Chapter 15: The Autonomic Nervous System

Question 1

compare the structural and functional differences between the somatic and autonomic parts of the nervous system.

Answer 1

SOMATIC

somatic nervous systems

1. sensory neurons convey input from receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations) and from receptors for the special senses (sight, hearing, taste, smell, and equilibrium)
2. All these senses are normally consciously perceived
3. Then somatic motor neurons innervate skeletal muscles - the effectors of the somatic nervous system, and produce both reflexive and voluntary movements.
4. When a somatic motor neuron stimulates the muscle, it contracts. The effect is always excitatory.
5. If somatic motor neurons stop stimulating a muscle, the result is paralysis (limp muscle with no tone)
6. Respiratory muscles are voluntary, although we are usually not conscious of breathing, we just do it.
7. A few muscles, ex. middle ear, cannot be contracted voluntarily, are controlled by reflexes.

AUTONOMIC

**Preganglionic sympathetic neurons - shortest axons

autonomic sensory neurons

1. Main ANS input comes from autonomic sensory neurons (Monitor internal environment)
2. interoceptors – sensory receptors located in blood vessels and viscera that provides info about the body’s internal environment. AKA Visceroceptor. 1. Ex. Chemoreceptors that monitor blood CO2 level, mechanoreceptors that detect degree of stretch in walls of organs or blood vessels.
3. These sensory signals are not usually consciously perceived but intense activation of interoceptors may produce conscious sensations 1. Ex. Pain sensations from damaged viscera and angina pectoris from inadequate blood flow to heart.
4. Some sensations monitored by somatic sensory and special sensory neurons can also influence the ANS 1. Ex. Pain can cause dramatic changes in some autonomic activities.

autonomic motor neurons – regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, glands). Ex of autonomic motor responses: change in pupil diameter, dilation and constriction of blood vessels, changes in heart rate and force of contraction.

Somatic motor neuron – recall, the axon of a single, myelinated somatic motor neuron extends from the CNS to the skeletal muscle fibers in its motor unit.

Autonomic motor pathway – most consist of two motor neurons in series, pre and postganglionic neurons

autonomic ganglion – A cluster of cell bodies of sympathetic or parasympathetic neurons located outside the CNS. Recall a ganglion is a cluster of neuronal cell bodies in the PNS

Two groups of autonomic ganglia: sympathetic ganglia, parasympathetic ganglia

Sympathetic ganglia – sites of synapses between sympathetic pre and

postganglionic neurons

Two major types of sympathetic ganglia:

1. Sympathetic trunk ganglia – AKA vertebral chain ganglia or paravertebral ganglia ii.

• Lie in a vertical row on either side of the vertebral column and are connected to one another forming a chain on each side.
• Extend from base of skull to coccyx
• Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above the diaphragm (head, neck, shoulders, heart)
• Sympathetic trunk ganglion in the neck have specific names: superior, middle, and inferior cervical ganglia; the rest do not have special names vi. These ganglia are near the spinal cord, so most sympathetic preganglionic axons are short and most sympathetic postganglionic axons are long Prevertebral ganglia – AKA collateral ganglion (**endocrine gland is directly innervated by sympathetic preganglionic axons = adrenal gland)
• Lies anterior to the vertebral column, close to the large abdominal arteries.
• Generally, postganglionic axons of prevertebral ganglia innervate organs below the diaphragm.
• 5 major prevertebral ganglia:

1. The celiac ganglion – on either side of the celiac trunk (an artery just inferior to the diaphragm)
2. The superior mesenteric ganglion – near the beginning of the superior mesenteric artery in the upper abdomen
3. The inferior mesenteric ganglion – near the beginning of the inferior mesenteric artery in the middle of the abdomen
4. The aorticorenal ganglion and
5. The renal ganglion Near the renal artery of each kidney
6. Parasympathetic ganglia – pre and post ganglionic neurons synapse in terminal ganglia

• Terminal ganglia – AKA intramural ganglia; Cluster of cell bodies of parasympathetic postganglionic neurons either lying very close to the visceral effectors or located within the calls of the visceral effectors.
• Terminal ganglia in the head have specific names:

1. Ciliary ganglion – very small ganglion, preganglionic axons come from oculomotor (III) nerve, postganglionic axons carry nerve impulses to the ciliary muscle and sphincter muscle of the iris
2. Pterygopalatine ganglion – cluster of cell bodies of parasympathetic postganglionic neurons ending at the lacrimal and nasal glands
3. Submandibular ganglion
4. Otic ganglion Have long preganglionic axons, short postganglionic axons.

