What role does calcium play in the activity of neurons and muscle cells?

Calcium plays a vital role in the excitability of neurons and muscle cells. In neurons, calcium ions are crucial for neurotransmitter release at synapses, which is essential for the transmission of signals between nerve cells. When an action potential reaches the end of a neuron, calcium channels open, allowing calcium ions to flow into the cell. This influx triggers the process that leads to the release of neurotransmitters into the synaptic cleft, facilitating communication within the nervous system.

In muscle cells, calcium is pivotal for muscle contraction. When a muscle cell is stimulated, calcium is released from the sarcoplasmic reticulum into the cytoplasm. The presence of calcium ions allows for the interaction between actin and myosin, the proteins responsible for muscle contraction. This process is essential for all types of muscle activity, including voluntary movements and involuntary actions such as the heartbeat.

The other options presented do not accurately capture the primary role of calcium in relation to neurons and muscle cells. While calcium is involved in blood coagulation, blood pressure maintenance, and blood viscosity, these functions are not directly related to the excitability and physiological processes of neurons and muscle cells.