Ch15

• Somatic Nervous System


  • Effectors or target organ = skeletal muscle
  • Efferent pathways and ganglia = cell body of the lower motor neuron is in the central nervous system (spinal cord ventral horn) and axon synapses with skeletal muscles
  • Neurotransmitter = acetylcholine is excitatory to skeletal muscles
  • Neurons are highly myelinated
• Autonomic Nervous System


  • Effectors or target organs = cardiac muscle, smooth muscle, and glands
  • Effector pathways and ganglia = the motor unit of the ANS is a two neuron chain
  • Preganglionic neurons = acetylcholine
  • Postganglionic neurons = acetylcholine or norepinephrine
  • Preganglionic neurons are lightly myelinated while postganglionic neurons are unmyelinated
• Divisions of the Autonomic Nervous System

  • Sympathetic (fight or flight)
  • Parasympathetic (rest and digest OR feed and breed)
• Parasympathetic Nervous System


  • Also called the Craniosacral Division because of the origin of the nerves
  • Utilizes acetylcholine at both synapses
• Parasympathetic Nervous System - Cranial outflow

  1. Oculomotor Nerve (III) - Innervate smooth muscles in the eyes that cause the pupils to constrict

  2. Facial Nerve (VII) - Stimulates lacrimal gland (tear production) and stimulates salivary glands (salivation)

  3. Glossopharyngeal Nerve (IX) - Stimulates the parotid salivary gland (salivation)

  4. Vagus Nerve (X) - Contains 75% of parasympathetic nerve fibers, decreases/steadies heart rate, constricts coronary veins and arteries, constricts bronchioles, reduces breathing rates, expels bile, stimulates secretion of enzymes and increases contraction and motility of stomach, stimulates insulin secretion and enzyme secretion, increases motility (peristalsis) of intestines
• Parasympathetic Nervous System - Sacral outflow


  Preganglionic fibers (also called pelvic splanchnic nerves) S2-S4 synapse in ganglia within their target organs: Distal large intestines and rectum=promote defecation, Bladder=contraction of smooth muscle of bladder to promote urination, Genitalia=causes erection
• Sympathetic Nervous System


  • Also called the Thoracolumbar Division because of origin of the nerves
  • More complex than the parasympathetic division because it innervates more organs which are both visceral and somatic
  • Preganglionic neuron axons (between spinal cord and ganglion) are small in diameter and myelinated = white ramus communicans
  • Postganglionic axons (unmyelinated) leave chain and pass through gray ramus communicans and reenter spinal nerve
  • Innervate sweat glands, smooth muscle in blood vessels of skin or bones, and arrector pili in skin
  • Innervate heart muscle, thoracic blood vessel smooth muscle, smooth muscle of esophagus and lungs and, from a sympathetic nerve plexus near the carotid artery, the head and neck sweat glands, salivary glands, smooth muscle in blood vessels, the eye, and arrector pili
  • Some preganglionic axons originating at T5-12 enter chain ganglia but exit at the same or different level, without synapsing, as splanchnic nerves going to collateral (=prevertebral) ganglia to synapse with postganglionic neurons
  • Collateral ganglia are in the abdomen close to where major arteries (for which they are named) arise from the abdominal aorta: celiac, superior mesenteric, and inferior mesenteric
  • Innervate abdominopelvic structures: smooth muscle in walls of blood vessels and organs or glands (e.g., pancreas, liver, prostate)
  • Innervation of adrenal medulla = Composed only of preganglionic neurons whose axons synapse with cells of adrenal medulla, Eighty percent of cells secrete epinephrine and twenty percent secrete norepinephrine into blood stream
• Acetylcholine


  Released by all ANS preganglionic axons, all parasympathetic postganglionic axons, cholinergic axon fibers
• Norepinephrine


  Released by most sympathetic postganglionic axons, adrenergic axon fibers
• Cholinergic receptors


  • Nicotinic receptors= on all ganglionic neurons, adrenal medulla, and skeletal muscle
  • Muscarinic receptors=found on all parasympathetic target organs and select sympathetic organs (Eccrine sweat glands and blood vessels of skeletal muscle)
  • When ACh binds to nicotinic receptors it is always excitatory
  • When ACh binds to muscarinic receptors it may be excitatory or inhibitory
• Adrenergic receptors


  • Alpha=located in blood vessels serving skin, mucosae, visceral organs, kidneys, salivary glands, and all sympathetic organs except the heart
  • Beta=located within heart and coronary blood vessels, kidneys, liver, adipose tissue, lungs, and many other sympathetic target organs
  • When NE (or epinephrine) binds to alpha adrenergic receptors it is generally stimulatory while binding to beta adrenergic receptors is generally inhibitory
  • There are exceptions, Beta1 adrenergic receptors of the heart are actually stimulatory rather than inhibitory like other classes of beta receptors
• Drugs


  • Atropine, an anticholinergic drug blocks parasympathetic effects
  • Neostigmine inhibits acetylcholinesterase, therefore ACh accumulates in the synapse
  • Albuterol dilates bronchioles
  • Ephedrine is a sympathetic-mimicking drug that stimulates alpha adrenergic receptors
  • Beta-blockers are drugs that inhibit the cardiac Beta1 adrenergic receptors