CHAPTER 6

THE MUSCULAR SYSTEM

Overview of Muscle Tissues

Similarities Of All Muscles:– All muscle cells are elongated and are called

MUSCLE FIBERS– All muscle cells have the ability to contract.– All muscle cells have special prefixes, which

allow us to know that muscle is being referred to.

Muscle Types

Skeletal Muscle– Forms the muscles attached to the skeleton,

which move the limbs and other body parts.– Cells are long, striated, and contain many

nuclei.– Controlled voluntarily.– SKELETAL MUSCLES MAKE UP THE

MUSCULAR SYSTEM!

Muscle Types

Smooth Muscle– Cells are spindle-

shaped and have one nucleus

– Controlled involuntarily.

Muscle Types

Cardiac Muscle– Cells are striated

and are arranged in spiral bundles.

– Found only in the heart.

– Controlled involuntarily.

Muscle Functions

Producing Movement Maintaining Posture Stabilizing Joints Generating Heat

Microscopic Anatomy of Skeletal Muscle

The plasma membrane of the skeletal muscle cell is called the SARCOLEMMA.

The ribbon-like organelles are called MYOFIBRILS. Each myofibril is enclosed by a specialized ER

called the SARCOPLASMIC RETICULUM. Alternating LIGHT and DARK BANDS along the

myofibrils give the muscle cell its striped appearance.

Microscopic Anatomy of Skeletal Muscle

Myofibrils are also arranged in a banding pattern with 2 types of myofilaments:– Thick Filaments – also called myosin filaments

because they contain the protein myosin– Thin Filaments – also called actin filaments

because they contain the protein actin When a muscle contraction occurs the

myosin and actin filaments slide on top of each other and overlap.

Skeletal Muscle Activity

Stimulation and Contraction of Single Skeletal Muscle Cells– The Nerve Stimulus and the Action Potential

Skeletal muscles must be stimulated by nerve impulses to contract.

One motor neuron may stimulate a few muscle cells or hundreds of them.

– One motor neuron and all the skeletal muscle cells it stimulates are a MOTOR UNIT.

Skeletal Muscle Activity

Stimulation and Contraction of Single Skeletal Muscle Cells Continued…– When the nerve impulse reaches the end of the

motor unit, a chemical referred to as a NEUROTRANSMITTER is released. This neurotransmitter is called ACETYLCHOLINE.

– The neurotransmitter generates an ACTION POTENTIAL which eventually causes the muscle to contract.                                               

Skeletal Muscle Activity

Mechanism of Muscle Contraction: THE SLIDING FILAMENT THEORY– The actin and myosin filaments slide across one

another when activated by the nervous system.– The sliding movement is energized by ATP.

Contraction of a Skeletal Muscle as a Whole

Graded Responses – In skeletal muscles, the “ALL-OR-NONE” law

applies to the muscle cell. It states that a muscle cell will contract to its fullest extent when it is stimulated adequately; it never partially contracts.

Contraction of a Skeletal Muscle as a Whole

Graded Responses – Skeletal muscles react to stimuli with GRADED

RESPONSES which can be produced 2 ways: By changing the FREQUENCY of muscle stimulation. By changing the NUMBER of muscle cells being

stimulated.

Contraction of a Skeletal Muscle as a Whole

Graded Responses – Muscle Response to Increasingly Rapid

Stimulation: MUSCLE TWITCHES – single, brief, jerky contraction COMPLETE TEATANUS – when the muscle is

stimulated so rapidly that no evidence of relaxation is seen and the contraction is sustained.

Contraction of a Skeletal Muscle as a Whole

Graded Responses – Muscle Response to Stronger Stimuli:

When only a few cells are stimulated, the contraction of the muscle as a whole will be slight.

In the strongest contractions, when all the motor units are active and all the muscle cells are being stimulated, the muscle contraction is as strong as it can get.

Contraction of a Skeletal Muscle as a Whole

Providing Energy (page 173)

– Direct Phosphorylation of ADP by Creatine Phosphate

Creatine phosphate is found in muscle fibers and helps generate ATP.

No oxygen required.

Contraction of a Skeletal Muscle as a Whole

Providing Energy (page 173) – Aerobic Respiration

At rest and during light exercise, 95% of the ATP used for muscle activity comes from aerobic respiration.

