The USDA created the food pyramid to encourage a healthy diet, but the USDA also has a mission to encourage agricultural products grown in the US. There can be a LOT of conflict between those two goals. So the food pyramid (ahem, I mean MyPlate) is not simply an objective summary of the available evidence.
The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length. When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases.
The knee-jerk reflex involves a sudden kicking movement of your lower leg after the tendon just below your kneecap has been tapped. Doctors often trigger this reflex to test the function of your nervous system. If the reaction is exaggerated or absent, it may indicate a damage to the central nervous system.