Autonomic Nervous System

*Neurotransmitter and receptor type* in ganglion of sympathetic vs parasympathetic divisions

*Sympathetic:* - Ach - Nicotinic *Parasympathetic:* - Ach - Nicotinic

*Neurotransmitter* in effector organs of sympathetic vs parasympathetic divisions

*Sympathetic:* - Norepinephrine (except sweat glands, which are ACh) *Parasympathetic:* - Acetylcholine

*Receptor types in effector organs* of sympathetic vs parasympathetic divisions

*Sympathetic:* - alpha1, alpha2, beta1, beta2 *Parasympathetic:* - Muscarinic

Innervation of sympathetic fibers that *exit trunk without synapsing*, as *splanchnic nerve* - Synapse in prevertebral ganglion - Can also ascend and descend

- Abdominopelvic vasculature and viscera

What receives most of parasympathetic outflow?

- Almost exclusively to *cranial, thoracic, abdominal, and pelvic viscera*

Innervation of sympathetic fibers that *synapse immediately* at same level in sympathetic trunk?

- Blood vasc. of skin & skeletal m. - sweat glands - erector pili mm. in their zone of supply

Innervation of sympathetic fibers that *descend trunk* to synapse at lower sympathetic trunk ganglion?

- Blood vessels and skin of lower limb - Some abdominopelvic viscera

What nerves make up the cardiac plexus?

- Cervical cardiac branch of vagus nerve (parasympathetic) - Cervical cardiac nerves (sympathetic)

Innervation of sympathetic fibers that *ascend trunk* to synapse at higher sympathetic trunk ganglion?

- Iris - Salivary glands - Lungs, heart, esophagus - Head and thoracic blood vessels

Superficial vs deep cardiac plexus

- Superficial: mostly composed of left vagus nerve, on aortic arch - Deep: Mostly composed of sympathetic cardiac nerves, anterior aspect of the trachea

Where is the post-ganglionic neuron contained in the sympathetic nervous system?

- Sympathetic chain/trunk - Prevertebral ganglia

Baroreceptors

- Vagal afferent cardiac reflex, runs with parasympathetic efferent fibers - Found in *walls of great veins* as they enter the heart: Help regulate *cardiac output* according to venous filling - Found in *walls of aortic arch, branches of aorta, carotids*: Sense *changes in blood pressure* and adjust if necessary

Chemoreceptors

- Vagal afferent cardiac reflex, runs with parasympathetic efferent fibers - Found on *external surface* of great vessels, many between ascending aorta and pulmonary trunk, associated with vagal afferents - Initiate *respiratory and cardiac reflexes in response to lowered oxygen tension* in aortic blood

3 divisions of autonomic nervous system

- parasympathetic - sympathetic - enteric

Pathway of sympathetic fibers from IMLCC to sympathetic trunk

1. IMLCC (only in T1-L2/3) 2. Ventral rootlets 3. Ventral roots 4. Spinal nerve 5. White rami communicantes 6. Sympathetic trunk 7. 4 potential routes

Why is pain from an ischemic episode of the heart referred to the left arm (among other places)?

1. Pain caused by myocardial ischemia is sensed by pain receptors of the heart 2. Signal sent via afferent visceral fibers, which run with sympathetic efferent fibers (cervical, thoracic cardiac nerves), which normally descend from T1-T4/5 3. Afferents enter T1-4/5 dorsal horn 4. Visceral and somatic afferents *converge on same neurons* 5. Pain referred to somatic area of left upper limb (dermatomes supplied by T1, T2) and upper part of thorax (dermatomes T2-T5)

What are the types of unmyelinated visceral afferent fibers that hitchhike on ANS efferent fibers (from PNS to CNS)?

1. Parasympathetic fibers: Unconscious input - *BP, HR, oxygen pressure, etc* 2. Sympathetic fibers: - *Pain (and some reflexes)*

What are the 4 pathways sympathetic fibers can take within the sympathetic trunk?

