Читать книгу Abnormal Psychology - William J. Ray - Страница 76

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.