Module 4

• Autonomic nervous system (ANS)


  Regulates the activity of smooth muscle, cardiac muscle, and glands
• Autonomic nervous system


  • Contributes to homeostasis by sending motor output from the central nervous system to smooth muscle, cardiac muscle, and glands for appropriate responses to integrated sensory information
  • Usually operates without conscious control
• Autonomic reflexes


  Regulated by centers in the hypothalamus and brainstem
• Autonomic motor pathways


  • Consist of two motor neurons in series
  • Axon of the first motor neuron extends from the CNS and synapses in an autonomic ganglion with the second neuron
  • Second neuron synapses on an effector
  • Preganglionic fibers release acetylcholine and postganglionic fibers release acetylcholine or norepinephrine
• Divisions of the autonomic nervous system


  • Sympathetic
  • Parasympathetic
• Preganglionic neuron


  • First of two autonomic motor neurons
  • Cell body is in the brain or spinal cord
  • Myelinated axon, called a preganglionic fiber, passes out of the CNS as part of a cranial or spinal nerve, later separating from the nerve and extending to an autonomic ganglion where it synapses with the postganglionic neuron
• Postganglionic neuron


  • Second neuron in the autonomic motor pathway
  • Cell body and dendrites are located in an autonomic ganglion, where it makes synapses with one or more preganglionic fibers
  • Axon of a postganglionic neuron, the postganglionic fiber, is unmyelinated and terminates in a visceral effector
• Sympathetic preganglionic neurons


  • Cell bodies are in the lateral gray horns of the 12 thoracic and first 2 or 3 lumbar segments
  • Axons are known as the thoracolumbar outflow
• Parasympathetic preganglionic neurons


  • Cell bodies are in cranial nerve nuclei (III, VII, IX, and X) in the brainstem and lateral gray horns of the second through fourth sacral segments of the cord
  • Axons are referred to as the craniosacral outflow
• Sympathetic ganglia


  • Include the sympathetic trunk or vertebral chain or paravertebral ganglia and the prevertebral or collateral ganglia
  • Sympathetic preganglionic neurons synapse with postganglionic neurons in ganglia of the sympathetic trunk or prevertebral ganglia
• Parasympathetic ganglia


  • Are the terminal or intramural ganglia that are located very close to or actually within the wall of a visceral organ
  • Examples include the ciliary, pterygopalatine, submandibular, and otic ganglia
  • Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia
• Autonomic plexuses


  • Tangled networks of sympathetic and parasympathetic neurons
  • Major autonomic plexuses include the cardiac, pulmonary, celiac, superior mesenteric, inferior mesenteric, hypogastric, and renal
• Pathway from spinal cord to sympathetic trunk ganglia

  1. Sympathetic preganglionic neurons are part of the lateral gray horns

  2. Axons pass into the anterior root of a spinal nerve and enter a short pathway called a white ramus
• Organization of sympathetic trunk ganglia

  • Sympathetic trunk ganglia are arranged anterior and lateral to the vertebral column
  • Developmentally, the adrenal medulla is a modified sympathetic ganglion and its cells are similar to sympathetic postganglionic neurons
• Pathways from the sympathetic trunk ganglia to visceral effectors

  • Spinal nerves
  • Cephalic periarterial nerves
  • Sympathetic nerves
  • Splanchnic nerves
• Cranial outflow of parasympathetic division

  • Consists of preganglionic axons that extend from the brainstem in four cranial nerves
  • The cranial outflow components are four pairs of ganglia and the plexuses associated with the vagus nerve
• Sacral outflow of parasympathetic division

  Consists of preganglionic axons in anterior roots of the second through fourth sacral nerves
• Cholinergic neurons


  Release the neurotransmitter acetylcholine and include all sympathetic and parasympathetic preganglionic neurons, all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervate most sweat glands
• Cholinergic receptors


  • Nicotinic and muscarinic receptors
  • Activation of nicotinic receptors causes excitation of the postsynaptic cell
  • Activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors
• Adrenergic neurons


  Release norepinephrine and include most sympathetic postganglionic neurons
• Adrenergic receptors


  • Main types are alpha and beta receptors, which are further classified into subtypes
  • Depending on the subtype, activation of the receptor can result in either excitation or inhibition
• Location of cholinergic and adrenergic receptor subtypes and their responses


  • Summarized in Table 15.2
• Agonist


  A substance that binds to and activates a receptor, mimicking the effect of a natural neurotransmitter or hormone
• Antagonist


  A substance that binds to and blocks a receptor, preventing a natural neurotransmitter or hormone from exerting its effect
• Dual innervation


  • Most body structures receive fibers from both the sympathetic and parasympathetic divisions
  • Usually one division causes excitation and one causes inhibition resulting in autonomic tone
• Sympathetic responses


  • Prepare the body for emergency situations (the fight-or-flight responses)
  • Stimulated by the four E's (Emergency, Embarrassment, Exercise, Excitement)
  • Effects are longer-lasting and more widespread than those of parasympathetic stimulation
  • Norepinephrine is degraded more slowly than acetylcholine
  • There are more postganglionic neurons in this division
• Specific sympathetic responses


  • Pupils dilate
  • Heart rate and blood pressure increase
  • Blood flow to the kidneys and gastrointestinal tract diminishes
  • Blood flow to the skeletal muscles, liver, heart, and adipose tissue increases
  • Liver cells release glucose and adipose cells break down fat to increase available energy
• Parasympathetic responses


  • Regulate activities that conserve and restore body energy (energy conservation-restorative system)
  • Effects last for a shorter time and are less widespread than those of the sympathetic stimulation
  • Acetylcholine is degraded more rapidly than norepinephrine
  • There are more postganglionic neurons in this division
• Parasympathetic responses


  • Salivation
  • Lacrimation
  • Urination
  • Digestion
  • Defecation (collectively known as "SLUDD")
  • Decreased heart rate
  • Decreased airway diameter
  • Decreased pupil diameter
• Glands, cardiac muscle, and smooth muscle's responses to stimulation by the ANS


  • Summarized in Table 15.4
• Visceral autonomic reflex


  • Adjusts the activity of a visceral effector, often unconsciously
  • Consists of a sensory receptor, sensory neuron, integrating center motor neurons, and visceral effector
• Autonomic control by higher centers

  • The hypothalamus is the major control and integration center of the ANS
  • Control of the ANS by the cerebral cortex occurs primarily during emotional stress
• Autonomic dysreflexia is an exaggerated response of the sympathetic division of the ANS that occurs in about 85% of individuals with spinal cord injury at or above the level of T6
• Raynaud's phenomenon is due to excessive sympathetic stimulation of arterioles within the fingers and toes, resulting in diminished blood flow to the digits
• Dysautonomia


  Refers to a group of signs and symptoms that result from damage to the ANS
• Preganglionic fibers synapse with ganglia near target organs, while postganglionic fibers directly innervate effector cells.
• Sympathetic preganglionic axons are myelinated, whereas parasympathetic preganglionic axons are unmyelinated.
• Sympathetic neurons are preganglionic, while parasympathetic neurons are postganglionic.