1. Synapse immediately, same level 2. Ascend trunk 3. Descend trunk 4. Exit as splanchnic nerve (without synapsing until prevertebral trunk)

Categories of sympathetic preganglionic neuron synapses

1. Synapses on postganglionic neurons within sympathetic chain 2. Passes through sympathetic chain and synapses in *prevertebral ganglia* (celiac, superior mesenteric, inferior mesenteric) - Supplies visceral organs, glands, and enteric nervous system of GI tract

How many motor neurons are in series in the ANS?

2 efferent neurons

What types of fibers are contained in autonomic plexuses?

ANS visceral efferent and afferent (sensory) fibers Examples: - Cardiac plexus - Pulmonary plexus - Esophageal plexus

Receptors for norepinephrine

Adrenoreceptors

What function does the unmyelinated visceral afferent fibers running with sympathetic efferent fibers serve?

Afferents running with sympathetic cardiac nerves (eg cervical and thoracic cardiac nerves) *conduct pain sensation* from the heart, may also be involved with *reflexes*

What function does the unmyelinated visceral afferent fibers running with parasympathetic efferent fibers serve?

Afferents running with vagal parasympathetic nerves, concerned with cardiac reflexes involving *baroreflexes and chemoreceptors* - Unconscious input

What controls the body viscera?

All viscera (glandular tissue, smooth or cardiac muscle) is controlled by the *autonomic nervous system* (specifically the hypothalamus)

Autonomic reflexes

Baroreceptors and chemoreceptors, etc: - blood pressure - oxygen pressure - smooth muscle stretch, etc

Parts of sympathetic trunk

Bilateral, paravertebral ganglia: - Cervical - Thoracic - Lumbar - Sacral

Where is the pre-ganglionic neuron contained in the parasympathetic nervous system?

Brainstem: CN III, VII, IX, X and lateral horn of sacral region (S2-S4)

Receptors for acetylcholine

Cholinoreceptors

General visceral efferent

Conducting cells that carry information from the central nervous system - motor information from the ANS

General somatic efferent

Conducting cells that carry information from the central nervous system - motor supply to tongue, extraocular muscles, majority of muscles of the human body [but not SCM or trapezius as they are SVE]

Location of parasympathetic division of ANS

Craniosacral: - Brainstem: CNIII, CNVII, CN IX, CN X - Sacral outflow: S2-S4 Examples: - Vagus nerve - Pelvic splanchnic nerve

What spinal segments do preganglionic sympathetics innervating the heart arise from?

IMLCC T1-T4/5 - Cervical sympathetic ganglia

Where is the pre-ganglionic neuron contained in sympathetic nervous system ?

IMLCC of lateral horn

What is the autonomic nervous system?

Involuntary system that controls and modulates the function primarily of viscera organs - Each pathway consists of a *preganglionic* and *postganglionic* neuron - Cell body of preganglionic neuron: in CNS - Cell body of postganglionic neuron: Autonomic ganglia outside ANS - All preganglionic neurons release *ACh* (cholinergic) - Postganglionic neurons release *ACh or norepinephrine, sometimes neuropeptides* (cholinergic or adrenergic)

What other somatic areas can receive referred pain?

Irritation of inferior surface of central tendon of diaphragm, innervated by phrenic nerve (C3-5) - Referred to skin on shoulder: Supraclavicular nerve (C3-4)

Where is the post-ganglionic neuron contained in the parasympathetic nervous system?

Near target organ

General somatic afferent

Nerves that carry sensory information from the body to the central nervous system - sensory fibers from muscles and skin

General visceral afferent

Nerves that carry sensory information from the body to the central nervous system - sensory fibers from viscera (i.e. thorax and abdominopelvic organs)

Why does referred pain occur?

Occurs when sensory information comes to the spinal cord from one location but is interpreted by the CNS (usually supraspinal levels) as coming from another location innervated by the same spinal cord level - Usually happens when *pain information comes from a region with low amount of sensory output* and *afferents converge in dorsal horn with high amount of sensory input* (eg also from skin/muscle) - Pain from normally low output region is interpreted as coming from normally high output region

What is referred pain?

