Physiological Concept of Behavior, Neuroendocrine Integration for Behavioral Manifestation
Behavior is an integrated manifestation or interplay of the neuroendocrine system.
27.3.1 Neurobiology of Behavior
Behavior is a complex biological phenomenon mediated through the nervous system consisting of brain, spinal cord, and sensory and motor neurons.
The cerebral cortex, especially the neocortex, in the brain contains the centers for the integration of sensory stimuli, conscious reasoning, memory, and reflection. The limbic system in the cerebral cortex consisting of hippocampus, olfactory cortex, parts of thalamus and hypothalamus, is involved in the immediate control of basic behavioral programs related to feeding, aggression, and sexual behavior. The limbic system also connects to sensory areas in the neocortex and is responsible for attaching emotions and feelings to behavior.Stimuli from the surroundings constantly flow across the individual. All the sensory stimuli (visual, olfactory, auditory, and mechanic) perceived by the sense organs, are transmitted to the various areas of the brain where they are interpreted in a meaningful and purposeful way to the animal. The hippocampus, the parietal, and prefrontal cortex areas of the brain also play a vital role in the spatial learning process, and in particular the number of dendritic spines in these regions of the brain is correlated with learning ability. Key stimuli are usually linked to specific behavioral responses. Motor activity is the central aspect of all behavior occurring as a result of synchronized pathways of muscle contraction which causes the animal to move in a functional manner in response to the stimuli.
27.3.2 Neurophysiology of Behavior
Behavior is primarily controlled by the central nervous system involving brain and spinal cord and is also regulated by endocrine factors. The cerebral cortex involving all four cortical lobes (parietal, temporal, frontal, and occipital lobes) exerts control over cranial and spinal regulation of motor actions and controls all kinds of behavior.
The main cortical areas controlling the behavior involve hypothalamus, amygdala, limbic system, basal nuclei, and hippocampus. Prefrontal cortex (PFC) (anterior part of the frontal lobe) is the most important cortical part involved in the control of executive, social, emotional, or instinctive behaviors.The hypothalamus has one of the most complex circuitries of any brain region. There are both neural interconnections and also extensive non-neural communication pathways between the hypothalamus and other brain regions and the periphery. Hypothalamus mediates the control of behavior through several circuitries and pathways. Hypothalamic inputs to various motor pattern generators may increase the probability of specific behaviors. For example, when hungry, most animals need to forage for food, then explore it by sniffing and licking, and then finally consume it. The hypothalamus may reduce the threshold for activating motor pattern generators for locomotion, sniffing, and oral behaviors that are involved in the ingestion of food. Descending outputs from the hypothalamus to the sensory system may sensitize or desensitize them. Finally, hypothalamic control of autonomic responses may cause signals that reach higher cognitive regions to elicit the appropriate behavior. Similarly, hypothalamic regulation of endocrine systems is also interlinked and exhibits feedback control on the brain centers. For example, many neurons in the brain have receptors for steroid hormones involved in stress responses, reproduction, or salt depletion, and changes in these hormones may alter the likelihood of expression of various complex behaviors regulated by those neuronal systems.
The amygdala is a central processing area which accords the level of priority to a given stimulus. It then sends projections to the hypothalamus for further integration and coordination with the brain stem areas to initiate the body’s response including the “fight or flight” responses (e.g., increase in respiratory rate, blood pressure, etc.)
The limbic system of the brain controls a variety of behaviors that are essential for survival.
The limbic system predominantly regulates appropriate responses to stimuli having emotional, motivational, or social importance, which includes innate behaviors such as mating, aggression, and defense. The innate behaviors which ensure the survival of the individual or its offspring and species propagation include the establishment of social hierarchy, mating, maternal care, and defense. These behaviors are regulated and modulated by sensory stimuli such as sound, touch, and, most importantly, smell in rodents.The neural circuitry that regulates innate behaviors are intricately influenced by endocrine factors, primarily sex hormones such as testosterone and estrogen. Both circulating and local brain levels of estrogen and testosterone are expressed in a sex-dependent manner and they refine the neural circuits involved in sexually dimorphic behaviors. Most of the limbic circuitry structures express estrogen receptors in both females and males. Estrogen is the primary hormone in the induction of maternal care in females. Virgin female rats inhibit their aversion and show attraction to pups after supplementation with estradiol, behaving more like nursing females. While estrogens shape the programming of sexually dimorphic circuits, testosterone acts directly through the androgen receptor and is essentially required for the activation and modulation of male-typical displays such as territorial aggression, urine marking, and mating.
Basal nuclei are a set of subcortical gray matter collections located in the vicinity of the diencephalon having an essential role in rewarding value-guided or motivated behavior by deciding the choice of behavior that will be more rewarding to follow. Hippocampus involved in the processing and retrieval of memory also exerts its influence on behavior. Autonomic nervous system plays a key role in the expression of innate as well as learned behavior.
27.3.3 Endocrine Moderation of Behavior
Behavior is a complex response mediated by the nervous system and is modulated by the endocrine system.
