Dendritic spines

ScienceDaily has an item on a paper in Nature by Harnett and others, Synaptic amplification by dendritic spines enhances input cooperativity; a very interesting subject to me.

Not all that many years ago, neurons were thought of as simple switches and relays. Then they were thought of as logic gates; then little computers. But now we know that they were quite complicated. This paper looks at the spines on the dendrites of a neuron. A neuron has a ‘bush’ or ‘tree’ of dendrites. Along the length of these dendrite processes are short little spines and at the end of each spine are one or more synapses.

These tiny membranous structures protrude from dendrites’ branches; spread across the entire dendritic tree, the spines on one neuron collect signals from an average of 1,000 others. … Dendritic spines come in a variety of shapes, but typically consist of a bulbous spine head at the end of a thin tube, or neck. Each spine head contains one or more synapses and is located in very close proximity to an axon coming from another neuron. … Scientists have gained insight into the chemical properties of dendritic spines: receptors on their surface are known to respond to a number of neurotransmitters, such as glutamate and glycine, released by other neurons. … the spines’ incredibly small size — roughly 1/100 the diameter of a human hair …

Why do neurons have their incoming synapses at the end of little stalks. It isolates them chemically and electrically from other synapses. We can think of spines like the brackets in math and logical statements. The processing within the bracket is done before the result interacts with other terms. The spines also can amplify the signal before it leaves the spine. Further because the spines have high impedance they offer differing resistance depending on the frequency of the signal. The spines add an important layer of signal manipulation to the neuron.

Here is the abstract:

Dendritic spines are the nearly ubiquitous site of excitatory synaptic input onto neurons and as such are critically positioned to influence diverse aspects of neuronal signaling. Decades of theoretical studies have proposed that spines may function as highly effective and modifiable chemical and electrical compartments that regulate synaptic efficacy, integration and plasticity. Experimental studies have confirmed activity-dependent structural dynamics and biochemical compartmentalization by spines. However, there is a longstanding debate over the influence of spines on the electrical aspects of synaptic transmission and dendritic operation. Here we measure the amplitude ratio of spine head to parent dendrite voltage across a range of dendritic compartments and calculate the associated spine neck resistance for spines at apical trunk dendrites in rat hippocampal CA1 pyramidal neurons. We find that neck resistance is large enough (~500 MΩ) to amplify substantially the spine head depolarization associated with a unitary synaptic input by ~1.5- to ~45-fold, depending on parent dendritic impedance. A morphologically realistic compartmental model capable of reproducing the observed spatial profile of the amplitude ratio indicates that spines provide a consistently high-impedance input structure throughout the dendritic arborization. Finally, we demonstrate that the amplification produced by spines encourages electrical interaction among coactive inputs through a neck resistance-dependent increase in spine head voltage-gated conductance activation. We conclude that the electrical properties of spines promote nonlinear dendritic processing and associated forms of plasticity and storage, thus fundamentally enhancing the computational capabilities of neurons.

There is another aspect of spines to my mind. They allow glial cells to make better contact with synapses. They are also part of the complex processes in and around synapses.

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