Ch. 17-Anatomy

• Somatic Nervous System


  Axons of lower motor neurons extend from the CNS to contact and exert direct control over skeletal muscles
• Autonomic Nervous System (ANS)


  Innervates visceral effectors
• Information processing in the ANS

  Afferent sensory information is processed in the CNS, then efferent impulses are sent to effector organs
• Afferent pathways in the ANS

  • Originate in visceral afferent receptors, connect to visceral effector organs
• Innervation in the ANS

  Visceral motor neurons in the CNS (preganglionic neurons) send axons to synapse on ganglion neurons located outside the CNS, postganglionic fibers then innervate peripheral tissues/organs
• Subdivisions of the ANS


  • Sympathetic division
  • Parasympathetic division
  • Enteric division
• Sympathetic division


  Often called the "fight or flight" system, generally stimulates tissue metabolism and increases alertness to prepare the body for emergencies
• Parasympathetic division


  Often called the "rest and repose" system, conserves energy and promotes sedentary activities like digestion
• The two divisions of the ANS often have opposing effects, but they may also work independently or together to control different stages of a complex process
• Components of the Sympathetic division

  • Preganglionic neurons located between T1-L2 of the spinal cord
  • Ganglionic neurons in ganglia near the vertebral column (sympathetic chain ganglia and collateral ganglia)
  • Specialized neurons in the adrenal medulla
• Sympathetic chain ganglia


  • Innervate effectors in the body wall, head, neck, and limbs
  • Receive preganglionic fibers from the white ramus of the corresponding spinal nerve
• Collateral ganglia


  • Preganglionic fibers pass through without synapsing, converge to form splanchnic nerves
  • Generally reduce blood flow and energy use by visceral organs not important for short-term survival
• Suprarenal medulla


  • Receives preganglionic fibers that do not synapse in the sympathetic chain, releases epinephrine and norepinephrine when stimulated
• Sympathetic activation


  Increases alertness, energy, cardiovascular/respiratory activity, muscle tone, and mobilization of energy reserves
• Sympathetic preganglionic fibers release acetylcholine at their synapses with ganglionic neurons, while most postganglionic sympathetic terminals release norepinephrine
• Suprarenal Medullae


  Part of the adrenal glands
• Sympathetic stimulation


  1. Preganglionic fibers pass through sympathetic chain without synapsing

  2. Fibers proceed to suprarenal medullae

  3. Medullae release neurotransmitters epinephrine (E) and norepinephrine (NE)
• Effects of sympathetic stimulation

  • Increased alertness
  • Feeling of energy or euphoria
  • Temporary insensitivity to painful stimuli
  • Increased activity in cardiovascular & respiratory centers
  • Increased heart rate, contraction strength, elevated blood pressure, increased breathing rate and depth
  • Elevation in muscle tone
  • Mobilization of energy reserves
• Cholinergic synapses


  Sympathetic preganglionic fibers release acetylcholine (Ach) at their synapses with ganglionic neurons
• Neurotransmitter vesicles


  Each branch of the neuron is packed with mitochondria and neurotransmitter vesicles
• Effects of NE


  Longer in duration because blood doesn't contain enzymes to break it down and tissues have low concentrations of these enzymes
• Sympathetic receptors


  Alpha receptors and beta receptors, each with 2 or 3 subtypes
• Epinephrine and norepinephrine


  Epinephrine stimulates both alpha and beta receptors, norepinephrine primarily stimulates alpha receptors
• Sympathetic division of ANS

  • Induces two chains on each side of the vertebral column
  • Preganglionic fibers are short, postganglionic fibers are long
  • Shows extensive divergence, a single sympathetic motor neuron can control a variety of peripheral effectors
  • All preganglionic neurons release Ach, most postganglionic fibers release NE, a few release Ach
• Parasympathetic division


  Preganglionic neurons located in brainstem and sacral segments of spinal cord, ganglionic neurons in peripheral ganglia close to or within target organs
• Parasympathetic preganglionic fibers


  • Oculomotor
  • Facial
  • Glossopharyngeal
  • Vagus
  • Sacral
• Functions of parasympathetic division

  • Constriction of pupils
  • Secretion by digestive and other glands
  • Secretion of hormones that promote cellular action
  • Increased smooth muscle activity in digestive tract
  • Stimulation and coordination of defecation
  • Contraction of urinary bladder during urination
  • Constriction of respiratory passageways
  • Reduction in heart rate and force of contraction
  • Sexual arousal and stimulation of sexual glands
• Parasympathetic neurotransmitter


  All preganglionic and postganglionic fibers release acetylcholine (Ach) at their neuroeffector junctions
• Parasympathetic neuroeffector junctions


  Small with narrow synaptic clefts, effects of stimulation are short-lived, any Ach diffusing into surrounding tissue is deactivated by cholinesterase
• Parasympathetic receptors


  Nicotinic receptors on ganglionic neurons, muscarinic receptors at all cholinergic neuroeffector junctions
• Parasympathetic division


  • Cranial motor nuclei in brainstem
  • Sacral segments of spinal cord
  • Ganglionic neurons in intramural ganglia or closely associated ganglia
  • Innervates structures in head, thoracic & abdominopelvic cavities
  • All parasympathetic neurons are cholinergic
• Dual innervation


  • Many vital organs receive instructions from both sympathetic and parasympathetic divisions
  • Preganglionic fibers mingle at plexuses in thoracic and abdominopelvic cavities
• Visceral reflexes are the simplest functional units in the autonomic nervous system, providing automatic motor responses that can be modified by higher centers
• Visceral reflexes


  • Polysynaptic, consisting of a receptor, sensory nerve, processing center, and visceral motor neurons
  • May be long reflexes or short reflexes
  • Long reflexes are equivalents of polysynaptic reflexes, short reflexes bypass the CNS and involve sensory neurons and interneurons within autonomic ganglia