Autonomic Nervous System

Question 1

What are the anatomical differences between the somatic and autonomic nervous systems?

Answer 1

Sensory portion: both have only one neuron involved, a pseudounipolar neuron whose cell body is located in a DRG.

Efferent/motor portion:

Somatic nervous system has one neuron, a multipolar neuron whose cell body is located in the ventral horn gray matter of the spinal cord.

Autonomic nervous system has TWO neurons. Preganglionic neuron with cell body in the CNS and one with cell body in the periphery somewhere. Both neurons are multipolar.

For the sympathetic nervous system, the preganglionic neuron cell bodies are located in the intermediolateral cell column of T1 through L2.

For the parasympathetic nervous system, the preganglionic cell bodies are located in the brain stem or in the the intermediolateral cell column of the sacral spinal cord.

Question 2

What are the functional differences between the sensory somatic and autonomic nervous systems?

Answer 2

Somatic nervous system sensory: full range of sensations e.g. warmth/coolness, pressure, texture, proprioception, vibration, pain, and is well-localized.

Autonomic nervous system sensory: pain or distention only; poorly localized.

Question 3

What are the tissue types innervated by the somatic nervous system?

Answer 3

The somatic afferents innervate (sense) the five “somatic tissues” -

1. skin
2. skeletal muscle
3. bone
4. joints
5. dura mater.

The somatic motor neurons innervate SKELETAL MUSCLE. VOLUNTARY MOVEMENTS

Question 4

What are the tissue types innervated by the autonomic nervous system?

Answer 4

Visceral nervous system senses and controls the involuntary activities of

1. glandular epithelia
2. smooth muscle (this includes that of blood vessels)
3. cardiac muscle

Question 5

Name the 3 major divisions of the autonomic nervous system.

Answer 5

The autonomic nervous system is the EFFERENT division of the visceral nervous system, and has 3 divisions itself:

1. sympathetic
2. parasympathetic
3. enteric

Question 6

Explain how the sympathetic and parasympathetic divisions of the autonomic nervous system affect the enteric nervous system.

Answer 6

The pacemaker neurons of the enteric nervous system are the targets of the sympathetic and parasympathetic nervous system - sympathetic slows them down, parasympathetic speeds them up.

Question 7

Name the 3 regions of the CNS containing preganglionic neuron cell bodies and which type (sympathetic, parasympathetic) they contain.

Answer 7

T1 - L2: sympathetic preganglionic cell bodies

Brainstem and sacral spinal cord: parasympathetic preganglionic cell bodies

Question 8

Describe the pathways taken by PREGANGLIONIC fibers of the sympathetic division of the autonomic nervous system.

Answer 8

In general, preganglionic neurons send their axons

through the ventral root

through the white rami communicantes

into the sympathetic ganglion

Then can follow one of 4 paths:

1. form synapse within that sympathetic ganglion
2. pass through that ganglion and enter thoracic splanchnic nerves to travel to prevertebral ganglia
3. pass through that ganglion and ASCEND in the sympathetic trunk to make synaptic contact with ganglionic neurons in a more rostral sympathetic ganglion (most of the preganglionic neurons that ascend like this originate in T1-T3 and are ascending to make contact in a cervical sympathetic ganglion)
4. pass through that ganglion and DESCEND in the sympathetic trunk to make synaptic contact with ganglionic neurons in a more caudal sympathetic ganglion (most of the preganglionic neurons that descend like this originate in T12-L2 and are descending to make contact in a lumbar or sacral sympathetic ganglion)

Question 9

Describe the pathways taken by POSTGANGLIONIC fibers of the sympathetic division of the autonomic nervous system as they innervate the smooth muscle of hair follicles, blood vessels and sweat glands of trunk and limbs; smooth muscle, cardiac muscle and lungs in the thorax; smooth muscle of the abdomen; and smooth muscle of the pelvis.

Answer 9

1. Targets in the body walls and limbs: ganglionic neurons that innervate structures in the body wall and limbs send their axons along the GREY RAMI COMMUNICANTES to re-enter the spinal nerves. Then the axons follow the DORSAL OR VENTRAL RAMI of spinal nerves to reach their targets.
2. Targets in the viscera:
   Cardiopulmonary targets - synapse in a sympathetic chain ganglion and postganglionic fiber travels out the cardiopulmonary splanchnic nerve to the target

Abdominopelvic targets - preganglionic fiber travels out the abdominopelvic splanchnic nerve to synapse in a prevertebral ganglion and the postganglionic fiber travels out to the target

ganglionic neurons that innervate structures in the viscera generally send their axons along blood vessels to reach their targets. In the case of the heart and lungs, however, these post-ganglionic fibers contribute to the cardiac and pulmonary plexi (which also contain preganglionic parasympathetic fibers as well as sensory fibers).

Question 10

Name the 5 prevertebral sympathetic ganglia and describe their anatomical locations.

