A good book on the thalamus

Exploring the Thalamusby S. Murray Sherman and R.W. Guillery.Academic Press, 2000. £39.95 (xvii + 300 pages)ISBN 0 12 305460 5Murray Sherman and Ray Guillery, with eight decades of experience between them, have written a book about the thalamus. It is a scholarly study with great depth, touching not only its immediate subject but also addressing broad questions about the whole organization of the brain.The project arose from discussions between Sherman and Guillery during Sherman's 1985 sabbatical at Oxford. One imagines them sitting together at tea – it is believed that Oxonians spend much of their time at tea – and beginning a conversation that was to last for 15 years. The book retains the character of a conversation. It is not a textbook but a report from the trenches, a down-and-dirty look at the brain as it appears to two observers who have seen it from close at hand.The preface states the book's point of view baldly: ‘We wrote this book for ourselves.’ The rest of us are privileged to listen in. The dialog holds tight to the bedrock of structural facts and they are wonderful facts, hinting at a hundred subtleties that we only start to apprehend. A beginner will find here no multi-colored diagrams, a modeler will find few ‘canonical circuits’. The thalamus is presented warts and all, full of unexplained complexity and seen through the lens of incomplete data. Neither will the experienced neurobiologist find a grand synthesis. Instead, he or she will find unanswered questions. Why is the corticothalamic feedback so massive? What happens inside a glomerulus? Is it full of astrocyte, or only partly full? Do all classes of cortical neurons feed back to the thalamus, or does this only occur with selected neuron types? Why are there different numbers of interneurons in the different thalamic nuclei? Each of these and dozens more questions could found a PhD thesis.The book begins with an inventory of the thalamic neurons, followed by chapters on their afferent and efferent connectivity. The authors point out that the thalamus is pretty much in the center of things, not only geometrically but from the point of view of its connections. Through it pass the pathways of vision, hearing and touch. But its major connections are the great streams of axons to and from the cortex. As Sherman and Guillery point out, this radiating pattern is surely key to the role of the thalamus in the greater plan of the brain and it is remarkable how clueless we are to that plan.Later chapters move on to electrophysiology and biophysics. Here, too, the discussion is grounded in biological structure. For example, the authors point out the apparent paradox that the physiology of the sensory nuclei, in waking animals, is dominated by the sensory input yet those inputs make up only a small minority of the total synapses upon the principal relay neurons. ‘A numerically large input might be modulatory with a relatively subtle or finely graded effect, whereas a powerful driver input if it is critically located can have a dominant effect even though numerically it forms only a small proportion of the terminals’. If nothing else, this puts the brakes on the facile notion that physiological responses will soon be predictable from counting synapses.The book is not for neophytes, but it belongs on the shelves of all serious students of the CNS. Reading it reminded me of reading Cajal – after all, Sherman and Guillery are Cajal's scientific grandchildren. The joy of Cajal's writing is in the passion of his thinking – Cajal wrote dialogues, with real or imagined adversaries when possible, with himself when none were available. Exploring the Thalamus has the same flavor, and just as Cajal was usually right so will the questions raised by Sherman and Guillery turn out to be crucial ones. Their book is dense with ideas. It deserves to be read a few pages at a time, with thoughtful breaks, perhaps for tea.