Section Summary

Neurons are bilingual, using two inter-related but distinct signaling systems. Communication between neurons primarily uses the ancient language of chemical and receptor that all your cells use, though with adaptations for highly targeted and reliable communication. Grafted onto this ancient system is an electrical signaling system that transfers information within a neuron with a level of speed and precision that is relatively unique to the animal kingdom. An engineer probably wouldn’t have designed such a complex system. Instead, the hybrid nature of neural signaling reflects the piecemeal adaptation of nervous system functioning through a long evolutionary history. From this complexity emerges neural processing. The back-and-forth-and back-again transformation from chemical to electrical to chemical signaling enables neurons to synthesize, filter, and transform information in complex and fascinating ways.

Communication between neurons allows them to form circuits that can generate complex rhythms and behaviors. In the Tritonia swim network, attack from a predator activates sensory neurons as well as inhibitory feedback that generates cycles of contraction and relaxation that “swim” the animal away from danger. This example typifies some of the key properties of neural networks: they are highly efficient, operate in parallel, feature extensive and complex forms of feedback, but are susceptible to malfunction from either too much or too little activity. The field of computational neuroscience explores mathematical models of neurons that can be simulated on a computer; this field is succeeding in producing artificial networks that can mimic many of the remarkable behaviors produced by animal nervous systems.

This section was a crash course in the physics of neural signaling. If you made it through, you’ve seen that pumps are protein machines that use energy to build up concentration gradients of 4 key electrolytes, pushing K⁺ into neurons and Na⁺, Cl^–, and Ca²⁺ out of neurons. The concentration gradients produced by pumps function as chemical batteries: they provide a “push” for K⁺ to and Cl^– to charge the neuron towards negative potentials and for Na⁺ and Ca²⁺ to charge the neuron towards positive potentials. The cell membrane holds back this push of diffusion. Ion channels, on the other hand, can open to allow the pressure for a specific ion to be partly released, producing currents that charge the neuron to that ion’s equilibrium potential. Ca⁺ and Na⁺ produce currents that charge a neuron to a positive potential. K⁺ and Cl^– produce currents that charge a neuron to a negative potential. While ion currents can produce big changes in a neuron’s potential, they usually involve only a fraction of the ions the pumps have stored in or out of the neuron. The physics of neural electricity can be a bit daunting, but it provides a foundation from which we have built a detailed and powerful understanding of neural signaling.

Neural signaling represents an incredible ballet of electrolytes, pumps, and ion channels. The resting potential in neurons occurs due to the expression of leak K⁺ neurons. Because pumps concentrate K⁺ inside the neuron (along with balancing negative charges), leak channels allow diffusion to push some K⁺ out of the neuron, a departure of positive charge that pulls the neuron towards a negative resting potential. Chemical messages from partner neurons disturb this rest, binding to ligand-gated channels to produce post-synaptic potentials: small, transient local changes in membrane potential that push a neuron towards threshold (EPSP, due to Na⁺ or Ca²⁺ conductance) or away from threshold (IPSP, due to Cl^– conductance). When threshold is reached, an action potential is generated by a precisely timed sequence of Na⁺ entering through inactivating voltage-gated Na⁺ channels followed by K⁺ departure through voltage-gated K⁺ channels. This produces a rising phase (from Na⁺ entry) that propagates down the axon followed by the falling phase and undershoot (from K⁺ departure) that helps reset the neuron for the next action potential while also helping to prevent over-excitation. Myelin can speed up action potential propagation by preventing the leak of current over short sections of axon. Return to Table 2.4, which organizes the different types of electrical signals in neurons and their mechanisms.

Neuroscientists have developed a rich and detailed understanding of neural signaling that is helping us better understand and treat disorders of the nervous system. While this progress is encouraging, there is a daunting level of complexity to nervous system function and many important mysteries left to unravel.