A

Cerebellum Glomerulus

FIGURE 1.1-5 (A) Schematic sections through the cerebellar cortex of a primate brain. It has three anatomical layers and five types of interneuron. (B) Schematic structure of a cerebellar glomerulus found in the granular layer. In the glomerulus, intimate synaptic contact is made between a mossy fiber synapse, granule cell dendrites and a Golgi cell axon (not the GTO). (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

FIGURE 1.1-5 (A) Schematic sections through the cerebellar cortex of a primate brain. It has three anatomical layers and five types of interneuron. (B) Schematic structure of a cerebellar glomerulus found in the granular layer. In the glomerulus, intimate synaptic contact is made between a mossy fiber synapse, granule cell dendrites and a Golgi cell axon (not the GTO). (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

One factor that differentiates each different type of cell in the body are the many proteins and glycoproteins made by the internal biochemical machinery of the cell and embedded in the UM of the cell. Most of these large molecular weight proteins pass through the UM, projecting parts both on the inside and on the outside of the cell. The glycoproteins embedded in the UM of a cell have many functions. Some contain molecular receptors (binding affinity regions) on the outside of the cell where signaling molecules can dock and trigger configurational changes in the glycopro-teins. In the case of neurons, some signaling substances are neurotransmitters released by other neurons. Neurotransmitters can either trigger direct configurational changes of the glycoproteins, allowing certain external or internal ions to pass easily through the membrane, or trigger a cascade of intracellular chemical reactions in which a second-messenger molecule reacts inside the cell with a target glycoprotein, causing it to pass certain ions through the membrane. As will be seen, the passage of ions such as Na+, K+, Cl-, Ca++, etc., across the cell membrane will change its resting potential, either hyperpolarizing the membrane (driving the inside more negative with respect to the outside) or depolarizing it (making the inside less negative), depending on the ionic currents.

Not all membrane-bound glycoproteins are associated with gating ions. Some are adenosine triphosphate (ATP)-driven pumps that actively expel ions such as Na+ from inside the cell, or actively transport low-molecular-weight signaling substances to the inside of the cell. Still other membrane-bound glycoproteins are associated with electrical synapses, also known as gap junctions.

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