Chapter 2: The Biological Basis of Behavior
Neurons: The Messengers
Synapses and Drugs
The Central Nervous System
The Limbic System
The Cerebral Cortex
Tools for Studying the Nervous System
The Spinal Cord
The Peripheral Nervous System
The Endocrine System
Genes, Evolution and Behavior
This module presents the basic biological processes that are at the root of our thoughts, feelings, and actions. The body possesses two systems for coordinating and integrating behavior: the nervous system and the endocrine system.
The billions of neurons, or nerve cells, that underlie all the activity of the nervous system form a communication network that coordinates all the systems of the body and enables them to function. Neurons usually receive messages from other neurons through short fibers, called dendrites, that pick up messages and carry them to the neuron's cell body. The axon carries outgoing messages from the cell. A group of axons bundled together makes up a nerve. Some axons are covered with a myelin sheath, made up of glial cells. The myelin sheath increases neuron efficiency and provides insulation.
A typical myelinated neuron .
NEURONS: THE MESSENGERS
The neural impulse --communication within the neuron.
Electrical changes during the action potential.
Neurotransmitter molecules, released by synaptic vesicles, cross the tiny synaptic space (or cleft) between the axon terminal (or synaptic knob) of the sending neuron and the dendrite of the receiving neuron, where they latch on to a receptor site, much the way a key fits into a lock. This is how they pass on their excitatory or inhibitory messages.
Synaptic transmission – communication between neurons.
Click here to view the Major Neurotransmitters and Their Effects tableSynapses and Drugs
Experience and Neurons
The brain has plasticity, that is, it can be physically and chemically altered by experience. In a pioneering study of the influence of the environment on the brain, researchers found that rats that had been raised in a stimulating environment had more synaptic connections than rats that had been raised in cages that offered them no opportunities to explore or to manipulate objects.
Brain growth and experience.
THE CENTRAL NERVOUS SYSTEM
A schematic diagram of the divisions of the nervous system and their various subparts .
The hindbrain is found in even the most primitive vertebrates. It is made up of the cerebellum, the pons, and the medulla. The medulla is a narrow structure nearest the spinal cord; it is the point at which many of the nerves from the left part of the body cross to the right side of the brain and vice versa. The medulla controls such functions as breathing, heart rate, and blood pressure. The pons, located just above the medulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain our sleep-wake cycle. The cerebellum is divided into two hemispheres and handles certain reflexes, especially those that have to do with balance. It also coordinates the body's actions.
The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight. The forebrain is supported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists of the thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incoming messages from the sense receptors—except those for smell. The hypothalamus governs motivation and emotion and appears to play a role in coordinating the responses of the nervous system in times of stress.
Parts of the Brain and Their Functions
Click here to view the Parts of the Brain and Their Functions tableThe Limbic System
The Cerebral Cortex
These four lobes are both physically and functionally distinct. Each lobe contains areas for specific motor sensory function as well as association areas. The association areasareas that are free to process all kinds of information—make up most of the cerebral cortex and enable the brain to produce behaviors requiring the coordination of many brain areas.
The four lobes of the cerebral cortex .
The cerebral hemispheres .
Tools for Studying the Nervous System
EEG recording of one person's brain waves; EEG electrode attachment .
The Spinal Cord
The spinal cord and reflex action .
THE PERIPHERAL NERVOUS SYSTEM
The Somatic Nervous System
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.
The Autonomic 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 and parasympathetic divisions of the autonomic nervous system .
THE ENDOCRINE SYSTEM
The Thyroid Gland
The thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is under active.
The Parathyroid Glands
Within the thyroid are four tiny pea-shaped organs, the parathyroids, that secrete parathormone to control and balance the levels of calcium and phosphate in the blood and tissue fluids. This, in turn, affects the excitability of the nervous system.
The Pineal Gland
The pineal gland is a pea-sized gland that apparently responds to exposure to light and regulates activity levels over the course of the day.
The pancreas lies in a curve between the stomach and the small intestine and controls the level of sugar in the blood by secreting insulin and glucagon.
The Pituitary Gland
The pituitary gland produces the largest number of different hormones and therefore has the widest range of effects on the body's functions. The posterior pituitary is controlled by the nervous system. It produces two hormones: vasopressin, which causes blood pressure to rise and regulates the amount of water in the body's cells, and oxytocin, which causes the uterus to contract during childbirth and lactation to begin. The anterior pituitary, often called the "master gland," responds to chemical messages from the bloodstream to produce numerous hormones that trigger the action of other endocrine glands.
These reproductive glandsthe testes in males and the ovaries in females, and, to a lesser extent, the adrenal glandssecrete androgens (including testosterone) and estrogens.
The Adrenal Glands
The two adrenal glands are located above the kidneys. Each has two parts: an outer covering, the adrenal cortex, and an inner core, the adrenal medulla. Both influence the body's responses to stress. For example, in response to a stressful situation, the pituitary gland may release beta endorphin and ACTH, which, in turn, prompt the adrenal cortex to release hormones. Meanwhile, the autonomic nervous system stimulates the adrenal medulla to secrete hormones such as epinephrine into the bloodstream.
The glands of the endocrine system .
GENES, EVOLUTION, AND BEHAVIOR
Genetics is the study of how plants, animals, and people pass on traits from one generation to the next through genes. The transmission of traits is referred to as heredity. Each gene is lined up on tiny threadlike bodies called chromosomes, which are made up predominantly of deoxyribonucleic acid (DNA). Members of a gene pair can be either dominant or recessive genes. In polygenic inheritance, several genes interact to produce a certain trait.
Transmission of eye color by dominant (B) and recessive (b) genes .
In 1859 Charles Darwin proposed the theory of natural selection to account for evolutionthe idea that groups of organisms change over time. In modern terms, the theory of natural selection states that organisms best adapted to their environment tend to survive, transmitting their genetic characteristics to succeeding generations, whereas organisms with less adaptive characteristics tend to disappear.
Evolutionary psychology analyzes human thoughts, traits, and behaviors by examining their adaptive value from an evolutionary perspective. It has proved useful in explaining many cross-cultural commonalities in human behavior.
The study of behavior genetics and evolutionary psychology makes many people uneasy. With the development of amniocentesis and chorionic villus sampling, prospective parents can often detect genetic abnormalities in a fetus, leading to questions about the rights of a child versus those of the parents. Some fear that research in evolutionary psychology will undermine movements toward social equality by attempting to justify the adaptive value of certain forms of social injustice. Others fear that it will make people feel that genetics is destiny -- that who we are is written in some kind of permanent ink before we are born.
Average risk of schizophrenia among biological relatives of people with schizophrenia .
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