5.3: Comparison of Somatic and Autonomic Efferent Systems

In the SNS, a motor neuron originating in the brainstem or the ventral horn of the spinal cord connects directly to the skeletal muscle, bypassing any synapses in between. Conversely, in the ANS, a two-neuron chain is at play: the first motor neuron begins in the brainstem or spinal cord, and the second motor neuron arises from a ganglion to supply cardiac muscle, smooth muscle, or glands.

A notable difference between somatic and visceral reflexes resides in their efferent branches (Figure 1). In somatic reflexes, such as the withdrawal reflex, the output involves the lower motor neuron situated in the ventral horn of the spinal cord, which directly projects to a skeletal muscle, prompting its contraction. In contrast, the output of visceral reflexes follows a two-step pathway. It begins with the preganglionic neuron emerging from the lateral horn in the spinal cord or a cranial nucleus in the brainstem, leading to a ganglion. Subsequently, the postganglionic neuron projects to the target effector (Figure 1). Furthermore, there is a distinction in the neurotransmitters used by motor neurons in these systems. Motor neurons of the SNS release ACh. In contrast, motor neurons of the ANS release ACh, NE, or E.

The afferent branch of a reflex arc can also vary between somatic and visceral systems in specific instances. For example, specialized mechanoreceptors known as baroreceptors exist in the walls of the aortic and carotid sinuses. Baroreceptors sense the stretch of these structures when there is an increase in blood volume or blood pressure. They play a crucial role in cardiovascular and, particularly, vasomotor reflexes.

The ANS presides over regulating numerous internal organs, maintaining equilibrium through a delicate balance between its two divisions. These divisions are known as the sympathetic division, associated with the fight-or-flight response, and the parasympathetic division, often characterized as the rest-and-digest mode. Homeostasis is achieved through the interplay of these two systems. Dual innervation determines the activity at each target effector. For instance, the heart receives connections from both the sympathetic and parasympathetic divisions. Sympathetic activation accelerates the heart rate, while parasympathetic action decelerates it.¹

---