Chapter 6

The Muscular System

• The essential function of a muscle is to shorten or contract

• As a result of this ability, muscles are responsible for almost all body movement and can be viewed as the “machines of the body”

Overview of Muscle Tissues

Muscle Types - Three

1. Skeletal2. Cardiac3. Smooth

The 3 Types of Muscle Tissue differ in:

• Cell structure• Body location• How they are stimulated to contract

The three types of muscle tissue are similar because:

1. All muscle cells are elongated – called muscle fibers

2. The ability of muscle to shorten or contract depends on 2 types of myofilaments (part of the cytoskeleton)

Skeletal Muscle

1. Fibers are packaged into skeletal muscles that attach to the skeleton

2. Cigar-shaped3. Multinucleate (many nuclei)4. Largest of the muscle fibers

5. Striated muscle (appear striped)6. Voluntary muscle (conscious

control)7. Form smoother contours of the

body

Key words to think of for skeletal muscle:

• Skeletal, striated, voluntary

• Skeletal muscles are very fragile, but they are capable of exerting tremendous power.

• They are able to do this because:thousands of fibers are bundled together with connective tissue – these bundles are then bundled together

Tendons

• Attach muscle to bone

Figure 09.02

Functions of the tendons

1. Anchor muscles2. Provide durability and conserve

space3. Crossover bony projections

Smooth Muscle

• No striations• Involuntary• Found in the walls of hollow

digestive organs• Propels substances along a definite

path

• Spindle shaped• Single nucleus• Arranged in sheets or layers• Muscle contraction is slow and

sustained

Key words:

• Visceral, non-striated, involuntary

Cardiac Muscle

1. Found only the heart2. Striated and involuntary

Key Words:

• Cardiac• Striated• involuntary

Muscle Functions

• Muscles play four important roles in the body.

1. Produce movement

• Moves the body

2. Maintain posture

• Allow you to remain in an erect or seated posture despite gravity

3. Stabilize Joints

• Muscle tendons are extremely important in reinforcing and stabilizing joints

4. Generating Heat

• Heat is a by product of muscle activity

• ATP used as power – ¾ escapes as heat

Microscopic Anatomy of Skeletal Muscles

• Skeletal muscle contains both actin and mysosin filaments

• The overlapping pattern of thick and thin filaments is responsible for the light and dark bands seen in skeletal striated muscle

• The thick filaments are made up of a protein called myosin

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

• Sarcomere: contractile unit

Skeletal Muscle Activity

The 2 special functional properties of muscles:

1. Irritability – ability to receive and respond to a stimuli

2. Contractility – ability to shorten with adequate stimuli

• Skeletal muscles must be stimulated by nerve impulses to contract.

Motor Unit

• One neuron (nerve cell) and all the skeletal muscles it stimulates

Figure 09.09

How a muscle contracts

• A nerve impulse reaches the end of the nerve a neurotransmitter is released.

• The neurotransmitter that stimulates skeletal muscle is acetylcholine (Ach)

• When enough acetylcholine is released, sodium ions (Na+) will rush into the muscle.

• This rush of ions creates an electrical current known as the action potential.

• The action potential travels over the entire muscle causing it to contract.

The events that return a muscle to its resting state:

1. Diffusion of K+ (potassium) out of the cell

2. Activation of the Na+/K+ pump

The Sliding Filament Theory

• Muscle fibers contract when the sarcomere shortens.

• The sarcomere shortens when the actin fibers slide past the myosin filaments

• Myosin moves the actin.

• The sliding filament theory refers to the movement of actin in relation to myosin.

• ATP supplies the energy for muscle contraction.

• Myosin filaments do all the work. The actin filaments just sit there.

Figure 09.04

• Myosin filaments breakdown ATP and have crossbridges that pull the actin filaments toward the center of the sarcomere.