sympathetic division – of the ANS, having cell bodies of preganglionic neurons in the lateral gray columns of the thoracic segment and the first two or three lumbar segments of the spinal cord

• Primarily concerned with processes involving the expenditure of energy.
• AKA the thoracolumbar division.
• AKA Fight-or-flight division
• A single sympathetic preganglionic fiber has many axon collaterals, may synapse with 20+ postganglionic neurons This is an example of divergence, explains why many sympathetic responses affect almost entire body simultaneously.

parasympathetic division – of the ANS, having cell bodies of preganglionic neurons in nuclei in the brain stem and in the lateral gray horn of the sacral portion of the spinal cord

• Primarily concerned with activities that conserve and restore body energy
• AKA the craniosacral division.
• AKA Rest-and-digest division
• Axons of parasympathetic division pass to terminal ganglia near or within a visceral effector. The presynaptic neuron usually synapses with only 4-5 postsynaptic neurons, all of which supply a single visceral effector

Parasympathetic responses are localized to a single effector

• dual innervation – most organs of the body receive impulses from the sympathetic and parasympathetic neurons Sometimes impulses from one division of the ANS stimulate while the other division inhibits an organ’s activity Ex. Increased nerve impulses from sympathetic division increases heart rate, increased nerve impulses from parasympathetic division decreases heart rate.

Question 2

describe preganglionic and postganglionic neurons of the autonomic nervous system.

Answer 2

anatomy of autonomic motor pathways

preganglionic neurons – the first of two motor neurons in any autonomic motor pathway.

• Cell body and dendrites in the brain or spinal cord, myelinated axon ending at an autonomic ganglion where it synapses with a postganglionic neuron
• Axon exits the CNS as part of a cranial or spinal nerve
• Axon is a small-diameter, myelinated type-B fiber
• Usually terminates at an autonomic ganglion.

thoracolumbar division – AKA the sympathetic division.

a. In the sympathetic division, the preganglionic neurons have their cell bodies in the lateral horns of the gray matter in the 12 thoracic segments and first 2-3 lumbar segments of the spinal cord.

thoracolumbar outflow – the axons of sympathetic preganglionic neurons, which have their cell bodies in the lateral gray horns of the thoracic segments and first 2- 3 lumbar segments of the spinal cord.

craniosacral division – AKA parasympathetic division.

• Cell bodies of preganglionic neurons are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord.

craniosacral outflow – the axons of parasympathetic preganglionic neurons, which have their cell bodies in nuclei in the brain stem and in the lateral gray matter of the sacral portion of the spinal cord.

autonomic ganglia (no specifics on each individual ganglia) - see above

• sympathetic ganglia – of the sympathetic division of the ANS
• sympathetic trunk ganglia (or vertebral chain ganglia or paravertebral ganglia)
• prevertebral ganglia (or collateral ganglia)
• parasympathetic ganglia – of the parasympathetic ANS
• terminal ganglia (or intramural ganglia)

postganglionic neurons – the second autonomic motor neuron in an autonomic pathway. releases neurotransmitter that binds to the effector cell.

• Cell body and dendrites located in an autonomic ganglion and its unmyelinated axon ending at cardiac muscle, smooth muscle, or a gland.
• Lies entirely outside of the CNS.
• Axon is small-diameter, unmyelinated type C fiber
• Terminates in a visceral effector
• Axons of sympathetic preganglionic neurons may connect with postganglionic neurons in one of 4 ways:

1. Axon may synapse with postganglionic neurons in the ganglion it reaches first
2. Axon may ascend or descend to a higher or lower ganglion before synapsing with postganglionic neurons. The axons of incoming sympathetic preganglionic neurons that pass up or down the sympathetic trunk collectively form the sympathetic chains; the fibers on which the ganglia are strung
3. Axon may continue, without synapsing, through the sympathetic trunk ganglion to end at a prevertebral ganglion and synapse with postganglionic neurons there.
4. Axon may also pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cells of the adrenal medullae that are functionally similar to sympathetic postganglionic neurons.

autonomic plexuses – a network of sympathetic and parasympathetic axons; several examples:

• cardiac, pulmonary, celiac (LARGEST), superior and inferior mesenteric; hypogastric, renal
• Many of which lie along major arteries and named for same.

May also contain sympathetic ganglion and axons of autonomic sensory neurons

Question 3

compare the anatomical components of the sympathetic and parasympathetic divisions of the autonomic nervous system.

Answer 3

STRUCTURE OF THE SYMPATHETIC

white ramus (plural is rami)—white rami communicantes – provides the pathway from anterior ramus of spinal nerve to ganglia of sympathetic trunk.

