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.