the muscular system chapter 8 p. wilson human anatomy & physiology1

The Muscular System

Chapter 8

P. Wilson Human Anatomy & Physiology 1

III. Introduction

The 3 types of muscle are:1. Skeletal (aka: striated, voluntary)2. Smooth (aka: visceral, involuntary)3. cardiac


III. Structure of a Skeletal MuscleA. Kinds of Tissue

• Connective • Muscle • Nervous• Blood


III. Structure of a Skeletal MuscleB. 1 through 31. A skeletal muscle is held in position by layers

of fibrous connective tissue called fascia.2. This tissue extends beyond the end of a

skeletal muscle to form a cordlike tendon.3. When this tissue extends beyond the muscle

to form a sheet-like structure, it is called an aponeurosis.


III. Structure of a Skeletal Muscle4. Fascicles are bundles

of muscle fibers (cells)Thick (myosin) & thin (actin)

fibers

Myofibrils

Muscle fiber (cell)

Fascicle

Muscle P. Wilson Human Anatomy & Physiology 5

III. Structure of a Skeletal MuscleC. 1. Describe a single muscle fiberA single muscle fiber (muscle cell) is a thin, elongated cylinder

• The cell membrane of a muscle fiber is called the sarcolemma

• The cytoplasm of a muscle fiber is celled the sarcoplasm and it contains many nuclei, mitochondria, 7 other cellular structures; also in the sarcoplasm are many myofibrils that lie parallel to each other.

• Myofibrils are made up of bands of thick myosin fibers (“A” bands) and thin actin fibers (“I” bands) form myofibrils.


III. Structure of a Skeletal MuscleC. 2. Describe the structure & function of a sarcomere.

A sarcomere is the segment of a myofibril from one “Z” line to the next “Z” line. The striations of skeletal muscle are due to the structure of myofibril:•The light-colored “I” bands are thin actin fibers which are attached to Z lines•The dark-colored “A” bands contain the thick myosin fibers; •in the part of the “A” band closest to Z lines the myosin fibers overlap the actin fibers; •the “H” band portion of the “A” band there are no actin fibers but there is a central thickening known as the “M” line



• The structure of a sarcomere allows the actin & myosin filaments to slide with respect to each other when stimulated by the presence of ATP & Ca++ ions.

• This is called the sliding filament mechanism

• The contraction of a muscle results from the simultaneous contraction of all of its sarcomeres


III. Structure of a Skeletal MuscleC. 3. & C.4.

3. The network of membranous channels in the cytoplasm/sarcoplasm of muscle fibers is the sarcoplasmic reticulum. A second set of channels is the transverse tubules which extend inward from the fiber’s membrane. Each tubule opens to the outside of the fiber.

4. The sarcoplasmic reticulum and the transverse tubules activate the muscle contraction mechanism when the fiber is stimulated.


III. Structure of a Skeletal MuscleD. Label see also figure 8.5 on page 172 of text


IV. Skeletal Muscle ContractionA. Describe the roles of actin & myosin in muscle contraction.

• Myosin filaments have cross bridges along their length

• Actin fibers have binding sites for myosin cross bridges

• The 2 proteins troponin & tropomyosin act together to expose the actin binding sites in response to an influx of Ca2+

• The myosin cross bridges bind to actin filaments, pulling on the actin filaments, causing the sarcomere to shorten (the Z lines are pulled closer together)


IV. Skeletal Muscle ContractionB. Describe the transmission of a nerve impulse across…


• When a nerve impulse reaches the end of a motor neuron axon, vesicles in the neuron’s cytoplasm release acetylcholine (neurotransmitter) into the synaptic cleft between the neuron & the motor end plate of the muscle

IV. Skeletal Muscle ContractionC.1. Describe the interaction between acetylcholine & Ca2+…


• Acetylcholine stimulates a muscle impulse which passes in all directions over the surface of the muscle fiber, traveling to the sarcoplasmic reticulum

• The sarcoplasmic reticulum releases Ca2+ ion allowing cross bridges of myosin to bind to actin

IV. Skeletal Muscle ContractionC. 2. & C.3.


C.2. A cross bridge is formed when the projecting parts of the myosin filament can occupy a binding site on the actin filament and pull itself along the actin filament.

C. 3. The action of acetylcholine is halted by the enzyme acetylcholinesterase.

IV. Skeletal Muscle ContractionD. 1. & D.2.

D.1. ATP (adenosine triphosphate) stores energy in a high-energy chemical bond. When energy is needed for a muscle contraction, an ATP molecule releases energy and is degraded to an ADP (adenosine diphosphate) molecule.ATP Energy + ADP + P

D.2. Only small amounts of ATP are stored in a muscle so ATP must be regenerated in order for muscle contraction to continue. Creatine phosphate stores energy generated by the mitochondria then releases that energy to maintain a steady supply of ATP


Creatine phosphate

IV. Skeletal Muscle ContractionE. Oxygen supply & cellular respiration

1. Myoglobin is molecule synthesized in muscle cells that is capable of storing oxygen temporarily.

2. Cellular respiration is an aerobic (requires oxygen) process that releases energy from glucose & forms ATP. In other words, oxygen is necessary for formation of ATP - muscle contraction requires ATP -so… oxygen is necessary for muscle contraction.

3. In the absence of oxygen, a form of anaerobic respiration called lactic acid fermentation supplies the energy required for muscle contraction.


