The Biology of Behavior
PSYC 1301
Heredity influences much of behavior and experiences, but it does not operate on thought and behavior in a deterministic way.
We often hear references to DNA in today's media. DNA, or deoxyribonucleic acid, is a large coiled double-stranded nucleotide that is in every cell in the body except red blood cells. This DNA contains an organism's entire genetic code.
Inside the cells, DNA is packaged together to form structures known as chromosomes. Small segments of this DNA, known as genes, contain the plans for the production of proteins. Different forms of the same gene are called alleles. All of this genetic information in most of our cells is contained in the genome.
Dominant genes show their effect even if there is only one copy of the gene in the pair, whereas, recessive genes show their effects when both pairs are the same.
Behavioral genetics studies the extent to which behaviors run in families. They also study segments of DNA associated with particular behaviors as well as common behaviors shared by identical twins. Characteristics such as skin color, risk taking, and metabolism are said to be caused by the interaction of genes and are the product of polygenic transmission. Traits determined by a single gene are passed on by monogenic transmission. The extent to which observed differences in a trait are due to underlying genetic differences is known as heritability. Heritability refers to actual measurements drawn from actual samples. Twin studies, adoption studies, and twin-adoption studies are examples of research into heritability. The question is not whether a trait is caused by genetics but the manner in which genetic factors and environment work together. Thus, heritability is a measure of the degree of variance attributable to genetic makeup. Genetic psychology seeks to understand the relationship between genetics and behavior.
Events in the environment influence how and when genes are activated or deactivated. Epigenetics occurs when there is a change in the way genes get expressed, activated, or deactivated, without changing the sequence of DNA. For example, a mother's behavior may influence the expression of genes in her children.
The nervous system and the endocrine system are the body's two coordinating and integrating systems. The system that relays messages in the form of electrochemical impulses through the body is called the nervous system whereas; the system that coordinates and integrates behavior by secreting chemicals into the bloodstream is called the endocrine system.
Why study these systems in a psychology class?
We study these systems because psychology is the study of behavior and behavior is, in part, the consequence of our feelings, memories, and actions. This area of psychology that focuses on the ways in which biological processes affect our behavior is known as psychobiology. To understand how information from the environment gets into the body, we need to understand how the nervous system carries chemical and electrical signals from the body to the brain and vice versa. The nervous system consists of the central nervous system and the peripheral nervous system. The smallest unit in the nervous system is a neuron. The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of all the neurons that connect the nervous system to the rest of the body.
The central nervous system (CNS) includes the brain and the spinal cord.
The second major division of the nervous system, the peripheral nervous system, carries messages to and from the central nervous system. It consists of all of the nerve cells that are not in the brain or spinal column. It is comprised of two parts: the somatic and the autonomic nervous systems.
The somatic nervous system is composed of the sensory (afferent) neurons that carry messages to the central nervous system and the motor (efferent) neurons that carry messages from the central nervous system to the skeletal muscles of the body. Every deliberate action a person makes involves neurons in the somatic nervous system.
The autonomic nervous system carries messages between the central nervous system and the internal organs. It is broken into two parts: the sympathetic and parasympathetic divisions. The first acts primarily to arouse the body, the second, to relax and restore the body to normal levels of arousal.
The sympathetic division generally acts to arouse the body, preparing it for "fight or flight" or quick action in an emergency. The parasympathetic division calms the body after it experiences this fight or flight feeling.
The central nervous system is made up of glial cells and neurons.
Glial Cells
Glial cells hold the central nervous system together by providing structural support, promoting communication between neurons, and removing cellular waste products.
Neurons
The smallest unit in the nervous system is the neuron. Neurons are the building blocks of the nervous system underlying the activity of the entire nervous system. All major structures of the brain are composed of neurons. Three major parts of the neuron are the cell body, dendrites, and axon. The soma, or cell body, contains a nucleus and other components needed for cell maintenance. Genes that direct neural change and growth are located within the nucleus. An axon is a long projection from the soma which transmits electrical impulses toward the adjacent neuron. The neuron's spiny projections that receive messages from other neurons are called dendrites. Axons that become myelinated are insulated in a fatty myelin sheath so that the impulse travels more efficiently.
Neurons specialize in communication. Information comes in from the outside through our sense receptors such as our eyes and ears. Transmission of this information through the neuron is an electrical process. The cell membrane of the neuron is semi-permeable. The passage of a nerve impulse through a neuron starts at a dendrite, and then travels through the cell body down the axon to an axon terminal. Axon terminals lie close to the dendrites of neighboring neurons. When the nerve impulse reaches an axon terminal, terminal buttons release chemicals known as neurotransmitters into the gap between neurons, known as the synaptic cleft.
