Final answer:
The semicircular canals are part of the inner ear's balance system, detecting head movements, while muscle spindle afferents detect muscle stretch. Both contribute to proprioception and coordination of movement.
Step-by-step explanation:
The semicircular canals are structures within the inner ear that play a pivotal role in our sense of balance and spatial orientation. They operate by detecting rotational movement. When the head turns, the fluid inside the semicircular canals shifts, which bends the stereocilia of hair cells in structures known as ampullae, thereby sending signals about angular acceleration or deceleration to the brain. Specifically, when you turn your head or when your body changes speed or direction, these hair cells are stimulated. In contrast, muscle spindle afferents detect the stretch or lengthening of muscles.
Both the semicircular canals and muscle spindles are part of the body's proprioceptive system, which helps the brain know the position of different parts of the body. The semicircular canals are related to the vestibular system that helps with balance and orientation in 3-D space, while muscle spindles provide feedback to the central nervous system about muscle length and are pivotal for coordinated movement.