1. Chapter 9 – The Muscular System

2. Introduction There are 3 types of muscle tissue
in the muscular system:
Skeletal muscle:
Attached to bones of skeleton
Voluntary (consciously controlled)
Cardiac muscle:
Makes up most of the wall of the
heart
Involuntary (non-consciously-
controlled)
Responsible for pumping action
of the heart
Smooth muscle:
Found in walls of internal organs,
such as those of digestive tract

3. 9.1: Structure of a Skeletal Muscle
Skeletal Muscles:
Attach to bones, and skin of face
Under conscious control (voluntary)
A skeletal muscle is an organ of the muscular system
Skeletal muscles are composed of:
Skeletal muscle tissue
Nervous tissue
Blood
Connective tissues

4. 9.2: Skeletal Muscle Contraction
Contraction of a muscle fiber:
Requires interaction from several chemical and cellular components
Results from a movement within the myofibrils, in which the actin and myosin filaments slide past one another, shortening the sarcomeres
Muscle fiber shortens and pulls on attachment points

5. Neuromuscular Junction
A type of synapse
Also called a myoneural junction
Site where an axon of motor neuron
and skeletal muscle fiber interact
Skeletal muscle fibers contract
only when stimulated by a motor
neuron
Parts of a NMJ:
Motor neuron
Motor end plate
Synaptic cleft
Synaptic vesicles
Neurotransmitters

6. Oxygen Supply & Cellular Respiration
Anaerobic Phase :
In the absence of oxygen
Glycolysis
Occurs in cytoplasm
Produces little ATP
Aerobic Phase:
In the presence of oxygen
Occurs in the mitochondria
Produces the most ATP

7. Oxygen Debt Anaerobic (Lactic Acid) Threshold: Oxygen debt:
During rest or moderate exercise, respiratory & cardiovascular systems supply enough O2 to support aerobic respiration
Anaerobic (Lactic Acid) Threshold:
Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O2. Lactic acid is produced.
Oxygen debt:
Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.

8. Muscle Fatigue Muscle Fatigue: Muscle Cramp:
Inability to contract muscle
Common causes of muscle fatigue:
Decreased blood flow
Ion imbalances across the sarcolemma
Loss of desire to continue exercise
Accumulation of lactic acid
Muscle Cramp:
Sustained, involuntary muscle contraction
May be caused by changes in electrolyte concentration in extracellular fluids in the area

9. Heat Production
Heat is a by-product of cellular respiration in active cells
Muscle cells are major source of body heat
More than half the energy released in cellular respiration becomes heat; less than half is transferred to ATP
Blood transports heat throughout body core

10. 9.3: Muscular Responses
Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of the muscle fiber.
Electrical stimulator promotes the contractions

11. Threshold Stimulus Threshold Stimulus:
Minimum strength of stimulation of a muscle fiber required to cause contraction
When strength of stimulus reaches threshold, an action potential is generated
Impulse spreads through muscle fiber, releasing Ca+2 which begins to contraction process
One action potential from a motor neuron releases enough Ach (neurotransmitter) to produce threshold stimulus in muscle fiber, causing a muscle impulse

12. Recruitment of Motor Units
Increase in the number of motor units activated, to produce more force
Certain motor units are activated first, and others are activated only when the intensity of stimulus increases
As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

13. Sustained Contractions
Smaller motor units (smaller diameter axons) - recruited first
Larger motor units (larger diameter axons) - recruited later
Summation and recruitment can produce sustained contractions of increasing strength
Whole muscle contractions are smooth movements
Muscle tone (tonus): Continuous state of partial contraction in resting muscles

14. Types of Contractions
Isotonic: muscle contracts and changes length; produces movement
Walking
Moving a body part
Isometric: muscle contracts but does not change length; produces no movement
Sitting up
Standing
Maintaining posture

15. 9.4: Smooth Muscle
Compared to skeletal muscle fibers, smooth muscle fibers are:
Shorter
Single, centrally located nucleus
Elongated with tapering ends
Myofilaments randomly organized
Lack striations

16. Types of Smooth Muscle 2 types of smooth muscle:
Multi-unit Smooth Muscle:
Cells are less organized
Function as separate units
Fibers function independently
Iris of eye, walls of blood vessels
Stimulated by neurons, hormones
Visceral Smooth Muscle:
Single-unit smooth muscle; cells respond as a unit
Sheets of spindle-shaped muscle fibers
Fibers held together by gap junctions
Exhibit rhythmicity
Conduct peristalsis
Walls of most hollow organs
More common type of smooth muscle

17. Smooth Muscle Contraction
Resembles skeletal muscle contraction in these ways:
Interaction between actin and myosin (components of muscle fibers)
Both use calcium and ATP
Both are triggered by membrane impulses (motor neurons)
Different from skeletal muscle contraction in these ways:
Hormones can stimulate or inhibit smooth muscle
Stretching can trigger smooth muscle contraction
Smooth muscle slower to contract and relax
Smooth muscle more resistant to fatigue

18. 9.5: Cardiac Muscle Cardiac Muscle: Located only in the heart
Striated muscle cells
Muscle fibers joined together by intercalated discs
Fibers branch, contain a single nucleus
Network of fibers contracts as a unit (conduction system)
Rhythmic contractions
Longer refractory period (time where muscle is unresponsive) than skeletal muscle
No sustained contractions

19. Characteristics of Muscle Tissues

20. 9.6: Skeletal Muscle Actions
Skeletal muscles generate a great variety of body movements
The action of each muscle mostly depends upon
- the type of joint it is associated with
- the way the muscle is attached on either side of the joint

21. Body Movement
When bones or body parts move, bones and muscles act as levers.
4 basic components of levers:
Rigid bar or rod (bones)
Fulcrum or pivot on which bar moves (joint)
Object moved against resistance (weight)
Force that supplies energy for movement (muscles)

22. Origin and Insertion
One end of a skeletal muscle is more fixed, and the other end is more movable: Origin: less movable end Insertion: more movable end When a muscle contracts, insertion is pulled toward origin

23. Interaction of Skeletal Muscles
Most skeletal muscle function in groups. Roles of muscles in performing certain actions:
Agonist: muscle that causes an action
Prime mover: agonist primarily responsible for movement
(In some cases, the terms “agonist” and “prime mover” are used
interchangeably)
Synergists: muscles that assist agonist / prime mover
Antagonist: muscles whose contraction causes movement in the opposite direction of the prime mover

24. 9.7: Major Skeletal Muscles
Anterior View of the Superficial Skeletal Muscles:

25. Major Skeletal Muscles
Posterior View of the
Superficial Skeletal
Muscles:

26. 9.8: Lifespan Changes
Myoglobin, ATP, and creatine phosphate decline, starting in the 40s
Connective tissue and adipose cells replace some muscle tissue
By age 80, almost half of muscle mass has atrophied (wasted away)
Muscle strength decreases, and reflexes become slower
Exercise helps to maintain muscle mass and function