UNAM LAS LENGUAS EXTRANJERAS EN LA DIFUSIÓN DE LAS CIENCIAS BIOLÓGICAS Y DE LA SALUD

English for Medicine

Brain Facts

CHAPTER 1: Brain Basics

In this Chapter

Neurotransmitters and Neuromodulators

1The ability of a neuron to generate an electrical impulse depends on a difference in charge between the inside and outside of the cell. When a nerve impulse begins, a dramatic reversal in the electrical potential occurs on the cell’s membrane, as the neuron switches from an internal negative charge to a positive charge state. The change, called an action potencial, then passes along the axon’s membrane at speeds up to several hundred miles per hour. In this way, a neuron may be able to fire impulses multiple times every second.

2When these voltage changes reach the end of an axon, they trigger the release of neurotransmitters, the brain’s chemical messengers. Neurotransmitters are released at nerve terminals, diffuse across the synapse, and bind to receptors on the surface of the target cell (often another neuron, but also possibly a muscle or gland cell). These receptors act as on-and-off switches for the next cell. Each receptor has a distinctly shaped region that selectively recognizes a particular chemical messenger. A neurotransmitter fits into this region in much the same way that a key fits into a lock. When the transmitter is in place, this interaction alters the target cell’s membrane potential and triggers a response from the target cell, such as the generation of an action potencial, the contraction of a muscle, the stimulation of enzyme activity, or the inhibition of neurotransmitters release.

neurotransmitter
(10)

3An increased understanding of neurotransmitters in the brain and knowledge of the effects of drugs on these chemicals — gained largely through animal research — comprise one of the largest research efforts in neuroscience. Scientists hope that this information will help them become more knowledgeable about the circuits responsible for disorders such as Alzheimer’s and Parkinson’s diseases.

The synthesis, packaging, secretion, and removal of neurotransmitters
(11) The synthesis, packaging, secretion, and removal of neurotransmitters. (A) The life cycle of transmitter agents entails (1) neurotransmitter synthesis, (2) packaging into vesicles, (3) fusion of vesicles resulting in neurotransmitter release, and (4) activation of postsynaptic receptors. Neurotransmitters are then removed from the synaptic cleft (5). In many cases, the neurotransmitter and/or a breakdown product is reused for neurotransmitter synthesis. (B) Small-molecule neurotransmitters are synthesized at nerve terminals. The enzymes necessary for neurotransmitter synthesis are made in the cell body of the presynaptic cell (1) and are transported down the axon by slow axonal transport (2). Precursors are taken up into the terminals by specific transporters, and neurotransmitter synthesis and packaging take place within the nerve endings (3). After vesicle fusion and release (4), the neurotransmitter may be enzymatically degraded. The reuptake of the neurotransmitter (or its metabolites) starts another cycle of synthesis, packaging, release, and removal (5). (C) Peptide neurotransmitters, as well as the enzymes that modify their precursors, are synthesized in the cell body (1). Enzymes and propeptides are packaged into vesicles in the Golgi apparatus. During fast axonal transport of these vesicles to the nerve terminals (2), the enzymes modify the propeptides to produce one or more neurotransmitter peptides (3). After vesicle fusion and exocytosis, the peptides diffuse away and are degraded by proteolytic enzymes (4).

4Sorting out the various chemical circuits is vital to understanding the broad spectrum of the brain’s functions, including how the brain stores memories, why sex is such a powerful motivation, and what makes up the biological basis of mental illness.

5There are many different kinds of neurotransmitters, and they all play an essential role in the human body. The next section provides a summary of key neurotransmitters and neuromodulators, chemicals that help shape overall activity in the brain.

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