Hormones determine and influence the probability that a specific sensory input leads to a specific behavioral response. Hormonal changes might modify some ongoing behavior by increasing or decreasing the frequency or duration of that behavior, or they might trigger the onset or end of a behavior or behavioral sequence. Hormones do not initiate or inhibit any behavior by themselves; however, they influence the sensitivity of the neural circuits involved in exhibiting the various behaviors. In addition, hormones might prime animals so that they are more or less likely to behave in a specific way in a specific environment. For example, when baseline levels of testosterone are high, males are primed for aggressive behavior and display aggression when encountering another male. Testosterone shows a hormonal-behavioral feedback loop. Also if an animal wins a fight, partly as a result of behaviors resulting from high baseline levels of testosterone, the act of winning may in turn increases the probability of winning future fights by further increasing testosterone levels or by lowering the level of stress hormones such as cortisol. Similarly, oxytocin has a direct behavioral effect by calming an aggressive animal.Hormones also affect the organization of behavior systems during embryonic development. For example, female mice gestate many fetuses at the same time. If a developing male mouse fetus is surrounded by female fetuses, it is often exposed to low levels of circulating testosterone and high levels of estrogen in the uterus. When such males mature, they tend to be less aggressive and less sexually active than males that were surrounded by male fetuses in utero. This shows how a specific behavior could be fundamentally altered by hormonal influence early in development.
Similarly, in rodents, the sex of the two siblings surrounding an individual in utero can have dramatic effects on testosterone levels and an individual’s behavior after birth. Males that were surrounded by two other males in utero are more aggressive and mark their territories by scent and mount more females than males that were surrounded by two females.
Also, as a result of high testosterone levels, these males tend to exhibit less parental care. This relationship between in utero testosterone exposure and subsequent effects on brain activity especially on the preoptic area exhibits the tight connection between the hormonal and neu- robiological basis of behavior.The fight or flight response is another example showing the influence of hormone on the animal’s behavior. When an individual confronts stress or a predator, the hypothalamus initiates adrenaline and cortisol release. Adrenaline has a wide spread effect on all parts of the body including, an increase in cardiopulmonary activity thus delivering increased glucose and oxygen to the brain, skeletal muscles, and heart. These effects enable the animal to quickly flee from a predator or perhaps to fight against the danger.
27.3.4 Endocrine Influence on Social Behavior
The social behavior in animals is influenced by hormones which in turn are genetically controlled. Vasopressin and oxytocin are the two related hormones which play a central role in reproduction and parental care in mammals. For example, Prairie voles are monogamous, both males and females have a single mate for a given breeding season and males often display parental care and guard their mates. Meadow voles have a polygynous mating system, wherein males mate with multiple females during a breeding season, and males display very little parental care behavior toward their mates. One of the major differences in the male behavior towards their offspring and their mate is centered on the vasopressin system in these two species. The Prairie voles had higher numbers of vasopressin receptors (which are controlled by a gene known as avpr1a) in the ventral pallidium area of their brain than in the Meadow voles and this is considered to be responsible for the difference in male social behavior in Prairie versus meadow voles.
The relationship between behavior, hormones, and the nervous system is very intricate.
One such dramatic example is the “Bruce effect,” named after Hilda Bruce. Pregnant female mice abort their litters and reabsorb the embryos if a strange male mouse (not the father of the litter) comes in contact with them. Just the smell of a strange male is sufficient to induce abortion; it happens even when females come into contact with the litter soiled by another one. The pregnancy-blocking effect is brought about by inhibition of prolactin secretion and consequent reduction in the progesterone which is needed to maintain pregnancy.27.3.5 Behavioral Plasticity
The plasticity of behavior is an array of behavioral responses to varying environmental conditions. Animals respond to environmental change by dispersing/migrating, adjusting through phenotypic plasticity, or adapting through genetic changes. Phenotypic plasticity involves the tendency of a particular genotype to produce different phenotypes under altered environmental conditions. It allows an animal to adjust its behavior to suit the conditions of its immediate environment and, in so doing, increase its fitness. Phenotypic plasticity is the ability of an organism with a given genotype to change its phenotype in response to changes in the environment. Plastic responses can have adaptive significance for organisms in unpredictable environments, migratory species, and organisms in novel environments. The ability of individuals, populations, or species to switch between behaviors across situations can have significant ecological and evolutionary implications. For example, phenotypic plasticity can play a role in the diversification process and species range-expansion.
Broadly, three types of behavioral plasticity can be identified: differences in ontogenetic development, adjustments through learning, and the innate ability to respond to a variety of stimuli. Differences in the ontogeny of behavioral patterns may be due to varying social environments. The ontogeny of behavior is similar to that of morphological plasticity because it does not represent an immediate response and is not usually reversible. Learning which is defined as the modification of behavior by experience, results in behavioral plasticity, with an immediate and reversible response. Plasticity is expected to evolve by means of natural selection when it provides plastic individuals with a fitness advantage over less plastic individuals. For example, natural selection favors individuals that adjust their activity levels in response to changes in predation risk.
Behavior induces plastic changes in the brain components part, e.g., increase in size, connections, dendritic arborization, axonal sprouting, spine density, synaptic organization and formation, and neurogenesis (in hippocampus).
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Behavioral ecology refers to the study of how animal behavior develops its evolution and its contribution to reproductive success and survival. It deals with the individual animal’s behavior in relation to the environment and its ecological pressures. Young animals can be considered as miniature adults, gradually growing in size, but their behavioral responses must change as well to keep pace with their growth and the ever-changing environment. The behavioral and morphological changes occur in young animals which may be totally different from that of adults. Similarly, some specialized infantile behavior patterns do not always disappear but may return in a slightly different context. For example, Baby Meerkats (an African mongoose, Suricata suricatta) limp and behave passively when an adult seizes them by the scruff of the neck. This reflex facilitates them to be moved without injury. Similarly, during copulation, the adult female Meerkats relax when seized in a neck bite by the male. The neural basis for the reflex remains beyond infancy and even if they are not used again, some juvenile reflexes remain during the latter part of their life also.
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