Anatomy and Physiology Lecture Notes

Unit 6 - The Muscular System

A. Muscular System Introduction

- Muscles convert chemical energy into mechanical energy.

- Muscles make up the bulk of the body and account for about one-third of its

weight.

- Their ability to contract not only enables the body to move, but also provides the

force that pushes substances, such as blood and food, through the body.

- Without the muscular system, none of the other organ systems would be able to

function.

B. Types of Muscle Tissue

1. Skeletal Muscle Cells are striated (striped) with multiple nuclei; are arranged as

many, long, cylindrical cells bundled together; with several bundles enclosed in

tough connective tissue sheath to form a muscle; they are responsible for

voluntary movement, and are generally connected to bones via tendons

2. Smooth Muscle Cells are not striated, have a single nucleus; the cells are

tapered at both ends, are held together by tight junctions; and are located in the

walls of "hollow organs" i.e., blood vessels, stomach, bladder, and other internal

organs; they are responsible for involuntary movements i.e., peristalsis, and

although they are wrapped by connective tissue, they are not connected by

tendons. Although considered involuntary muscles, they do respond to

psychological states such as stress and excitement

3. Cardiac Muscle Cells are striated with multiple nuclei per cell like skeletal

muscles; however, the cells are branched and connected end-to-end by junctions

(fused cell membranes) that allow electrical current flow; they are located only in

the heart wall. Although also considered involuntary muscles, they do respond to

psychological states such as stress and excitement, and significant evidence

suggests that some degree of voluntary control can be gained over cardiac

muscles, through practices such as meditation and biofeedback

C. Functional Organization of Skeletal Muscle

1) Single Muscle Fibers

1. A muscle fiber is a single, multinucleated muscle cell.

2. A muscle is made up of hundreds or even thousands of muscle fibers,

depending on the muscles size.

3. Although muscle fiber makes up most of the muscle tissue, a large amount of

connective tissue, blood vessels, and nerves are also present.

4. Connective tissue covers and supports each muscle fiber and reinforces the

muscle as a whole.

5. The health of muscle depends on a sufficient nerve and blood supply. Each

skeletal muscle has a nerve ending that controls its activity (innervation), and an

individual system to supply and drain blood (vascularization).

2) Bundles of Muscles

6. Muscle Fibers consist of bundles of threadlike structures called myofibrils.

7. Each myofibril is made up of two types of protein filaments- Thick ones and

Thin ones.

8. The thick filaments are made up of a protein called myosin.

9. The thin filaments are made of a protein called actin.

10. Myosin and Actin filaments are arranged to form overlapping patterns, which

are responsible for the light and dark bands (striations) that can be seen in skeletal

muscle.

11. Thin actin filaments are anchored at their midpoints to a structure called the

z-line.

12. The region from one z-line to the next is called a sarcomere, which is the

functional unit of muscle contractions.

D. How Muscles and Bones Interact

1. Skeleton muscles generate force and produce movement only by contracting or

pulling on body parts.

2. Individual muscles can only pull; they cannot push.

3. Skeleton muscles are joined to bone by tough connective tissue called tendons.

4. Tendons attach muscle to bone; the origin is the more stationary bone, the

insertion is the more movable bone.

5. Tendons are attached in such a way that they pull on the bones and make them

work like levers. The movements of the muscles and joints enable the bones to act

as levers.

6. Most skeletal muscles work in pairs.

7. When one muscle or set of muscles contracts, the other relaxes.

8. The muscles of the upper arm are a good example of this dual action: a flexor,

is a muscle that bends a joint, while an extensor is a muscle that straightens a

joint.

a. when the biceps muscle (on the front of the upper arm, flexor)

contracts, it bends or flexes the elbow joint.

b. when the triceps muscle (on the back of the upper arm, extensor)

contracts, it opens, or extends, the elbow joint.

c. a controlled movement requires contraction by both muscles.

E. Names of Skeletal Muscles

Individual muscles are named on the basis of several criteria, each of which focuses on a particular structural or functional characteristic. Paying close attention to these cues can greatly simplify your task of learning muscle names and actions:

• Direction of the muscle fibers. Some muscles are named in reference to some imaginary line (usually the midline) on the body. When a muscle name includes the term rectus (straight), its fibers run parallel to that imaginary line. The term oblique (slanted) in a name tells you that the muscle fibers run at an angle to the imaginary line.

The rectus femoris is a straight muscle of the thigh. The external oblique is a muscle slanted across the abdomen.

• Relative size of the muscle. Such terms as maximus (largest), minimus (smallest), and longus (longest) are often used in muscle names.

The gluteus maximus is the largest muscle of the gluteus muscle group.

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

The temporalis and frontalis muscles overlie the temporal and frontal bones of the skull.

• Number of origins. Some muscle names begin with a prefix indicating the number of origins.

The biceps, triceps, and quadriceps have two, three, and four origins respectively.

• Location of the muscle's origin and insertion. Some muscles are named for their attachment sites.

The sternocleidomastoid muscle has its origin on the sternum (sterno) and clavicle (cleido) and inserts on the mastoid process of the temporal bone.

• Shape of the muscle. Some muscles have a distinctive shape that helps to identify them.

The deltoid muscle is roughly triangular. (Deltoid means "triangular".)

• Action of the muscle. Action terms like flexor, extensor, and adductor appear in the names.