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

In the Autonomic Nervous System, the innervation is through two neurons. The neuron arising directly from the CNS is called the Preganglionic neuron, viz, the neuron before the ganglion, and the neuron with which the preganglionic neuron synapses and which supplies the tissues, is called the Postganglionic neuron. All the preganglionic neurons in the Autonomic Nervous System have acetylcholine as their neurotransmitter and are myelinated for faster conduction of the nerve impulses. The sympathetic fibres in the preganglionic neurons are shorter and make more synapses than the parasympathetic fibres.

In the Postganglionic neurons, the sympathetic nerve fibres contains norepinephrine as their neurotransmitter while the parasympathetic nerve fibres contains acetylcholine as their neurotransmitters. There are two exceptions in this:-

1. The sympathetic fibres in the sweat glands releases acetylcholine for the activation of muscarinic receptors in contrast to the norepinephrine.

2. Chromaffin cells of the adrenal medulla acts as a modified sympathetic ganglion.