Preganglionic and postganglionic fibers in the parasympathetic division secrete

Most-specifically, the term Autonomic Nervous System (ANS) refers to efferent two-neuron pathways that innervate smooth muscle, cardiac muscle & glands. Most-generally, ANS includes both afferent & efferent visceral innervation, as well as the Central Nervous System (CNS) structures that control visceral innervation. Visceral organs vary in their dependence on CNS neural control (e.g., enteric function is least dependent whereas micturition is highly dependent on the CNS). For a presentation of CNS components associated with visceral innervation click here: CNS Control.

Neurotransmitters & Receptors

Autonomic preganglionic neurons synapse on postganglionic neurons. The preganglionic neurons release acetycholine (ACh) which binds to nicotinic receptors (nAChR) on postganglionic neurons. The nicotinic receptors are cation channels; when open, they enable depolarization (excitation) of postganglionic neurons.

Autonomic postganglionic neurons release either acetycholine (ACh) or nor-epinephrine (NE), the former neurons are designated cholinergic, the latter adrenergic. All parasympathetic postganglionic neurons are cholinergic. The great majority of sympathetic postganglionic neurons are adrenergic (except that neurons innervating sweat glands, arrector pilli muscles, and some vessels are cholinergic). Cells of the adrenal medulla, which are modified postganglionic neurons, release mainly epinephrine (adrenalin).

The two major categories of adrenergic receptors (AR) are designated alpha and beta. Each has sub types: Receptor α1 involves a Gq protein that elevates C++, resulting in smooth muscle contraction. Receptor α2 involves a Gi protein that decreases cAMP and C++ influx which inhibits neurotransmission. Receptors β1, β2 & β3 involve a Gs protein and cAMP elevation that increases C++ for smooth muscle contraction (β2 can be linked to a Gi protein that decreases cAMP resulting in smooth muscle relaxation). Thus the effect of ACh and NE can be either excitory or inhibitory, based on the receptor distribution within visceral organs.

The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The autonomic nervous system provides unconscious control over visceral functions and has two divisions: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is activated in stressful situations to prepare the animal for a “fight or flight” response. The parasympathetic nervous system is active during restful periods. The sensory-somatic nervous system is made of cranial and spinal nerves that transmit sensory information from skin and muscle to the CNS and motor commands from the CNS to the muscles.

Exercises

1. Which of the following statements is false?
   1. The parasympathetic pathway is responsible for relaxing the body, while the sympathetic pathway is responsible for preparing for an emergency.
   2. Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
   3. Slowing of the heartbeat is a parasympathetic response.
   4. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while sympathetic neurons are responsible for releasing acetylcholine.
2. Activation of the sympathetic nervous system causes:
   1. increased blood flow into the skin
   2. a decreased heart rate
   3. an increased heart rate
   4. increased digestion
3. Where are parasympathetic preganglionic cell bodies located?
   1. cerebellum
   2. brainstem
   3. dorsal root ganglia
   4. skin
4. ________ is released by motor nerve endings onto muscle.
   1. Acetylcholine
   2. Norepinephrine
   3. Dopamine
   4. Serotonin
5. What are the main differences between the sympathetic and parasympathetic branches of the autonomic nervous system?
6. What are the main functions of the sensory-somatic nervous system?

Answers

1. D
2. C
3. B
4. A
5. The sympathetic nervous system prepares the body for “fight or flight,” whereas the parasympathetic nervous system allows the body to “rest and digest.” Sympathetic neurons release norepinephrine onto target organs; parasympathetic neurons release acetylcholine. Sympathetic neuron cell bodies are located in sympathetic ganglia. Parasympathetic neuron cell bodies are located in the brainstem and sacral spinal cord. Activation of the sympathetic nervous system increases heart rate and blood pressure and decreases digestion and blood flow to the skin. Activation of the parasympathetic nervous system decreases heart rate and blood pressure and increases digestion and blood flow to the skin.
6. The sensory-somatic nervous system transmits sensory information from the skin, muscles, and sensory organs to the CNS. It also sends motor commands from the CNS to the muscles, causing them to contract.

Glossary

What is secreted by parasympathetic postganglionic fibers?

What neurotransmitter is released by both preganglionic and postganglionic parasympathetic neurons?

What does parasympathetic postganglionic neuron release?