There are three kinds of neurons. Sensory neurons receive incoming sensory information from the senses (eyes, nose, ear, skin, tongue). Motor neurons carry commands from the brain to muscles of the body. Interneurons are neurons that communicate only with other neurons. Interneurons are the most common kind of neurons in the brain.
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Over the last 20 years, researchers have tracked neural activity in relation to certain stimuli. Some experts, however, suggest that research is too focused on a single neuron rather than populations of neurons listening to other groups of neurons. |
For further discussion by Dr. Thomas Albright, Salk Institute for Biological Studies, and Dr. Gilles Laurent, California Institute of Technology, click here. |
Neurotransmitters are chemical messengers that travel across the gap (synapse) with the message to the dendrite of a neighboring neuron. Some neurotransmitters excite and others inhibit. An impulse travels one way from the dendrites long the axon and away from the soma. This process is both electrical and chemical. The fluid inside and outside the cell contains chemically charged particles called ions. The language used by neurons to communicate involves simple "yes-no," "on-off"" electrochemical impulses. Neurons will fire an impulse when they are stimulated. When neurotransmitters attach to receptor sites on a neuron's dendrite, they can increase (hyperpolarization) or decrease (depolarization) membrane potential. This is known as action potential. During depolarization, membrane potential decreases from –70 milivolts. This value is the resting potential of the neuronal membrane. If depolarization reaches –50 milivolts, an action potential is triggered. During the action potential the membrane potential reaches +40 millivolts. Thresholds are points of no return. There is no halfway. Triggering an action potential is like squeezing the trigger of a gun. If enough pressure is applied, the gun will fire. If too little pressure/depolarization is applied, neither the gun nor the neuron will fire; but it is all or nothing. While the neuron is returning to the resting state, it becomes super negatively charged. During this refractory period, the neuron cannot generate another action potential. Thus, the "all or none" law is the principle stating that a neuron fires at full strength or not at all. Our behavior is, in part, the consequence of millions of cells talking to each other via these chemical and electrical processes.
Information always travels in one direction in the neuron – from the dendrites to the soma to the axon to the synapses. When the action potential arrives at the terminal buttons of a neuron, it triggers the second phase in neural transmission – the release of neurotransmitters into the synaptic cleft to pass on the impulse to other neurons. Synaptic vesicles are sacs in the terminal button which contain neurotransmitters. Not all neurotransmitter molecules that are released bind with receptors. Excess neurotransmitters need to be removed either through enzymatic degradation or reuptake. In the process of enzymatic degradation, enzymes bind to a specific neurotransmitter to destroy excess. During the process of reuptake, excess neurotransmitters are returned to the presynaptic neuron.
More than 60 neurotransmitters have been identified. Acetylcholine, epinephrine, norepinephrine, dopamine, serotonin, GABA, and glutamate appear to have the most relevance for the study of human thought and behavior. Acetylcholine (ACh) controls muscle movement and plays a role in mental processes such as learning, memory, attention, sleeping, and dreaming.
Both epinephrine and norepinephrine have energizing and arousing properties. We commonly call epinephrine "adrenaline." Norepinephrine is involved in being alert.
Dopamine is released in response to behaviors that make us feel good or are rewarding to the individual.
Gamma-aminobutyric acid, or GABA, is an inhibitory neurotransmitters that tell the postsynaptic neurons not to fire. GABA is the brakes of the central nervous system that keep it from running out of control. Without GABA, the nervous system would never cease activity.
Glutamate, a major excitatory neurotransmitter, is important in learning, memory, neural processing, and brain development.
Researchers now believe that production of too much or too little of one or more neurotransmitters or a breakdown in the communication between nerve cells plays a significant part in mental health. Dopamine, for example, is believed to play a role in the voices and delusions of schizophrenics. Glutamate doesn't function properly in people with schizophrenia causing them to become confused. New treatments for schizophrenia focus on restoring glutamate function. Because dopamine activity makes us feel good, some drug addictions involve increased dopamine activity. Cocaine, for example, blocks reuptake of dopamine, thereby increasing its availability in the synaptic cleft and creating a feeling of euphoria. Depression is now thought to result in part from a deficiency of serotonin. A common treatment of depression is to block the reuptake of serotonin at the synapse making more of it available for binding with postsynaptic neurons. People with low attention-deficit/hyperactivity disorder (ADHD) tend to have low levels of epinephrine.
