The trochlea is a term used for certain anatomical features in the human body that resemble a pulley. This is a simple device consisting of a wheel possessing a groove through which a rope or chain passes to lift objects. The term, which is of Latin origin, actually means pulley.
One of the more common uses of trochlea is in the term trochlea of humerus. This is the surface of the humerus — the lone long bone of the upper arm, which extends from the shoulder to the elbow — that articulates with the ulna at the elbow. The ulna is one of the two long bones that comprise the lower arm, or forearm.
Thus, the union of the trochlea of humerus and ulna creates a hinge joint so that the forearm can move, but in one plane that consists of forwards and backward. The part of the ulna that connects with this section of the humerus is referred to as the trochlear notch. It also goes by other terms, such as the trochlear notch of ulna, the semilunar notch, and the greater signmoid cavity.
The trochlea of humerus itself is best described as a deep depression at the bottom end of the humerus. It is directly below the coronoid fossa. This is a depression that bears the ulna’s circular coronoid process for the forearm’s flexion.
Trochlea can also be found in the term trochlear process, which is also known as the fibular trochlea of calcaneus. This is an elevation of the heel bone’s lateral side between the tendons of two muscles of the leg’s lateral compartment. These are the peroneus longus and peroneus brevis, muscles responsible for turning the sole of the foot inside out.
Another major use for the term trochlea is in guides for certain muscles of the body. The trochlea of superior oblique, for instance, acts as the eye’s pulley. It is named for the muscle it guides, which is the superior oblique muscle. Also known as the obliquus oculi superior, it is responsible for depressing and rotating the eyeball, particularly when the eye is moved to the middle of the eye socket.
The obliquus oculi superior is the origin for the naming of the trochlear nerve. It is responsible for providing the trochlea of superior oblique with innervation. The trochlear nerve is also called the fourth cranial nerve or fourth nerve.