Pain perceived in a region other than where the stimulus originates - Vague pain, non-localized, deep, cannot pinpoint - Usually referred to surface of body: Dermatomal representation

Where do the preganglionic fibers of the splanchnic nerves synapse?

Postganglionic cell bodies of abdominal ganglia, after they pierce diaphragm and enter abdomen

What fibers generally make up the aortic plexus?

Postganglionic sympathetic fibers - Generally, after the preganglionic sympathetic fibers synapse, the postganglionic fibers can enter their respective plexus or descend into the aortic plexus

What are splanchnic nerves?

Preganglionic sympathetic fibers that run anteriomedially and inferiorly from sympathetic trunk - Carry visceral (eg abdominal) efferent and afferent fibers of the ANS - synapse at pre vertebral ganglion - Run anteriomedially from sympathetic trunk, anterior to vertebral bodies: *unnamed thoracic splanchnic nerves*, turn into *named thoracic splanchnic nerve*, pierces diaphragm to enter abdomen

Named thoracic splanchnic nerves

Preganglionic sympathetic fibers that run anteriomedially and inferiorly from sympathetic trunk - Greater splanchnic, T5-9 - Lesser splanchnic, T10-11 - Least splanchnic, T12

What is the function of the vagus nerve?

Responsible for much of the body's parasympathetic innervation - *Thorax* - *part of GI system* Exit to abdomen (as vagal trunks) via esophageal hiatus, T10

What is contained in the lateral horn?

Spans T1-L2/3 - Intermediolateral cell column (IMLCC) - IMLCC contains: *Visceral motor neurons*: Cell bodies (origin) of *preganglionic sympathetic neurons*

Adrenal medulla synapse

Specialized sympathetic ganglion whose pre ganglion neurons originate in thoracic spinal cord (T5-T9), pass through sympathetic chain and celiac ganglion without synapsing, and travel in greater splanchnic nerve (pre vertebral synapse) to adrenal gland - Develops from *neural crest cells* - Cells secrete *norepinephrine and epinephrine*

Special somatic and visceral afferent

Strictly for cranial nerves - *Somatic:* special sensory information from the special senses of sight [retina] and vestibulocochlear apparatus - *Visceral:* sensory information from the special senses of taste and olfaction

Special visceral efferent

Strictly for cranial nerves - motor information to muscles of pharyngeal arch origin

What preganglionic sympathetic nerves innervate the heart? Where do these nerves arise from?

Superior, middle, and inferior cardiac nerves - From cervical sympathetic ganglia (IMLCC T1-T4/5)

Where does the rami communicantes attach?

There is debate on whether the rami communicants attach to the *spinal nerve or the ventral ramus,* however on dissection it appears as though the rami communicantes attach to the *ventral ramus*

Location of sympathetic division of ANS

Thoracolumbar: T1-L2/L3

Learning Objectives

Understand the general arrangement of the autonomic nervous system Compare and contrast afferent and efferent nerves Compare and contrast the sympathetic and parasympathetic nervous systems Describe the central and peripheral pathways of the sympathetic and parasympathetic preganglionic fibers and postganglionic fibers Define the sympathetic, parasympathetic, and sensory innervation of the thoracic organs Understand the general visceral afferent pathways implicated in autonomic regulation and pain Understand referred pain how it relates visceral and somatic areas

Spinal cord branches

Unidirectional information: - Dorsal root to dorsal rootlets - Ventral rootlets to ventral root

White rami communicantes vs gray rami communicantes

White rami communicantes carry presynaptic sympathetic fibers from the ventral primary rami to the sympathetic trunk and exist only between spinal cord segments T1 and L2 - *Distal, sympathetic trunk entryway* - *Myelinated* - Located from *T1-L2/3* Gray rami communicantes carry postsynaptic sympathetic fibers from the sympathetic trunk to all spinal nerves. Above T1 and below L2, the sympathetic trunk is attached to the ventral primary ramus of the spinal nerve only by gray rami communicantes. - *Proximal, sympathetic trunk exit* - *Unmyelinated* - Located *throughout sympathetic trunk*

Ganglion

collection of neuronal cell bodies outside the CNS