CHAPTER TWO

Neurons and functional control of behavior

Axons are the basic cellular units of the nervous system. Axons transmit electrical or chemical signals from one location in an animal's body to another and, in the central nervous system, interconnecting axons house memory, decision-making, and other integrative functions.

Vertebrate axons are covered with a myelin sheath, which allows rapid transmission of electrical impulses from one end of the axon to to the other. Invertebrate axons lack a myelin sheath; the speed of transmission is proportional to the diameter of the axon in invertebrates. You can think of this as being like an electrical wire; a skinny wire has more resistance to electricity than a fat wire. Invertebrate axons which have critical functions in fast escape responses have large diameters and are called giant axons.

Messages are usually carried between axons by neurotransmitters. The most common neurotransmitter is acetylcholine, which also often is the messenger between axons and muscles. When an action potential, or nervous impulse, reaches the end of the axon, the transmitter is released and causes depolarization of the membrane of the next axon or of the muscle cells. An enzyme, acetylcholinesterase, breaks down the acetylcholine once the message is transmitted. Other commonly discussed neurotransmitters are octopamine, serotonin and dopamine, which more commonly have functions in the central nervous system.

Nervous systems

Axons are organized into a nervous system. In the simplest animals there is no centralized area of the nervous system which coordinates the animal's functions, but most animals have a "brain" which receives inputs from sensory nerves and organizes behavioral responses. The brain also receives information about the animal's physiological state and serves as a command center for translating physiological needs into behavioral responses.

Neurotransmitters modulate general behavioral state at the level of the central nervous system. Octopamine, for example, appears to be associated with general arousal in insects; injecting or feeding octopamine to insects results in high levels of activity. In humans serotonin and norepinephrine levels in the brain are associated with depression, while dopamine is related to pychosis. Antidepressant drugs, such as Prozac, affect serotonin levels in the brain, while antipsychotics affect dopamine levels. For learning and memory to function appropriately acetylcholine must be present in the brain at appropriate levels. The new drugs, such as Aricept, for Alzheimer's disease are acetylcholinesterase inbitors; they interfere with the breakdown of acetylcholine in the brain, causing levels to rise so that normal learning and memory functions can operate.

Neuropeptides provide a link between the brain and the body's other physiological systems. Acting as hormones, neuropeptides regulate many functions, including some behaviors.

Endocrine systems

The hypothalamus is the main neuroendocrine interface andin vertebrates. It coordinates the production of many hormones and can have a central role in the control of behavior. Neuropeptides from the hypothalamus induce the pituitary gland, which is suspended from the hypothalamus to produce a number of hormones. Perhaps the most important pituitary hormones which ultimately affect behavior are the luteinizing hormone (LH) and follicle stimulating hormone (FSH), which activate the ovaries and testes of vertebrates, stimulating the production of estrogens and testosterone. **Oxytocin**

Behaviorally, the most important vertebrate hormones are the estrogens and testosterone. These hormones affect the expression of mating behavior, parental behavior, and aggression and territoriality. Migratory impulses are at least partially under hormonal control.

For invertebrates, the best understood hormone with behavioral effects is juvenile hormone, a product of the corpora allata in insects. Juvenile hormone affects mating behavior, pheromone secretion, parental behavior, in adult insects, as well as worker behavior in eusocial insects, such as honey bees.

Neural and endocrine integration in the development of behavior

After hatching or being born, an animal's nervous system likely is incompletely formed. Nevertheless, it coordinates the activities needed to sustain the young animal's life, such as seeking food or its mother, hiding from potential predators, and finding the appropriate environmental conditions. Vertebrate species are termed either altricial--needing intensive parental care at hatching or birth--or precocial--being relatively independent at birth. Baby sea turtles, upon hatching on a beach,, are programmed to seek the water, where they then can maintain themselves without parental assistance. Mouse pups, on the other hand, have incompletely developed eyes and lack the ability to feed themselves; they must be carefully nurtured by their mother.

During development the endocrine system plays a critical role in the expression of age-appropriate behavioral patterns. For vertebrates, estrogen and testosterone have critical roles, while juvenile hormone is important in insects. These hormones affect both physical and behavioral development; because they affect both of these developmental areas, physical and behavioral development are appropriately coordinated.

Sensing the environment