Autonomic Nervous System

ANS: Sympathetic Division

"flight or fight" response for stress or emergency situations. However, most responses are not "stressful." 1. T1-L2 region of spinal cord; hence, anatomically called the thoracolumbar division. 2. Cell bodies of preganglionic motor neurons are in the lateral horns of spinal cord. 3. Pre- and postganglionic fibers synapse in one of the ganglia of the sympathetic trunk ganglia near the spinal cord, or in prevertebral ganglia in the visceral cavity. 4. Preganglionic fibers release ACh at the synapses in the ganglia, which are stimulatory. 5. Most sympathetic postganglionic fibers release norepinephrine [NE]. [Common name for NE is adrenalin]. These fibers are stimulatory and the NE is longer-lasting than ACh.

ANS: Parasympathetic Division

"resting and digesting" system 1. Nuclei associated with CN III, VII, IX and X, and sacral nerves, S2-S4; hence, anatomically called the craniosacral division. 2. Cell bodies of sacral preganglionic neurons are in the lateral horns. 3. Pre- and postganglionic fibers synapse in terminal ganglia near the target or intramural ganglia within the target tissue. 4. Like the sympathetic, all preganglionic fibers release ACh at the synapses in the ganglia, which stimulate the postganlionic fibers.. 5. Postganglionic fibers release ACh but may be stimulatory or inhibitory

Anatomy and ANS function

(Even though the visceral afferents start the visceral arc,) they are concentrated on the visceral efferent system or "motor outflow." Hence, the rest of this discussion is about the anatomy of the visceral motor neurons and how they function.

Somatic sensory

(afferent) fibers cause somatic motor (efferent) responses. -These somatic efferents belong to the "voluntary" peripheral nervous system [PNS] whose axons cause skeletal muscle to contract. -These axons branch into motor units that will contact muscle fibers at motor end plates [neuromuscular junction]. -The neurotransmitter, acetylcholine [ACh], is released at the motor end plate to cause the contraction. ACh is the same transmitter released at all somatic PNS synapses. -Cranial nerves are also part of the PNS and also have ACh at their synapses; however, the autonomic nervous system has a role in cranial nerves that you will learn about later.

Parasympathetic Division anatomy: Preganglionic

1. The preganglionic neurons associated with cranial nerves III, VII, IX and X each has its own specific nucleus adjacent to the nucleus for the somatic part of the cranial nerve. Preganglionic fibers initially travel with the somatic efferents towards the effector, then branch away to synapse in the terminal or intramural ganglia close to the effector. The relatively short postganglionic fibers leave the ganglia to reach the effector tissue. The effect is more specific and localized than the sympathetic postganglionic fibers.

Parasympathetic Division anatomy: Cranial nerves

2. Each parasympathetic part of cranial nerves [CN] III, VII, and IX each has a specific ganglion. CN X- has terminal or intramural ganglia spread throughout most of the internal viscera.

Parasympathetic Division anatomy: Sacral spinal cord

3. In the sacral spinal cord, the preganglionic fibers begin in the lateral gray horns, form splanchnic nerves before synapsing with ganglia near the target tissue.

Relationships between the two divisions : Organs or tissues

A. Organs or tissues may have dual, antagonistic innervations from both systems.

Divisions of the ANS

A. Sympathetic Division B. Parasympathetic Division

Both divisions of the ANS innervate specific tissues or visceral organs by:

A. Targeting smooth muscle, cardiac muscle, exocrine glands and the adrenal glands. B. Using TWO sequential motor neurons from the spinal cord or brain stem to the target tissue or gland. The first neuron synapses with the second neuron in a ganglion. 1. preganglionic neuron 2. postganglionic neuron

Relationships between the two divisions: Physiological effect

B. Physiological effect will depend on the length of time of innervation by the sympathetic and whether there is antagonism. Sometimes they work together, e.g.,a sexual response.

Relationships between the two divisions: ACh

C. ACh is short-term, lasting only a few seconds, NE may last 30 seconds. However, the adrenal gland, when stimulated as part of the stress response, releases large amounts of NE/epinephrine into the blood that lasts much longer than 30 seconds!

