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Reading Notes: The Neuronal Network

My Digital Brain?

The network made of neurons is digital. That’s what I took from this statement about neuron firing: “A neuron can only fire or not fire; there is no ‘slightly activated’ signal from a neuron” (“Neurobiology” 2013, p. 6). I took this activity to be analogous to digital 0s and 1s: a digitally transmitted signal is either on or off (which explains why digital televisions have either perfect pictures or terrible pictures — there are no in-between states of digitally-transmitted data).

Okay, so the brain is not exactly digital. The neuronal network is a composite of electro-chemical impulses within highly specialized organic cellular structures. But I’m intrigued by the idea that neurons either fire or don’t fire. This suggests that brain functions use on/off switches like digital networks; by extension, this suggests that my ability or inability to remember something is not strictly a neuronal function. Other aspects of brain function beyond synaptic firing are at work when it comes to memory retrieval. That’s not exactly relevant to this summary, but it’s an interesting perspective on brain function as it relates to neuron firing.

The Network

The neuronal network is remarkable. I’ll break down the network into its composite parts as I understand them.

Nodes: When I took notes on the reading, I identified cells in the brain as nodes — neurons, glial cells, and other matter (p. 2). But as I reflect on the network, I’ve decided that the nodes in the neuronal network are more likely sensory inputs and physical or chemical outputs: brain function “can be broken down into three basic functions: (1) take in sensory information, (2) process information between neurons, and (3) make outputs” (p. 1). Neurons throughout the body, concentrated in the brain, represent potential pathways for connective activity (processing information) among network nodes. Signals from sensory inputs travel through these pathways into and out of the brain and back to outputs like organs and muscles, which react to sensory inputs.

Connections: This is where neurons excel. Neurons transmit signals from node to node in the network. Transmission is remarkably complex, involving electrical and chemical impulses along with ionic activity along chemical pathways, assisted by myelin, other glial cells, and neurotransmitters along the way. The signals carried along these neuron pathways are physical, chemical, and other responses to sensory inputs.

Meaning: Within the neuron itself, “meaning” is ionic. Neurons send and receive nerve impulses, or “action potential” (p. 4), within their cell structures, then fire such impulses across synapses using neurotransmitters and neural receptors (p. 5-6) to communicate “messages” to and from sensory, mechanical, and chemical nodes. Those messages are encoded from the nodes as chemical or electrical impulses, but within the neuron itself, those impulses travel as ions. Between the neurons, in their own network of axons and dendrites, impulses travel as neurotransmitters and engage many neurons in the activity of transmitting meaning among input and output nodes: “the activation of a single sensory neuron could quickly lead to the activation of inhibition of thousands of neurons” (p. 6).

Visualization of action potential moving through a neuron's axon.

Action potential movement through an axon. Image archive in Unit 10: Neurobiology chapter, part of the Rediscovering Biology online textbook.

Framework and Activity: Neurons represent the framework in which the network is activated. Without sensory input or some kind of output (chemical, physical, emotional), the network is inactive. The framework exists to enable activity, and that framework actually grows or repairs itself over time by generating new neurons from neuronal stem cells (p. 13). Activity in the network is defined as meaning being sent and received through the neuronal connections by input and output nodes. Activity within the neuron itself is defined by chemical processes. As a result, sensory deprivation could halt the network — but so could a lack of neurotransmitter or destruction of myelin or other glial cells. In other words, the network can cease functioning as a result of deactivating nodes or deactivating connectors.

Memory and Learning

Another node I need to consider, aside from sensory inputs and mechanical or chemical outputs, is the hippocampus, “a structure through which all information must pass, before it can be memorized” (p. 12). Based on this description from the text, I’d suggest that, in network terms, the hippocampus might be considered a node with many incoming connections (a la Latour, 2005) or a network gateway that routes (perhaps mediating along the way) neuronal impulses toward memory storage areas. Memory and learning “involve molecular changes in the brain” (p. 10) and represent a chemical output (and input) node in the neuronal network. Sensory input translated into ionic and neurotransmitter meaning travels through the neuronal network through the hippocampus node to be inscribed as molecular change in the brain; this molecular change (memory) becomes a node that can be activated by other sensory inputs to elicit the output response, “I remember!”

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Training the brain. Not sure I buy the Luminosity premise, but if effective, Luminosity probably results in opening new pathways through the hippocampus toward molecular change (memory). Screen capture from Luminosity landing page (tracked via Google Analytics through Google Adwords as a result of selecting a Google search ad).

Application

I was interviewed by a nurse this morning to help determine my eligibility for an insurance product. Much of the interview appeared to relate to my ability to remember things: 4-, 5-, and 6-digit strings of random numbers, series of words, and random words that I incorporated into sentences. In basic terms, I’m pretty sure my neuronal network never transmitted any of these impulses through my hippocampus. That is, I never committed these items to any kind of memory. The impulses (aural, via telephone) were translated into meaning, which I then output vocally (also via telephone), but they did not result in molecular changes in my brain. The tests, I believe, were not intended to test long-term memory, but to ascertain whether I could process data from sensory input through neural transmission to vocal outputs. In short, is my neuronal network functioning as expected? The answer to that question, coupled with my age, profession, eating and exercise habits, and level of alertness will be used in underwriting the policy to determine whether I will be an acceptable risk for the insurance product. I, of course, believe myself quite eligible — but then again, I didn’t remember those ten random words well.

References

Latour, B. (2005). Reassembling the social: An introduction to actor-network-theory. Oxford, UK: Oxford University Press. Clarendon Lectures in Management Studies

Neurobiology: Unit 10. (2013). Rediscovering biology: Molecular to global perspectives [Online textbook PDF]. Retrieved from http://www.learner.org/courses/biology/pdf/10_neuro.pdf

[Header image: Neural Network Visualization (made from Babybel cheese bag). CC licensed image from Flickr user Rick Bolin]