45. Nervous System
Neurons and Action Potentials
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Our brains are constantly processing information, which requires communication between billions of nerve cells. Signals are passed from a sending neuron to a receiving neuron at a junction called a synapse. An action potential in the sending neuron travels down the axon until it reaches a synaptic terminal. The narrow gap between the synaptic terminal and the receiving neuron is called the synaptic cleft. The synaptic terminal of a sending neuron contains numerous vesicles filled with neurotransmitters, chemicals that carry information across the synaptic cleft. When an action potential reaches the synaptic terminal, the vesicles fuse with the plasma membrane of the sending neuron, releasing neurotransmitters into the synaptic cleft. The neurotransmitters affect the receiving neuron, changing the distribution of charge across its membrane. Let’s take a closer look. An action potential is propagated down an axon by the opening and closing of sodium and potassium channels. When an action potential arrives at the synaptic terminal, it causes the opening of calcium channels, shown in green. Calcium ions enter the synaptic terminal through the calcium channels. Calcium ions bind to the vesicles containing neurotransmitters. This causes the vesicles to fuse with the plasma membrane of the sending neuron, releasing neurotransmitters into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and bind to receptors in the plasma membrane of the receiving neuron. Here, the receptors are ion channels that open. Ions move across the membrane, changing the distribution of charge across the membrane. The neurotransmitters are quickly removed from the synaptic cleft, ending their effect on the receiving neuron. A single neuron can receive signals from many sending neurons. The signals arriving from the axon on the left are excitatory -- they make the receiving neuron more likely to generate an action potential, as indicated by the green glows. Signals from the axon on the right are inhibitory -- they make the receiving neuron less likely to generate an action potential, as indicated by the red glows. The two sets of signals cancel each other out, and no action potential is generated. Now, only excitatory signals are transmitted, and the receiving neuron generates an action potential down its axon. The signals that pass between these and countless other neurons constitute our thoughts and coordinate our activities.
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