The thalamus coordinates sensory and emotional inputs, serving as a gateway and relay between the body, limbic system, and cerebral cortex. Thalamic relay nuclei coordinate the projection of sensory information from the body and sensory organs, directing it to the appropriate somatosensory portions of the cerebral cortex—the most recent and "conscious" addition to the brain. The amount of the cortex reserved for any particular body area or function depends on its overall use, sensitivity, and relative importance for the animal's survival. Further, the area of the cerebral cortex allotted to any particular function corresponds proportionately to the relative size of the thalamic sensory nuclei relaying to it. Animals can be categorized into three basic types depending on which sensory function dominates: beholders, feelers, and listeners (Welker, 1973). Within this scenario, dogs probably fit into the listener-type category, indicating that a disproportionately large area of the canine cortex and thalamus is devoted to the representation and analysis of auditory information.
Besides relaying sensory and emotional input, the thalamus plays an important role in the expression of attentional behavior. In contrast to the general arousal functions served by the reticular formation, the thalamus mediates a more selective, "informed" attentional response toward sensory inputs. The thalamus enables a dog to selectively concentrate and focus on one thing at a time, whereas the reticular formation facilitates general alertness, causing all sensory inputs reaching an effective threshold to capture attention.
Unlike all other sensory inputs, which travel first to the thalamus before being relayed to other parts of the brain, olfactory sensory input moves directly from the olfactory bulb via the olfactory tract to the primary olfactory cortex (paleocortex). From the olfactory cortex, the olfactory input projects to the medial dorso nucleus of the thalamus, from where it is relayed to neocortical destinations (orbitofrontal cortex) for cognitive (associative) processing and the conscious perception of smell. A second major olfactory pathway originating in the primary olfactory cortex projects to the preoptic/lateral hypothalamus. Another important limbic destination of olfactory information is the amygdala. The corticomedial nucleus of the amygdala receives afferent input directly from the olfactory bulbs as well as forming connections with the olfactory cortex. Secondary olfactory projections terminate in various other related limbic areas, including the septum and hippocampus (Thompson, 1993). Clearly, many areas of the dog's brain receives olfactory information via parallel and interacting circuits. These various neural circuits serve such diverse functions as food and mate selection, kinship recognition, sexual behavior, memory, imprinting, motivation, emotion, and learning. Not surprisingly, a proportionately larger area of a dog's brain than the human brain is devoted to analyzing olfactory information.
The hypothalamus performs many regulatory functions over basic biological activities, including appetite, thirst, and various homeo-
static functions like blood pressure, temperature regulation, and blood sugar levels. Besides controlling basic appetitive/homeostatic drives and regulating the expression of emotional behavior, hypothalamic nuclei also control sexual drive. Hypothalamic activity is intimately connected with the endocrine system and the regulation of the pituitary gland—the so-called master gland of the body. The hypothalamus exercises direct chemical regulatory control over the pituitary by the manufacture and secretion of releasing factors. Hypothalamic releasing factors circulate via the portal blood supply to the anterior pituitary, causing it to release various tropic hormones involved in growth, sexual behavior, maternal behavior, metabolism, and general biological stress reactions. The hypothalamus also controls the ANS, which is composed of two subsystems: the sympathetic nervous system and the parasympa-thetic nervous system. Together the sympathetic division and parasympathetic division perform numerous complementary functions designed to achieve biological homeostasis (Fig. 3.2). The sympathetic division provides immediate physiological preparation for emergency freeze-flight-fight reactions. Sympathetic arousal is regulated by the posterior hypothalamus, which when appropriately stimulated evokes a bodywide neuroendocrine preparation for vigorous action. Besides directly activating the biological systems needed for emergency action, sympathetic arousal stimulates the adrenal medulla to release the peripheral hormones epinephrine and NE into the bloodstream. Epinephrine reinforces and sustains ANS-triggered stimulation of such stress-related bodily changes as increased heart rate and respiration. This interaction between the hypothalamus and the adrenal medulla is known as the sympathetic-adrenomedullary system.
