Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS)

• Key to anticipating the effects and side effects of drugs on patients.
• Consists of motor neurons that innervate smooth muscles, cardiac muscle, and glands.
• Adjusts body activities by shunting blood, and regulating heart rate, blood pressure, etc.
• Operates via subconscious control.
• Also called the involuntary nervous system or general visceral motor system.

ANS vs Somatic Nervous System

• Both contain motor fibers, but differ in effectors, efferent pathways/ganglia, and target responses

Effectors

• Somatic nervous system: innervates skeletal muscles.
• ANS: innervates cardiac muscle, smooth muscle, and glands.

Efferent Pathways and Ganglia

• SNS: cell body is in the CNS, and a single, thick myelinated Group A axon extends directly to skeletal muscle via spinal or cranial nerves.
• ANS: uses a two-neuron chain.
• Preganglionic neuron: cell body is in the CNS, features a thin, lightly myelinated preganglionic axon extending to the ganglion.
• Postganglionic (ganglionic) neuron: Located outside the CNS; a cell body synapses with the preganglionic axon in an autonomic ganglion, featuring a nonmyelinated postganglionic axon to the effector organ.

Neurotransmitter Effects

• Somatic nervous system: all somatic motor neurons release acetylcholine (ACh), always with a stimulatory effect.
• ANS: preganglionic fibers release ACh, while postganglionic fibers release norepinephrine or ACh at effectors. The effect can be stimulatory or inhibitory based on receptor types.

Overlap Between Somatic and Autonomic Function

• Higher brain centers coordinate both systems.
• Spinal/cranial nerves contain somatic/autonomic fibers.
• Adaptations involve skeletal muscles and visceral organs.
• Active muscles require more oxygen and glucose; thus, ANS nerves increase heart rate and open airways.

Divisions of the Autonomic Nervous System

• Two arms of ANS: parasympathetic and sympathetic.
• The Parasympathetic division promotes maintenance and conserves energy.
• The Sympathetic division mobilizes during activity.
• Dual innervation: all visceral organs are served by both divisions, causing opposite effects.
• Dynamic antagonism between divisions maintains homeostasis.

Role of the Parasympathetic Division

• Keeps body energy use low during maintenance activities; directs digestion, diuresis, and defecation.
• Referred to as the "rest-and-digest" system.
• Relaxing example: blood pressure, heart rate, and respiratory rates are low, gastrointestinal activity is high, and pupils are constricted with lenses accommodated for close vision

Role of the Sympathetic Division

• Mobilizes the body during activity.
• Referred to as the "fight-or-flight" system.
• Exercise, excitement, emergency, and embarrassment activate the sympathetic system, increasing heart rate, causing dry mouth and cold, sweaty skin, and dilating pupils.
• During physical activity, blood is shunted to skeletal muscles/the heart, bronchioles dilate, and the liver releases glucose.

Key Anatomical Differences between the Parasympathetic and Sympathetic Divisions

• Three main differences including sites of origin, relative lengths of fibers, and location of ganglia.

Sites of Origin

• Parasympathetic fibers are craniosacral, originating in the brain and sacral spinal cord.
• Sympathetic fibers are thoracolumbar, originating in the thoracic and lumbar regions of the spinal cord.

Relative Lengths of Fibers

• Parasympathetic has long preganglionic and short postganglionic fibers.
• Sympathetic has short preganglionic and long postganglionic fibers.

Location of Ganglia

• Parasympathetic ganglia are located in or near their visceral effector organ.
• Sympathetic ganglia lie close to the spinal cord.

Parasympathetic Division

• Also called the craniosacral division because fibers originate in the brain stem and sacral regions/cord.
• Long preganglionic fibers extend from the CNS almost to target organs. These synapse with postganglionic neurons in terminal ganglia, which are close to or inside the target organs.
• Short postganglionic fibers synapse with effectors.

Cranial Part of Parasympathetic Division

• Cell bodies are located in the brain stem
• Preganglionic Fibers
  • Oculomotor nerves (III): Control smooth muscle of eye, cause pupils to constrict and lenses to bulge for focusing.
  • Postganglion cell bodies located in ciliary ganglia within eye orbitals.

Cranial Part of Parasympathetic Division

• Facial nerves (VII): stimulate large glands in the head.
• Fibers that activate nasal and lacrimal glands have synapse in pterygopalatine ganglia.
• Fibers that activate submandibular and sublingual salivary glands synapse in submandibular ganglia.
• Glossopharyngeal nerves (IX): stimulate parotid salivary glands; fibers synapse in otic ganglia.

