The Ecology of Humans (26-27) Astrocytes

 Astrocytes

The historical notion that astrocytes are cushions for the neurons to feel comfortable or protected is not the case. ~ Hungarian neurobiologist Tamas Horvath

Astrocytes were first described by German neuroanatomist Otto Deiters in an 1850s unfinished manuscript.

This star-shaped cell was named astrocyte in 1891 by Hungarian anatomist Mihály Lenhossek. Neuroglia was the vogue catchall term for non-neuron brain cells in the 19th century.

Astrocytes were previously called spider cells. Lenhossek thought all the glia should be lumped together as spongicytes and then further divided, one cell type being the astrocyte.

Neurobiologists long ignored the physical keeper of intelligence. This is a typical textbook description: “astrocytes are small cells involved in forming a structural and functional barrier between the blood and the brain.”

Astrocytes are the most abundant cell in the human cortex. There 4 times as many astrocytes in the human brain as there are neurons. Astrocytes also line the spinal cord.

The center of physiological intelligence processing lies in the astrocytes. Astrocytes are the knowledge workers in the city of smarts reached by neuronal highways.

Early work in exploring the intelligence system relied upon measuring electrical impulses for localizing brain functions. As neurons are electrical conduits, the facile conclusion was to hold those cells responsible for thought.

Positron emission tomography (PET) scans are used to look at organ functioning. PET scans show that the areas of the brain that Penfield identified as responsible for mentation have increased blood flow. For example, speaking flushes the left temporal cortex with blood. Astrocytes, not neurons, have their end feet on blood vessels.

The brain is a voracious energy consumer: requiring 10 times more oxygen and nutrients than other organs. The dense network of brain blood vessels are set up by radial glia cells early in life. Radial glia cells are stem cell progenitors to astrocytes, oligodendrocytes, and neurons.

Astrocytes drive the master clock in the brain. ~ neurobiologist Marco Brancaccio et al

Astrocytes manage the brain blood supply, nourishing neurons with both oxygen and energy. They break down glucose from capillaries into lactate, which nerve cells can absorb for energy consumption within their mitochondria. Astrocytes maintain a glucose energy reserve for their neurons, for use when the metabolic rate of neurons in the area surges owing to increased inter-astrocyte communication demand.

Sleep is necessary for memory retention, by affording glial calcium wave processing and regeneration without external stimulus.

Astrocytes play pivotal, sleep-dependent roles in ‘cleaning the brain’ during sleep. ~ American neurobiologist Philip Haydon

Astrocytes nestle some of their pointed projections against neurons, determining how nerve cells connect.

Astrocytes, the most abundant cells in the central nervous system, promote synapse formation and help to refine neural connectivity. ~ American cytologist Anna Molofsky et al

Just as they are instrumental in nerve cell generation, astrocytes regulate the replacement of aged neurons.

Astrocytes are self-sufficient, self-signaling, and self-replicating. Neurons have no reason to exist except to support astrocytes.

Mature neurons do not function alone, whereas mature astrocytes often function without neural input. In a petri dish, neurons quickly expire without astrocytes, while astrocytes survive just fine.

Sensory input is transmitted by neurons to astrocytes, which process the data and signal appropriate motor action that is communicated by neurons. Simple reflexes bypass astrocyte processing, having been hardwired via prior astrocyte programming of neurons.

Astrocytes are the physiological form for thought processing, including pattern matching, memory recall and storage. Any and all decisions involve astrocyte activity.

In studying the effects of cannabis in the brain, researchers discovered the cells most affected in getting stoned.

The starting point for this phenomenon – the effect of marijuana on working memory – is the astroglial cells. Astrocytes modulate working memory. ~ Chinese neurobiologist Xia Zhang

From neuron to astrocyte, neurotransmitters get their message across, then a transmitter is absorbed by the astrocyte, broken down, and resynthesized for reuse. This happens with all transmitters: glutamate, dopamine, serotonin, and so on.

Astrocyte receptors at a neural synapse match the neuron’s transmitter use. In the cortex, that is frequently glutamate. In the basal ganglia, dopamine predominates.

At gap junctions where astrocytes interconnect, astrocytes selectively employ transmitters, though the functions of these channel releases are not yet understood. Astrocyte employment of transmitters differs somewhat from neuron use.

Astrocytes rule the cortex: controlling nourishment from the blood, releasing transmitters at a synapse to control neuronal firing, and dictating the type of signaling that a neuron is to perform. Neurons can fire at different frequencies. Astrocytes tend to invoke short signaling, suppressing long signals at the synapse.

Astrocytes receive signals from quick response messenger neurons, processing the input data into information, and selectively storing the result as memory. In turn, astrocytes stimulate neurons to send information elsewhere, and invoke muscular action.

Glia cells also affect neuron release of vasopressin and oxytocin: hormones that regulate whole body fluid balance, among other things.

Astrocytes maintain synapses: strengthening their connection through transmitter use and growth factors. Without astrocyte maintenance, neurons are stunted and produce few connections. Glia can also eliminate synapses. In short, neurons serve astrocytes, not the other way around.

Astrocyte growth can be a lifelong process. In the lower areas of the brain, such as the olfactory bulb and hippocampus, astrocytes readily regenerate and sprout – some turning into neurons to provide a fast link to cortical astrocyte centers where new knowledge nuggets can be processed and stored.

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The cognitive facilities of humans owe in part to astrocyte evolution.

Astrocytic complexity has permitted the increased functional competence of the adult human brain. ~ American neurobiologist Nancy Ann Oberheim et al

Human astrocytes are 2.6 times longer than those in mice. They carry calcium ion waves through the brain 5 times faster. Humans also have astrocyte subtypes which mice lack.

In one experiment, human glial progenitor cells were placed into mouse brains, where they multiplied and matured into astrocytes. Over several months, the introduced human astrocytes started to replace mice astrocytes. As the human astrocytes took over, the level of calcium signals in the brain increased 3-fold.

Mice with human astrocytes had enhanced neural communication. When tested on a battery of learning and memory tasks, the mice with human glial cells outperformed their mouse-brained counterparts.

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The hippocampus is the brain area especially active when forming new memories. ~80% of large neural synapses in the hippocampus are surrounded by astrocytes. Astrocytes physiologically store memories – not neurons, as commonly supposed.