• It is the portion of a preganglionic sympathetic axon that branches from the anterior ramus of a spinal nerve to enter the nearest sympathetic trunk ganglion.
• “white” indicates they contain myelinated axons
• Occur in thoracic and first 2-3 lumbar nerves

gray ramus (plural is rami)—gray rami communicantes – contain sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to spinal nerves

• A short nerve containing axons of sympathetic postganglionic neurons
• Cell bodies of the neurons are in a sympathetic chain ganglion, and the unmyelinated axons extend via the gray ramus to a spinal nerve and then to the periphery to supply smooth muscle in blood vessels,
• arrector pili muscles, and sweat glands.
• Outnumber the white communicantes because there is a gray ramus leading to each of the 31 pairs of spinal nerves

Horner’s syndrome – sympathetic innervation to one side of the face is lost due to an inherited mutation, an injury, or a disease

• Affects sympathetic outflow through the superior cervical ganglion
• Symptoms: occur on affected side, include ptosis (drooping of upper eyelid), miosis (constricted pupil)

STRUCTURE OF THE PARASYMPATHETIC

cell bodies of parasympathetic neurons found in nuclei in the brain stem and in the lateral gray matter of the second through fourth sacral segments of spinal cord. Axons emerge as part of a cranial nerve or as part of the anterior root of a spinal nerve.

cranial parasympathetic outflow – consists of preganglionic axons that extend

from the brain stem in four cranial nerves

• Has four pairs of ganglia and the ganglia associated with the vagus (X) nerve.
• The four pairs of cranial parasympathetic ganglia innervate structures in the head, located close to the organs they innervate.

1. Ciliary ganglia – close to each optic (II) nerve, near posterior aspect of orbit Preganglionic axons pass with the occulomotor (III) nerves to the ciliary ganglia. Postganglionic axons from the ganglia innervate smooth muscles fibers in the eyeball
2. Pterygopalatine ganglia – lateral to the sphenopalatine foramen

• Preganglionic axons from facial (VII) nerve
• Postganglionic axons to nasal mucosa, palate, pharynx, lacrimal glands

1. Submandibular ganglia – near ducts of submandibular salivary glands

• Receive preganglionic axons from facial nerves
• Postganglionic axons to submandibular and sublingual salivary glands

1. Otic ganglia – just inferior to each foramen ovale

• Receive preganglionic axons from the glossopharyngeal (IX) nerves
• Send postganglionic axons to parotid salivary glands
• Preganglionic axons of the vagus (X) nerve carry nearly 80% of all craniosacral outflow. Vagal axons extend to many terminal ganglia in the thorax and and: heart, lung airways, liver, gall bladder, stomach, pancreas, small intestine, part of large intestine

sacral parasympathetic outflow – consists of preganglionic axons in anterior roots of S2-S4 sacral spinal nerves.

• The preganglionic axons of both cranial and sacral outflows end in terminal ganglia, where they synapse with postganglionic neurons

pelvic sacral (splanchnic) nerves – branches off preganglionic axons as they course through the sacral spinal nerves.

• Synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera.
• From the terminal ganglia, parasympathetic postganglionic axons innervate smooth muscle and glands in the walls of the colon, uterus, bladder, and reproductive organs.
• C. enteric division - of the GI tract – the specialized collection of nerves and ganglia forming a complex, integrated neuronal network within the wall of the GI tract, pancreas, and gallbladder.
• Contains roughly 100 million neurons
• Contains sensory neurons capable of monitoring tension in the intestinal wall and contents of intestines.

1. These sensory neurons send input to interneurons within enteric ganglia.
2. Interneurons establish a network that processes incoming signals and generates regulatory output signals to motor neurons throughout plexuses within digestive organ walls.
3. Motor neurons carry the output signals to the smooth muscle and glands of the GI tract to exert control over its motility and secretory activities.

• Can and does function independently of CNS but can receive controlling input from CNS.
• The surface of the GI tract is considered the external environment, it responds to surrounding stimuli to generate proper homeostatic controls

myenteric plexus (AKA Plexus of Auerback) – largest plexus, positioned between the outer longitudinal and circular muscle layers from upper esophagus to anus. Communicates extensively with the smaller submucosal plexus

submucosal plexus (AKA Plexus of Meissner) – occupies the gut wall between the circular muscle layer and the muscularis mucosae, runs from stomach to anus.

Question 4

describe the neurotransmitters and receptors involved in autonomic responses.

