Dervla O’Malley, University College Cork, Ireland
With his long-standing research career focused upon hypothalamic function, Gareth Leng, a Professor of Experimental Physiology at the University of Edinburgh, illuminates, entertains and informs the reader on the multifunctional actions of the hypothalamus. Far from being the evolutionarily primitive organ that it is often regarded as, Leng explains that the hypothalamus is in fact a highly evolved structure, which regulates all of the base drives: hunger, thirst, metabolism, body rhythms, stress, growth and reproduction. The book title refers to the hypothalamus as “The Heart of the Brain”, and indeed, it also drives irrational behaviours such as passions, biases and instincts: essentially, all of the things that make us human.
Apparently, if one curls the tongue back as far as it will go and presses it against the roof of the mouth, the hypothalamus will almost be resting on its tip. Leng alludes to the complex nature of the hypothalamus by describing it as the “Europe” of the brain, as it encompasses a confusion of small nations (neuronal nuclei controlling different functions), each one noisy, heterogeneous and sometimes strident. Each nation contains multiple clans (neuronal clusters) that use a variety of languages (neurotransmitters and neuropeptides) and other signals that act at diverse spatial and temporal scales to communicate with other clans and neuronal nations. The clan members differ from each other but are more similar to each other than to members from other clans. Leng goes on to contrast the actions of neuropeptides, which are striking in their ability to orchestrate complex behaviours by coordinating different systems, with the release of neurotransmitters. He eloquently describes neurotransmitters as “whispered secrets that pass from one neuron to another at a very specific time and place” whereas, “peptides are public announcements, broadcast to whole populations.”
On the topic of obesity, Leng aims to dispel the dogma that the prevalence of obesity is due to over eating and under exercising. Indeed, actual caloric intake has not changed that much since the 1940s, and whilst sedentariness has certainly increased, more people now consciously exercise. Leng explains that obesity is primarily a disease caused by hypothalamic dysfunction, and is primarily down to our genes. When we are in a state of starvation, the hypothalamus defends our body weight by reducing metabolic rate and energy expenditure by inhibiting reproduction and immune activity, and by focusing on the search for food. In obese individuals, resistance to leptin is often accompanied by low mood, lethargy, susceptibility to minor infections and a continuous hunger. Leptin resistance is difficult to reverse and is probably exacerbated by weight-loss dieting.
However, Leng’s interest in oxytocin neurons is the common thread throughout the book. The cell bodies of oxytocin neurons are aggregated in hypothalamic nuclei and their axons extend down to the pituitary gland, which more closely resembles a “chickpea” in size rather than a pea, as Wikipedia states. Leng describes the long and winding path which finally led to understanding the random complexity needed in oxytocin neural nuclei in order to regulate the milk-ejection reflex, an icon of neuroendocrinology. However, he often refers back to this neural cell type as he explores appetite and social behaviours.
The book aims to celebrate the hypothalamus, and in particular its role in regulating our sometimes erratic behaviours. However, Leng also wanted to use this book to display the imaginative part of science: how ideas begin, how theories arise from observations and how they are then tested. Although Leng hopes that his book should be comprehensible by “any thinking person willing to listen”, the terminology and language used means that this book will be of most interest to the specialist reader. As a fellow lecturer in neurophysiology, each year I try to parallel the complexity of the hypothalamus with the mundanity of the functions that it controls. This book is both educational and enjoyable to read, and many of my learnings, both about the workings of the hypothalamus and the scientific process, will be shared with future neurophysiology students. In a quote that would resonate with many undergraduate students, Leng states that, “A message is only a message if it can be understood by its recipient. Neurons can’t decipher long and complex sentences. They have a short attention span, are easily distracted, and much of the time they aren’t even listening.”