A neuroendocrine system associated with stressful arousal and homeostasis is formed by the hypothalamus, pituitary, and the adrenal cortex. Under conditions of stress, the hypothalamus secretes corticotropin-releasing factor (CRF), which signals the pituitary gland to secrete a tropic hormone—adrenocorti-cotropic hormone (ACTH)—into the bloodstream. ACTH stimulates the adrenal cortex
to release various steroidal hormones, including cortisol (corticosterone). Cortisol serves many biological functions (regulation of blood pressure, control of glucose levels in the blood, and acceleration of the breakdown of protein into amino acids) to help an animal cope effectively with stress, injury, or defense. The release of cortisol into the bloodstream completes the circuit when it reaches the hypothalamus, where it restrains CRF production and thereby inhibits ACTH production by the pituitary. The reduction of circulating ACTH causes the adrenal cortex to decrease production and secretion of cortisol. This slower stress-activated system is known as the hypothalamic-pituitary-adreno-cortical (HPA) system.
The parasympathetic branch of the ANS shadows the actions of the sympathetic system, but with an opposing calming influence more specifically targeted on the various organs and systems of the body activated by the sympathetic division. Although autonomic activity is aimed at achieving homeostatic balance of sympathetic and parasympathetic influences, some individuals appear to be genetically predisposed in one direction or the other (Kagan et al., 1987; Kagan and Snid-man 1988). Some dogs are sympathetically dominant (prone to emotional reactivity and biological stress), whereas others, the parasympathetically dominant type, are inherently more calm and enjoy a more precise biological adaptation. The sympathetically dominant temperament type is more prone to develop behavior problems involving emotional reactivity and psychosomatic disorders than the parasympathetically dominant counterpart. Tests devised to evaluate relative sympathetic versus parasympathetic ANS reactiv ity in puppies could be potentially useful in conjunction with puppy temperament-testing procedures. Measures of heart rate, blood pressure, respiration, and cortisol levels under various conditions of stress could provide a reliable means to assess ANS dominance and temperament reactivity.
Gunnar (1994) reviewed several studies showing a correlation between cortisol levels and relative dominance-assertiveness in young children. This correlation is an interesting finding, since HPA activation is generally associated with emotional distress and fear. She reports an experiment performed by de Haan and colleagues (1993) demonstrating a direct relationship between dominance-aggressiveness and cortisol levels in 2-year-old children. In this experiment, salivary cortisol levels were measured during the first few days of nursery school. Subsequent interviews with the teacher and parents of the children tested revealed several positive correlations between high cortisol levels and a child's tendency to become a group leader, engage in aggressive behavior in school, and exhibit an "angry temperament" at home. On the other hand, more socially retiring children exhibiting shyness, with a tendency toward solitary play, and other signs of behavioral inhibition did not show significant cortisol indicators of HPA system arousal.
Studies with various animal species have shown a link between increased HPA system activity and stress. Sapolsky (1990) studied the dynamic interaction between social status, the stress response, and a variety of correlated hormonal changes exhibited differentially by dominant and subordinate free-ranging baboons. He found that resting cortisol levels are higher in subordinate males, but that, under acute stress, cortisol levels in dominant males overshoot that exhibited by subordinates under the same conditions:
Cortisol is responsible for much of the double-edged quality of the stress response. In the short run it mobilizes energy, but its chronic overproduction contributes to muscle wastage, hypertension and impaired immunity and fertility. Clearly, then, cortisol should be secreted heavily in response to a truly threatening situation but should be kept in check at other times. This is precisely what occurs in dominant males. Their resting levels of cortisol are lower than those of subordinate males yet will rise faster when a major stressor does come; exactly how this speedier rise is accomplished is not understood. (1990:120)
Similarly, Manogue and colleagues (1975) found that, among squirrel monkeys, individuals destined to play a dominant role in the group exhibited higher cortisol levels than subordinates during the early phases of the group's organization. Once the group became stable, the dominant monkey's cortisol level dropped below that of the subordinates. Under conditions of external distress, however, the dominant monkey exhibited emergency cortisol levels that quickly overshot that of subordinate group members. McLeod and coworkers (1995) have shown that urinary cortisol levels are increased under the influence of social stress among captive wolves. For example, they found that the lowest-ranking females exhibited the highest levels of cortisol in their urine. The presence of high levels of cortisol in the urine of subordinate females may help to account for the natural inhibition of estrus in such females via stress-mediated suppression of hypothalamic secretion of gonadotropin-releasing hormone. Haemisch (1990) investigated cortisol levels in guinea pigs undergoing social conflict in familiar and unfamiliar environments. Under conditions of social conflict occurring between an offensive individual and defensive individual, the defensive guinea pig exhibited significantly higher levels of plasma cortisol (about four times as much) under familiar environmental conditions than the offensive animal. When confrontations took place in an unfamiliar environment, the difference between the offensive and defensive individuals was not significant.