Answer 4

ANS neurotransmitters and receptors

cholinergic neurons - a neuron that liberates ACh as its neurotransmitter

• acetylcholine (ACh) - neurotransmitter released by many PNS and some
• CNS neurons.
• Acetylcholine is released by parasympathetic postganglionic neurons and is removed from the synaptic cleft at a faster rate than norepinephrine
• • .Excitatory at NMJs but inhibitory at some other synapses.
• In the ANS, cholinergic neurons include: 1) all sympathetic and parasympathetic preganglionic neurons, 2) sympathetic postganglionic neurons that innervate most sweat glands and 3) all parasympathetic postganglionic neurons

cholinergic receptors - integral membrane proteins in the postsynaptic plasma membrane. 2 types:

1. nicotinic receptors – named because Nicotine activates these receptors but does not activate muscarinic receptors for ACh

• Found on both sympathetic and parasympathetic postganglionic neurons, on skeletal muscle in the motor end plate, and chromaffin cells of the adrenal medullae
• Activation by ACh causes depolarization and therefore excitation

1. muscarinic receptors – named because muscarine (mushroom poison) activates these but does not activate nicotinic receptors

• Found on all effectors innervated by parasympathetic postganglionic axons, sweat glands innervated by cholinergic sympathetic postganglionic axons.
• Activation by ACh may be excitatory or inhibitory

1. acetylcholinesterase (AChE) - quickly deactivates Ach Therefore, cholinergic neuron effects are brief

adrenergic neurons - a neuron that releases epinephrine (adrenaline) or norepinephrine (noradrenaline) as its neurotransmitter

• In the ANS, they release norephinephrine

norepinephrine (NE) AKA noradrenalin – stored in synaptic vesicles, released by exocytosis.

• Molecules diffuse across the synaptic cleft, bind to specific
• adrenergic receptors on the postsynaptic membrane
• Cause either excitation or inhibition of effector cell
• Can be released as a neurotransmitter by sympathetic postganglionic neurons or released as a hormone into blood by chromaffin cells of the adrenal medullae
• (Epinephrine is released as a hormone)

adrenergic receptors - bind both norepinephrine and epinephrine.

a. alpha receptors and beta receptors – two main types of adrenergic receptors

1. Found on visceral effectors innervated by most sympathetic postganglionic axons
2. Classified into subtypes, based on responses they elicit and by selective binding of drugs that activate/block them.

receptor agonists – a substance that binds to and activates a receptor

• In the process, mimics the effect of a natural neurotransmitter or hormone
• Ex: phenylephrine – adrenergic agonist at alpha1 receptors. Common ingredient in cold and sinus meds. Constricts blood vessels in nasal mucosa, decreasing mucus production, relieving nasal congestion
  
  • Prevents a natural neurotransmitter or hormone from exerting its effect.

receptor antagonists – a substance that binds to and blocks a receptor

• Ex. Atropine – blocks muscarinic ACh receptors, dilates pupils, reduces glandular secretions, relaxes smooth muscle in GI tract. Is used during eye exams, to treat smooth muscle disorders, and as an antidote for chemical warfare agents that inactivate AChE.
• Another example: selective and non selective beta-blockers. 1. Ex. Propanolol – non selective 2. Metoprolol – selective to beta 1

Question 5

describe the major responses of the body to stimulation by the sympathetic and parasympathetic divisions of the ANS

Answer 5

physiological effects of the ANS

1. autonomic tone – balance between sympathetic and parasympathetic activity
   * Regulated by the hypothalamus
2. The two divisions of ANS work in opposition to one another. Typically, hypothalamus turns up one tone at same time as it turns down the other tone.
3. The two ANS divisions can affect body organs differently because their postganglionic neurons release different neurotransmitters and because the effector organs possess different adrenergic and cholinergic receptors.
4. few structures receive only sympathetic innervation: sweat glands, arrector pili muscles, kidneys, spleen, most blood vessels, and adrenal medullae. Therefore in these structures, no opposition from the parasympathetic division But still, an increase in sympathetic tone has one effect, and decrease in sympathetic tone has opposite effect
5. sympathetic responses - sympathetic division dominates during physical or emotional stress
   b) Stimulates sweat glands
   c) Synapses with smooth muscle in blood vessel walls
   d) Short preganglionic neurons
   e) Thoracolumbar output

• High sympathetic tone favors body functions that can support vigorous physical activity and rapid ATP production.
• At the same time, sympathetic division reduces body functions that favor energy storage.
• Effects of sympathetic stimulation are longer lasting and more widespread than parasympathetic stimulation for 3 reasons:

1. Sympathetic postganglionic axons diverge more extensively
2. AChE quickly inactivates ACh but norepinephrine lingers in the synaptic cleft for a longer time
3. Epinephrine and norepinephrine secreted into the blood from the adrenal medullae intensify and prolong the responses cause by NE liberated from sympathetic postganglionic axons. These blood-borne hormones circulate throughout the body, affecting all tissues that have alpha and beta receptors. In time, NE and epinephrine are destroyed by enzymes in the liver.

