The majority of motor functions in our body are driven by electrical currents originating in the brain motor cortex and conducted through the spinal cord and peripheral nerves to the muscles, where the electrical impulse is converted to motion by the contraction and retraction of specific muscles. For example, to bend the arm at the elbow joint, the biceps muscle contracts and the triceps relaxes. This seemingly simple movement is the result of the cumulative activity of many brain cells in the area of the cortex in charge of arm movement. The neurons, following a cognitive decision to bend the arm, generate an electric impulse through the peripheral nerves, causing the correct muscles to contract or relax.
The term used for neuronal activity is "action potential." Action potential occurs when an electric impulse shoots through the long shaft of the neuron, called the axon. Each neuron has one axon but is connected to many other neurons through chemical connections called synapses, and can influence other neurons or be influenced itself by the activity of adjacent neurons, creating an extremely complex network of neural cells.
The action potential in a neuron can be measured by inserting an extremely thin electrode adjacent to the axon, where the passing electric current can be detected. The electrode measures the neuron’s rate of action potential in one second, thus measuring its activity.
Most neuroscientists agree that the rate or frequency of the firing constitutes a sort of code for brain activity. For instance, if a certain group of neurons fires action potentials at a high frequency together, the result is the movement of a limb.
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