Answers, BIO 2310, Autonomic Nervous System


1.Somatic efferent involves one motor neuron and travels to skeletal muscle. Visceral efferent (a.k.a. ANS) involves a preganglionic and postganglionic neuron and travels to smooth muscle, cardiac muscle and glands. Somatic is voluntary, ANS is involuntary.

2.Preganglionic neuron travels from CNS to ganglion and is myelinated. Postganglionic neuron travels from ganglion to effector (smooth muscle, cardiac muscle, glands) and is gray.

3.These are sympathetic ganglia and the Celiac ganglion sends postganglionic fibers to upper abdominal structures such as stomach & liver; the Superior Mesenteric ganglion sends postganglionic fibers to middle abdominal structures such as the small intestine; the Inferior Mesenteric ganglion sends postganglionic fibers to lower abdominal/pelvic structures such as large intestine.

4.In Sympathetic Division, the preganglionic neuron travels with the spinal nerve. After leaving the vertebra, it branches away from the spinal nerve and this is the white ramus communicans. At this point it can synapse at the same level and the postganglionic neuron branch that REJOINS the spinal nerve is the gray ramus communicans. Don’t forget that the preganglionic neuron can also travel up or down the sympathetic trunk and have a synapse at any level with the postganglionic neuron.

5.Pertaining to viscera.

6.Sympathetic division is for fight or flight responses. Parasympathetic division is for rest and relaxation responses.

7.The preganglionic neuron exits the spinal cord between T1-L2. Preganglionic neurons tend to be quite short & postganglionic neurons are long. The sympathetic trunk is a chain of interconnected ganglia so that the sympathetic response is widespread and shows divergence. It allows the stimulation that may have begun at L2 to have an effect at the level of your head.

8.The preganglionic neurons exit the spinal cord in the S2-S4 region as well as from the brainstem along Cr. nerves III, VII, IX and X. There is no interconnection between these spinal cord segments and the individual cranial nerve segments. Isolated responses are the norm for parasympathetic. The preganglionic neurons tend to be long and the postganglionic neurons are quite short.

9.Adrenal medulla, which is part of the Sympathetic Division

10.See above. Sympathetic has much more divergence. It is rare to have and isolated sympathetic response such as increased heart rate without other sympathetic responses. However, it is common to have isolated parasympathetic responses such as only a decreased heart rate.

11.Cholinergic fibers release acetylcholine as the neurotransmitter for the synapse. Both the preganglionic and postganglionic neurons are cholinergic in the PNS. Only the preganglionic neuron is cholinergic in the SNS. The postganglionic neuron in SNS releases norepinephrine and is therefore adrenergic.

12.Two types of receptors receiving acetylcholine(ACH). Muscarinic respond to a drug called muscarine and are found on the receptors on the effectors in PNS. For example, a drug that would stimulate muscarinic receptors would encourage responses in the effectors in PNS (responses such as slowing of heart rate, constricting of pupil, increasing salivation). Nicotinic receptors also respond to ACH and nicotine as well. They are found on the postganglionic neurons receiving the ACH from the preganglionic neurons in SNS & PNS.


Two basic types of receptors exist for norepinephrine. Alpha receptors (1 & 2, 1s are most common) Stimulation of these receptors works with both norepinephrine and epinephrine (which is released from the adrenal medulla during a fight or flight situation) and the response is excitatory. Example: contraction of the smooth muscle in the wall of a cutaneous blood vessel.


The Beta receptors are receptors for ephinephrine primarily but also for norepinephrine too. The beta 1 receptors are found in the heart and are stimulatory causing increased rate and force of contraction when stimulated. The beta 2 receptors tend to be inhibitory. If these are activated, there tends to be a relaxation response such as the relaxation of the smooth muscle surrounding an airway during fight or flight. This response would dilate the airway.


14. Atropine is antimuscarinic and thus blocks the PNS

15.Heart: SNS increases rate and force of contraction, PNS opposite

Airways & breathing rate: SNS dilates airways and increases resp. rate, PNS opposite.

Digestion: SNS inhibits digestive secretions (including saliva) and movement, PNS opposite – You salivate from PNS stimulation

Pupil: SNS dilates pupil, PNS constricts pupil

Sweat glands: SNS causes sweat, PNS doesn’t go to skin structures

Adrenal medulla: SNS causes release of epinephrine which exaggerates and prolongs the fight or flight response, PNS doesn’t go there

Cutaneous & Abdominal Blood vessels: SNS causes constriction and PNS doesn’t go there.

Skeletal muscle blood vessels: SNS causes dilation; PNS doesn’t go there.

Urinary Bladder: SNS causes contraction of sphincter (makes you feel like you have to urinate) and relaxation of bladder wall. PNS causes relaxation of sphincter (when you are voluntarily urinating this happens) and contraction of bladder wall.