5 Tricks of Neurons
Neurons are cells. As cells, they contain components common to all animal cells, such as the nucleus and Golgi apparatus. However, neurons have other features unique to neurons, or at least, not common in other cells. These unique features exist because neurons are specialized for processing and communicating information.
This specialization evolved because it allowed organisms to increase their survival chances by moving within their environment based on sensing things like food, toxins, temperature, and predators.
Neurons are so specialized compared to any other cells that the study of neurons and neural organization comprises many schools of neuroscience. If you’re interviewing for a job in a neuroscience lab, you will impress the lab head by knowing about the five neural cell attributes covered here.
Getting the biggest bang for the buck with dendritic spines
Neuronal dendrites are not merely smooth cylindrical surfaces between the branch points. Rather, they are covered with little mushroomlike appendages called spines. Spines are the site of much of the synaptic input to the dendrites, particularly excitatory input.
Here’s the interesting thing about spines: They can appear and disappear and dynamically change their shape based on activity in both the presynaptic and postsynaptic neuron. In other words, spine shape changes appear to be a mechanism for dynamic changes in synaptic strength that underlie learning and plasticity in the nervous system.
The shapes of spines impact at least two mechanisms that affect synaptic efficacy:
- The length and diameter of the spine neck affect the amount of current that reaches the main dendritic shaft from the synapse at the spine head.
- The spine volume appears to affect ionic concentration changes associated with synaptic current, such that small volumes may cause locally high concentrations of ions like calcium that mediate changes in plasticity.
Ligand-gated receptors: enabling neurons to communicate chemically
Ligand-gated receptors are the protein complexes on the postsynaptic side of synapses. They connect the world outside the neuron with the world inside by allowing ions to move through channels in the membrane. The receptor is selective to particular kinds of messages from other neurons, based on its affinity for the neurotransmitter released by the presynaptic neuron. The ion channel that is opened when the receptor binds a neurotransmitter is the message that is communicated to the rest of the cell as synaptic input current.
Ligand-gated ion channels known as ionotropic receptors are different from another type of receptor called metabotropic receptors. The metabotropic receptor functions by having a ligand binding site on the exterior of the membrane, like the ionotropic receptor, but it has no channel in its receptor complex. Instead, ligand binding causes the release of an intracellular messenger that was bound to the interior side of the complex, that activates (usually by opening) a channel somewhere else on the membrane (usually nearby).
Ligand-gated ionotropic ion channels usually mediate fast synaptic transmission underlying behavior. Metabotropic receptors are typically slower and mediate modulatory or homeostatic responses, although there are a few cases of fast metabotropic receptor channels. They make it possible for a single neurotransmitter to have different effects in different postsynaptic neurons.
Getting specialized for the senses
During the evolution into multicellular life forms, neurons became specialized for sensing aspects of the environment. This specialization was the result of the development of specialized membrane receptors or intracellular organelles in single cells. Neural sensory receptors are cellular transducers that respond to energy, forces, or substances in the external or internal environment and convert the detection into electrical activity, often through modulating the release of a neurotransmitter. Here are some examples:
- Photoreceptors in vertebrate eyes have structures derived from cilia that capture light photons. Capturing these photons causes a G-protein intracellular cascade that closes sodium-permeable ion channels, hyperpolarizing the cell, and reducing the release of glutamate, the photoreceptor neurotransmitter.
- In the ear, auditory hair cells also have cilia that, when bent, open ion channels that depolarize the cell and cause action potentials in the auditory nerve.
Computing with ion channel currents
Neural membranes, like those of most animal cells, are virtually impervious to the flow of water and most ions. Neural membranes differ from those of other cells, however, in that they have many different ion channels that can be activated by ligands or voltage.
When ion channels are open and allow sodium ions to flow through, the neuron is excited. It is inhibited when potassium or chloride channels are open. Neurons have thousands of ion channels of different types in their membranes that open in complex, time-varying combinations.
The neuron computes the interaction of all the ion flows of all the channels in the neuron. For this computation to occur, a certain number of excitatory inputs have to be simultaneously active, while a certain number of inhibitory inputs must not be active. Given 10,000 different inputs, the number of unique combinations a neuron can discriminate is an astronomically large number.
The structure of the neuron’s dendritic tree is crucial in this computation. Researchers know that less current injected at distal synapses reaches the cell body than that injected at proximal synapses, and the time course of the distal current is slower. A branching dendritic tree yields many more synaptic sites on the many distal branches than on proximal ones, which partly makes up for the reduction in current magnitude from individual distal synapses. It does not make up for the slower time course of distal versus proximal synapses, however. Because of this, inputs to neurons that tend to dominate the postsynaptic neurons’ fast activity tend to occur on proximal synapses, while distal synapses are primarily modulatory.
Keeping the signal strong across long distances
The most remote dendrites in a neural dendritic tree are rarely more than a few hundred micrometers from the cell body. Synaptic inputs at this distance are severely weakened by the time they reach the cell body by what is called electrotonic spread over the membrane. But even though the weakening is significant, there are enough synapses on the most distant dendrites to result into an effective input at the cell body.
It’s a different story with the axons, however. Most cells have only one axon leaving the cell body, and this axon may travel a meter through the body and then branch into hundreds or thousands of axon terminals.
The single most important invention of the nervous system is the action potential. The action potential uses transient, voltage-gated sodium channels in the axonal membrane to create a voltage pulse that, by causing a chain reaction across adjacent cells, can send a signal to the most distant synapses without weakening. Each action potential pulse is essentially identical, and the shape of the action potential is more or less the same as at the terminal as it was at its origin.