The autonomic nervous system or ANS regulates involuntary functions such as cardiac output, glandular secretion, and digestion, which are essential for homeostasis. The ANS is subdivided into sympathetic and parasympathetic divisions. Each division includes two efferent neurons: preganglionic neurons originating from the central nervous system or CNS and postganglionic neurons innervating the organs. The sympathetic preganglionic neurons begin from twelve thoracic segments and three lumbar segments. These thoracolumbar neurons end in paravertebral ganglia near the spinal cord or prevertebral ganglia in the abdominal cavity. The parasympathetic preganglionic neurons originate from four cranial nerves and three sacral nerves. These craniosacral neurons end in the parasympathetic ganglia, located within the target organ. Autonomic ganglia of both divisions contain cell bodies of postganglionic neurons, which innervate visceral organs. Generally, paravertebral ganglia neurons innervate organs above the diaphragm and prevertebral ganglia neurons innervate organs below the diaphragm.