Negative feedback relay

Most anterior pituitary hormones are secreted in response to a negative feedback mechanism whereby increases in the secretions of the target glands inhibit further pituitary hormone production. The hypothalamus is important in the negative feedback chain as it synthesizes link or releasing hormones which reach the anterior pituitary by the hypothalamohypophysial portal system to stimulate hormone secretion. There is no information yet of the effect of injury on most of the releasing hormones of the hypothalamus. The anterior pituitary is an important relay center in the response to injury. Destruction of the median eminence in experimental animals prevents the release of ACTH in response to stress (Hll). 

In the circuit of Fig. 12.7, there is an action that "feeds back" and makes the operation of the relay weaker, rather than stronger. This action is simple and direct — it is the disconnection of power to the coil. In the first experiment, the capacitor is not attached. A high pitched buzz is heard, because the armature vibrates (oscillates) at a fairly high frequency. The negative feedback occurs too soon for the armature to get all the way down to the NO contacts. 

If a transistor can be arranged to have delayed negative feedback, similar to the relay buzzer example on page 138, it can oscillate continuously. This is called "astable," meaning not stable. That is, it will not remain in one state, but instead it flips back and forth between "on" and "off."... 

The switching transition, once the positive feedback starts to take effect, is much faster than the charging of the capacitors. Therefore the switching action can be completed, long before the negative feedback from other half of the circuit will reverse things. This is similar to the relay circuit, where the armature motion is much faster than the charging of the capacitor. 

---