Pre Admission Biology

Chapter 7

The Muscular System


I. Muscle Tissue Types

A. Skeletal


NOTE: This chapter will focus primarily on skeletal tissue.

B. Cardiac

C. Smooth


II. Structure of Skeletal Muscle

(See page 135, figure 6-2)

A. Macroscopic level

1. Most muscles attach to two bones that have a movable joint in between them.

2. Origin: End of the muscle that is attached to the more stationary bone of the two bones.

3. Insertion: End of the muscle that is attached to the more movable bone of the two bones.

4. Tendon: anchors muscle to bone

5. Bursae:small fluid-like filled sacs lying between tendons and the bones beneath them

6.Synovial membranes: They line bursae and produce synovial fluid (a lubricant).

B. Microscopic level

(See page 136, figure 6- 3) 

1.     Muscle tissue consists of muscle fibers

2.     Each muscle fiber consists of :

a. Thin myofilaments called actin

b. Thick myofilaments called myosin

3.Sarcomere:The basic functional or contractile unit of muscle

4. Z line: The dark bands that separate adjacent sacromeres

5. Sliding filament model explains how skeletal muscles contract:

Actin and myosin myofilaments attach to one another by bridges in the presence of calcium. The bridges pull the myofilaments past each other. This requires ATP.


III. Functions of Skeletal Muscle

A. Movement

1.     Muscles work by pulling on bones

2.     Skeletal muscles are under voluntary control

3.     Skeletal muscles work in pairs called antagonistic pairs

a. Prime mover

One (of several muscles) that is primarily responsible for producing a particular movement by contracting

b. Synergists

Other muscles that help the prime mover by contracting

c. Antagonist

The muscle that relaxes when the prime mover and the synergists contract

E.g. Movement of forearm "upwards"

Biceps brachii (prime mover)_________ Contracts

Brachii (synergist)___________________ Contracts

Triceps brachii (antagonist)____________ Relaxes

B. Posture

1. Muscles are made up of many muscle fibers.

2. Not all the muscle fibers in a muscle contract at the same time(usually).

3. Thus a muscle can have degrees of contraction.

4. Tonic contraction :

a. Only a few muscle fibers contract so the whole muscle does not shorten(contract)

b. Holds the body parts in positions that favor good functioning

(Good posture)

c. Muscle tone maintains posture by counteracting the effect of gravity.

C. Heat production

1. The contraction of muscle fibers produces most of the heat required to maintain body temperature.

2. Normal body temperature is 37 degrees C or 98.6 degrees F

3. Hypothermia: when the body temperature drops way below normal

4. Fever: when the body temperature is higher than normal


IV. Muscle Stimulus and Muscle Contraction

A. Muscle Stimulus

1. Threshold stimulus:

 Minimal level of stimulation required to cause a muscle fiber to contract completely

2. All or none"

 When a muscle fiber is subjected to a threshold stimulus, it contracts completely.

  NOTE:    Whole muscles do not contract completely when given a threshold stimulus

3. Muscle fibers are stimulated by motor neurons. Their point of contact is called a neuromuscular junction. All of these structures make up the motor unit (see page 139, figure 6-4 in 11th edition).

4. Maximum stimulus:

The stimulus that beyond it the degree of contraction of whole muscle does not increase.

B. Types of Skeletal Muscle Contraction

1. Tonic contraction:

Maintains muscle tone and posture

2. Tetanic contraction:

A more sustained and steady response

Where contractions together to produce a sustaines contraction (tetanus) as a result of repeated stimuli (on a graph).

Twitch is a quick, jerky response to a stimulus and occurs in isolated muscles. Twitch plays a minimal role in normal muscle activity.

3. Isotonic contraction:

Where the muscle shortens and the insertion end moves towards the origin end

Causes movement at a joint

(E.g.)    walking, running, lifting

4. Isometric:

Where the muscle's tension increases, but the muscle does not shorten and no movement results

Repeated isometric contractions make muscles grow longer and stronger.

(E.g.)    pushing against a wall

 5.  Fatigue:

  If a muscle is repeatedly stimulated beyond tetanic contraction without a period of rest, the muscle contraction decreases and eventually loses its ability to contract.

C. Effects of Exercise on Skeletal Muscles

1. During exercise ATP is used for muscle contraction. Aerobic respiration of nutrients (w. O2) is required to replace the ATP. Sometimes due to the intensity and/or duration of the exercise, the muscles blood supply cannot keep up. When the O2 supply runs low, the muscle switches to anaerobic fermentation. This results in lactic acid build up which produces muscle soreness.

2. Oxygen Debt:

After strenuous physical activity the oxygen required in the recovery period, in addition to that required while resting, to oxidize the excess lactic acid produced and to replenish the depleted stores of ATP and phosphocreatinase. (Taber's Cyclopedic Medical Dictionary 17th edition.)

3. Disuse atrophy:

Muscles shrink in mass due to prolonged inactivity

4. Hypertrophy:

 Muscles increase in mass due to exercise

5. Endurance / Aerobic training:

Exercise that increases the muscle's ability to sustain moderate activity over a long period and does not usually result in muscle hypertrophy.

It increases the number of blood vessels in a muscle and the number of mitochondria (ATP)

V. Movements Produced by Skeletal Muscle Contractions

A. Flexion

1. Makes the angle between two bones at their joint smaller

2. Bending movement

B. Extension

1. Makes the angle between two bones at their joint larger

2. Straightening or stretching movements

C. Abduction

1. Moving body part away from midline

D. Adduction

1. Moving body part towards the midline

E. Supination

1. Hand is palm or face up

F. Pronation

1. Hand is palm down or face down

G. Dorsiflexion

1.Top of foot elevated with toes pointing up

H. Plantar flexion

1. Bottom of foot is downward pointed so you stand on your toes

I. Rotation

1. Circular motion around a longitudinal axis