Contraction of Skeletal Muscle as a

whole

Figure 09.06

Graded Responses

• Different degrees of shortening• Different numbers of muscles contract

Graded muscle contractions can be produced in two ways:

1. By changing the speed of muscle stimulation

2. By changing the number of muscle cells being stimulated

Providing Energy for Muscle Contraction

1. Direct phosphorylation of ADP by creatine phosphate (CP)

• CP gives a phosphate to ADP to make ATP

• ATP is regenerated in a fraction of a second

• CP supplied energy used in 20 seconds

2. Aerobic Respiration – ATP is made by aerobic respiration

• 1 glucose – 36 ATP• Fairly slow – needs continuous

supply of oxygenAdenosine Triphosphate

                                                  

3. Anaerobic Respiration and Lactic Acid Formation

• No oxygen• 2 ATP per glucose• Lactic acid is made and builds up in

muscles• 5X faster than aerobic• 30-40 seconds of strenuous

exercise

• Problems: needs lots of glucose– Small amount of ATP produced

per glucose– Lactic acid

3. Anaerobic Respiration and Lactic Acid Formation

• No oxygen• 2 ATP per glucose• Lactic acid is made and builds up in

muscles• 5X faster than aerobic• 30-40 seconds of strenuous

exercise

• Problems: needs lots of glucose– Small amount of ATP produced

per glucose– Lactic acid

Muscle Fatigue and Oxygen Debt

Muscle Fatigue

• Occurs when muscles are exercised strenuously

Fatigued

• When a muscle is unable to contract even though it is being stimulated

Oxygen Debt

• The volume of oxygen needed after exercise to get rid of the lactic acid formed during exercise

• The major factor that effects the work that a muscle can do and how long it can do work without becoming fatigued is:

• How good the blood supply is

When muscles lack oxygen:

1. Lactic acid builds up2. Muscles also run out of ATP

So lack of ATP and lactic acid build up cause a muscle to stop contracting

The Two Types of Muscle Contractions

Isotonic contractions

• When the force of the muscle contraction is greater than the force that is resisting the contraction

• Ex. Weight is lifted

Isometric Contractions

• When the resistance to the contraction is equal to the force generated in the muscle tissue (muscle do not contract or only a little)

• Pulling on a stationary bar

Effect of Exercise on Muscles

• Muscle inactivity always leads to: muscle weakening and wasting away

• Regular exercise increases muscle: size, strength and endurance

Atrophy

• Diminish in size and become weaker • result of no exercise

Hypertrophy

• Increase in size and strength • Result of exercise

• Anabolic steroids are sometimes taken by athletes to promote muscle growth

Undesirable effects of anabolic steroid use:

• Cardiovascular disease• Liver and kidney dysfunction• Impotency• Sterility• Increase in rash behavior “roid

rage”

Slow Twitch Muscle Fibers - Legs

• Steadier tug• More endurance• Aerobic energy production – tire

when fuel is gone• Sports useful in: long distance

running, biking, jogging, swimming• Color: dark• Many mitochondria, dense capillary

beds which draw more blood and oxygen

Fast Twitch Muscle Fibers - Breasts

• Anaerobic – tire quick because of lactic acid build up

• Designed for strength• Explosions of energy• Sports useful in: sprinting, weight

lifting• Color: white• Few mitochondria, few blood cells

Aerobic or Endurance Excercise

• Stronger more flexible muscles with greater resistance to fatigue

• Blood supply increases as cells form more mitochondria and store more oxygen

Body Benefits

1. Overall body metabolism is more efficient

2. Improves digestion3. Skeleton stronger4. Heart pumps more blood with each

beat5. Fat deposits cleared6. Lungs are more efficient at gas

exchange

Resistance or Isometric Exercise

• A few minutes every other day• Muscles increase in size because

muscle cells increase in size (not increase in number of cells)

• Amount of reinforcing connective tissue also increases

Types of Muscles

• Muscles cannot push. They can only pull.

• So most body movements are a result of the activity of pairs or teams of muscles acting together or against each other

• Muscle groups are arranged on the skeleton so that whatever one muscle can do, another group of muscles can do in reverse

Naming Skeletal Muscles

Skeletal muscles are given names based on:

1. Size – gluteus maximus2. Shape – deltoid3. Location4. Direction of fibers – rectus – up and

down – transverse – horizontal5. Number of attachments – biceps – 2

attachments6. Action

Figure 09.22

Figure 09.23

Figure 09.24

Figure 09.25a

Figure 09.27b

Figure 09.27c

Figure 09.27d

Figure 09.28

Figure 09.29d

Figure 09.31b

Figure 09.34

Figure 09.35a

Figure 09.37a

Figure 09.37b

Figure 09.37c

Figure 09.38c

Figure 09.39a

Figure 09.39a

Figure 09.39b

Figure 09.40

The End