Relationships between the two divisions: hypothalamus

D. The hypothalamus is the major control/integration system for ANS. Emotional or physical stimuli from the cortex, reticular formation, limbic system and memory affect both divisions.

autonomic nervous system [ANS]

Different than the somatic sensory/motor portion of the PNS; innervates the visceral part of your body, i.e., your internal organs, the specific effectors being the exocrine glands, adrenal glands, smooth muscle and cardiac muscle. Further, the two divisions of the ANS, the sympathetic and its antagonist, and the parasympathetic. -was defined originally as "involuntary and autonomous;" however, the cerebral cortex, reticular formation, limbic system, and memory do modify its functions; in other words, we can physically and mentally override the ANS.

Anatomy of the sympathetic and parasympathetic divisions: Sympathetic Division

Four possible efferent routes from the lateral horn: They all begin with a myelinated preganglionic fiber leaving the lateral horn and entering the "white ramus communicans", which is the entrance to the sympathetic ganglia, or the prevertebral [collateral] ganglia. 1. One route 2. Another route 3. To reach thoracic organs 4. Abdominal and pelvic viscera 5. The adrenal gland

Sympathetic Division: Another route

The preganglionic fiber continues superiorly in the sympathetic trunk to one of the cervical ganglia in the neck before synapsing; then the postganglionic exits to the target, e.g., salivary glands.

Anatomy of the sympathetic and parasympathetic divisions: All spinal nerves and some cranial nerves

contain both somatic sensory and somatic motor axons but they also contain autonomic fibers. 1. Sympathetic preganglionic fibers are relatively short with a longer postganglionic fiber. 2. Parasympathetic preganglionic fibers are relatively long with a shorter postganglionic fiber.

postganglionic neuron

has its cell body located in the ganglion; its unmyelinated axon will reach the target tissue.

Sympathetic Division: The adrenal gland

has modified postganglionic neurons in its medulla [inner part] that secrete NE and epinephrine directly into the blood. Preganglionic fibers exit, without synapsing, from the sympathetic trunk to these modified postganglionic neurons in the medulla to innervate the release of NE and epinephrine

visceral reflex arc

implies that there are unconscious sensations constantly sent by visceral afferents in order for visceral motor efferents to respond and innervate the effectors that maintain the homeostatic mechanisms of the cardiovascular, respiratory, digestive, excretory and reproductive systems, and integrative mechanisms that maintain body temperature.

Ganglion

is a group of neuron cell bodies located OUTSIDE the CNS.[Exception: adrenal gland has only one motor neuron reaching it].

hypothalamus

is the "headquarters" of the ANS. It has nuclei dedicated to the sympathetic and the parasympathetic divisions. The hypothalamus and visceral receptors monitor all vital systems. Changes in blood chemistry or pressure, for example, are interpreted which iniitiates visceral efferents to target specific effectors to remedy the imbalance. For example, running up the stairs will increase heart rate followed by a change in blood pressure and respiratory rate increase [Well, at least for most of us ol' timers.]. Visceral afferents sense the homeostatic needs and send appropriate signals to the heart, smooth muscle lining of blood vessels and lung airways, and perhaps increase glucose needs for skeletal muscle.

Sympathetic Division: One route

is to synapse in one of the sympathetic trunk ganglia. The postganglionic fiber [unmyelinated] then exits through the "gray ramus communicans" to either capillaries or sweat glands.

preganglionic neuron

its cell body is in the brain stem or spinal cord; its myelinated axon will synapse with the second neuron, the postganglionic neuron, in an autonomic ganglion.

ANS: Parasympathetic

maintain the homeostatic balance of your internal environment.

Sympathetic Division: Abdominal and pelvic viscera

receive postganglionic fibers from a prevertebral ganglion. The preganglionic fibers exit directly from the white ramus communicans to become the "splanchnic nerve." These preganglionic fibers synapse in the prevertebral ganglion. The postganglionic fibers go to the organs.

ANS' visceral sensory afferents

take information into the CNS and either start a visceral reflex arc or initiate an arc that will include centers in the medulla oblongata or the hypothalamus.

Sympathetic Division: To reach thoracic organs

the preganglionic fiber synapses in a sympathetic trunk ganglion and the postganglionic fiber exits to the target, e.g., lungs. A variation on the route to the thorax if for the preganglionic fiber to synapse in the superior cervical ganglion followed by the postganglionic fiber going to the heart. [ e.g., "cardiac nerve"].