The Languages of Neurons: an Analysis of Coding Mechanisms by Which Neurons Communicate, Learn and Store Information
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Entropy 2009, 11, 782-797; doi:10.3390/e11040782 OPEN ACCESS entropy ISSN 1099-4300 www.mdpi.com/journal/entropy Article The Languages of Neurons: An Analysis of Coding Mechanisms by Which Neurons Communicate, Learn and Store Information Morris H. Baslow Nathan S. Kline Institute for Psychiatric Research, Center for Neurochemistry, 140 Old Orangeburg Road, Orangeburg, NY 10962, USA; E-Mail: [email protected]; Tel.: +1-845-398-5471; Fax: +1-845-398-5472 Received: 17 August 2009 / Accepted: 30 October 2009 / Published: 4 November 2009 Abstract: In this paper evidence is provided that individual neurons possess language, and that the basic unit for communication consists of two neurons and their entire field of interacting dendritic and synaptic connections. While information processing in the brain is highly complex, each neuron uses a simple mechanism for transmitting information. This is in the form of temporal electrophysiological action potentials or spikes (S) operating on a millisecond timescale that, along with pauses (P) between spikes constitute a two letter “alphabet” that generates meaningful frequency-encoded signals or neuronal S/P “words” in a primary language. However, when a word from an afferent neuron enters the dendritic-synaptic-dendritic field between two neurons, it is translated into a new frequency-encoded word with the same meaning, but in a different spike-pause language, that is delivered to and understood by the efferent neuron. It is suggested that this unidirectional inter-neuronal language-based word translation step is of utmost importance to brain function in that it allows for variations in meaning to occur. Thus, structural or biochemical changes in dendrites or synapses can produce novel words in the second language that have changed meanings, allowing for a specific signaling experience, either external or internal, to modify the meaning of an original word (learning), and store the learned information of that experience (memory) in the form of an altered dendritic-synaptic-dendritic field. Keywords: biosemiotics; codes; cognition; language; learning; memory; neurons; semiosis Entropy 2009, 11 783 1. Introduction Semiosis is broadly defined as any form of activity, conduct or processes that involve signs including the production of meaning, and semiotics is broadly defined as a study of these processes and their significance in communication. A semiotic system has also been more narrowly defined [1,2] as consisting of signs, meanings, a code and a codemaker and it has been proposed that this code model is a first step toward a scientific understanding of biosemiotics, or biology interpreted as a sign system study, a classification based on the general assertion that all living organisms are semiotic systems. Further, that biosemiotics as a recognizable scientific discipline requires proof that the individual cell is a semiotic system [1]. Based on the more narrow definition, the codemaker is described as the genetic code which determines the nature of proteins synthesized by cells, and that signs and meanings are codemaker-dependent entities. In this article, both the broader and more narrow definitions of semiotics at the cellular level are considered, and an attempt is made to bring together current research in the natural sciences on how cells communicate, learn and store information. In multi-cellular animals, neurons are cells specialized for rapid communication of meaningful information, and their interactions on the cellular level form the basis for semiosis at all higher levels of complexity in all animal forms of life. In this paper evidence is provided that individual neurons possess language (production of meaningful signs), based on codemaker generated “sensors” and that the basic unit for communication (interpretation of signs) consists of two neurons and their field of interacting dendritic and synaptic connections. In order to better understand biosemiotic interactions between neurons there are several premises which form the basis for such an analysis. These are: 1. Encoded messages sent by afferent neurons are meaningful. 2. Messages received by efferent neurons can be interpreted. 3. Messages received by efferent neurons can be acted upon. 4. There is a feedback system between neurons to ascertain successful communication. 5. Meaningful messages can be changed to new meaningful messages that reflect useful changes induced by either external or internal experiences. 6. Communication is an energy consuming process. While the structure of neural codes in complex systems remains to be resolved [3,4], in the following sections, coding methods used by individual neurons to communicate are explored, and possible mechanisms whereby messages can be altered by experience (learning) and the learned information of that experience (memory) can be stored over long periods of time for use in higher cognitive functioning. 