Answer
Origin and insertion of muscle
A muscle is usually attached to two bones. Generally, when the muscle contracts, it narrows the angle between the two bony attachments -- origin and insertion. In this angle-narrowing process, one point of attachment is relatively mobile, and the other remains relatively stable. The site of attachment that remains relatively stable is called the origin of the muscle; the attachment point that is relatively mobile is called the insertion (point).
Example: In the case of the biceps brachii, its origin is on the scapula, and it inserts on the radius.
Belly of muscle. This term has various definitions: the fleshy middle part of the muscle; the middle thicker part of the muscle; the part of the muscle between origin and insertion. Most muscles taper at both ends, but some do not ; also, the definitions of origin and insertion may change operationally, dependent on the action of the muscle. For example, origin and insertion of quadriceps femoris reverse in the following actions : kicking a soccer ball ( tibia is insertion ); sitting on a chair--tibia functions as origin.
It should be noted that not all muscles insert on bones-- some insert in fascia ( biceps brachii), and some in skin (CN VII).
Work Step by Step
Action of muscle
The effect produced by the contraction of a muscle is called its action. The effect may be to produce a movement or to prevent one.
Innervation of muscle
A nerve (or nerves) that stimulates a muscle is considered its innervation. The recognition or identification of the innervation of a muscle facilitates diagnosis of injury to brain stem, spinal cord or nerve, based on muscle function or dysfunction. This enables the healthcare provider to set relevant attainable goals for rehabilitation.