A study involving HPA activity in pointers found that genetically nervous dogs possessed significantly larger adrenal glands than normal controls (Pasley et al., 1978). Adrenal hypertrophy is commonly associated with chronic HPA-mediated stress. However, a subsequent experiment performed by Klein and colleagues (1990) failed to show a significant difference between nervous and normal pointers in terms of HPA system activity. The authors speculate that the lack of difference between the two strains of pointers may have resulted from the testing method employed (a static single-point baseline comparison), which may have missed significant differences in HPA activity occurring episodically at other times during the day. Another interesting finding relevant to hypothalamic-pitu-itary interaction involves differences in so-matomedin/insulin-like growth factor (IGF-I) levels found in nervous and normal pointers (Uhde et al., 1992). Nervous dogs exhibit lower plasma levels of growth factor (GF) than normal controls. Nervous dogs appear to be smaller, perhaps a direct physical outcome of GF insufficiency. Further, nervous pointers are more prone to exhibit compulsive behaviors (especially oral ones like excessive licking, biting, and pulling) than normal pointers. Uhde and colleagues suggest that replacement GF might provide some therapeutic benefit for acral lick dermatitis (to date, an untested possibility).
intense sympathetic arousal may precipitate pronounced parasympathetic rebound effects like diarrhea and urination resulting from increased alimentary and urinary motil-ity. Another common outcome following intense sympathetic arousal is opponent-processed parasympathetic reduction of heart rate. Church and colleagues (1966) demonstrated that dogs stimulated with shock experience an initial sharp rise in heart rate (sympathetic arousal), but with the cessation of shock the subjects' heart rates fall far below their original quiet preshock levels. Konorski (1967) reported experiments in which a dog and several rabbits were caused to experience intense fear by being shot with noninjurious paper projectiles from a sham gun. Whereas the dog experienced an increase in blood pressure after being shot, the rabbits exhibited a sharp fall in blood pressure, with some experiencing an increase after the cessation of stimulation. A few of the stimulated rabbits died as the result of a precipitous and lethal fall in blood pressure. Another relevant study with respect to parasympathetic rebound effects on cardiac function was performed by Richter (1957), who immersed rats in water and observed their swimming behavior under various experimental conditions. one group of swimmers had their whiskers (vibrissae) cut off before being placed in the swim tank. in rats, the whiskers are a very important sensory accessory for providing information about their immediate surroundings. According to Welker's nomenclature, previously discussed above, rats are feelers whose thalamo-cortical world is dominated by sensory information provided by the whiskers. The rats without whiskers panicked, apparently responding to the tank situation as though it were inescapable without the aid of whiskers; they swam frantically for a minute or so, before giving up and sinking to the bottom of the tank. Subsequent necropsies showed that the rats had not drowned but had suffered cardiac arrest. Ordinarily, rats can swim for long durations without stopping (up to 48 hours). The dewhiskered rats, however, were seized with intense sympathetic activation rapidly followed by an equal but opposed parasympathetic rebound, resulting in the loss of heart activity. in subsequent studies, Richter found that if he repeatedly immersed and rescued a rat before cutting off its whiskers, the animal did much better than controls not pretreated before exposure to immersion. Pretreated rats appeared to be partially immunized against an apparent "helplessness" effect generated by the removal of their whiskers.
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Getting a new puppy is a fun and interesting time. You probably went to a breeder or pet store or maybe just saw an ad on the Internet or the newspaper, for puppies, and decided just to check it out. Before you knew it those little eyes and fluffy puppy fur had your heart melting and you were headed home with him or her in your arms. If you are like most new pet owners you had visions of playing fetch with your dog, of watching him frolic at the lake, and of cuddling up on cold nights.