The human brain
Imagine, if you will, a journey that began with the intricate workings of the human frame itself. In his earlier exploration, "The Human Body," Isaac Asimov laid bare the structures and operations that make us a physical entity. But a body, however wonderfully constructed, is merely a collection of parts unless these parts are coordinated into a seamless whole. It is to this marvel of organization and control that Asimov turns his attention in "The Human Brain: Its Capacities and Functions".
Our story begins not within the folds of the brain itself, but with the body's first system of control: the chemical messengers, or hormones. As Asimov guides us, we see how glands like the pituitary, adrenal, and thyroid, under the brain's watchful eye, dispatch these molecules to regulate a myriad of bodily activities. He introduces us to substances like secretin, a protein that orchestrates the pancreas' digestive juices. Even for such a seemingly straightforward task, the underlying chemistry involves intricate protein molecules, composed of chains of amino acids arranged in precise sequences. Asimov, with his knack for clarity, demystifies these complex structures, providing just enough detail for understanding without overwhelming the reader. We learn of insulin, a hormone produced by the islets of Langerhans in the pancreas, and its crucial role in managing the sugar levels in our blood. The isolation and structural determination of insulin, a landmark achievement in biochemistry, showcases the painstaking efforts to unravel the body's chemical intricacies.
Having established the foundational layer of chemical control, our narrative shifts to the more rapid and intricate network of the nervous system. Asimov leads us step by step, from the fundamental unit, the nerve cell or neuron, with its projections called axons and dendrites, to the complex organization of the spinal cord and the various divisions of the brain. He explains how nerve impulses, electrochemical signals that travel along these fibers, facilitate communication throughout the body. The speed of these impulses, a topic once considered beyond human measurement, varies depending on the nerve fiber. We encounter the giant axons of the squid, a biological marvel that allowed early neurologists to probe the mysteries of nerve function. The crucial role of chemical transmission at the synapse, the junction between two neurons, is elucidated through the action of acetylcholine. Substances that interfere with this transmission, like the arrow poison curare and nerve gases, demonstrate the delicate balance of this system.
As our story progresses, we ascend to the pinnacle of the nervous system: the brain itself. Asimov guides us through its major components, from the ancient brain stem and cerebellum, responsible for fundamental functions like balance and coordination, to the grand cerebral hemispheres, the seat of higher thought and consciousness. He touches upon the cerebrospinal fluid that bathes and protects this vital organ. Within the cerebrum, the outer layer, or cerebral cortex, is where sensory information is processed and motor commands are initiated. Asimov introduces the concepts of sensory and motor areas, painting a vivid picture of how different parts of the body are mapped onto the cortex. The electrical activity of the brain, detectable through the electroencephalogram (EEG), reveals the constant underlying activity of this complex organ, even during sleep. Conditions like epilepsy highlight the delicate balance of neural activity and the potential for disruption.
Finally, our narrative arrives at the most enigmatic aspect of the human being: the mind. Asimov delves into the capacities that arise from our highly developed cerebrum, exploring the nature of senses, reflexes, and the intricate process of learning. He considers the difference between innate behaviours and those that are learned through conditioning and even a more abstract form of "etherealized trial-and-error" that we might call reason. The role of memory as a cornerstone of higher cognitive functions is emphasized, highlighting the vast storage capacity of the human brain. Asimov contemplates what sets the human mind apart, suggesting that it is not merely the possession of reason but perhaps more significantly the development of speech, allowing for the sharing and accumulation of knowledge across individuals and generations. He even touches upon the emerging field of psych biochemistry, hinting at the chemical underpinnings of mental states.
Throughout this exploration, Asimov maintains his signature style: clear, informal, and meticulously attentive to detail. He tackles a difficult subject with simple words, ensuring that the layman can grasp the fundamental concepts. Like a seasoned storyteller, he builds his narrative logically, moving from the simpler systems of bodily control to the breathtaking complexity of the human brain and mind. While not revealing any dramatic "plot" in the fictional sense, the book unfolds as a fascinating journey of discovery, revealing the intricate mechanisms that make us who we are. It builds upon the foundation laid in "The Human Body" by explaining the "how" behind the "what," and it subtly foreshadows deeper explorations into the molecular basis of life and heredity, themes further developed in works like "The Genetic Code". The journey concludes not with a definitive answer to the mysteries of the human mind, but with a sense of wonder at its unrealized potentials and the ongoing quest to understand ourselves.
