Review of the Sympathetic and Parasympathetic Divisions: Videos & Practice Problems

The sympathetic and parasympathetic nervous systems play crucial roles in regulating the heart's function through a process known as dual innervation, where most organs receive input from both systems. Understanding the anatomy and function of these systems is essential for grasping how they influence heart rate and overall physiological responses.

Starting with the sympathetic nervous system, its fibers originate from the spinal cord segments T1 to L2. The preganglionic fibers are relatively short, extending from the spinal cord to the sympathetic trunk, where they synapse with postganglionic fibers. These postganglionic fibers then innervate the heart, leading to an increase in heart rate, which is part of the body's fight or flight response. Additionally, some sympathetic fibers, known as splanchnic nerves, bypass the sympathetic trunk and synapse in collateral ganglia, particularly for innervating organs like the bladder.

In contrast, the parasympathetic nervous system's preganglionic fibers originate from the brainstem and sacral region. These fibers are longer, as they travel almost directly to their target organs, synapsing either at terminal ganglia close to the effectors or directly on the effectors themselves. The parasympathetic activity promotes a decrease in heart rate, embodying the rest and digest response.

Both systems share similarities, such as the presence of preganglionic and postganglionic fibers that convey nerve impulses to effectors, with synapses occurring between these fibers. However, the key differences lie in their origins and synapse locations: sympathetic fibers originate from T1 to L2 and synapse in sympathetic trunk or collateral ganglia, while parasympathetic fibers arise from cranial and sacral regions, synapsing in terminal ganglia near or on the effectors.

Functionally, these systems exhibit opposing effects on organ function, highlighting their complementary roles in maintaining homeostasis. Understanding these dynamics is vital for comprehending how the body responds to various stimuli and maintains balance in physiological processes.

The sympathetic division of the autonomic nervous system features a ganglionic trunk, which runs parallel to the spinal cord. This structure is crucial as it is where many preganglionic fibers synapse, facilitating communication within the sympathetic nervous system. In contrast, the parasympathetic division lacks a similar trunk structure. Instead, its preganglionic fibers synapse in terminal ganglia, which are located close to the effector organs. This distinction highlights the different pathways and synaptic arrangements between the sympathetic and parasympathetic divisions, emphasizing the role of the ganglionic trunk in the sympathetic system and the terminal ganglia in the parasympathetic system.