The brain is the most complex structure of the human body and is responsible for thought, memory, emotion, and language. It weighs about three pounds and is made up of approximately 100 billion neurons. The three major regions of the brain are the hindbrain, the midbrain, and the forebrain.
Hindbrain: The hindbrain is directly connected to the spinal cord and regulates breathing, heart rate, arousal, and basic functions of survival. The hindbrain is made up of the medulla, the pons, and the cerebellum. These three are collectively known as the brain stem. The medulla regulates breathing, heart rate, and blood pressure. The pons relays information from the lower to the higher brain regions. The cerebellum is responsible for body movement, balance, coordination, fine motor skills, and is important in cognitive activities such as language and learning.
Midbrain: Different parts of the midbrain control eye muscles, process both auditory and visual information, and initiate voluntary body movement.
Forebrain: The forebrain is the largest part of the brain made up of the cerebrum, thalamus, limbic system and other structures. Regions of the forebrain control cognitive, sensory, and motor function and regulate temperature, reproduction, eating, sleeping, and expression of emotions. Most forebrain structures are bilateral, located on each side of the brain. Within the forebrain, the thalamus receives input from the senses. All sensory signals go first to a relay station in the thalamus, a central structure named after the Greek word for "couch" because the cerebral hemispheres seem to rest comfortably on it. The thalamus relays sensory impulses to the cortex. The cortex controls thought, decision, memory, and emotional response. About 85 percent of all neurons in the body are in the cortex, also known as the cerebrum or cerebral cortex. The prefrontal cortex oversees thought and logical thinking and has been linked with depression and schizophrenia.
Limbic System: The limbic system is located directly around the thalamus and is important in emotion and motivation. The system includes the hypothalamus, the amygdala, and the cingulated gyrus. The amygdala and the hippocampus help form new memories as well as influence motivation and emotion. Our ability to read the facial expressions of emotion in other people is registered primarily in the limbic system. The hypothalamus regulates almost all major drives including hunger, thirst, temperature, and the sex drive. It also controls the pituitary gland and the production of hormones. Despite its numerous vital functions, the hypothalamus in humans accounts for only 1/300 of total brain weight, and is about the size of an almond. The basal ganglia are a collection of structures surrounding the thalamus and are involved in voluntary motor control. The cingulated gyrus is a belt-like structure which plays an important role in attention and cognitive control.
Cerebrum and Cerebral Cortex: The outer, convoluted layer of the brain is known as the cerebral cortex. A large portion of human thought, planning, perception, and consciousness take place in the cerebral cortex. The cerebrum consists of four lobes. Functions of the frontal lobes include recognizing consequences, attention, planning, abstract thinking, control of impulses, creativity, and social awareness. The parietal lobes play a role in sensation and perception of touch. The temporal lobes house the auditory cortex which controls hearing and they are involved in memory and emotion. Visual information is processed in the visual cortex of the occipital lobes. The insula is located deep inside the cerebrum and is active in the perception of sensations, emotional states, empathy, and addictive behaviors.
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A rendered image of how the rod went through Phineas' head.
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One of the earliest documented cases of severe brain injury to the frontal lobes in which the individual survived was the case of Phineas Gage, a railroad foreman who sustained a frontal lobe injury in September, 1848. Although Gage lived 13 years after the injury, he suffered immediate and obvious changes to his personality. For a more detailed description of the case of Phineas Gage, click here.
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Since the 1990s, principles of brain plasticity have emerged. Neuroplasticity describes the brain's ability to adopt new functions, reorganize itself, and make new neural connections as a function of experience. Research has shown that almost every major structure of the neuron is capable of change based on experience; however, not all regions of the brain are equally plastic. Brain plasticity gradually decreases with age which explains why it is easier for children to learn to speak a language than adults.
Nature vs. Nurture Consideration: The argument for life-long learning – throughout life, the brain can adopt new functions, reorganize itself, or make new neural connections as a function of experience. Neurogenesis is the process of developing new neurons. The growth and formation of new dendrites is known as aborization. Both contribute to synaptogenesis, the formation of entirely new synapses or connections with other neurons that is the basis of learning.
In more authoritarian times, scientists believed that the brain had a strictly hierarchical organization. Each relay station was supposed to send increasingly complex information to a higher level until it reached the very top, where everything would somehow be put together.
