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.