PSY 3740 Module 1 Introduction and Nerve Cells Paper: The Action Potential, Synaptic Release and How Many Cells Make a Brain Example

Eighty-Six Billion Messengers: How a Nerve Cell Sends a Signal

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Bachelor of Arts in Psychology, Thomas Edison State University

PSY-3740: Physiological Psychology

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Eighty-Six Billion Messengers: How a Nerve Cell Sends a Signal

Introduction

At the sleep lab where I work nights, I attach electrodes to patients' scalps and watch waves scroll across the screen. A patient once asked me whether I could see his thoughts. I could not, and I realized I did not fully understand what I was seeing. This course begins with the basic unit of the nervous system, the nerve cell, and this paper explains how neurons send signals, how they communicate with each other and how many of them the brain actually contains.

Parts of a Nerve Cell

Most neurons share three main parts. Dendrites receive signals from other neurons. The cell body, or soma, contains the nucleus and integrates incoming signals. The axon carries signals away from the cell body to other neurons, muscles or glands, ending in terminals that release chemical messengers. Glial cells coat many axons in myelin, a fatty sheath that speeds conduction by allowing the signal to jump between gaps called nodes of Ranvier.

The Resting Potential

A neuron at rest holds a charge of roughly minus 70 millivolts inside compared with outside. This resting potential is maintained by differences in ion concentrations across the membrane, with more sodium outside and more potassium inside, held there because a sodium-potassium pump keeps exchanging ions and the membrane lets some ions cross more easily than others.

The Action Potential

Hodgkin and Huxley (1952) used the giant axon of the squid, large enough to insert electrodes into, and a technique called the voltage clamp to measure the currents that flow across the membrane during a nerve impulse. They showed that the action potential results from rapid, voltage-dependent changes in the membrane's permeability to sodium and potassium. When the membrane is depolarized past a threshold, sodium channels open and sodium rushes in, driving the inside positive. Within about a millisecond, sodium channels close and potassium channels open, letting potassium flow out and returning the membrane to its negative state. They expressed these changes in a set of equations that predicted the shape and speed of the action potential with remarkable accuracy, work that earned them a Nobel Prize.

All or None

Because an action potential is triggered only when the threshold is reached and then proceeds at full size, it is described as all or none. The strength of a stimulus is coded not by the size of individual action potentials but by how often a neuron fires and how many neurons fire together.

Synaptic Transmission

When an action potential reaches an axon terminal, it must cross a tiny gap, the synapse, to influence the next cell. Südhof (2013) reviewed how this happens. The arriving action potential opens voltage-gated calcium channels, and calcium flows into the terminal. Calcium binds to a sensor protein, synaptotagmin, which triggers a set of proteins, including the SNARE complex, to fuse vesicles filled with neurotransmitter with the cell membrane, releasing their contents into the synapse. The entire process, from calcium entry to release, takes less than a millisecond. The neurotransmitter then binds to receptors on the next neuron, nudging that neuron toward or away from firing.

Excitation and Inhibition

Some neurotransmitters, such as glutamate, typically excite the receiving neuron; others, such as GABA, typically inhibit it. Each neuron receives thousands of excitatory and inhibitory inputs and fires only when their combined effect pushes it past threshold. Many drugs act at synapses, mimicking, blocking or altering neurotransmitters, which is why understanding synaptic transmission is central to understanding how psychoactive drugs and many psychiatric medications work.

How Many Cells?

Textbooks long claimed that the human brain contains about 100 billion neurons and ten times as many glial cells. Azevedo et al. (2009) tested these figures by dissolving brain tissue from four adult men into a uniform suspension of cell nuclei and counting neurons and other cells directly. They found about 86 billion neurons and about 85 billion nonneuronal cells, an approximately equal number rather than ten to one. Most neurons were in the cerebellum, with about 16 billion in the cerebral cortex. They concluded that the human brain is a scaled-up primate brain, not an exception to primate patterns.

Glial Cells

Glial cells, once thought to be mere glue, perform many functions. Oligodendrocytes and Schwann cells form myelin. Astrocytes regulate the chemical environment around neurons, support synapses and help control blood flow. Microglia act as the brain's immune cells. Azevedo and colleagues' count shows that these cells are about as numerous as neurons, and research continues to reveal their role in brain function and disease.

What EEG Records

The electrodes I apply do not record individual action potentials. EEG mainly reflects the summed postsynaptic potentials of large numbers of cortical neurons, especially pyramidal cells aligned in the same direction, firing in synchrony. When millions of neurons fluctuate together, as during deep sleep, the EEG shows large, slow waves; when activity is less synchronized, as during alert wakefulness, waves are smaller and faster. EEG cannot read thoughts, but it can reveal the brain's state and detect abnormal activity such as seizures.

The Ten Percent Myth

A related myth holds that people use only ten percent of their brains. Brain imaging shows activity throughout the brain over the course of a day, and damage to almost any area produces some deficit. The brain's high energy use, about a fifth of the body's energy despite its small size, also argues against large unused regions.

Limits

Hodgkin and Huxley studied a squid axon, and human neurons show more varied channel types. Azevedo and colleagues counted cells in only four brains.

Conclusion

Hodgkin and Huxley showed how sodium and potassium currents produce the action potential, Südhof showed how calcium triggers the machinery that releases neurotransmitter within a millisecond and Azevedo and colleagues showed that the human brain contains about 86 billion neurons, with nearly as many glia and other cells. The waves on my screen are the combined echo of billions of these messengers working in rhythm.

References

Azevedo, F. A. C., Carvalho, L. R. B., Grinberg, L. T., Farfel, J. M., Ferretti, R. E. L., Leite, R. E. P., Jacob Filho, W., Lent, R., & Herculano-Houzel, S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology, 513(5), 532-541. https://doi.org/10.1002/cne.21974

Hodgkin, A. L., & Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. Journal of Physiology, 117(4), 500-544. https://doi.org/10.1113/jphysiol.1952.sp004764

Südhof, T. C. (2013). Neurotransmitter release: The last millisecond in the life of a synaptic vesicle. Neuron, 80(3), 675-690. https://doi.org/10.1016/j.neuron.2013.10.022