Cranial Part of Parasympathetic Division

• Vagus nerves (X): account for ~90% of all preganglionic parasympathetic fibers in the body; serve all thoracic and abdominal viscera.
• Preganglionic fibers arise from the medulla and synapse in terminal ganglia (intramural ganglia) in the walls of target organs.
• Cardiac plexus: slows heart rate.
• Pulmonary plexus: serves lungs and bronchi.

Sacral Part of Parasympathetic Division

• Originates from neurons in $S_2-S_4$, serves pelvic organs and the distal half of the large intestine.
• Axons travel in the ventral root of spinal nerves and then branch off, forming the pelvic splanchnic nerves.
• Synapses with: -Ganglia in pelvic floor (inferior hypogastric [pelvic] plexus). -Intramural ganglia in walls of the distal half of the large intestine, urinary bladder, ureters, and reproductive organs.

Sympathetic Division

• It is more complex and innervates more organs than the parasympathetic.
• Some structures, including sweat glands, arrector pili muscles of hair follicles, and smooth muscles of all blood vessels.
• Also called the thoracolumbar division. Contains preganglionic neurons in spinal cord segments T₁-L₂ forming the lateral horns of the spinal cord.

Key Anatomical Differences Between the ANS

• Preganglionic fibers pass through white rami communicates and enter the sympathetic trunk (chain or paravertebral) ganglia.
• Paravertebral ganglia vary in size, position, and number.
• 23 paravertebral ganglia exist in the sympathetic trunk (chain).
• 3 Cervical.
• 11 Thoracic.
• 4 Lumbar.
• 4 Sacral.
• 1 Coccygeal.

Location of the Sympathetic Trunk

• Short preganglionic fibers can follow one of three pathways after entering the sympathetic trunk ganglion.

Synapse in Trunk Ganglia

• Synapse with ganglionic neuron in the same trunk ganglion.
• Ascend or descend the sympathetic trunk to synapse in another trunk ganglion.

Synapse in Collateral Ganglia

• Pass through the trunk ganglion and emerge without synapsing in the trunk (only in the abdomen and pelvis). Synapse outside of the trunk in collateral ganglia.

Summary of Autonomic Ganglia

• Terminal ganglia are in the parasympathetic nervous system and are located within the wall of the organ served (intramural ganglia), or close to the organ.
• Sympathetic trunk ganglia are in the sympathetic nervous system and are paired, beside the spinal cord.
• Collateral (prevertebral) ganglia are in the sympathetic nervous system and are unpaired, anterior to the spinal cord.

Sympathetic Pathways

• Postganglionic axons enter ventral rami via gray rami communicantes (nonmyelinated postganglionic fibers.)
• White rami communicantes: myelinated preganglionic
• Contain sympathetic system neurons.
• These fibers innervate sweat glands, arrector pili muscles, and vascular smooth muscle via.

Pathways to the Head

• Fibers emerge from T₁ to T₄ and synapse in the superior cervical ganglion. -Innervate the skin and blood vessels of the head. -Stimulate dilator muscles of the iris. -Inhibit nasal and salivary glands. Innervate smooth muscle of the upper eyelid. -Branch to the heart.

Pathways to the Thorax

• Preganglionic fibers emerge from T₁ to T₆ and synapse in cervical trunk ganglia.
• Postganglionic fibers emerge from middle and inferior cervical ganglia and enter nerves C₄ to C₈. -Innervate the heart via the cardiac plexus. -Thyroid gland and the skin. -Lungs and esophagus.

Pathways with Synapses in Collateral Ganglia

• Most fibers from T₅ to L₂ synapse in collateral ganglia outside of the trunk, forming splanchnic nerves.

Pathways to the Abdomen

Preganglionic fibers from T₅ to L₂ travel through thoracic splanchnic nerves, synapse in celiac and superior mesenteric ganglia. Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys.

Pathways to the Pelvis

Preganglionic fibers originate from T₁₀ to L₂ and travel through the trunk of lumbar and sacral ganglia. some synapse with postganglionic fibers run in lumbar and sacral splanchnic nerves.

Pathways with Synapses in the Adrenal Medulla

• Some preganglionic fibers pass directly to the adrenal medulla without synapsing.
• Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood.
• Sympathetic ganglia and the adrenal medulla arise from the same tissue, with the adrenal medulla considered a "misplaced" sympathetic ganglion

Visceral Reflexes.

