S2C6: The Nervous System

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a) Transmitting information

Survival, reproduction, information – it seems to be all about them isn’t it. That holy trilogy. In fact, survival is overrated and isn’t an end unto itself, only the means to buy the time to replicate, after which it doesn’t really matter anymore what happens to the organism from the perspective of the fundamental replicators, the genes, except to the extent the parents of the offspring still fulfil a protective role. If reproduction is the aim, then manipulating information is the ultimate competitive advantage in the evolutionary arms race, it provide organisms that can store it, transmit it and process it with superior control over their own complexity and take actions which improve their odds of survival and of reproducing.

As one would expect, complex organisms have therefore evolved ever more sophisticated software and hardware to deal with information. Arguably, the first places to observe information management in action is not us solving complex mathematical problems, it is homeostasis and body movements. Over the course of the five preceding chapters in this Series 2 we have seen countless examples of processes, literally cascades of chemical reactions enabling us to survive, one minute at a time. Not trying to sound dramatic here but when a major disruption occurs, such as our heart ceasing to beat or immune system either overdoing it or proving deficient, then the clock is running down fast. Therefore, maintaining homeostasis may not rise to the level of our consciousness but it needs to happen, continuously.

As we have seen with the endocrine system in particular, regulation is key and can be broken down into three components: the receipt of information, its processing, and a resulting action. I will not go into the mechanics of information processing right away; however it is necessary to understand the fact that a decision coming out of this processing introduces the concept of timing and the fact that the processing is linked to receipt of information, quite often from various parts of the organism, introduces that of coordination. The importance of timely and coordinated action becomes transparent when we think of movement, and even in the micro-adjustments in muscle tension enabling us to remain stable instead of losing our balance. How does this all work then? More specifically, how is information transferred across the organism and between which types of cells? We’ll look first at the mechanics underlying the transfer of information and in the subsequent sections we’ll direct our gaze towards the various organic information networks within our body.

Amidst the chemical processes we previously explored in relation with the musculoskeletal system, we learned in S2 Section 3.d about the concepts of neurotransmitters and membrane polarization. Neurotransmitters are potent and crucial but not dissimilar to other signalling biomolecules, their main distinction is their birth pedigree since they are secreted by neurons. Phenotypically, these cells are characterized by the presence of a downstream projection called axon and a tree of upstream fibres called dendrites. The driving force behind this morphology is without doubt its function in integrating action potential and firing one or none in turn, a behaviour known in the aggregate as “taking a decision”. Because have no doubt about it, even though most of our conscious decisions happen at scale and involve massive neural networks, each node in the network is involved in the overall network output so each neuron decision adds something to or subtracts something from the overall outcome. We’ll revert to this integrated computing aspect in section d) when providing an overview of the process of learning.

This raises the question of the nature of the signal conveying information and the answer is actually quite straightforward, it is the propagation or not of an electrical signal, and the pattern created by such binary on/off bits of information. This should be reminiscent of the way electrical current is used in telecommunication, as briefly described in S1 Section 8.a except the signal neurons cannot modulate the intensity of a signal but a network of them can by firing simultaneously. And if you are unconvinced by what patterns of on/off messages can achieve in terms of information volume, detail and usefulness, just remember computers rely on 0s and 1s.

Physiologically, axons and dendrites are highly susceptible to changes in membrane potential, the difference in electric charge concentration between the interior and exterior of the cell. At rest, the membrane potential of a neuron is normally slightly negative (about -70mV) as some of the potassium ions (K+) diffuse out of the cell through the membrane. However, some stimulus can lessen the membrane potential, a process called depolarization, and if this crosses a threshold then a chemical cascade ensues where ions channels in the membrane open to let sodium cations (Na+) out or another set of channels let chloride anions (CL^–) in, or both get activated. This creates a hyperpolarization and therefore an electric current propagating along the membrane. If triggered, a spike in electrical current will travel at the axon hillock, an area of the cell body, and travel down the axon and causes the release of neurotransmitters stored in a vesicle on the axon-side of the synapse, the space between the branches of an axon and those of a dendrite. On the dendrite side are receptors that will activate if and when the specific neurotransmitters bind to them after crossing the synapse and this may generate either an excitatory or an inhibitory post-synaptic action potential. All these inputs are then integrated and an action potential may or may not propagate to the neuron cell body. This is why I described the dendrite as being upstream, the axon as being downstream and the individual neuron outputs as adding to or subtracting from the overall network output.

A diagrammatic representation of a motor neuron is enclosed in Figure 3, it shows the dendrites, axon and cell body with a zoom-in insert illustrating the synaptic neurotransmitters and receptors.

