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Neuroscience

Neurophilosophy

Action waves in the
brain

A new theoretical model describes the nervous impulse as an electromechanical
wave

Beneath the Great Wave off Kanagawa, by Katsushika Hokusai,
circa 1831. Illustration: Alamy

Mo
Costandi

@mocost

Friday 1 May 2015 04.00 EDT

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Two researchers from Princeton University have developed a theoretical model
describing the nervous impulse as an electromechanical wave that travels along
nerve fibres, explaining curious experimental observations and challenging basic
assumptions about how the brain works.

The mechanism of the nervous impulse was made clear in a series of
experiments carried out by Alan Hodgkin and Andrew Huxley from the late
1930s onwards. They prepared segments of giant squid axon, placed them in salt
water solution, and then impaled them with microelectrodes, with which they
could both inject electrical current into the fibre and record its voltage. This
enabled them to control the voltage across the membrane and also measure the
movements of current responsible for producing the impulse.

Resting nerve cells have a lower concentration of sodium ions, and a higher
concentration of potassium ions, than the spaces surrounding them, so that the
inside of the membrane is negatively charged with respect to the outside. This
transmembrane voltage is called the resting potential; most nerve cells have a
resting potential of about -70 millivolts.

Hodgkin and Huxley discovered that the nervous impulse is caused by the flow
of sodium ions into the cell, followed almost immediately by the flow of
potassium ions out. The ions move in and out through channel proteins that
traverse the membrane, and open briefly in response to changes in membrane
voltage, allowing first one ion species in, then the other out, in just one
thousandth of a second. The influx of sodium ions reverses the transmembrane
voltage, but then the potassium ion efflux quickly reverts it to its resting
state. Hence, neuroscientists refer to nervous impulses as action potentials.

3D model of a nerve terminal in atomic detail | Mo
Costandi

Read more

As these charges move along the membrane, they activate sodium channels in
adjacent regions, carrying the impulse along the axon towards the nerve terminal. Upon arriving at the terminal,
they cause synaptic vesicles to fuse with the terminal membrane and release
neurotransmitter molecules into the synapse, relaying the signal to another
cell, which then produces its own impulses.

Hodgkin and Huxley developed a mathematical model describing how nervous
impulses are produced and propagated along the nerve fibre. They published their
work in a series of classic 1952 papers, and were later awarded
the 1963 Nobel Prize in Physiology or Medicine for
their work. Their technique for recording the movement of ionic currents across
nerve cell membranes (called voltage-clamping) is still used widely by
electrophysiologists today.

The Hodgkin-Huxley model remains highly influential, but since its
publication other researchers have noticed that nervous impulses are accompanied
by various mechanical perturbations of the axon fibre. Some have observed small
rapid changes in axon diameter and pressure, while others have recorded changes
in fibres’ optical properties but, failing to see how or
why this might happen, dismissed the observations as meaningless by-products of
the nervous impulse, or as experimental artefacts.

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The nervous impulse (action potential, AP) and accompanying
action wave (AW) together make up an electromechanical pulse that travels along
the axon. From El Hady & Machta (2015).

The new model, developed by Ahmed El Hady and Benjamin Machta, is based on
data about biophysical parameters such as axon diameter, elasticity of the nerve
cell membrane, and density and viscosity of the axoplasm (the cytoplasm inside
the axon), gleaned from other experiments.

It states that initiation of a nervous impulse displaces the membrane inwards
just behind it. This triggers an accelerating mechanical wave that precedes the
impulse as it is propagated along the axon and constricts the fibre as it moves,
causing tiny ripples in both the axoplasm and the fluid outside the cell.

“We call them action waves,” says El Hady, “and we think they are driven by
action potentials. They’re a bit like the surface waves you get when you throw a
stone into water - the stone is like a travelling electrical wave that induces
this mechanical displacement of the membrane.”

Before moving to Princeton, El Hady and his colleagues had been using
state-of-the-art super-resolution microscopy to examine the actin cytoskeleton, a dynamic network of
filamentous proteins, in live neurons. Recently, a team of researchers in China
showed that in axons the cytoskeleton is arranged as a series of concentric
rings, and El Hady suspects that action waves may be produced by the
successive constriction of these rings.

White matter might matter much more than we thought
| Mo Costandi

Read more

The action wave is purely theoretical, existing only as equations describing
the mechanical changes that occur at the nerve cell membrane as a nervous
impulse passes along it. Recently, several research groups have independently
reported the surprise finding that ultrasonic stimulation can induce nervous
impulses, suggesting that the electrical and mechanical properties of the
membrane do indeed interact. Sodium channels are modulated by membrane
mechanics, and may therefore mediate this interaction.

El Hady and Machta have used the model to make predictions that are
consistent with others’ research findings, and are now planning to test it
experimentally themselves. “The next step will be to see how these actin rings
are moving,” says El Hady, “and we also want to vibrate neurons with fine
piezoelectric elements to see if we can elicit an electrical response.”

They believe action waves are required for propagating impulses along nerve
fibres and for carrying information, and are also planning experiments involving
artificial constriction of axons, to see if this hinders the movement of
impulses. They also argue that the waves contribute information processing, too,
perhaps by helping to position synaptic vesicles at the nerve terminal, prime
ion channels for the incoming electrical impulse, or help to synchronise their
opening and closing.

“Our main aim is to make people think about this,” says El Hady, “and
ultimately we hope more people will come on board to do experiments that test
the model.”

Reference: El Hady, A. & Machta, B. B.
(2015).Mechanical surface waves accompany action potential
propagation. Nat. Comm. DOI: 10.1038/ncomms7697

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