What Causes a Neural Impulse to Be Fired?

Neurons are nerve cells that communicate with each other, allowing you to experience and interact with the world. The adult brain contains 10 billion neurons, each connecting to thousands more at connections called synapses. Specialized cells in your sensory organs convert sensations into electrical signals called action potentials. The action potential causes neurons to release neurotransmitters, chemicals that stimulate electrical signals in cells connected at the synapse.

Membrane Potential

• Neurons possess an electrical voltage across their cell membrane. When neurons are not firing an action potential they rest at a level of -70 to -60 mV. Membrane voltage develops because the neuron controls the concentration of electrolytes inside and outside the cell. Positively charged sodium and negatively charged chloride are very high outside the cell. Positively charged potassium and negatively charged proteins are concentrated inside the cell. Positively charged calcium is relatively low outside the cell and extremely low inside the cell.

Action Potential

• If a neuron's membrane potential rises to a threshold level, voltage-gated sodium channels open, allowing sodium to rush into the cell. This causes the membrane potential of the neuron to rush upwards to +40 mV. At the positive peak, sodium channels close and potassium channels open, returning the membrane potential to just below resting membrane potential. This process initially occurs at a small patch of membrane but spreads quickly throughout the neuron. After the action potential is a refractory period where the neuron cannot fire again until the sodium and potassium channels reset.

Neurotransmitters

• When the action potential reaches the synapse, voltage-gated calcium channels allow calcium to enter the synaptic terminal, causing neurotransmitter to be released into the synapse. Neurotransmitters bind to receptors on the cell on the other side of the synapse, called the post-synaptic cell. These receptors are normally sodium channels that directly cause an action potential in the post-synaptic cell.

Special Sensory Cells

• In the eyes, ears, skin, tongue and nose, special receptors turn energy in your environment to action potentials that can spread to your brain. Photoreceptors in the eyes turn light into action potentials. Hair cells in the ear fire action potentials in response to vibrations. Receptors in your skin translate different kinds of pressure and vibration into action potentials. Your tongue and nose sense levels and combinations of chemicals in your mouth and the air.