Understanding the structure of a muscle cell More free lessons at: http://www.khanacademy.org/video?v=uY2ZOsCnXIA

The role of the sarcoplasmic reticulum in controlling calcium ion concentrations within the muscle cell. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/test-prep/nclex-rn/rn-muscular-system/rn-th… Missed the previous lesson?

How calcium ion concentration dictates whether a muscle contracts or not. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/test-prep/nclex-rn/nclex-rn-circulatory-sys… Missed the previous lesson?

How myosin and actin interact to produce mechanical force. . Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/test-prep/nclex-rn/nclex-rn-circulatory-sys… Missed the previous lesson?

How one neuron can stimulate (or inhibit) another neuron at a chemical synapse. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-a… Missed the previous lesson?

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How electrotonic and action potentials propagate down cells

Correction to "Sodium-potassium pump" video. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-a… Missed the previous lesson?

How a sodium potassium pump can maintain a voltage gradient across a cell or neuron's membrane. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/test-prep/mcat/cells/transport-across-a-cel… Missed the previous lesson?

Neurons (or nerve cells) are specialized cells that transmit and receive electrical signals in the body. Neurons are composed of three main parts: dendrites, a cell body, and an axon. Signals are received through the dendrites, travel to the cell body, and continue down the axon until they reach the synapse (the communication point between two neurons). Created by Sal Khan.

Hemoglobin and its role in the circulatory system. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-a… Missed the previous lesson?

Introduction to the circulatory system and the heart. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-a… Missed the previous lesson?

Oxygen uptake by hemoglobin in red blood cells. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/human-anatomy-a… Missed the previous lesson?

The pulmonary system including the lungs, larynx, trachea, bronchi, bronchioles, alveoli and thoracic diaphragm. Created by Sal Khan. Watch the next lesson: https://www.khanacademy.org/science/health-and-medicine/respiratory-sys… Missed the previous lesson?

Learn about diffusion, osmosis, and concentration gradients and why these are important to cells. Watch the next lesson: https://www.khanacademy.org/science/biology/membranes-and-transport/pas… Missed the previous lesson?