Autonomic Nervous system | Anatomy2Medicine
autonomic nervous system

Autonomic Nervous system

    • The Autonomic Nervous System (ANS)
      • is a general visceral efferent (GVE) motor system
      • controls and regulates smooth muscle, cardiac muscle, and glands.
      • has three division

 

  • the sympathetic
  • the parasympathetic
  • enteric.

 

      • consists of two types of projection neurons
        • preganglionic neurons
        • postganglionic neurons
        • sympathetic ganglia have interneurons
      • output that is influenced by the hypothalamus.
    • Divisions of the Autonomic Nervous System
      • Sympathetic division
        • is also called the thoracolumbar, or adrenergic, system.

 

  • Preganglionic neurons

 

          • are located in the intermediolateral cell column (T1–L3).
          • project via ventral roots and white communicating rami to the sympathetic trunk or via splanchnic nerves to prevertebral (collateral) ganglia
          • They synapse at both locations with postganglionic neurons.

 

  • Postganglionic neurons

 

          • are located in the
            • sympathetic trunk (paravertebral ganglia)
            • prevertebral (collateral) ganglia.
          • in the sympathetic trunk, project via gray communicating rami to spinal nerves
            • innervate blood vessels, arrector pili muscles, and sweat glands.
          • in prevertebral ganglia, project to abdominal and pelvic viscera.

 

  • Interneurons

 

          • are called small intensely fluorescent (SIF) cells.
          • are located in sympathetic ganglia.
          • are dopaminergic and inhibitory.

 

  • Neurotransmitters
  • Acetylcholine (ACh)

 

            • is the neurotransmitter of preganglionic neurons

 

  • Norepinephrine

 

            • is the neurotransmitter of postganglionic sympathetic neurons, with the exception of sweat glands and some blood vessels that receive cholinergic sympathetic innervation.

 

  • Epinephrine

 

            • is produced by the chromaffin cells of the adrenal medulla

 

  • exists in a 4:1 ratio to norepinephrine.
  • Dopamine

 

            • is the neurotransmitter of the SIF cells.

 

  • Vasoactive intestinal polypeptide (VIP)
  • is co-localized with ACh in some postganglionic parasympathetic fibers.

 

            • is a vasodilator.
      • Parasympathetic division
        • is called the craniosacral, or cholinergic, system.
        • uses ACh as the neurotransmitter for both preganglionic and postganglionic synapses

 

  • Cranial division is associated with four cranial nerves.
  • Oculomotor nerve (CN III)
  • Facial nerve (CN VII)
  • Superior salivatory nucleus
  • projects preganglionic fibers to the pterygopalatine and submandibular ganglia.
  • Pterygopalatine ganglion
  • projects postganglionic fibers to the lacrimal gland and to the mucosa of the nose and palate.
  • Submandibular ganglion

 

              • projects postganglionic fibers to the submandibular and sublingual glands.

 

  • Glossopharyngeal nerve (CN IX)
  • Inferior salivatory nucleus

 

              • projects preganglionic fibers to the otic ganglion.

 

  • Otic ganglion

 

              • projects postganglionic fibers to the parotid gland.

 

  • Vagal nerve (CN X)
  • Dorsal motor nucleus

 

              • projects preganglionic fibers to intramural (terminal) ganglia within or adjacent to visceral organs.

 

  • Intramural (terminal) ganglia
  • innervate, via short postganglionic fibers, viscera of the thorax and abdomen as far as the left colic flexure.
  • Nucleus ambiguus
  • projects preganglionic fibers to the intramural ganglia of the heart (sinoatrial and atrioventricular nodes).
  • Sacral division

 

          • originates from the sacral parasympathetic nucleus of sacral segments S2 to S4.

 

  • postganglionic neurons lie on, near, or in the wall of the innervated viscus (intramural ganglia).
  • innervates via pelvic nerves the lower abdomen and pelvic viscera, including the colon distal to the left colic flexure, urinary bladder (detrusor muscle), and genital viscera.

 

          • is involved with micturition, defecation, and sexual function.

 

  • Neurotransmitters
  • ACh
  • VIP
  • is co-localized with ACh.

 

            • stimulates secretomotor neurons and vasodilator neurons.

 

  • Nitric oxide

 

            • responsible for relaxation of smooth muscle.
            • is responsible for penile erection

 

  • Enteric division

 

        • consists of
          • intramural (enteric) ganglia
          • plexuses of the gastrointestinal tract, including the
            • submucosal (Meissner) plexus
            • myenteric (Auerbach) plexus.
        • is influenced by postganglionic adrenergic sympathetic input.
        • is influenced by preganglionic cholinergic parasympathetic input.
        • functions independently when deprived of central nervous system (CNS) innervation.
        • plays a major role in the control of gastrointestinal motility.

 

  • Visceral Afferent Fibers and Pain
  • All sympathetic and parasympathetic nerves contain both general visceral afferent (GVA) and GVE fibers.

 

      • GVA fibers and innervated structures

 

  • Most visceral reflexes and organic sensations are mediated by parasympathetic afferent fibers.
  • GVA cell bodies

 

        • are found in

 

  • dorsal root ganglia

 

          • inferior ganglia of the glossopharyngeal nerve (CN IX)
          • the vagal nerve (CN X),
          • geniculate ganglion of the facial nerve (CN VII).

 

  • GVA pain fibers

 

        • are found in the white communicating rami.
        • accompany sympathetic nerves exclusively.
        • have their cell bodies in the dorsal root ganglia of the thoracolumbar region (T1–L3).

