The nervous system is regularly considered by the cognitive sciences to be an information processing hub that analyses incoming sensory information and creates an output in return (Keijzer, van Duijn, & Lyon, 2013). This carries a very distinct, sequential and directional view of interactions between an organism and its environment, and is regularly referred to as the Input-Output View (Keijzer et al., 2013). The neuron doctrine lies in the foundation of this view, as well as many similarities drawn between neural activation and logic gates found in computers (Gardner, 1985). Challenges have began to mount against these foundational stances as more and more evidence contradicts their assertions. Contradictory to the neuron doctrine, neurons have been shown to fire bidirectionally, interact with each other through purely electrical gap junctions, and to be affected by substances other than neurotransmitters such as neuropeptides and cytokines (Keijzer et al., 2013).
The reflex arc is a popular basic representation of the input-output view (Parker, 1919). This representation is believed to show evidence of how nervous systems evolved from more primitive organisms, as it breaks down nervous system activity to it’s component parts. Parker (1919) described how independent effectors must have first appeared to aid the organism, followed by further connections with receptors to guide action. Over time, large conducting nerve cells evolved to connect the two over longer distances and allow for greater varieties of movement. Pantin (1956) criticised this view heavily, proposing that the benefit of effectors is not their individual power, but comes instead from the patterns of contraction over a field of tissue. The coordinated nature of the contractions allow for key, complex movements that individual cells cannot accomplish.
An alternative interpretation of the evolution of the nervous system comes from the Skin Brain Thesis (SBT; Holland, 2003). Nervous systems are not clearly defined and it can become difficult to differentiate them from other conductive tissues (Mackie, 1970). The current defining characteristics of nervous systems are physiological or anatomical, yet functional difference is ignored (Guillery, 2007). The Skin Brain Thesis attempts to create this functional specification and creates a narrative of the possible, developmental sequence of the nervous system. It does so by focusing on the benefits that a nervous system holds over other types of connective tissue, which would have led to it’s evolution (Keijzer et al., 2013).
Within SBT, the myoepithelium is believed to be the precursor of a central nervous system (Arendt, 2008; Miller, 2009). This external surface can act as a conductor of activations in all directions, as all cells are connected in a unbroken sheet of gap junctions (Mackie, 1970). As such, the sheet acts as a sensory, conductive and effector system all at once. This system can be effective for motility, and allows for complex movements without the need for any nervous system (Seipel & Schmid, 2005). However, as the size of the organism grows, this method of movement becomes unviable. Larger animals require more robust structures for motility, such as muscle-based movement. Furthermore, a conductive epithelium only allows the body itself to be the antagonist to contracted tissue, as the diffused activations don’t allow for local specialisation (Mackie, 2004).
For local specialisation to occur, a shift was required from electrical transmission to chemical transmission (Mackie, 2004). Chemical transmission allows for excitation speeds to differ across a medium, and also allows for inhibition. It is hypothesised that this shift towards a chemical transmitter epithelium allowed for the development of nervous systems as we know them today (Keijzer et al., 2013).
Conductive tissue is only functional if there is a continuous unbroken sheet of cells to carry the activations. However, certain organisms would receive benefits from being able to carry excitation signals through certain tissues without activating it. Thus, the development of centralised neurons that could carry information over longer distances were adaptively useful (Jeltsch et al., 1997). Long-distance connections allow for wave like patterns to travel over much larger surfaces with ease, as well as removing the need for unbroken surfaces. With this change, the specialisation of certain muscle groups were possible, creating much more flexible structures.
When looking at the development of the nervous system through these lens, we can see that the reflex arc is not a simple presentation of a primitive nervous systems, but just a simplification of a complex and well-developed self-sustaining structure (Keijzer et al., 2013). Moving from the input-output view towards SBT changes our interpretation of organisms from automated reactors to self-sustaining systems that modify their endogenous activity to adapt to outside perturbations (Passano, 1963). The nervous system comes about from a series of additions and adaptations that allowed for more successful self-sustaining patterns of activity within the organism, and not because it increased the power of information processing (Keijzer et al., 2013).
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