|The Biological Foundations of Behavior|
The Big Picture: Chapter Overview
The brain is a complex, versatile, and flexible network that controls our behaviors and mental processes. The evolutionary psychology approach, which emphasizes the importance of adaptation, reproduction, and natural selection in explaining psychology, considers how the human nervous system has evolved to its complex present state. Most scientists believe that behavior is determined by the interaction of the environment and the organism's biological inheritance. This chapter focuses on neuroscience, the area of specialization in psychology that studies the nervous system.
The nervous system is made up of interconnected nerve cells that transmit information throughout the body. There are four defining characteristic of the nervous system: (1) it communicates via electrochemical transmission, (2) it is characterized by its complexity, as the brain alone is composed of billions of nerve cells, (3) it can integrate information from many sources and create a coherent psychological experience, and (4) it has a great capacity to adapt to changes in the environment and the body. The capacity of the brain to adapt is termed plasticity.
Cells that carry input to the brain are called afferent neurons and those that carry output from the brain are called efferent neurons. Most of the communication in the nervous system takes place through neural networks, which are nerve cells that integrate sensory input and motor output.
The nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The peripheral nervous system connects the brain and spinal cord to the other parts of the body. The peripheral nervous system is divided into the somatic nervous system, which contains sensory and motor nerves, and the autonomic nervous system, which monitors the body's internal organs.
There are two types of nerve cells: neurons and glial cells. The neurons are in charge of communication and the glial cells support and nourish the neurons. A neuron is made of (1) a cell body, which regulates the cell's growth and maintenance; (2) dendrites, which collect information for the neuron; and (3) an axon, which carries information away from the cell body to other cells. Most axons are covered with a layer of fat cells called the myelin sheath, which insulates the axon and speeds up the impulse.
Neurons send information down the axon in the form of waves of electricity called the action potential. The neuron has a cell membrane that allows certain substances to enter the cell and other substances to exit the cell. When a neuron is resting it is not communicating, and it has more negative ions inside than outside. This slightly negative charge inside the cell is termed the resting potential. When a neuron is stimulated, the cell membrane allows positive ions to enter the cell. When the inside of the cell becomes positively charged, the cell membrane allows other positive ions to exit the cell, restoring the slightly negative inside charge. This exchange of electrochemical charges occurs one spot at a time in the axon. Once a spot has allowed positive ions in and out, it stimulates the next spot; this is what is understood as the electrochemical wave that moves across the axon. Picture a wave such as the one that develops at football games: you stay sitting until the people next to you stand and raises their hands. When you stand up with your arms in the air, you are like that spot in the axon where the cell membrane allows positive ions in and out; this state is the action potential. When you sit again, you are like that spot in the axon, returning to its resting potential. When you stand up and raise your hands you stimulate the person next to you to do the same, and so on and so forth. The action potential operates according to the all-or-none principle.
Each axon ends in numerous terminal buttons. Each terminal button stores neurotransmitters. When the electrochemical wave arrives at the terminal button, the neurotransmitter is released onto the synapse, the tiny gap between neurons. The neurotransmitters carry the message across the synapse to the receiving dendrite or cell body of the next neuron. Dendrites and some soma have receptor sites, which are neurotransmitter specific. The most common analogy is that of a lock and key. The neurotransmitter is the key and the receptor site is the lock. When the neurotransmitter latches onto a receptor site, it initiates an electrochemical wave in the receiving neuron. This is how neurons communicate! However, some neurotransmitters are inhibitory, which means that when they latch onto a receptor site, they keep the next neuron from starting an action potential. The neurotransmitters that stimulate other neurons to start the action potential are referred to as excitatory. Chapter Three includes a discussion of six neurotransmitters that are very important in the human nervous system: acetylcholine, GABA, norepinephrine, dopamine, serotonin, and endorphins. Some drugs called agonists mimic or increase a neurotransmitter's effect; antagonists are drugs that block a neurotransmitter's effects. Glial cells provide support and nutritive functions for neurons.
The neural communication is the foundation of our psychology. Whenever we have an experience, say stepping on a sharp stone, a number of neurons are stimulated and neural communication takes place throughout the nervous system. Some of those neurons will control your movements as you retrieve your foot and regain your balance; they will communicate again in the future when you recall the event and when a similar experience occurs.
Contemporary technology allows neuroscientists to explore the structure and function of the brain very effectively. Some of the techniques that are used are the study of naturally occurring brain lesioning in humans and lesioning of the brains of laboratory animals, the staining of selective cells to distinguish some nerve cells from others, electrical recording, such as the electroencephalograph, and brain imaging, such as the CT scan, the PET scan, and the MRI. The brain consists of the hindbrain, midbrain, and forebrain. The hindbrain is the lowest portion of the brain and consists of the medulla, cerebellum, and pons. The midbrain is an area where many nerve-fibers ascend and descend and relay information between the brain and the eyes and ears. An important structure of the midbrain is the reticular formation. The highest region of the brain is the forebrain. Its major structures included the limbic system, thalamus, basal ganglia, hypothalamus, and cerebral cortex. Each performs certain specialized functions involving emotion, memory, senses, movement, stress, and pleasure. The cerebral cortex comprises the largest part of the brain and consists of two hemispheres (left and right) and four lobes (occipital, temporal, parietal, and frontal). The cerebral cortex consists of the sensory cortex, motor cortex, and association cortex. Two important areas in the cerebral cortex involved in language are Broca's area and Wernicke's area. The two hemispheres are connected by the corpus callosum. No complex function can be assigned to one single hemisphere or the other. There is interplay between the two hemispheres.
A number of important body reactions produced by the autonomic nervous system result from its action on the endocrine glands. The endocrine system is a set of glands (pituitary, thyroid, parathyroid, adrenal, pancreas, and the ovaries in women and testes in men) that regulate the activities of certain organs by releasing hormones into the bloodstream. The anterior part of the pituitary is called the master gland; it is controlled by the hypothalamus. The adrenal glands play an important role in mood, energy, and stress.
Plasticity refers to the brain's capacity for modification and reorganization following damage. The amount of damage is a key factor in determining the degree of recovery. Collateral sprouting, substitution of functioning, and neurogenesis are repair mechanisms that can lead to the brain restoring some lost functions. Brain grafts and implants may help individuals with brain damage such as in Parkinson's disease and Alzheimer's disease.
Our psychology has genetic and evolutionary foundations. The last part of Chapter Three explores the basic concepts of genetics and heredity. The nucleus of each human cell contains 46 (23 pairs) of chromosomes that contain DNA. Genes, the units of hereditary information, are short segments of chromosomes. Genes combine with other genes to determine our characteristics. There are dominant and recessive genes. Polygenic inheritance is the effect that multiple genes have on behaviors and mental processes. The study of genetics has progressed from the basic experiments of Mendel to molecular genetics and the development of genomes. The Human Genome Project strives to describe the complete set of instructions for making a human being. There are great expectations for this project to contribute to the understanding of physical disease and mental disorders. Genetic methods include selective breeding and behavior genetics. Psychologists now face the challenge of finding theoretical frameworks that successfully integrate the biological foundations of psychology and research in genetics and neuroscience with the wealth of psychological theories that explore the influences of the environment and experiences on human psychology.