Arrangement of Muscle Fibers
|Parallel Muscles|| |
Have fibers that run parallel to each other along the axis of the muscle. When parallel muscles contract, the entire muscle shortens equally.
EXAMPLE--Most muscles of the body are Parellel
Biceps Brachii, Hamstrings
Almost parallel, ending in flat tendons.
EXAMPLE--Digastric muscle, spindle shape muscle in neck with two bellies connected by a tendon
|Circular Muscles|| |
Have fibers that are arranged in a circle around an opening. Contraction of a circular muscle closes or reduces the size of the opening--relaxation makes opening wider--guard against entrances and exits of internal passages
EXAMPLE--muscles around mouth (orbicularis oris), muscles around anus (sphincter ani)
|Triangular Muscles (Convergent)|
Have a broad base but attach at a common site. The muscle fibers are arranged like a fan. The muscle may pull at a tendon or a connective tissus sheet--can be manipulated to pull at different directions by stimulating specific groups of cel fibers at any given moment.
EXAMPLE--Pectoralis Muscle, Trapezius
Shaped like feathers, fasicles are arranged obliquely, forming a common angle with a central tendon that may extend along almost the entire length of the muscle. The fascicles pull on the tendon at an angle, they do not move the tendon as far as a parallel muscle; facilitates the accommodation of more muscle fibers in a unit area compared with a parallel muscle, making it possible to exert more force than a parallel muscle.
EXAMPLE: extensor digitorum (a muscle that extends the fingers--a unipennate muscle)
Rectus Femoris--Extensor of the knee (bipennate)