ANS Neuro

• The autonomic nervous system (ANS) regulates the activity of smooth muscle, cardiac muscle, and glands
• The ANS 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
• The ANS usually operates without conscious control and its reflexes are regulated by centers in the hypothalamus and brainstem
• Comparison of Somatic and Autonomic Nervous Systems:

  • Somatic nervous system: contains both sensory and motor neurons, with sensory input consciously perceived and motor neurons innervating skeletal muscle for voluntary movements
  • Autonomic nervous system: contains autonomic sensory and motor neurons, with sensory input not consciously perceived and motor neurons regulating visceral activities by either increasing or decreasing ongoing activities of cardiac muscle, smooth muscle, and glands
• All autonomic motor pathways consist of two motor neurons in series, with preganglionic fibers releasing acetylcholine and postganglionic fibers releasing acetylcholine or norepinephrine
• Anatomy of Autonomic Motor Pathways:

  • Preganglionic neurons: located in the brain or spinal cord, with axons passing out as part of a cranial or spinal nerve to synapse in an autonomic ganglion with postganglionic neurons
  • Autonomic ganglia: sympathetic ganglia include the sympathetic trunk or vertebral chain, while parasympathetic ganglia are terminal or intramural ganglia located close to or within the wall of a visceral organ
  • Postganglionic neurons: connect with preganglionic neurons within ganglia, sympathetic chains, prevertebral ganglia, or adrenal medulla
• Autonomic Plexuses are tangled networks of sympathetic and parasympathetic neurons, with major plexuses including the cardiac, pulmonary, celiac, superior mesenteric, inferior mesenteric, hypogastric, and renal
• The adrenal medulla is a modified sympathetic ganglion with cells similar to sympathetic postganglionic neurons
• In Horner’s syndrome, the sympathetic innervation to one side of the face is lost
• Cholinergic neurons release acetylcholine and include all sympathetic and parasympathetic preganglionic neurons, all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervate most sweat glands
• Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane, with nicotinic and muscarinic receptor types
• Activation of nicotinic receptors causes excitation, while activation of muscarinic receptors can cause either excitation or inhibition depending on the cell
• Adrenergic neurons release norepinephrine and include most sympathetic postganglionic neurons
• The main types of adrenergic receptors are alpha and beta receptors, which can result in either excitation or inhibition depending on the subtype
• Sympathetic responses prepare the body for emergency situations, stimulated by the four E’s: Emergency, Embarrassment, Exercise, Excitement
• Specific sympathetic responses include: pupils dilate, heart rate and blood pressure increase, blood flow to the kidneys and gastrointestinal tract diminishes, while blood flow to skeletal muscles, liver, heart, and adipose tissue increases
• The parasympathetic division regulates activities that conserve and restore body energy, with responses like salivation, lacrimation, urination, digestion, and defecation (SLUDD)
• Parasympathetic responses include decreased heart rate, airway diameter, and pupil diameter
• 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