Answer 10

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In the abdomen, there is a second set (first is the paravertebral ganglia of the sympathetic chain) of small-to-medium sized sympathetic ganglia located
ANTERIOR to the ABDOMINAL AORTA at its junction with the large blood vessels of the abdomen.

1. celiac ganglion (celiac trunk)
2. aorticorenal ganglion (renal arteries)
3. superior mesenteric ganglion (superior mesenteric artery)
4. inferior mesenteric ganglion (inferior mesenteric artery)
5. diffuse sets of very small ganglia that make up the superior and inferior hypogastric plexi

Question 11

Describe the white & gray rami communicantes.

• -name the type of fibers that form each ramus
• -name the spinal cord levels associated with each and why
• -state which direction nerve impulses are traveling in each

Answer 11

White: myelinated nerves that connect the spinal nerves with their sympathetic ganglia.

White rami are only associated with T1 - L2 (because they carry PREGANGLIONIC axons, and this is where preganglionc nerves are found).

White rami carry nerve impulses TOWARD ganglia.

Gray: unmyelinated nerves that connect the sympathetic ganglia with spinal nerves.

Gray rami are associated with ALL of the spinal nerves! C1 - Coccyx1 (because they carry POSTGANGLIONIC axons).

Gray rami carry nerve impulses AWAY from ganglia

Question 12

Name the 3 main abdominopelvic splanchnic nerves and the spinal nerves they are associated with.

Answer 12

1. Greater splanchnic nerve (T5 - T9)
2. Lesser splanchnic nerve (T10 or T11)
3. Least splanchnic nerve (T12)

Question 13

Describe the location of parasympathetic preganglionic neurons.

Answer 13

Preganglionic neuron cell bodies are located in the BRAIN STEM and in the intermediolateral gray column of the spinal cord from S2, S3, S4.

Question 14

Describe the location of parasympathetic postganglionic neurons.

Answer 14

Postganglionic neuron cell bodies are located in hundreds of ganglia that are located within the wall of the organ being innervated (“intramural ganglia”) or nearby the wall of the organ (“terminal ganglia”).

Question 15

Describe the pathways taken by preganglionic and postganglionic parasympathetic fibers from the CNS to the effector organ.

Answer 15

For thoracic viscera and abdominal viscera up through the proximal large intestine:

Preganglionic nerve originates in brainstem -> cranial nerve X (vagus nerve) -> near target organ

For abdominal viscera from left splenic flexure onward, and pelvic viscera:

Preganglionic nerve originates in S2, S3, or S4 -> pelvic splanchnic nerves -> near target organ

For all parasympathetic targets:
Postganglionic fiber travels a VERY SHORT distance, from synapse near target organ -> actual target organ innervation site

Question 16

Describe the effects (if any) of sympathetic nervous system activation on the following structures:

Eye
Heart
Blood vessels
Bronchi
Gut
Urinary bladder
Adrenal medulla
Erectile tissues
Ductus deferens

Answer 16

Sympathetic nervous system STIMULATES:

Eye: smooth muscle contraction in iris - dilates pupil = better night vision

Heart: cardiac muscle contraction - increases rate and strength of heart beat

Blood vessels: smooth muscle contraction in blood vessels of skin and GI tract- diverts blood to skeletal muscles

Gut, urinary bladder: smooth muscle contraction in sphincters of hollow viscera - sphincters close

Adrenal medulla: secretion of epi/norepi

Sympathetic nervous system INHIBITS:

Blood vessels: smooth muscle contraction in blood vessels of skeletal muscle - increases blood flow, facilitating muscular activity

Gut, urinary bladder: smooth muscle contraction in walls of hollow viscera - reduces motility

Bronchi: smooth muscle in wall of tracheobronchial tree - dilates airway

Question 17

Explain how the sympathetic nervous system participates in body thermoregulation.

Answer 17

95% of sympathetic postganglionic neurons secret norepinephrine at their terminals. 5% of sympathetic postganglionic neurons secrete acetylcholine, and these neurons innervate sweat glands of the skin and cause stimulation of secretion by sweat glands.

Question 18

Describe the effects (if any) of parasympathetic nervous system activation on the following structures:

Eye
Heart
Blood vessels
Bronchi
Gut
Urinary bladder
Adrenal medulla
Erectile tissues
Ductus deferens

Answer 18

Parasympathetic nervous system STIMULATES:

Eye: smooth muscle contraction in iris and lens - constricts pupil and thickens lens for near vision

Bronchi: smooth muscle contraction in wall of tracheobronchial tree - constricts airway

Gut, urinary bladder walls: smooth muscle contraction - increases motility

Erectile tissues: smooth muscle contraction in genital tissues (both genders) - erection

GI tract glands: secretion

Parasympathetic nervous system INHIBITS:

Heart: cardiac muscle contraction - reduces heart rate

Anal sphincters: smooth muscle contraction - aids defecation

Urinary bladder sphincter: aids micturition