Ch15: The Autonomic Nervous System

Structure of the Parasympathetic Division

-Cell bodies of parasympathetic preganglionic neurons=in nuclei in brain stem + in lateral gray matter of S2-4 of spinal cord. Axons emerge as part of a cranial nerve or part of anterior root of spinal nerve, end in terminal ganglia+ synapse w postganglionic neurons. Cranial outflow has 4 pairs of ganglia + the ganglia associated w vagus (X) nerve. They innervate structures in the head += close to the organs they innervate. -Preganglionic axons leaving brain as part of vagus (X) nerves carry 80% craniosacral outflow. Vagal axons extend to many terminal ganglia in thorax + abdomen. As vagus nerve passes through thorax, it sends axons to heart + lungs

ways that pre+post ganglionic neurons synapse

-Once axons of sympathetic preganglionic neurons pass to sympatheic trunk ganglia, axon: 1. By synapses w postganglionic neurons in the ganglion it first reaches 2. Ascend/descend to a higher or lower ganglion before synapsing w postganglionic neurons. Axons of incoming sympathetic preganglionic neurons passing up/down sympathetic trunk together= sympathetic chains (fibers where ganglia= strung) 3. Pass w/o out synapsing, through the sympathetic trunk ganglion to end at a prevertebral ganglion + synapse there. 4. Pass w/o synapsing through sympathetic trunk ganglion + a prevertebral ganglion + extend to chromaffin cells of adrenal medullae (similar to sympathetic postganglionic neurons)

Autonomic motor neurons

-Regulate visceral activities by increasing (Exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac/smooth muscle + glands). Examples of autonomic motor responses: pupil dilation, dilation + constriction of blood vessels+ adjustment of rate + force of heartbeat. Unlike skeletal muscle, tissues innervated by ANS fxn to some extent when nerve supply = damaged. -Most autonomic responses cannot be consciously altered greatly. Signals from somatic + special senses (via limbic system) influence responses of autonomic motor neurons.

Comparison of Somatic + Autonomic Motor Neurons

-SNS: axon extends from CNS to skeletal muscle fiber in its motor neuron. -ANS: most autonomic motor pathways consist of 2 motor neurons in series. 1)(preganglionic neuron) has cell body in CNS + its myelinated axon extends from CNS to autonomic ganglion. 2) cell body of postganglionic neuron = in same autonomic ganglion; unmyelinated axon extends from the ganglion to effector (Smooth/cardiac muscle or gland). All somatic motor neurons release ACh as neurotransmitter, but autonomic motor neurons release ACh or NE -2 ANS output: sympathetic+ parasympathetic. Most organs have dual innervation. Others: APs from 1 division of the ANS stimulate the organ to inc its excitation + impulses from the other division dec the organ's activity (inhibition)

Comparison of SNS and ANS

-Somatic Nervous System (SNS): sensory+ motor neurons. Sensory neurons convey input from receptors for somatic senses+special senses. All these sensations= conscious. Neurons innervate skeletal muscles+ produce reflexive + voluntary movements. When a somatic motor neuron stimulates, the effect is ALWAYS excitation if no stimulation, result = paralyzed, limp muscle w/ no muscle tone. -Autonomic Nervous System (ANS): autonomic (visceral) sensory neurons, associated w interoceptors (sensory receptors in blood vessels, visceral organs, muscles + the nervous system, monitoring internal environment). These sensory signals aren't consciously perceived. Input that influences ANS also includes some sensations monitored by somatic sensory + special sensory neurons.

Autonomic Control by Higher Centers

-Usually unconscious. -Hypothalamus= major control+ integration center of ANS. Receives input of visceral fxns, olfaction+ gustation, temp, osmolarity, levels of substances in blood+ input relating to emotions from limbic sys. Output influences autonomic centers in brain stem + spinal cord. -Posterior+ lateral parts of hypothalamus control sympathetic division. Stimulation inc heart rate+ force of contraction, rise in BP b/c constriction of blood vessels, inc in body temp, dilation of pupils+ inhibition of GI tract. -Anterior+ medial hypothalamus control parasympathetic division. Stimulation dec heart rate, lowers BP, constricts pupils+ inc secretion+ motility of GI tract.

