Question

Describe the relationship between the two motor neurons in the ANS pathways. How do they differ for sympathetic and parasympathetic pathways? Which neurotransmitter (ACh or NE) is being released by the pre- and postganglionic neurons for the sympathetic and parasympathetic pathways?

Answer 1

The peripheral nervous system is made up of the somatic nervous system (SNS) and the autonomic nervous system (ANS).

SNS stimulates skeletal muscles while ANS controls smooth muscles, cardiac muscles, and glands.

The relationship between the two motor neurons in the ANS pathways?

In SNS, only one type of neuron is present that connects CNS to skeletal muscles being stimulated. In ANS, two motor neurons connect CNS and its target i.e., preganglionic neuron and postganglionic neuron. The synapse between the neurons is located outside the CNS in the autonomic ganglia. Fibers from the CNS to the ganglion form the preganglionic fibers and fibers from the ganglion to the target organ form the postganglionic fibers. The preganglionic (or the first neuron) must first cross the synapse onto the postganglionic neuron before reaching the target organ. The preganglionic or the first neuron will begin at the outflow (spinal cord exit points) and then must cross a synapse at postganglionic or second neuron's cell body. The postganglionic neuron will then cross the synapse at the target organ.

How do they differ for sympathetic and parasympathetic pathways?

ANS is further divided into the sympathetic and parasympathetic nervous systems. One stimulates the target organ while the other inhibits it. The sympathetic system prepares the body for fight or flight situations (requiring alertness and strength) while the parasympathetic system is active during digestion and rest and maintains normal bodily activities.

All preganglionic fibers of ANS are myelinated for faster transmission of the signal.

Sympathetic preganglionic fibers are shorter when compared to parasympathetic preganglionic fibers because sympathetic ganglia are located closer to the spinal cord than the parasympathetic ganglia. Postganglionic fibers of sympathetic are longer than parasympathetic.

Ganglia of the sympathetic system is located in the paravertebral synthetic ganglion chain while in the parasympathetic system, the terminal ganglion is located near or embedded in the target tissue.

Since sympathetic fibers form more synapses with ganglionic neurons than parasympathetic fibers, the divergence factor (ratio of preganglionic to postganglionic fibers) is 1:20 in a sympathetic while it is 1:4 in parasympathetic system.  

Cell bodies of preganglionic neurons of the sympathetic system (thoracolumbar division) are located in the lateral horns of the grey matter of the 12 thoracic and first 2 lumbar segments of the spinal cord while the cell bodies of the preganglionic neurons of the parasympathetic system (craniosacral division) are located in the grey matter of the sacral segments S2 to S4 and in the brainstem (cranial nerves III, VII, IX, X).

Which neurotransmitter (ACh or NE) is being released by the pre- and postganglionic neurons for the sympathetic and parasympathetic pathways?

Preganglionic fibers of both sympathetic and parasympathetic systems are cholinergic i.e., they have acetylcholine as the neurotransmitter.

Postganglionic fibers of the sympathetic nervous system are adrenergic i.e., they have norepinephrine as the neurotransmitter while the postganglionic fibers of the parasympathetic system are cholinergic having acetylcholine as the neurotrnamitter.