Figure 3: Diagram of a myelinated vertebrate motor neuron

Credit: LadyofHats in the Public Domain

b) Voluntary and autonomous nervous systems

These networks of neurons can be arranged in a chain-like fashion with sometimes very long axons extending towards all parts of our body as part of the peripheral nervous system, named thus by opposition to the central nervous system located within our spinal cord and brain. Such chains of neurons with long axons are bundled together into nerves, not without being isolated from each other by a protective tissue called endoneurium.

There are three types of neurons when classifying them by function and it should be noted they can also be categorized depending on their morphology, their connectivity, or their neurochemistry. Going by function, the most common type is your traditional connector between two other neurons; it goes by many names, the most frequently used being “interneuron”. The specificity of the two other types of neurons is a direct consequence of information outside of the brain essentially flowing in two directions: stimuli are collected by our various sensory apparatuses and conveyed to the brain for processing and the output of said processing is at some stage sent back down towards particular groups of muscles to create contractions, which translate into movements or action. These bi-directional flows of information are made possible by sensory and motor neurons.

The role of sensory neurons, also called afferent neurons, is to convert a certain signal originating from a sensory receptor into an electrical signal that can then propagate along the nerves. This conversion process is called transduction and it generally involves the conversion of mechanical energy, such as the vibration of a membrane. We will explore the exact nature of transduction and sensory receptors underpinning all our senses in the following chapters of this series. When I stated the axons in the nerves could be long, this was no exaggeration since most of the cell bodies of sensory neurons are located within our spinal cord.

In contrast, motor neurons transfer information away from the central nervous system and are thus called efferent neurons – in this instance, as in many others, knowing the etymology does help remember technical terms since in Latin “adferens” means “bring toward” and “efferens” means “carry out of”. The motor neurons control glands, to induce the synthesis and secretion of chemical substances such as hormones, and they also innervate muscle fibres and through the emission of action potential, a phenomenon colloquially known as “firing”, they may initiate the chemical process resulting in muscle contraction, as we learned in S2 Section 3.b. Those motor neurons which signal directly to the muscles are sub-categorized as lower motor neurons and the upper motor neurons are those making the link from the brain, sometimes by way of interneurons.

This distinction between upper and lower motor neurons announces another, this time between the central nervous system and the rest of the nervous system collectively known as the peripheral nervous system. This division may seem somewhat arbitrary but functionally and physiologically it does have some merit. The central part takes charge of most of the processing and is the ultimate arbiter, it is where big decisions are taken, including the regulation of subservient organs and systems, whereas the peripheral components relay those orders and do not benefit from the same bunker-like protection afforded by the blood-brain barrier. To use a simple metaphor, the central nervous system is the commander-in-chief safely tucked in the head-quarters and the periphery is directed on the ground by the officers taking their strategic orders from HQ, yet they retain the ability to make immediate tactical decisions.

Biological sciences further sub-divide the peripheral nervous system into somatic and autonomic. The use of the word autonomic can be somewhat misleading and should not be understood in the sense of autonomous, rather it conveys the absence of volition and its unconscious character. The role of the autonomic sub-system is that of a local central nervous system specialized in the regulation of processes, mostly homeostatic in nature, which need either no heavy computing or no integration from sensory stimuli. Accordingly, a lot of the orders it emits in the form of neuron firing are reflexive in nature: if X occurs then do Y. This might be oversimplifying but the algorithms are seldom multi-layered as they are in the central nervous system. Think about almost any vital function your organism implements without you having to think about it in a conscious manner and it is likely to be regulated autonomically: heartbeats and the movement of muscles allowing us to breath should come to mind.

The second peripheral nervous system sub-division goes under the name of somatic or voluntary nervous system and comprises mostly of both the motor and sensory neurons linking the central nervous system with the muscles, glands and sensory receptors. In terms of anatomical organisation, this system is traditionally described based on the nerves it forms, about 70% of them are anchored in the spinal cord and the remaining 30% called cranial nerves connect to the brain stem.

c) Anatomy and functions of the central nervous system

It is now time to go beyond the references to spinal cord, brain stem and generally the brain and describe the anatomical makeup of the central nervous system. I will not dwell too much into the details and try to ascribe precise functions to each individual area because a lot of the more evolved capabilities of our brain are the result of distributed processing across various of these areas. One thing which should become clear is the evolutionary development chronology away from the brain stem where primitive and vital functions are carried out towards more complex cognitive functions performed in the forebrain, such as the prefrontal cortex that not only evolved fairly recently but only matures well after our teenage years.

Perhaps the easiest place to start is with the spinal cord and then we can make our way up to the brain stem and radiate outward within the brain. The spinal cord is housed within our vertebral column, it starts at the base of the brainstem, at the back of our skull, and descends all the way to the posterior part of our pelvis, between the first and second lumbar vertebrae of our backbone. As we have seen previously, it mainly serves as an attachment point for the various spinal nerves made of either efferent motor neurons or afferent sensory neurons. Yet, its strategic location, physically much closer from most parts of our organism than our brain is, makes it an ideal candidate for evolution to defer reflexive processing there when time is of the essence in order to shave off tens of milliseconds versus the alternative of transmitting the information all the way up to the brain and then back down to activate our muscles. And this is indeed what happened, no surprise there.