ANS neurotransmitters and receptors

-cholinergic neurons -acetylcholine (ACh) -cholinergic receptors -nicotinic receptors -muscarinic receptors -acetylcholinesterase (AChE) -adrenergic neurons -norepinephrine (NE) or noradrenalin -adrenergic receptors -alpha receptors and beta receptors -receptor agonists and receptor antagonists

autonomic ganglia

2 groups of autonomic ganglia: sympathetic (components of sympathetic division of ANS) + parasympathetic ganglia (components of parasympathetic division of ANS)

prevertebral ganglia (or collateral ganglia)

Anterior to vertebral column + close to large abdominal arteries. Innervate organs below the diaphragm. 5 major prevertebral ganglia: (1) celiac ganglion is on either side of the celiac trunk, an artery that is inferior to diaphragm. (2) superior mesenteric ganglion is near beginning of superior mesenteric artery in upper abdomen. (3) inferior mesenteric ganglion@ beginning of the inferior mesenteric artery in mid-abdomen. (4) aorticorenal ganglion + (5) renal ganglion are near the renal artery of each kidney.

thoracolumbar outflow

Axons of the sympathetic preganglionic neurons -thoracolumbar outflow.

AChE

Because ACh is quickly inactivated by AChE, effects triggered by cholinergic neurons are brief.

Adrenergic Receptors

Bind both NE and epinephrine. 2 types: alpha + beta receptors, on visceral effectors innervated by sympathetic postganglionic axons. Subtypes: alpha1, alpha2, beta1, beta2+ beta3= based on responses+ selective binding drugs that activate /block them. W exceptions, activation of alpha1 +beta1 receptors produce excitation+ activation of alpha2 +beta2 receptors causes inhibition of effector tissues. beta3 receptors= on cells of brown adipose tissue, where activation causes thermogenesis. Effectors cells have alpha or beta receptors; visceral effector cells have both. -NE stimulates alpha receptors more strongly than beta receptors; epinephrine is a potent stimulator of both alpha and beta receptors.

Sympathetic Responses

During physical or emotional stress, sympathetic division dominates parasympathetic division. High sympathetic tone favors body functions that can support vigorous physical activity + rapid production of ATP. Sympathetic division reducess functions that store E. Activation of sympathetic division + release of hormones by adrenal medullae start physiological responses: Fight-or-flight response. -Effects= longer lasting+ more widespread than parasympathetic stimulation for 3 reasons: 1) Sympathetic postganglionic axons diverge more 2) AChE quickly inactivates ACh, but NE lingers in synaptic cleft longer 3) Epinephrine + NE secreted into blood from adrenal medullae intensify + prolong responses caused by NE liberated from sympathetic postganglionic axons, until destroyed by enzymes in liver.

Anatomical Components of ANS

Each division of the ANS has 2 motor neurons: preganglionic neuron + post ganglionic neuron. Preganglionic neurons convey nerve impulses from the CNS to autonomic ganglia + postganglionic neurons relay the impulses from autonomic ganglia to visceral effectors.

thoracolumbar division

In sympathetic (thoracolumbar) division : cell bodies in lateral horns of gray matter in theT1-12 + L1-2 of the spinal cord.

Adrenergic Neurons

In the ANS, adrenergic neurons release NE (aka noradrenalin). Most sympathetic postganglionic neurons are adrenergic. Like ACh, NE is stored in synaptic vesicles + released by exocytosis. Molecules of NE diffuse across the synaptic cleft + bind to specific adrenergic receptors on the postsynaptic membrane, causing excitation or inhibition of the effector cell. -NE= released as a neurotransmitter by sympathetic postganglionic neurons or as hormone into blood by chromaffin cells of adrenal medullae; epinephrine= as a hormone. -Activity of NE at synapse ends when NE is taken up by the axon that released it or enzymatically inactivated by COMT or MAO. Compared to ACh, NE lingers in the synaptic cleft for a longer time.

Cholinergic Receptors

Integral membrane proteins in the postsynaptic plasma membrane. 2 types (both bind ACh): nicotinic receptors + muscarinic receptors. -Nicotine doesn't activate muscarinic receptors+ muscarine doesn't activate nicotinic receptors, but ACh activates both types of cholinergic receptors.

sympathetic trunk ganglia (or vertebral chain ganglia or paravertebral ganglia)

Lie in a vertical row on either side of the vertebral column. Innervate organs above diaphragm. Those in the neck have names (superior, middle, + inferior cervical ganglia), the others don't. B/c the sympathetic trunk ganglia = near spinal cord, most sympathetic preganglionic axons= short + sympathetic postganglionic axons = long. 3 cervical, 11-12 thoracic, 4/5 lumbar, 4/5 sacral + 1 coccygeal.