Various responses stimulate the sympathetic division: physical

exertion, fear, embarrassment, rage i. Think “E situations” - embarrassment, exercise, emergency, excitement = sympathetic responses

fight-or-flight response – the effects produced upon stimulation of the sympathetic division of the ANS

1. Pupils dilate
2. Heart rate, force of contraction, and BP increase
3. Airways dilate = faster movement of air in and out of lungs
4. Kidney and GI tract blood vessels constrict = decreases blood flow through these tissues, slowing urine formation and digestive activities, not essential during exercise
5. Skeletal, cardiac, liver, and adipose tissue blood vessels dilate = allow greater blood flow through these tissues
6. Release of glucose by liver = increases blood glucose level
7. Non-essential processes inhibited, Ex GI tract muscular activity

3. parasympathetic responses

a) Long preganglionic neurons
c) Vagus nerve output
d) Ganglia found near visceral effectors
e) Sacral spinal cord output

• rest-and-digest - body functions that conserve and restore body energy during times of rest and recovery.
• Between periods of exercise, parasympathetic impulses to digestive glands and smooth muscle of the GI tract predominate over sympathetic impulses, allowing food to be digested and absorbed.

Parasympathetic responses also reduce body functions that support physical activity.

Acronym SLUDD to remember the 5 parasympathetic responses:

• S – Salivation
• L – Lacrimation
• U – Urination
• D – Digestion
• D – Defecation
  
  1. Heartrate

3 important parasympathetic decreases:

1. Airway diameter (bronchoconstriction) - airway constriction
2. Pupil diameter (constriction)

Question 6

describe the components of an autonomic (visceral) reflex.

Answer 6

autonomic reflexes – occur when nerve impulses pass through an autonomic reflex arc. Key role in regulating controlled conditions by changing BP and adjusting motility and muscle tone of GI tract, and opening and closing of sphincters (urination, defecation)

• Receptor - the distal end of a sensory neuron, which responds to a stimulus and produces a change that will ultimately trigger nerve impulses. Autonomic sensory receptors are most closely associated with interoceptors
• sensory neuron - conducts nerve impulses from receptors to the CNS
• integrating center - interneurons within the CNS relay signals from sensory neurons to motor neurons. The main integrating centers for most autonomic reflexes are located in the hypothalamus and brain stem. Some autonomic reflexes, such as those for elimination have integrating centers in the spinal cord.
• motor neuron - nerve impulses triggered by the integrating center propagate out of the CNS along motor neurons to an effector. In an autonomic reflex arc, two motor neurons connect the CNS to an effector: the preganglionic neuron conducts motor impulses from the CNS to an autonomic ganglion, and the postganglionic neuron conducts motor impulses from an autonomic ganglion to an effector.
• Effector - in autonomic reflex arc, effectors are smooth muscle, cardiac muscle, and glands. And the reflex is called an autonomic reflex.

autonomic control by higher centers

• Not generally aware of muscular contractions of digestive organs or heart, or changes in blood vessel diameter. These integrating centers are in the spinal cord of the lower regions of the brain.
• The hypothalamus is the major control and integration center of the ANS.

1. It receives sensory input related to visceral functions, olfaction, and gustation, as well as changes in temp, osmolarity, and levels of substances in blood. Also receives input relating to emotions from the
2. limbic system.
3. Output from the hypothalamus influences autonomic centers in both the brain stem (such as the cardiovascular, salivation, swallowing, and vomiting centers) and the spinal cord (such as elimination reflex centers in the sacral spinal cord)
4. The hypothalamus is connected to both the sympathetic and parasympathetic divisions of the ANS by axons of neurons withdendrites and cell bodies in various hypothalamic nuclei (cluster of cellbodies within the brain/spinal cord)
5. The axons form tracts from the hypothalamus to parasympathetic and sympathetic nuclei in the brain stem and spinal cord thorugh relays in the reticular formation.
6. The posterior and lateral parts of the hypothalamus control the sympathetic division. Stimulation of these areas produces an increase in heart rate and force of contraction, rise in BP due to blood vessel constriction, increase body temp, dilation of pupils, inhibition of GI tract.
7. Anterior and medial parts of the hypothalamus control the parasympathetic division. Stimulation of these areas results in decrease heart rate, lower BP, pupil constriction, increased secretion and motility of GI tract.