 

  • GVA reflex fibers

 

        • accompany both sympathetic and parasympathetic nerves.
        • terminate centrally in the solitary nucleus
        • mediate the gag reflex.

 

  • Carotid sinus

 

        • is a slight dilation of the common carotid artery at the bifurcation.
        • It contains baroreceptors

 

  • when baroreceptors are stimulated cause bradycardia and a decrease in blood pressure.

 

        • is innervated by GVA fibers from CN IX.

 

  • Carotid body (glomus caroticum)

 

        • is a small structure just above the bifurcation of the common carotid artery

 

  • it contains chemoreceptors that respond to carbon dioxide, oxygen, and pH levels.

 

        • is innervated by GVA fibers from CN IX and CN X.

 

  • Autonomic Innervation of Selected Organs

 

      • Eye

 

  • Sympathetic input
  • Hypothalamic neurons project directly to the intermediolateral cell column at T1 and T2, the ciliospinal center of Budge.
  • The intermediolateral cell column (T1–T2) projects preganglionic fibers via the sympathetic trunk to the superior cervical ganglion.
  • The superior cervical ganglion projects postganglionic fibers via the internal carotid artery to the cavernous sinus.

 

          • Pupillodilator fibers reach the dilator pupillae muscle of the iris via the superior orbital fissure and via the nasociliary and long ciliary nerves (CN V)
          • How does Horner syndrome occur with otitis media.
            • Some pupillodilator fibers accompany the caroticotympanic nerves prior to entering the orbit;

 

  • Fibers to the tarsal muscles of Müller reach the eyelids via the ophthalmic artery (in the optic canal).
  • Interruption of sympathetic input to the eye at any level results in Horner syndrome.
  • Preganglionic versus postganglionic Horner syndrome

 

            • Instill 1% hydroxyamphetamine into the conjunctional sac.
              • A dilated iris indicates preganglionic (central) Horner syndrome
              • An undilated iris indicates postganglionic Horner syndrome.
              • This test is used to rule out other causes of miosis (e.g., anisocoria).

 

  • Parasympathetic input
  • The Edinger-Westphal nucleus projects preganglionic fibers via the oculomotor nerve (CN III) to the ciliary ganglion.
  • The ciliary ganglion projects postganglionic fibers via the short ciliary nerves to the

 

            • sphincter pupillae (which acts to contract the pupil)
            • ciliary muscle (which affects lens shape in accommodation).

 

  • Postganglionic fibers mediate the efferent limb of the pupillary light reflex.

 

          • Interruption of the parasympathetic input results in
            • internal ophthalmoplegia (a fixed unresponsive] and dilated pupil)
            • cycloplegia (paralysis of accommodation).
      • Blood vessels
        • receive their innervation from the sympathetic division of the ANS.

 

  • Arteries and arterioles

 

          • Constriction of cutaneous and splanchnic blood vessels results from sympathetic stimulation of alpha-receptors.

 

  • Dilation of skeletal muscle arteries results from sympathetic stimulation of Beta-receptors.

 

          • Blood vessels are not affected by parasympathetic stimulation.

 

  • Large veins and venules are moderately innervated.

 

        • Capillaries seem to have no innervation.

 

  • Cerebral blood vessels respond to circulating metabolites (carbon dioxide and oxygen).

 

      • Heart

 

  • Sympathetic input

 

          • The intermediolateral cell column (T1–T5)
            • projects preganglionic fibers to the upper thoracic ganglia and to the three cervical ganglia of the sympathetic trunk.
          • The rostral sympathetic trunk
            • projects postganglionic fibers via cardiac nerves to the ventricular and atrial walls and the pacemaker tissue.

 

  • Stimulation of cardiac nerves results in an increase in heart rate and in the force of cardiac contractility (via Beta 1-adrenergic receptors).
  • Parasympathetic input

 

          • The nucleus ambiguus of CN X projects preganglionic fibers via the vagal nerve to the intramural ganglia of the atria and the sinoatrial node.
          • Postganglionic fibers from the intramural ganglia innervate the heart.

 

  • Vagal stimulation lowers the strength and rate of cardiac contraction.
  • Bladder (control of micturition)

 

        • Control is predominantly parasympathetic.

 

  • Sympathetic input

 

          • is from T12 to L3 via the inferior mesenteric plexus and via the inferior hypogastric plexus to the detrusor muscle and the internal sphincter.

 

  • Damage to sympathetic fibers has no effect on bladder function.
  • Parasympathetic input

 

          • is from S2–S3 via the pelvic splanchnic nerves to the detrusor muscle and the internal sphincter.
          • Stimulation results in emptying the bladder.

 

  • Paralysis produces an atonic bladder, with no reflex or voluntary control.
  • Somatomotor input
  • is from S2 to S4 via the pudendal nerves to the external sphincter muscle.
  • Sensory input to spinal cord

 

          • is via hypogastric, pelvic, and pudendal nerves.

 

  • Damage results in an atonic bladder with overflow incontinence (dribbling).
  • Ascending pathway for bladder sensation

 

          • controls the urge to void.
          • is found with sacral fibers of the lateral spinothalamic tract.

 

  • Transection results in loss of the urge to void and overflow incontinence.
  • Upper motor neuron (UMN) input
  • controls volitional micturition.

 

        • is from the paracentral lobule via the corticosacral tract
        • Bilateral transection results in an uninhibited neurogenic bladder
          • Sensation is normal
          • patient has no control over voiding
          • bladder fills and suddenly empties without cortical control.