Hypothalamus sits at the mid-line of the brain just below the thalamus extending to the base of the brain, its size is not larger than the tip of a little finger and it weighs about less than one percent of the brain's entire weight.
Hypothalamus is very vital to both emotion and motivation, thus it is called “the brain within the brain". It is an important brain structure, which through its connections with autonomic nervous system controls glands and smooth muscles, blood vessels and the heart. It influences various kinds of emotional responses accompanied by physiological arousal.
The heart rate speeds up, blood circulations from internal organs carrying increased amount of glucose blood sugar to the muscles take place. These "fight" or "flight" emergency responses are the preparatory processes of body for appropriate kind of action and are regulated by the autonomic nervous system under the control of hypothalamus.
By stimulating a part of the hypothalamus in cats, Levison and Flynn (1966) have found that they turn into cold, silent killers of its victims. Flynn, (1967) by stimulating other part of hypothalamus found that the cats can be violent, aggressive emotionally, and can attack the victim that comes close. Thus aggressive behaviour can be elicited in animals by mild electrical stimuli of a particular part of the hypothalamus.
By implanting electrodes when a of a cat's hypothalamus is stimulated the animal hisses, its hairs stand on its body, its pupils become dilated and it attacks the rat or its victim ferociously in the experimental cage. Injected neurochemical stimulation of a hypothalamic region results in exactly the same violent attack response by the cat.
On other hand inhibitory neurochemical stimulation injected to the same part of hypothalamus produced temporary inhibitory responses. (Smith, King, & Hoebel, 1970). Later studies have shown that hypothalamus is the important center for the integration of emotional impulses.