ______________, and _______cranial nerves.
• The rest of the large intestine and the pelvic organs are served by the sacral outflow, which arises from neurons located in the lateral gray matter of spinal cord segments ___________

Sympathetic Division Anatomy (Thoracolumbar)

• Preganglionic Fibers originate in the lateral horns of _________
• The sympathetic division supplies the visceral organs in the internal body cavities but also all visceral structures in the somatic part of the body.
• The preganglionic fibers from T5 down synapse in collateral ganglia; thus these fibers enter and leave the sympathetic chains without synapsing.
• Some fibers of the thoracic splanchnic nerves terminate by synapsing with the hormone producing medullary cells of the adrenal cortex.
• Sympathetic Trunks and Pathways
  • When synapses are made in chain ganglia, the postganglionic axons enter the ventral (or dorsal) ramus of the adjoining spinal nerves by way of communicating branches called gray rami communicantes.

ANS Physiology

• Interactions of the Autonomic Divisions
  • Most visceral organs receive dual innervation by both ANS divisions, allowing for a dynamic antagonism between the divisions and precise control of visceral activity.
  • The sympathetic division will increase heart and respiratory rates during a fight-or-flight situation and decrease activity of digestive and elimination organs.
  • Sympathetic tone occurs in the vascular system, and parasympathetic tone occurs in the digestive and urinary tracts.
    • Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension
  • The parasympathetic and sympathetic divisions may work together to achieve a common purpose. For example, the parasympathetic division controls erection while the sympathetic division controls ejaculation.
  • The parasympathetic division exerts short-lived, localized control over its effectors, while the sympathetic division responds in a diffuse and interconnected way to cause a body-wide mobilization.
• Neurotransmitters and Receptors
  • Cholinergic receptors, nicotinic and muscarinic receptors, bind acetylcholine.
    • The effect of ACh binding to nicotinic receptors is always stimulatory
  • Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers
    • The effect of ACh binding:
      • Can be either inhibitory or excitatory
      • Depends on the receptor type of the target organ
  • Adrenergic receptors alpha and beta bind to epinephrine.
    • The two types of adrenergic receptors are alpha and beta
    • Each type has two or three subclasses (alpha1, alpha2, beta1, beta2 , beta3)
    • Effects of NE binding to:
      • alpa receptors is generally stimulatory
      • beta receptors is generally inhibitory
    • A notable exception - NE binding to beta receptors of the heart is stimulatory
• Affects of Drugs
  • Knowing the locations of the cholinergic and adrenergic receptor subtypes allows specific drugs to be prescribed to obtain desired inhibitory or stimulatory effects on target organs
  • Atropine - blocks parasympathetic effects
  • Neostigmine - inhibits acetylcholinesterase and is used to treat myasthenia gravis
  • Tricyclic antidepressants - prolong the activity of NE on postsynaptic membranes
  • Over-the-counter drugs for colds, allergies, and nasal congestion - stimulate alpha-adrenergic receptors
  • Beta-blockers - attach mainly to beta1 receptors and reduce heart rate and prevent arrhythmias

Control of Autonomic Functioning

• The brain stem appears to exert the most direct influence over autonomic functions.
• The hypothalamus is the main integration center for the autonomic nervous system.
• Cortical or voluntary control of the autonomic nervous system does appear to be possible.

Homeostatic Imbalances of the ANS

• Hypertension, or high blood pressure, may result from an overactive sympathetic vasoconstrictor response due to continuous high levels of stress.
• Raynaud's disease is characterized by intermittent attacks causing the skin of the fingers and the toes to become pale, then cyanotic and painful.
• Mass reflex reaction is a life-threatening condition involving uncontrolled activation of both somatic and autonomic motor neurons.
• Orthostatic hypotension is a form of low blood pressure that occurs when sympathetic vasoconstriction centers respond slowly to positional changes
• Hyperhidrosis - excessive sweating
• Hirschsprung's disease (megacolon) -digestive system malfunction

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