Bidirectional Interaction Between the Cortex and Subcortex
The formation of biological motivation requires the integration of various brain regions: the hypothalamus, thalamus, reticular formation, and cerebral cortex. This system shares general neurochemical properties (e.g., its excitation is not blocked by urethane) and relies on bidirectional communication:
- Ascending influences: Motivogenic structures of the hypothalamus specifically activate overlying areas, up to the cerebral cortex.
- Descending influences: The cerebral cortex and other structures selectively affect initiating hypothalamic centers.
The nature of descending control depends on the type of motivation and the specific cortical area. During feeding behavior, stimulation of the anterior cortex and dorsal hippocampus inhibits the 'hunger center' in the lateral hypothalamus, while stimulation of the occipital cortex lowers its excitation threshold (facilitates the reaction). In the case of defensive motivation, the reverse picture is observed: the anterior cortex exerts a facilitating influence, while the occipital cortex exerts an inhibitory one.
The Hypothalamus as a Motivational Pacemaker
According to the concepts of P.K. Anokhin and K.V. Sudakov, the hypothalamus plays the role of a pacemaker (rhythm driver) for motivational arousal, analogous to the sinus node of the heart. It keeps other structures in functional dependence.
Key properties of the hypothalamic pacemaker:
- Excitability gradient: Brain structures are organized into a strict hierarchy. Hypothalamic centers possess the highest excitability to chemical and electrical stimuli compared to the rest of the brain.
- High chemical sensitivity: Experiments with atropine showed that shutting down 'hunger' activation via the hypothalamus requires a concentration 100 times smaller than acting on the cortex. Moreover, injection into the hypothalamus produces a generalized effect throughout the brain, whereas application to the cortex yields only a local effect.
- Criticality for survival: Destruction of hypothalamic centers leads to complete disintegration of the system. With bilateral destruction of the lateral hypothalamus, an animal dies of starvation even in the presence of abundant food, as motivation is entirely eliminated (resulting in aphagia).
Transformation of Motivation into Behavior
The transition from an internal need to goal-directed action is realized sequentially at two levels:
- Hypothalamic level: Here, the neurohumoral signal regarding metabolic need is converted into neural excitation. Primary energetic activation of brain structures occurs.
- Cortical level: The excitation arriving from below is translated into goal-directed behavioral mechanisms. Meanwhile, the informational significance of the initial need is fully preserved.
In parallel, dominant motivation selectively increases the sensitivity (sensitizes) of peripheral receptors to make it easier for the organism to interact with the environment. Hunger sharpens taste, aggression increases the sensitivity of the trigeminal region (around the mouth), and sexual motivation activates the receptors of the genital organs.
Molecular Integration and Pharmacology
A motivational state has not only a structural but also a chemical architecture. Each need recruits its specific ensemble of protein molecules—oligopeptides—at different levels of the CNS.
- Pentagastrin stimulates feeding behavior.
- Cholecystokinin, conversely, inhibits it.
- Beta-lipotropin acts in a bidirectional manner: when injected into the brain ventricles of hungry rabbits, it suppresses feeding reactions, whereas in fed rabbits, it activates them.
Knowledge of pacemaker mechanisms determines pharmacological treatment tactics. Because motivational pacemakers have the most intense metabolism, drugs act on them first and in lower doses. This allows for the targeted disruption of cortico-subcortical integration during pathological dominant motivations.