• They send sensory signals regarding chemical changes stretch, temperature, and irritation of viscera.
• Receptors for visceral senses are free nerve endings widely scattered throughout viscera.
• Cell bodies are located in dorsal root ganglia and in sensory ganglia of cranial nerves.
• Axons travel in the same nerves as autonomic motor fibers (ex: parasympathetic fibers of the vagus nerve) as they monitor conditions or organs serviced by ANS nerves.

Visceral reflexes

• Visceral reflex arcs share components with somatic reflex arcs (receptor, sensory neuron, integration center, motor neuron, and effector) but have three distinct differences. -Visceral reflex arcs have two consecutive neurons in the motor pathway. -Afferent fibers are visceral sensory neurons. -Effectors are smooth muscle cardiac muscle, and glands rather than skeletal muscles.

Neurotransmitters

• Major neurotransmitters of the ANS are acetylcholine (ACh) and norepinephrine (NE).
• ACh (same as ACh used by somatic motor neurons) is released by cholinergic fibers at all ANS preganglionic axons and all parasympathetic postganglionic axons.
• NE is released by adrenergic fibers at almost all sympathetic postganglionic axons.

Cholinergic Receptors

• There are two types that bind to ACh: Nicotinic and Muscarinic receptors.
• They are named after drugs that bind to them and mimic ACh effects: nicotine and muscarine (Mushroom poison)

Nicotinic Receptors

• Found on all postganglionic neurons (sympathetic and parasympathetic), hormone-producing cells of the adrenal medulla, sarcolemma of skeletal muscle cells at the neuromuscular junction.
• The Effect of ACh at nicotinic receptors is always stimulatory; opens ion channels, depolarizing the postynaptic cell.

Muscarinic Receptors

• Located on all effector cells stimulated by postganglionic cholinergic fibers.
• The effect of ACh at muscarinic receptors can be either inhibitory or excitatory.

Adrenergic Receptors

• Two major classes that respond to NE or epinephrine: alpha (α) and beta (β) receptors.
• Divided into subclasses: α₁, α₂ and B1, B2, B3.

Effects Depend on Receptors on Target Organ

• Binding of NE to cardiac muscle B₁ receptors increases, whereas epinephrine causes bronchial relaxation when bound to B₂ receptors.

Parasympathetic and Sympathetic Interactions

• Most visceral organs have dual innervation.
• Action potentials fire down axons of both divisions, producing antagonism that precisely controls the visceral activity.
• Both ANS divisions are partially active, establishing basal sympathetic and parasympathetic control.

Sympathetic Tone

• Sympathetic tone: almost all blood vessel smooth muscle are innervated by only sympathetic fibers, meaning this division controls blood pressure, even at rest.
• vasomotor tone: a continual state of partial constriction of blood vessels increases constriction raises the the pressure drop means dilating them decreases blood pressure.

Parasympathetic Tone

• Division normally dominates the heart and smooth muscle of digestive and urinary tract organs, and it activates most glands(except adrenal and sweat glands.
• Slows the heart and dictates normal activity levels of digestive and urinary tracts.

Cooperative Effects

• Cooperation between divisions: the control of external genitalia. -Parasympathetic fibers cause vasodilation for erection of the penis/clitoris.
• Sympathetic fibers cause ejaculation of semen in males, along with reflex contraction of the female's vagina.

Unique Roles of the Sympathetic Division

• Adrenal medulla, sweat glands, arrector pili muscles, kidneys, and almost all blood vessels receive only sympathetic fibers.

Other Unique Functions of Sympathetic Division

• Thermoregulatory responses to heat body temps rise, sympathetic nerves dilate skin blood vessels, allowing heat to escape, and activate sweat glands body temps in drop blood vessels constrict.

Release of Renin

• Sympathetic system causes it to activate a system that increases blood pressure.

Metabolic Effects

• Increases the rates of cells raises blood glucose levels mobilise fats for fuells.

Localized Versus Diffuse Effects

• Parasympathetic division elicits short-lived and localized control over effectors; ACh is quickly destroyed by acetylcholinesterase.
• Sympathetic division provides longer-lasting, body-wide effects; NE is inactivated more slowly than ACh, and NE/epinephrine hormones from adrenal medulla have prolonged effects after sympathetic signals stop.

Control of Autonomic Nervous System

• Cerebral structures hypothalamus, spinal cord.
  • Cerebral input may modify input.
    • Cerebral.