Rat Hypothalamus

The hypothalamus may be regarded as the front part of the autonomic, or visceral motor, nervous system and also of the less well described visceral sensory system. It has complex connections, both afferent and efferent, with the cerebral system. It has complex connections, both afferent and efferent, with the cerebral hemispheres, and the thalamus. It is the most vascular part of the brain, and it probably responds directly to changes in the blood composition. Inject exceedingly small amounts of sodium chloride solution into the hypothalamus, and the animal will begin drinking when it has previously refused water. Electrical stimulation of parts of the anterior hypothalamus may influence the functioning of the gut, for instance, peristalsis; stimulation of posterior regions can alter blood pressure and other aspects of the internal state. The anterior is then especially related to parasympathetic function, the posterior to sympathetic.

The hypothalamus is not regarded as a “reflex center”: the visceral sensory tracts end lower in the brain stem, and the visceral motor tracts which pass to motor neurons in the spinal cord also take origin further back. The hypothalamus evidently activates or inhibit reflexes; but it does more. Local stimulation may set going complex patterns of behavior; one example is attack, which is associated with sympathetic stimulation; another is the whole sequence of activities involved in feeding (parasympathetic).

The hypothalamus is also involve in the inactivity of sleep; according to Nauta, the posterior hypothalamus contains a waking center, and the anterior, a sleep center (496). In Nauta’s experiments the hypothalamus of rats was completely cut across on both sides; the result was loss of the normal regulation of sleep. Another maintenance function, thermoregulation, also depends on the hypothalamus; stimulation of the anterior may activate changes which lead to loss of heat, such as peripheral vasodilation, sweating and panting, while the posterior directly regulates the increase of heat production and other process normally evoked by exposure of the blood. If a minute metal probe is used to alter the temperature of the hypothalamus, the animal behaves as if the temperature of the whole body had been raised or lowered.

Rat Reticular System

In most elementary account of the brain, the brain stem structure is described as an arrangement of well-defined reflex centers. The centers are highly specialized in function and can operate independently of the forebrain: a “midbrain animal” is one in which the forebrain has been destroyed; it still has an array of reflex responses. But paradoxically the midbrain also has a component of which the function are generalized and not specific and which operates in intimate relation to the forebrain. This component, the reticular activating system (RAS), consists of large numbers of cells, most of them without long axons, whose functions were for long an enigma. In 1949 Moruzzi & Magoun published observation which have given the RAS a prominent position in neurophysiology. The subject has been given reviewed by Jasper and colleagues and by Magoun himself.

The RAS is a matrix or network of cells extending from the anterior part of the spinal cord into the thalamus. It receives collateral fibers from afferent tracts as they proceed toward the thalamus, and all the receptors contribute inputs to it, directly or indirectly; hence it is a center, or group of centers, on which information from every sensory modality converges. The are also cortico-reticular tracts, both from the isocortex and the limbic system of fibers from the reticular substance to the whole of the cortex; this is in contract to the more familiar specific projection system carrying the sensory inputs, via the thalamus, to the special sensory regions.

THE FUNCTION OF EXPLORATION OF THE RAT

Wild rats once they leave the nest, are subject to predation, and their principal means of avoiding predators are the uses of pathways under cover, and flight to a burrow or other place of concealment. These actions depend on previous experience of the topography of their living space. Given such experience, they can run from any one point to any other, by the shortest route and in the last possible time . Rats are more vulnerable to predations by cat if they are in unfamiliar surroundings. But exploration does not lead only to learning where cover and nest sites are; it is accompanied by sampling of the edible or potable materials encountered , and so a rat is regularly informed of sources of food and water.

A further consequence of general movements is that odor trail are left on the routers regularly used, and these are detected by other rats.

The word ‘random ‘ is sometimes applied to exploratory rat behavior. A more appreciate term would be “unpredictable”; but even this is misleading. The directions of rat’s movement, even in unknown terrain, is influenced in an orderly way by many of the features it encounters. The inorganic stimuli may be tactile, visual, or olfactory . A similar variety of stimuli may be provide by other rats.

Thigmotaxis, formally studied in young rats by Crozier, has usually been regarded as behavior which appears regardless of the conditions in which the rats is reared or of any particular sort of experience. But Patrick & Laughlin raised rats in an environment without opaque walls, and they did not develop a tendency to move close to a wall.

Other guiding stimuli are more likely to lead to exposure; the odors of food or of other rats habit have an attractive effect which may overcome the shelter-seeking tendency.