Now scientists see the brain as a pattern processor and categorizer that recognizes which categories go together with a particular stimulus. Microelectrode techniques are used to study single neurons, whereas macroelectrode techniques are used to study overall activity in particular regions of the brain. Structural imaging techniques are used to map structures in the living brain. Functional imaging techniques observe neural activity as it reacts to sensory stimuli. A new field, computational neuroscience, studies neural networks and the interaction of neurons on behavior. As pointed out by Dr. Frederick Petty, a psychiatry professor at UT Southwestern, "You just can't do brain biopsies like you can do cardiac catheterizations." In the past 15 to 20 years, brain imaging technology has provided better pictures of mental illness and how drugs work. Brain scans have shown that people with depression have less nerve activity in the prefrontal regions that oversee complex thought. Brain imaging has also indicated that people with depression have a smaller hippocampus. Dr. Dwight German, also a professor at UT Southwestern, focused on a subregion of the thalamus and determined that people with schizophrenia have a million fewer nerve cells there. Dr. German is using a $1 million grant from the National Institute of Mental Health to map the thalamus. He indicated, however, that it is too early to determine if some mental illnesses are related to the structure of the brain. "We are just starting to scratch the surface really," German said in a recent article in the Dallas Morning News.
Dr. Dwight German, Professor, The Carl J. & Hortense M. Thomsen Chair in Alzheimer's Disease Research Department of Psychiatry UT Southwestern Medical Center.
Electroencephalography
Electroencephalography (EEG): An EEG is used to record electrical activity of the brain by placing electrodes on a person's scalp. It is better than some other imaging techniques in that it shows when brain activity occurs; however, it is not very accurate in locating where activity occurs.
Event-related potential (ERP): Electrical activity is taken from raw EEG data to measure cognitive processes. It will show brain activity linked with psychological tasks but exhibits poor spatial resolution.
Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) uses magnetic fields to produce finely detailed images of the brain's structure and soft tissues. It does not tell anything about activity, just structure.
Functional MRI (fMRI) provides information about brain activity occurring during assigned tasks by measuring blood oxygen.
Positron Emission Tomography (PET)
A PET scan measures blood flow to different areas of the brain so that researchers and medical personnel can determine which brain areas are active during certain situations.
Diffusion Tensor Imaging (DTT)
DTT is a method of brain measurement similar to MRI that measures white matter rather than gray matter. White matter refers to the long fibers of myelinated axons that are involved in communicating information among different areas of the brain. This allows examination of connections between brain regions rather than the regions themselves. DTI helps neurosurgeons locate important tracts to avoid during surgeries.
Near Infrared Spectrometry (NIRS)
NIRS uses light rather than magnets to produce images of brain tissue. This type of brain imaging is portable and can be used with infants.
Dr. Daniel Amen is a Christian psychiatrist whom I have heard speak on brain health several times at the bi-annual world conference of the American Association of Christian Counselors. Dr. Amen uses single photon emission computerized tomography, known as Brain Spect Imaging, in the diagnosis of mental and physical problems. For an atlas of brain images comparing normal brain and abnormal brain function, click here.
The endocrine system—the other communication system in the body—is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to guide such processes as metabolism, growth, and sexual development. Hormones are also involved in regulating emotion-experiencing pleasure, as well as for various motivational and emotional activities and emotional balances in general.
The endocrine system itself is a chemical communication network that sends messages throughout the nervous system via the bloodstream. The endocrine system communicates its messages at a slower speed than the nervous system. Although not a structure of the brain itself, the endocrine system is intimately tied to the hypothalamus because it controls the pituitary gland.
Like neurotransmitters, hormones communicate chemical messages throughout the body, although the speed and mode of transmission are different. Hormones are of interest to psychologists for two reasons. First, hormones trigger changes that occur in our bodies at certain stages of development. Puberty and menopause are two examples of hormone-triggered changes. Second, hormones activate behavioral responses such as aggressiveness, reactions to stress, sexual behavior, alertness, or sleepiness. Hormones can have a dramatic effect on mood, emotional reaction, and the ability to learn. Radical changes in hormones can contribute to serious psychological disorders such as depression.
Adrenal glands release hormones in response to stress and emotion. Catecholamines are a class of chemicals that includes the neurotransmitters dopamine, norepinephrine, and epinehprhine. Cortisol is responsible for maintaining the activation of bodily systems during prolonged stress.
Metabolism is the process by which the body converts nutrients into energy. The thyroid gland sits in the neck region and releases hormones that control metabolism rate.
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