The brainstem lies at the back and bottom of our brain, where the spinal cord connects to the rest of the central nervous system. It is subdivided into the medulla, the pons and the midbrain, the latter encompassing further smaller structures. Part of the brainstem functions are very similar to those of the spinal cord and have evolved to be nested there simply because it is so much closer to the area they control, namely the head and the neck through cranial nerves. Being at the root of the brain and therefore of the bulk of the central nervous system would suggest many homeostatic mechanisms, those constantly ensuring a living organism doesn’t turn into a dead one, are taking place there. This is indeed the case as it houses key regulating functions relating to breathing, sleeping and blood pressure in addition to being involved within the various sensory systems. Let’s take the example of respiration; a lot of information from various sources is integrated and then processed in the brainstem before carrying out the necessary adjustments in terms of blood pressure, pH level, gas pressure within the circulatory system, etc. This information originates from mechanical and chemical receptors as well as other parts of the brain like the hypothalamus.

Just behind the brainstem, at the back of the neck, lies the cerebellum (in Latin it can be translated as “little brain”). As far as research indicates through experiments and observations, the primary role of this brain area is to process sensory inputs received by way of the spinal cord and the brainstem and to coordinate motor activity. A standard modus-operandi for finding out the role of an organ and in particular when it comes to brain areas, since direct observation is impossible, is to take note of the various issues arising upon injury. In the case of the cerebellum, disorders include maintaining balance and deficiencies in motor learning. It also seems to play a central role in regulating emotions, behaviours and in cognitive processes including language-related tasks.

Above the cerebellum, part of the forebrain yet quite centrally located within the mass of the brain, is the thalamus. This region is often described as a sensory relay between the subcortical area (meaning below the cerebral cortex) and the cerebral cortex, though it clearly also does some data processing as well. One can imagine a time when the final processing was carried out there but, as our cognitive capabilities developed alongside the cerebral cortex, the thalamus started to direct more complex tasks to specialized areas located there. Going by the same logic, we shouldn’t expect the functions of the thalamus to be restricted to the domain of sensory information and indeed it is also involved in the sleep process and in regulating our level of alertness.

Around the thalamus is a set of structures clubbed into what is known as the limbic system; these include some components which may perhaps ring a bell such as the amygdala, the hippocampus and the hypothalamus. We have already come across the latter as a duo formed with the pituitary gland in the context of the neuromodulation of the autonomic nervous system and several key endocrine systems (refer to S2 Section 4.b). It has a regulatory hand in an extensive list of processes including sleep, the desire to eat or drink, and even our circadian rhythm – our body’s internal clock. The amygdala’s role is entirely different and it is most activated when fear arises, an emotion based on the association of certain memories with adverse events, which from an evolutionary standpoint would trigger a fight-or-flight type of behaviour. The involvement with respect to memory isn’t exclusively “negative” however and this area seems to have a pivotal role in overall memory consolidation, alongside the hippocampus. On this basis, the limbic system is often described as being the centre of our emotional state and having a strong influence on our behaviour. Fortunately, these emotions are often tampered and the behaviours overridden by our cerebral cortex, in particular an area called the pre-frontal cortex.

The cerebral cortex encompasses the outer layer of our brain and, as the last part of this organ to have evolved from an evolutionary standpoint, it is the one embedding the higher-order processing tasks and cognitive skills. For our purpose here, we should think of cognition as the ability to understand, learn and reason. I can’t possibly describe all the roles and zones but will simply mention they are classified into 4 lobes with their own sub-areas: the frontal, temporal, occipital and parietal lobes.

The occipital lobe is positioned at the back and acts as the primary location for visual processing;
the temporal lobe occupies the sides of the two hemispheres of our brain and deals with language processing and memory storing and retrieval;
the parietal lobe is at the top of our brain and focuses on the sense of touch and the somewhat related spatial awareness and proprioception; and
the frontal lobe is big on motor control and reasoning, which includes the understanding of social norms and context, and given the chance it will often try to overrule the lower-order instinctive reactions driven by the amygdala.

In summary, the central nervous system interprets sensory stimuli, it optimizes the response of the organism based on this information – and behaviour is an external instantiation of said response, it controls our muscles and modulates several systems involved in the body’s homeostasis. The optimization aspect entails several cognitive processes designed to improve the quality of our response such as memory, learning, assessing situations and the potential consequences of our action, and reasoning. As for emotions, it might be more perceptive to think of them more as an outcome or symptom rather than a process while feelings are altogether of a different nature and the phenomenology that makes them possible is still a matter of both scientific research and philosophical speculation. In my book Higher Orders, I have explained the nature of qualia, narrowed down the range of plausible and probable physical and chemical origins behind this phenomenon which permits us to see colour, feel pressure and hear sounds, and advanced personal hypotheses as to the integration process making unified and seamless representation possible after processing of external stimuli by our brain.