Autonomic Tone

Most organs receive innervation from both divisions of the ANS, working in opposition. Balance btwn sympathetic + parasympathetic activity (autonomic tone)= regulated by hypothalamus. Typically, it turns up sympathetic tone at the same time it turns down parasympathetic tone+ vice versa. Divisions affect organs differently b/c postganglionic neurons release diff neurotransmitters+ effector organs have diff adrenergic + cholinergic receptors. A few structures receive only sympathetic innervation—sweat glands, arrector pili muscles on hair follicles, kidneys, spleen, most blood vessels+ adrenal medullae. In these structures there is no opposition from the parasympathetic division. Still, an inc in sympathetic tone has 1 effect+ a dec in sympathetic tone produces the opposite effect.

post ganglionic neuron

Part of the Autonomic Nervous system. Cell bodies + dendrites= in an autonomic ganglion, where it synapses w preganglionic axons. The axon of a postganglionic neuron is a small-diameter, unmyelinated type C fiber that terminates in a visceral effector.

preganglionic neuron

Part of the Autonomic Nervous system. Cell body in brain or spinal cord, its axon exits the CNS as part of a cranial or spinal nerve. Axon =small-diameter, myelinated type B fiber extending to an autonomic ganglion, where it synapses w a post ganglionic neuron. -In sympathetic (thoracolumbar) division : cell bodies in lateral horns of gray matter in theT1-12 + L1-2 of the spinal cord. -In parasympathetic divisions: cell bodies in the nuclei of 4 cranial nerves in brain stem (III, VII, IX + X) + lateral gray matter of S2-4 of spinal cord. -1 sympathetic preganglionic fiber has many axon collaterals(branches) + may synapse w 20+ postganglionic neurons. -Axons of preganglionic neurons of parasympathetic division pass to terminal ganglia in/near visceral effector. In ganglion, presynaptic neuron synapses w 4 or 5 postsynaptic neurons, all supplying 1 visceral effector (localized to 1 effector)

Structure of the Sympathetic Division

Preganglionic axons leave spinal cord w somatic motor neurons through intervertebral foramina. Then the myelinated preganglionic sympathetic axons pass into anterior root of a spinal nerve + enter a short pathway (white ramus) before passing to nearest sympathetic trunk ganglion on same side. Axons leave sympathetic trunk in 4 ways: (1) Enter spinal nerves, (2) form cephalic periarterial nerves, (3) form sympathetic nerves (provide sympathetic innervation to heart + lungs) (4) form splanchnic nerves (to abdominopelvic organs, or adrenal medulla)

parasympathetic ganglia

Preganglionic axons of the parasympathetic division synapse with postganglionic neurons in terminal (intramural) ganglia. Close to or in the wall of a visceral organ.

Cholinergic Neurons

Release neurotransmitter ACh. In ANS: 1) all sympathetic+ parasympathetic preganglionic neurons 2) sympathetic postganglionic neurons innervating sweat glands+ 3)all parasympathetic postganglionic neurons. -ACh is stored in sympathetic vesicles+ released by exocytosis. It diffuses across synaptic cleft + binds w specific cholinergic receptors.

Autonomic Reflexes

Responses occuring when nerve APs pass through autonomic reflex arc. Regulate controlled conditions in body, eg blood pressure, by adjusting <3rate, force of ventricular contraction+ blood vessel diameter; digestion, by adjusting the motility+ muscle tone of GI tract+ defecation + urination, by regulating opening +closing of sphincters.

Parasympathetic Response

Rest-and-digest. Support body functions that conserve + restore body energy during times of rest and recovery. - Acronym SLUDD: salivation (S), lacrimation (L), urination (U), digestion (D)+ defecation (D). -Other impt parasympathetic responses = "3 decreases": dec heart rate, dec diameter of airways (bronchoconstriction), +dec diameter (constriction) of pupils.

sympathetic ganglia

Sites of synapses btwn sympathetic pre+ postganglionic neurons. 2 types of sympathetic ganglia: sympathetic trunk ganglia and prevertebral ganglia.

PNS Divisions

Somatic Nervous system, Autonomic Nervous systems (sympathetic + parasympathetic division) + Enteric Nervous system

Receptor Agonists

Substance that binds to + activates a receptor, mimicking effect of a natural neurotransmitter or hormone. Phenylephrine, an adrenergic agonist at alpha1 receptors= cold medications. B/c it constricts blood vessels in nasal mucosa, phenylephrine dec production of mucus.

