Defining the functional components of the thalamic gate

This chapter starts by summarizing the electron microscopic appearance of the retinogeniculate axons and their immediate environment. These form the functional components of the visual input to the thalamic gate. I then look at evidence that all major thalamic relay nuclei have a shared structure produced by a shared developmental and evolutionary origin. Each nucleus receives a small proportion of its synaptic inputs (<10%) for relay to the cortex; these are the drivers. Drivers are topographically organized with the topography representing body parts, sensory space, or parts of the brain. Some drivers come from sensory pathways or from subcortical regions of the brain, and these innervate first-order thalamic relays; another, major part of the thalamus receives its drivers from the cerebral cortex itself, and these form the higher-order relays to the cortex. These higher-order corticothalamic inputs are crucial for understanding cortical processing. A large proportion of synaptic inputs (>90%) are not relayed to the cortex and are classifiable as modulators. They contribute to controlling the gate. Some modulators match the topography of the drivers, thus relating to the parts of the body and the world; others do not show this specificity and have more global actions.

Edward George Gray. 11 January 1924 – 14 August 1999

George Gray was an early contributor to our knowledge of the electron microscopic appearance of the central nervous system. He was skilful with the difficult techniques for preparing the tissues, worked rapidly, and was an astute observer. Sitting with him in the dark, staring at a dim image that George was moving rapidly as he searched for significant detail, could be an exciting experience. He had clear ideas about features that mattered and could quickly relate the two-dimensional electron microscopic images to the three-dimensional neural structures under investigation. He is best known for his detailed and perceptive description of synaptic junctions in the mammalian neocortex, and his name is still linked to two distinct junctional types (Gray's type 1 and Gray's type 2), now recognized as generally distinguishing excitatory from inhibitory junctions. He studied a wide range of neural tissues, played a significant role in the early isolation of ‘synaptosomes’, contributed greatly to the rapid advance of knowledge that accompanied the early application of the electron microscope to neural tissues, and influenced a great many later fine-structural studies of the nervous system.