Development of the nervous system in humans in the lower part. Neuronal migration is the method by which neurons travel from their origin or birthplace to their final position in the brain. There most common means of migration are radial and tangential migration. Neuronal precursor cells proliferate in the ventricular zone of the developing neocortex. The first postmitotic cells to migrate from the preplate which are destined to become Cajal-Retzius cells and subplate neurons. These cells do so by somal translocation. Neurons migrating with this mode of locomotion are bipolar and attach the leading edge of the process to the pia. The soma is then transported to
Systole Systole of the heart is initiated by electrically excitable cells situated in the sinoatrial node. These cells are activated spontaneously by depolarization of the electrical potential across their cell membranes, which causes voltage-gated calcium channels on the cell membrane to open and allow calcium ions to pass through into the sarcoplasm (cytoplasm) of cardiac muscle cells. Calcium ions bind to molecular receptors on the sarcoplasmic reticulum (see graphic), which causes a flux (flow) of calcium ions into the sarcoplasm. Calcium ions bind to troponin C, causing a conformational (i.e., structural) change in the troponin-tropomyosin protein complex, causing the myosin head