Baskets and Chandeliers : Neurons that keep the balance

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Balance is key – be it in ballet, martial arts or optimal brain function. Different kinds of neurons in the brain perform a delicate job of balancing overall activity.

 

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Neurons are the building blocks of the brain that process information. Neurons have distinct parts that are responsible for different steps. The dendrite receives information from other neurons. The cell body or the soma processes this information and the axon carries forward this processed information to other neurons.

 

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There are two main types of neurons in brain – excitatory neurons and inhibitory neurons.

 

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True to their names – excitatory neurons excite all other connected neurons. Connected circuits of many neurons pass on excitation as a means of information transfer.

 

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Inhibitory neurons on the other hand, are the traffic cops on the neuronal world. They inhibit excitation, clamp down on excited neurons.

 

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The spread of excitation is the main method by which neurons communicate with each other. But excessive excitation can be dangerous. Imagine a runaway bicycle on a downward slope. Putting the brakes is crucial to stop this runaway excitation – be it a gentle stop with brakes on the bicycle or a violent stop with a pothole. Similarly, inhibitory interneurons generate inhibition in many different ways.

 

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Lets look at some of the key cells in the inhibitory interneuron family.
Basket cells are a famous example, so named because they resemble a delicate basket – with their axons positioned to cradle the somata of other excitatory neurons.

 

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Chandelier cells are named for their distinctive ‘light fixture’ like vertical projections at the tips of their axonal branches.

 

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Discovered in 1889 by Carlo Martinotti and named after him, these cells are critical to help stop runaway excitation in the cerebral cortex.

 

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Lets look at how these inhibitory neurons do what they do.

Basket cells provide strong inhibition to the cell body or soma of excitatory neurons.

 

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Chandelier cells use their long vertical arms to inhibit the axons of excitatory neurons. This effectively ‘chokes’ the excitatory neuron from spreading excitation to other neurons.

 

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Martinotti cells inhibit the dendrites of excitatory neurons, changing the way a neuron receives excitation from other neurons.

 

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Imagine that a neuron is like a participant in a game of Chinese whispers. It receives the excitatory message via the dendrites – just like the participant listens to the message with their ears. The message is processed by the soma – just as the message is understood by the brain. And finally, the excitation is dispatched through the axon, just as the message is spoken out by the participant.

 

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Now imagine a situation where an excitatory cell is sending excitation to another excitatory cell. Lets say that the potential for runaway excitation in this case is very high. How do the three different kinds of interneurons intervene to control this runaway excitation?

 

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Basket cells are strong and fast inhibitors that control the spread of excitation very effectively. The inhibition they provide at the soma of the excitatory neuron is like putting a bucket over its’ head. The message received is muffled at best and it is nigh impossible to pass on the excitation to nearby neurons.

 

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Chandelier cells also provide strong inhibition but at a specific cellular compartment. They inhibit the axons of the excitatory neurons. This prevents the excitatory neuron from spreading the runaway excitation. Much like putting a piece of duct tape on the whisperer’s mouth.

 

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Martinotti cells are the most elegant of all. They get to the problem right at the source. Similar to putting earplugs on someone, Martinotti cells modulate the way incoming excitation is received by inhibiting the dendrites of the excitatory neuron.

 

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Why is all this important? The balance of excitation and inhibition plays a major role in many neurological diseases. It is well known that the dysfunction of inhibitory interneurons is a major contributor in diseases like epilepsy, Obsessive Compulsive Disorder (OCD) and Autism.

Reference: Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex. Nat. Rev. Neuroscience 2008 vol.9 (7) pp.557-568

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