Protein Triggers Neurotransmitter Release

Biological Strategy

Animals

Eleanor Banwell

Image: Christoph Burgstedt / iStock / Some rights reserved

Transduce/Convert Signals

Sometimes a signal, such as sound or vibration, reaches a living system and needs to be converted into a different, usable form. This requires converting from one type of energy form to another. For example, organisms with eyes have photoreceptors at the back of the eyes that convert light into electrical pulses. These pulses travel to the brain and allows it to register color, shape, and detail.

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Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

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Differentiate Signal From Noise

The environment surrounding any living system is full of signals, such as smells, sounds, electricity, magnetism, and more. Too many signals at once can be overwhelming, making it difficult to locate and react to any particular signal. To address this, living systems must differentiate among all signals, or “noise,” so they can focus on those they really need. During mating season, for example, male frogs gather close together in large groups to call for females. Despite their overwhelming, collective noise, females are able to find a particular male whose call is attractive to her. This is because the males use desynchronized calling, enabling individual recognition.

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Encode/Decode

Organisms are constantly taking in new information, processing it, and storing that information as memories. Sensory information and stimuli such as sounds, smell, time, and spatial organization are encoded along with our memories. Whether we’re learning a new skill or repeating a task we’ve done before, we can easily call upon and decode that information when we need it.

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Compute

Computation allows an organism to take in and process information to make decisions. Whether it be calculating how fast prey is moving or deciding where to build a nest, computation is an important part of an organism’s life.

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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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Proteins in nerve cells of animals trigger neurotransmitter release in response to electrochemical signal.

Nerve cells are the core components of an animal’s nervous system. They transmit and process information using a combination of electrical and chemical signals, and are found in all animals with the exception of the simplest, like sponges. Nerve cells have a cell body, dendrites (which receive signals), and an axon (which transmits the signal). The dendrites and axon project from the cell body. Axons branch into blind ends called termini, and axon termini connect to dendrites of other nerves via synapses. When a nerve receives sufficient stimulation from its dendrites, the nerve fires an electrochemical signal along its axon, called the “action potential.”

Chemical synapses are the junctions between axon termini on one nerve cell and dendrites on a neighboring cell; they are narrow gaps, or clefts, only 20nm wide. Signals are transmitted across chemical synapses by neurotransmitters—small molecular chemical messengers. When an action potential reaches a chemical synapse, neurotransmitters are quickly released into the synaptic cleft. As they must only enter the cleft in response to an action potential, release is tightly controlled.

Neurotransmitters are made in large quantities in axon termini and packaged into membrane sacs called vesicles. When vesicles fuse with the cell membrane at the synapse, they release their contents into the synaptic cleft. Two proteins, “SNAREs” and “synaptotagmins,” are required for vesicle fusion. SNAREs act like velcro, with some bound to the vesicle and some to the cell membrane. When SNAREs come into contact with each other, they bind tightly, pulling the vesicle close to the cell boundary. Membrane fusion is dependant on the curvature of the membrane and without the action potential, curvature is too shallow for fusion to occur. The other proteins, the synaptotagmins, are also attached to the vesicles. They have large wedge shaped sections that can reach from the vesicle to the cell membrane. In the absence of an action potential, synaptotagmins have the same charge as the cell membrane and they are repelled. However, when the signal reaches the axon termini, calcium ions are released into the cell cytoplasm. Synaptotagmins bind calcium, which is positively charged and neutralises the repulsion. The proteins then embed themselves in the cell membrane creating an area of higher curvature. In combination with the pulling of the SNAREs, this is sufficient for the vesicle and cell membrane to fuse and for the neurotransmitters to be released.

Synaptotagmin activation, vesicle fusion, and neurotransmitter release all occur within 1 ms of the action potential arriving at the axon terminal.

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Last Updated April 20, 2018

References

“The fusion of synaptic vesicles and dense-core granules with the plasma membrane is essential for neurotransmitter and hormone release and therefore for efficient intercellular communication. Synaptic vesicle exocytosis is specialized in a way that allows neurons to communicate in a fast and controlled manner. These fusion events are tightly coupled to the influx of extracellular Ca2+ into the cell and are remarkably fast, with synaptic vesicle fusion occurring within less than 1 ms after Ca2+ influx” (Martens and McMahon 2008:549)

“According to studies of large dense-core granule exocytosis (and therefore, by implication, of synaptic vesicle fusion), the only molecules that are directly involved in the fusion step are synaptotagmins and the SNAREs, and manipulation of either has a direct effect on the kinetics of fusion” (Martens and McMahon 2008:550)

Journal article