the brain with an emphasis on the neuron.
Figure 2.5 The Brain in Terms of Structural and Functional Anatomy
Neurons and Neural Transmission
The brain’s function involves one basic element, the neuron. Although neurons come in a variety of sizes and shapes, there are some basic characteristics as shown in Figure 2.6:
1 The cell body contains a nucleus, which includes deoxyribonucleic acid (DNA) and other elements including mitochondria, which are involved in supplying energy.
2 The axon is a slender nerve that conducts electrical impulses away from the cell body. Axons can be fairly short, as found in the human brain, or 4 or 5 feet in length, such as those that go from the spinal cord to the arms and legs.
3 The dendrites receive information from other cells.
The dendrites receive information from other neurons, which connect at different locations on the dendrites. Although illustrations in textbooks usually show only a few connections between neurons, there are generally thousands of these connections. The terminal branches from these other neurons do not actually touch but make a biochemical connection through a small gap filled with fluid, which is referred to as a synapse. These biochemical connections can release molecules (ions) with an electrical charge.
Figure 2.6 Basic Characteristics of a Neuron
Source: Sobel & Li (2013).
As more of these electrical charges add together, it increases the size of the electrical potential. At a critical point, an action potential is produced at a location near the cell body, which travels quickly down the axon in one direction. An action potential is referred to as an “all or none” signal, since above the critical value an action potential is produced, whereas below the critical value, no electrical activity is sent down the axon.
The speed at which the action potential travels down the axon depends on two factors. The first is the width of the axon. For example, action potentials travel faster in larger diameter axons. The second factor relates to whether the axon is covered with an insulating material called the myelin sheath. Action potentials travel faster in axons surrounded by myelin. Thus, an axon with a larger diameter and wrapped in myelin would have the fastest conduction times. Disorders such as multiple sclerosis and autism show deficits in axonal connections.
Figure 2.7 Depiction of the Structures and Processes of Synapses
Source: Universal Images Group North America LLC/Alamy Stock Photo
It should be noted that there are two major types of synapses. One type, referred to as a chemical synapse, involves secretion from the previous neuron of various types of neurotransmitters. These neurotransmitters create a current flow. This changes the physiological state of the next (postsynaptic) neuron such that it is more likely (excitatory) or less likely (inhibitory) to create an action potential (see Figure 2.7). The second type of synapse is electrical in nature. Current flows through special channels that connect the gap between the two neurons.
How Does the Neuron Pass Information?
Passing information from one neuron to another involves a number of steps:
1 Neurotransmitters need to be created and stored.
2 An action potential travels down the axon to the terminal.
3 Through a variety of processes, a neurotransmitter is released into the gap between the two neurons.
4 The neurotransmitter then binds with specific proteins in the next neuron.
5 This either increases (excitatory) or decreases (inhibitory) the possibility that the next neuron will create an action potential.
6 The gap between the two neurons must be made neutral at this point by any of a number of mechanisms including making the neurotransmitter inactive, having the neurotransmitter taken up by the first neuron (referred to as reuptake), and removing the neurotransmitter from the gap between the two neurons.
It is these neurotransmitters that lead to anxiety processes in some cases but depression in others. Most medications used for treating mental illness influence the neurotransmitters at the synapses. Alzheimer’s disease, which results in memory loss, is caused by destruction of individual neurons throughout the brain (Nath et al., 2012). Most addictive drugs increase the amount of dopamine in the gap between the neurons.
Major Neurotransmitters
In the chemical synapse, neurotransmitters play a critical role. Neurotransmitters transmit signals from one neuron to another (see Figure 2.8). It is also the case that psychotropic medications largely have their influence at the site of the synapse. To date, more than 100 different neurotransmitters have been identified. Neurotransmitters have been classified both in terms of structure and in terms of function. Most neurons utilize more than one type of neurotransmitter for their functioning.
neurotransmitters: chemicals released into the synaptic space that are involved in increasing or decreasing the likelihood for action potentials to be produced; they also maintain the communication across the synapse. Their presence or lack is related to particular psychopathological disorders
Regarding their structure, neurotransmitters can be classified in terms of size (Purves et al., 2013). This results in two broad categories. The first type is small molecule neurotransmitters such as glutamate, which is excitatory, and GABA (gamma-aminobutyric acid), which is inhibitory. They are often composed of single amino acids. These small molecule neurotransmitters tend to be involved in rapid synaptic functions.
Glutamate is considered to be the most important neurotransmitter in terms of normal brain function. In abnormal conditions, the firing of rapid glutamate neurons can lead to seizures in a number of areas of the brain. GABA is inhibitory, and drugs that increase the amount of GABA available are used to treat such disorders as anxiety. The second type of neurotransmitter in terms of size is larger protein molecules referred to as neuropeptides. These can be made up of 3 to 36 amino acids. Neuropeptides tend to be involved in slower, ongoing synaptic functions.
In terms of function, neurotransmitters can also be categorized into three broad groups (Nadeau, 2004). The first group includes those neurotransmitters such as glutamate and GABA that mediate communication between neurons. The second group includes those neurotransmitters such as opioid peptides in the pain system that influence the communication of information. The third group includes those neurotransmitters such as dopamine, adrenaline, noradrenaline and serotonin that influence the activity of large populations of neurons. For example, cocaine blocks the ability of a neuron to remove the neurotransmitter dopamine from the synapse, which increases the experience of addiction. (See Table 2.1 for a description of various neurotransmitters.)
Most medications used for treating mental illness influence the neurotransmitters at the synapses.
(c) iStockphoto.com/BCFC
Encoding Information
Information is encoded by means of action potentials in terms of frequency. That is, a loud sound would be encoded by a series of action potentials from the cells sensitive to sound intensity. A soft sound would result in fewer action potentials being fired. When observed in relation to a stimulus, action potentials are also referred to as spikes, and a number of spikes over time are referred to as spike