Glucose provides the energy and is broken down to carbon dioxide and water

Occurs in the mitochondria and requires oxygen

Contraction of a Skeletal Muscle as a Whole

Providing Energy (page 173) – Anaerobic Glycolysis and Lactic Acid

No oxygen required. Glucose is broken down to pyruvic acid which

produces ATP. Pyruvic acid can be broken down to lactic acid which

causes muscle fatigue and muscle soreness.

Muscle Fatigue and Oxygen Debt

If we exercise our muscles strenuously for a long time, MUSCLE FATIGUE occurs.

A muscle is fatigued when it is unable to contract even though it is still being stimulated.

Muscle fatigue is believed to result from the OXYGEN DEBT that occurs during prolonged muscle activity: A person is not able to take in oxygen fast enough to keep the muscle supplied with the oxygen that they need.

Lactic acid will build up in the muscle cells and cause the muscles to completely stop working.

Types of Muscle Contractions

Isotonic – The myofilaments are successful in their sliding

movements, the muscle shortens, and movement occurs.– Example = bending the knee, rotating the arms, and

smiling

Isometric– The myofilaments do not slide successfully, the muscles

do not shorten, and the tension in the muscle keeps increasing.

– Example = pushing against a wall with bent elbows

Muscle Tone

The state of continuous partial contraction. Contraction is not visible, but the muscle

remains firm and healthy.

Effects of Exercise on Muscles

Muscle inactivity always leads to muscle weakness and wasting.

Regular exercise increases muscle size, strength, and endurance.

Muscle Movements, Types, and Names

Types of Body Movements (pages 177 – 178)– Flexion

Brings 2 bones closer together Example = Bending the knee and elbow

– Extension Increases the distance between 2 bones Example = straightening the knee or elbow

– Rotation Movement of a bone around an axis Example = shaking your head “no”

Muscle Movements, Types, and Names

Types of Body Movements (pages 177 – 178)– Abduction

Moving the limb away from the midline of the body Example = lifting arm or leg away from the body

– Adduction Moving the limb toward the midline of the body Example = bringing arm or leg in towards the body

– Circumduction A combination of flexion, extension, abduction, and

adduction (movement in a circle fashion) Example = Movement of the shoulder

Muscle Movements, Types, and Names

Special Movements – do not fit into the previous categories

– Dorsiflexion – lifting foot towards the shin– Plantar Flexion- pointing the toes– Inversion – turning the sole of the foot medially– Eversion – turning the sole of the foot laterally– Supination – rotate the forearm so that the palm faces

anteriorly– Pronation – rotate the forearm so that the palm faces

posteriorly– Opposition – moving the thumb to touch the tips of the

other fingers

Muscle Movements, Types, and Names

Types of Muscles – Muscle movement is the result of the activity of two or more muscles acting together or against each other. – Prime Mover - the muscle that has the major

responsibility for causing a particular movement

Muscle Movements, Types, and Names

Types of Muscles Continued…– Antagonists – muscles that oppose or reverse a

movement

**When a prime mover is active, its antagonist is stretched and relaxed**

– Synergists – help prime movers by producing the same movement or by reducing undesirable movements

– Fixators – are specialized synergists that hold a bone still so all the tension can be used to move another bone

Naming Skeletal Muscles

Use the following 7 criteria: – Direction of the Muscle Fibers – Are the fibers parallel or

slanted?– Relative Size of the Muscle – Are the muscles large or

small?– Location of the Muscle – Number of Origins – 2 (bi-); 3 (tri-); 4 (quad)– Location of the Muscle’s Origin and Insertion – Shape of the Muscle – Action of the Muscle

Developmental Aspects of the Muscular System

To remain healthy, muscles MUST be regularly exercised. Without exercise, they atrophy; with extremely vigorous exercise, they hypertrophy.

As we age, muscle mass decreases and the muscles become stringy. Exercise helps to retain muscle mass and strength.

Body weight tends to decline in the elderly person as this natural loss in muscle mass occurs. Strength also decreases by about 50% by the age of 80.

Muscular Disorders

Muscular Dystrophy– Muscle destroying disease that affects specific

muscle groups.– The muscles enlarge due to fat and connective

tissue deposit, but the muscle fibers degenerate and atrophy.

Muscular Disorders

Myasthenia Gravis– Affects muscles during adulthood– Characterized by drooping of the upper eyelids,

difficulty in swallowing and talking, and generalized muscle weakness and fatigue.

– Due to a shortage of acetylcholine receptors