6.4c&d Naming Skeletal Muscles and Arrangement of Fascicles

DIRECTION OF THE MUSCLE FIBERS

• Some muscles are named in reference to some imaginary line, usually the midline of the body or the long axis of a limb bone.

• Rectus (straight): it’s fibers or whole structure run parallel to that imaginary line

• Oblique: a muscle’s name tells you that the muscle fibers run obliquely (at a slant) to the imaginary line.

RELATIVE SIZE OF THE MUSCLE

• Maximus: largest

• Minimus: smallest

• Longus: long

LOCATION OF THE MUSCLE

• Some muscles are named for the bone with which they are associated.

• For example, the temporalis and frontalis muscles overlie the temporal and frontal bones of the skull, respectively.

NUMBER OF ORIGINS

• When the term biceps, triceps, or quadriceps forms part of a muscle name, you can assume that the muscle has two, three, or four origins, respectively.

LOCATION OF THE MUSCLE’S ORIGIN AND INSERTION

• Occasionally, muscles are named for their attachment sites.

• Sterno: sternum

• Cleido: clavicle

SHAPE OF THE MUSCLE

• Some muscles have a distinctive shape that helps to identify them.

• Deltoid: triangle

ACTION OF THE MUSCLE

• When muscles are named for their actions, terms such as flexor, extensor, and adductor appear in their names.

Arrangement of fascicles

• Skeletal muscles consist of fascicles, but fascicle arrangements vary, producing muscles with different structures and functional properties.

• A muscle’s fascicle arrangement determines its range of motion and power.

• The longer and the more nearly parallel the fascicles are to

  a muscle’s long axis, the more the muscle can shorten.

• Muscle power depends more on the total number of muscle fibers in the muscle.

CIRCULAR

• The fascicles are arranged in concentric rings.

• Circular muscles are typically found surrounding external body openings, which they close by contracting, creating a valve.

• Sphincters “squeezers”

• E.g. orbicular muscles surrounding the eyes and mouth

CONVERGENT

• The fascicles converge toward a single insertion tendon

• Triangular or fan-shaped

• E.g., the pectoralis major muscle of the anterior thorax

FUSIFORM

• Modification of the parallel arrangement

• A spindle-shaped muscle with an expanded belly (midsection) and tapered ends

• An example is the biceps brachii muscle of the arm.

PARALLEL

• The length of the fascicles runs parallel to the long axis of the muscle.

• Sartorius of the anterior thigh.

• Muscles are straplike

PENNATE

• (pen′āt; “feather”) pattern

• Short fascicles attach obliquely to a central tendon

• Unipennate: fascicles insert into only one side of the tendon

• Bipennate: fascicles insert into opposite sides of the tendon

What is the fascicle arrangement of the orbicularis oris muscle?

Circular