Parasympathetic nervous system act in a complementary way increasing or slowing the heart rate. In this context, the vagus nerve acts on sinoatrial node slowing its conduction thus actively modulating vagal tone accordingly. This modulation is mediated by the neurotransmitter acetylcholine and downstream changes to ionic currents and calcium of heart cells. The vagus nerve plays a crucial role in heart rate regulation by modulating the response of sinoatrial node, vagal tone can be quantified by investigating heart rate modulation induced by vagal tone changes. As a general consideration, increased vagal tone (and thus vagal action) is associated with a diminished and more
Lymph node range in size from a few millimeters to about 1–2 cm long. Each lymph node is surrounded by a fibrous capsule, which extends inside the lymph node to form trabeculae. The substance of the lymph node is divided into the outer cortex and the inner medulla. The cortex is continuous around the medulla except where the medulla comes into direct contact with the hilum. Thin reticular fibers of reticular connective tissue, and elastin form a supporting meshwork called a "reticulin" inside the node. B cells are mainly found in the outer (superficial) cortex where they are clustered together as follicular