2. Neuronal Languages 2.1. Energy requirements for communication The basic function of neurons is to communicate, and they do this by generating coherent intracellular electrophysiological signals, in the form of action potentials (AP’s) or “spikes”, at various encoded frequencies [5,6]. These signals are then translated into a variety of frequency-encoded neurochemical signals transmitted to other neurons at synapses, and subsequently interpreted at some Entropy 2009, 11 784 level in the central nervous system (CNS) neural network [7,8]. Since formation of spikes is an energy expensive process, calculated as using 2.2 × 109 adenosine triphosphate (ATP) molecules per spike [9], it has been proposed that AP’s are generated only when required for specific tasks and that neurons are electro-physiologically inactive most of the time 2.2. Neuronal language formats While information processing in the brain is highly complex, each neuron uses a simple code mechanism for transmitting information. This is in the form of temporal electrophysiological action potentials or spikes (S) of about a 1 millisecond (ms) duration that, along with pauses (P) between spikes constitute a two letter “alphabet” that generates meaningful frequency-encoded signals or neuronal S/P “words” in a primary language. The term “word” for neuronal activity in spikes/s has previously been used to describe the electrophysiological activity of the visual interneuron of the blowfly where each AP was considered a “short” word and trains of AP’s considered “longer” words [10]. In this article a neuronal word is defined as a single AP together with the pause before the next AP (the “short” word) in a ms timeframe. The “longer” word is defined as a “phrase” consisting of a repeated group of two or more short words preceded by, and followed by pauses, and a sentence as a temporal grouping of such words containing a subject, and a predicate that expresses what is stated about the subject. All neuron codes, whether continuous trains of spikes that are characteristic of information transmitted at low frequencies, or bursts of spikes that are characteristic of information transmitted at high frequencies, are made up of interactions between spikes and pauses. There are two theories regarding how these spikes carry encoded information [7]. One is the “spike rate code” which suggests that information is carried in the average rate at which the neuron fires, and that the timing of each spike is random. The other is the “spike timing code” that suggests that specific information is carried not only in the average rate at which a neuron fires, but also in the precise timings between each spike. While it is difficult to investigate how information might be encoded within the intact CNS [3], a study of how some receptors sense and transmit information is informative. In a study of the isolated salt receptor of the blowfly [11], it was observed that information about NaCl molarity was transmitted using a frequency code in the range of 2–120 Hertz (Hz), evident as a linear association between the mean receptor frequency response and the logarithm of molarity (spike rate code). From these results it is also possible to derive the S/P encoded words in a millisecond timeframe (spike timing code) that is associated with each NaCl molar concentration experienced by the receptor (Table 1). Coding mechanisms of the salt, as well as other taste receptors of the fruit fly are similar to that of the blowfly, and have been reviewed [13]. Similar responses have been observed in hamster fungiform taste receptor cells exposed to acid stimulation, where the frequency of spikes varies directly with increasing acidity between pH 2.5 to 5.5 [14]. In this case, the frequency was almost linear, with a mean frequency response of 3.5 Hz (S1P286) at pH 2.5, and a frequency of 0.25 Hz (S1P4000) at pH 5.0. In a study of the responses of receptor B-cells of the antennal taste sensilla of the ground beetle to NaCl and several other salts, the rate of spiking was observed to be increased directly with NaCl concentrations of 0.001, 0.01, 0.10 and 1.00 M resulting in spiking codes of approximately S1P1000; S1P71; S1P29 and S1P23 respectively [15]. Importantly, these cells were also able to distinguish between the different salts tested including NaCl, KCl, sodium acetate, CaCl2 and MgCl2 evident by their production of different Entropy 2009, 11 785 spiking codes for each salt. Thus, use of S/P language in this case was clearly quite rich in the meanings that could be transmitted. Table 1. Response of the isolated salt receptor of the blowfly to the molar concentration of NaCl at 25.5 °C and 80% relative humidity a. Relative NaCl Signal [Hz] S/P encoded wordc refractivenessb (M) (spikes/s) (ms time frame) (ms between spikes) 0.10 2 499 S1P499 0.20 22 45 S1P45 0.25 36 27 S1P27 0.50 52 18 S1P18 1.00 80 12 S1P12 2.00 104 9 S1P9 3.00 120 7 S1P7 a Adapted from [11] b Assumes the total spike length (SL) including an absolute refractory period is 1 ms [12] c P = [1,000 ms – (Hz × SL)]/Hz Single glucosensing neurons in the hypothalamus of the brain can sense the concentration of glucose (Glc) in their environment and respond with changes in spiking frequency [16-18].