That is quite the job scope for the central nervous system, isn’t it?

If we further abstract all these tasks, it all comes down to improving the odds of survival and reproduction by remaining alive through the combination of internal processes (homeostasis) and figuring out how to best interface with our environment (cognitive functions).

d) Memory, learning and intelligence

Before wrapping up this chapter I thought it would be odd not to at least discuss briefly some of the higher cognitive functions our brain is capable of, including the ability to read those words and understand the ideas they convey. Consequently, we shall go through an overview of the learning process and of the nature of intelligence. This section is going to be significantly more abstract than the previous ones and, as a disclaimer, a lot of the ideas expressed here are the result of my own analysis and conclusions, so reader beware.

Learning is making the connection between two events or data points, deducing or inferring relationships between them. This seems like an oversimplification but just as biological organisms are no more that the result of complexity applied to elementary particles and fundamental interactions, then any skill and understanding can be broken down into a chain of relationships. You learn to read by deciphering letters and eventually ascribing meaning to words and sentences, you learn to play the piano by figuring out how to move your fingers and apply pressure to the keys in a coordinated manner. Sometimes a one-off observation is sufficient to infer a relationship, though such inference may also occasionally turn out to be inaccurate, but most of the times it requires several experiences to notice a correlation. The next step in the build-up of knowledge at scale is our ability to generalize these relationships by applying them to proximal concepts, proceeding by analogy.

Our ability to generalize and infer relationship also relies on our ability to encode and store information, if that capability was not there then we could not discern patterns in repeated observations, nor could we store knowledge and enshrine skills in our brain by “burning them into the neural circuitry”. This information storage is what we call memory, and it is physically instantiated into particular firing patterns of specific networks of neurons. It is worth highlighting that memories can also be retrieved, including unconsciously, otherwise we couldn’t learn.

So what makes us intelligent then, is it only the capacity to remember and learn? The Cambridge dictionary defines intelligence as the “the ability to learn, understand, and make judgments or have opinions that are based on reason”. That’s very broad and it is not inaccurate, nonetheless it doesn’t convey the idea of degrees of intelligence because I trust it isn’t contentious to affirm intelligence is a continuum, not a binary property or skill.

Hence, to conclude this section I would like to quote from Higher Orders, a book in which I also discuss consciousness, qualia, and artificial intelligence: “Superior intelligence is not so much about higher speed in reasoning as connecting concepts into novel ways, detecting patterns and therefore discovering relationships between them. This allows for the creation of ever more abstract concepts such as mathematical symbols, for their manipulation, for counterfactual thinking (extrapolating what might or could have been), and finally for planning, which is a multi-step chain of reasoning.”

e) Trivia – Diving reflex

To conclude this chapter and Part A of this second series, we will look at a topic that is a little more trivial though still very much connected to what we have learned thus far: the diving reflex.

When it comes to survival, the single most important parts of complex organisms requiring protection are the central nervous system and the circulatory system since the latter distributes nutrients and oxygen to the rest of the body, making energy production possible and allowing the continuation of the most essential cell processes. Without it, homeostasis cannot be maintained and even near-term survival is out of the question. The diving reflex, most pronounced in aquatic mammals because of the liquid environment in which they are immersed when they are not breathing at the surface but present in land mammals as well, seeks to enhance the odds of survival of the animal when it is unable to breathe and bring in new oxygen molecules in circulation.

This is mostly achieved by a two-pronged physiological response: the slowing down of oxygen consumption through a reduction in the heartbeat rate and a preferential allocation of oxygenated blood to the critical organs mentioned earlier, namely the heart and the brain. In humans, the triggers are the wetting of the face and the touch of cold water with maximum response when both are combined and the face is immersed in cold water. If you happen to watch the movie “the Big Blue”, they do show a scene with the drop in heartbeat rate freedivers experience, similar to aquatic mammals.

The key underlying chemical and mechanical processes involve chemoreceptors providing information about the oxygen and carbon dioxide pressure suggestive of breath-holding, and a downregulation of the heartbeat ensues accompanied by peripheral vasoconstriction which redirects blood flow away from the extremities and towards the vital organs, a phenomenon known as blood shift. The information regarding bloodstream gas pressure provided by the chemoreceptors also leads to a release of oxygen-carrying red blood cells from the spleen. It’s all about survival.

a) Transmitting information

b) Voluntary and autonomous nervous systems

c) Anatomy and functions of the central nervous system

d) Memory, learning and intelligence

e) Trivia – Diving reflex

f) Further reading (S2C6)