Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Neurons and the Nervous System
Organisms, Stimuli, Receptors, Nervous Systems, Responses, and Behavior
Multicellular animals have nervous systems to generate behavior. Nervous systems are formed from specialized cells that conduct signals rapidly through the long cell extensions that make up nerves. The nerve cells communicate with each other by secreting specific excitatory and inhibitory molecules. In sense organs, specialized cells detect light, sound, and specific chemicals and enable animals to monitor what is going on in the world around them.
Like other aspects of an organismís biology, behaviors have evolved through natural selection. Behaviors often have an adaptive logic when viewed in terms of evolutionary principles.
Behavioral biology has implications for humans, as it provides links to psychology, sociology, and anthropology.
In animals the evolution of a nervous system that allowed for communication of the internal environment of an organism with external stimuli became highly adaptive. The development of a nervous system allowed animals the opportunity to move more easily, which in turn gave them advantages in activities such as food gathering and escaping from predators. These actions evolved into behavioral responses. As animals became more complex, so did their nervous systems and their responses to environmental stimuli.
Organisms respond to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organismís own species and others, as well as from environmental changes. The broad patterns of responses exhibited by organisms have evolved to ensure reproductive success. Organisms often live in unpredictable environments, and so their responses must be flexible enough to deal with uncertainty and change.
Like other aspects of an organismís biology, behavioral patterns have evolved through natural selection. The adaptive nature of many behavioral patterns is apparent when viewed in the context of evolutionary principles.
The nervous system is based on the structure of the neuron, or the nerve cell. Basically, neurons can be classified into two groupsCsensory neurons , which respond to light, heat, pressure, or chemicals in the environment; and motor neurons, which carry information to areas of the animalís body for appropriate response. Chemical compounds called neurotransmitters can excite or inhibit responses of nerve cells. By releasing neurotransmitters, nerve cells regulate the physiological functions of the organism.
A complex sensory system evolved in animals. This system allowed for the interpretation of many selective environmental stimuli. Without this system animals could not maintain homeostasis with their environments. The sensory system allowed for the innovation of what is commonly known as the five sensesCtouch, taste, smell, sound, and sight. Sensory systems have become highly adapted. In complex animals behavioral responses such as courtship, mating habits, nesting, food gathering, and communication have become integrated with sensory systems.
Behavioral patterns in animals are based on genes that have been selected because of their positive response to environmental stimuli. Behavior is a naturally selective process. In selection the "fit" characteristics in some way contribute to the survival of individuals and allow them to increase in number. Behavioral responses directly or indirectly parallel reproductive success.
A response can be instinctive, that is, genetically programmed, or it can be learned, which means that it has been acquired or eliminated as a result of experience. With instinctive behavior patterns, nervous reflex becomes an automatic response to a stimulus. A sense receptor in some way has been activated, and a reflexive act or what is known as a fixed action pattern occurs. It is understood, however, that this reflex is adaptive and has been naturally selected.
In learned behavioral responses, animals adjust their behavior patterns through experience. Examples of learning behavior are habituation, where organisms cease to respond to stimuli that are not important to them; conditional learning, where the organism learns by continuous exposure to one stimulus; and imprinting, where a learned response takes place at a brief critical period in the early life of an animal when there is exposure to a specific trait of one or both of the parents. Learned behavior is adaptive as well because it gives animals an opportunity to adjust to new environmental changes. Behavior has a genetic basis.
Nervous systems, neuron, motor nerves, sensory nerves, behavior, learning, stimulus, homeostasis
Neurotransmitters, voltage, depolarization, repolarization, stimulation, impulse, membrane potential, action potential, threshold, all-or-none response, imprinting, conditional reflex
Active transport, sodium pump, refractory period
Neurotransmitters, neuromuscular synapse, summation, CNS, PNS, sensory system, fixed action pattern, tropisms
Electrochemical transmission of synaptic signals, the Weber-Fechner law (the stimulus needed to be perceived is proportional to the stimulus acting), genetic and molecular bases of behavior
Sodium-potassium pump and membrane potential, proximate and ultimate causation, special behavior, kin selection, selfish gene, altruism