Receptor Antagonists

Substance that binds to+ blocks a receptor, preventing a natural neurotransmitter or hormone from exerting its effect. EG, atropine blocks muscarinic ACh receptors, dilates pupils, reduces glandular secretions+ relaxes smooth muscle in the GI tract, so used to dilate pupils during eye examinations, in treating smooth muscle disorders + as an antidote for chemical warfare agents that inactivate AChE.

Autonomic Plexuses

Tangled networks of sympathetic ganglia +axons of autonomic sensory neurons. -Plexuses in thorax: cardiac plexus, (heart)+ pulmonary plexus (bronchial tree). -Abdomen/pelvis: Celiac (solar) plexus= largest, Superior mesenteric plexus, Inferior mesenteric plexus, hypogastric plexus, renal plexus

terminal ganglia (or intramural ganglia)

Terminal ganglia in head have names: ciliary, pterygopalatine, submandibular + otic ganglion, the others don't. Close to or in the wall of the visceral organ. Parasympathetic preganglionic axons= long, in contrast w the short parasympathetic postganglionic axons.

craniosacral outflow

The axons of the parasympathetic preganglionic neurons

Nicotinic receptors

Type of Cholinergic Receptor. In plasma membrane of dendrites+ cell bodies of sympathetic + parasympathetic postganglionic neurons, plasma membranes of chromaffin cells of the adrenal medullae+ motor end plate at NMJ. -Activation of nicotinic receptors by ACh causes depolarization + thus excitation of postsynaptic cell, which can be a postganglionic neuron, an autonomic effector, or a skeletal muscle fiber. -Nicotine doesn't activate muscarinic receptors+ muscarine doesn't activate nicotinic receptors, but ACh activates both types of cholinergic receptors.

muscarinic receptors

Type of Cholinergic receptors. In plasma membranes of all effectors innervated by parasympathetic postganglionic axons. Most sweat glands receive their innervation from cholinergic sympathetic postganglionic neurons+ possess muscarinic receptors. -Nicotine doesn't activate muscarinic receptors+ muscarine doesn't activate nicotinic receptors+ ACh activates both types. -Activation of muscarinic receptors by ACh sometimes causes hyperpolarization (inhibition), depending on which particular cell bears the muscarinic receptors. E.g. binding of ACh to muscarinic receptors inhibits (relaxes) smooth muscle sphincters in the GI tract. By contrast, ACh excites muscarinic receptors in smooth muscle fibers in the circular muscles of the iris of the eye, causing them to contract.

Autonomic Nervous System

operates via reflex arcs. Structurally: autonomic sensory neurons, integrating centers in the CNS, autonomic motor neurons + the enteric division. Continual flow of nerve APs from autonomic sensory neurons in visceral organs + blood vessels propagate into integrating centers in CNS. APs in autonomic motor neurons propagate to effector tissues. Enteric division= network of nerves + ganglia forming an independent nerve network in the GI tract. ANS operates w/o conscious control, but centers in hypothalamus + brain stem regulate ANS reflexes.

dual innervation

receive impulses from sympathetic + parasympathetic neurons (both = part of the Autonomic Nervous System)

craniosacral division

the parasympathetic division

Components of an Autonomic Arc Reflex

•Receptor: sensory neuron distal end responds to a stimulus+ produces change to trigger nerve APs. Associated w interoceptors. •Sensory neuron: Conducts APs from receptors to CNS. •Integrating center: Interneurons in CNS relay signals from sensory - motor neurons. Main= in hypothalamus+ brain stem. Some in spinal cord •Motor neurons: APs leave CNS along 2 motor neurons to effector •Effector: smooth, cardiac muscle+ glands

fight-or-flight response

•The pupils of the eyes dilate. • Heart rate, force of heart contraction, and blood pressure increase. •The airways dilate, allowing faster movement of air into and out of the lungs. •The blood vessels that supply the kidneys and gastrointestinal tract constrict, which decreases blood flow through these tissues, slowing urine formation+ digestive activities. •Blood vessels that supply organs involved in exercise/fighting off danger dilate •Liver cells: glycogenolysis (break glycogen to glucose)+ adipose tissue cells -> lipolysis. •Liver releases glucose+ inc blood glucose level. •Processes not essential for meeting stressful situation= inhibited