IV. Skeletal Muscle ContractionE. Oxygen supply & cellular respiration

4. Oxygen debt equals the amount of oxygen liver cells require to convert accumulated lactic acid into glucose, plus the amount of oxygen muscle cells need to restore ATP & creatine phosphate to their original concentrations.

??? How was lactic acid accumulated? During prolonged, strenuous exercise the muscles so not have sufficient oxygen to do aerobic respiration & must use anaerobic respiration to obtain energy. The anaerobic respiration causes an accumulation of lactic acid in muscles which diffuses into bloodstream & eventually reaches the liver.


IV. Skeletal Muscle ContractionF. Muscle Fatigue

1. When a muscle is exercised strenuously for a prolonged period it may lose its ability to contract. Causes?

• Most likely an accumulation of lactic acid as a result of anaerobic respiration; could also be caused by

• An interruption in the muscle’s blood supply, or• A lack of acetylcholine in the motor neuron axons

2. Less than half the energy released by cellular respiration is available for metabolic processes. The rest is lost as heat.


V. Muscular ResponsesA.

1. Threshold stimulus is the minimum strength needed for a stimulus to produce a muscle contraction.

• When a muscle fiber is exposed to a single stimulus of threshold strength it will contract once, then relax = “twitch”

2. All-or-nothing response means that when a muscle contracts, it contracts to its fullest extent; there is no such thing as a partial muscle contraction.


V. Muscular Responses A.

3. Summation: A muscle fiber receiving a series of stimuli of increasing frequency reaches a point when it is unable to relax completely and the force of individual twitches combine by the process of summation.If the sustained contraction lacks any

relaxation, it is called a tetanic contraction.


V. Muscular Responses A.

4. Recruitment : as the stimulus intensity increases, the number of motor units responding to the stimulus increases (remember that a muscle fiber responds in an all-or-nothing manner).


V. Muscular ResponsesC.

• Fast-twitch fibersproduce strong

contractions but easilyare activated during

high-intensity exercise that requires strength (ex: weight lifting)

• Typical people are about 50-50;

• Olympic sprinters >80% fast twitch;

• Olympic marathoners >90% slow twitch

• Slow-twitch fibersare fatigue

resistantare activated

during low-intensity exercise when endurance is more important than strength (ex: swimming, running)


V. Muscular Responses D. Contractions

Sustained Contraction• The result of both recruitment

and summation producing sustained contractions of increasing strength. The smaller motor units contract first, followed by the larger motor units (which can contract more forcefully).

• Sustained contractions of whole muscles produce the movement necessary to perform everyday functions.

Muscle Tone• Even when a muscle appears

to be at rest, its fibers undergo some sustained contraction as a response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers.

• Muscle tone is important in maintaining posture. If muscle tone is suddenly lost the body collapses (ex: when a person loses consciousness)


VI. Smooth MusclesA. Compare Smooth & Skeletal Muscle Fibers

The mechanisms of contractions are the same for both smooth & skeletal muscles.Both have actin & myosin fibers. But…

• Smooth muscle fibers have a more random arrangement of the myosin & actin fibers so that smooth muscle lacks the striated appearance of skeletal muscle.

Also, the sarcoplasmic reticulum of smooth muscle is less well-develop that that in skeletal muscle


VI. Smooth MusclesB. Types of Smooth Muscles

Multi-unit Smooth Muscle

• muscle fibers are separate – not organized in sheets

Visceral Smooth Muscle

• muscle fibers are composed of sheets of spindle-shaped cells in contact with each other




• found in irises of eye and

• in the walls of blood vessels


• found in the walls of hollow organs (stomach, intestines, bladder, uterus, intestines)




• typically contracts only in response to stimulation by motor nerve impulses or certain hormones


• Visceral muscle fibers can stimulate other visceral muscle fiber (self-exciting)

• also display rhythmicity – a pattern of repeated contractions

• These 2 properties allow peristalsis to occur


Visceral Smooth Muscles & Peristalsis

• Self-excitation and rhythmicity are largely responsible for a wave-like motion called peristalsis that occurs in certain tubular organs like the intestines.

• Peristalsis helps force the contents of these organs along their lengths.


VI. Smooth MusclesC.1 & C.2. Smooth Muscle Contraction

Skeletal muscles• One

neurotransmitter – acetylcholine

Smooth Muscles• Two

neurotransmitters – acetylcholine and norepinephrine

• Each neurotransmitter can stimulate contractions in some muscles and inhibit contractions in others


VI. Smooth MusclesC.3. Compare Smooth & Skeletal Muscle Contractions

• Smooth muscle is slower to contract & relax than skeletal muscle

• Smooth muscle can maintain a forceful contraction longer than skeletal muscle

• Smooth muscle can change length without changing tautness – this means smooth muscles in stomach & intestinal walls can stretch as these organs fill, yet maintain the pressure inside these organs


VII. Cardiac Muscle A. Sarcoplasmic Reticulum

The sarcoplasmic reticulum of cardiac muscle is distinctive in 2 ways: •larger transverse tubules and •less well-developed cisternae that store less calcium. •The larger transverse tubules contain calcium obtained from outside the muscle fibers and release larger numbers of calcium ions. This results in a longer cardiac muscle twitch than found on skeletal muscle.


VII. Cardiac MuscleB. & C.

B. Opposing ends of cardiac muscle fibers are connected by intercalated discs which allow muscle impulses to pass freely so that they travel rapidly from cell-to-cell.

C. When one portion of the cardiac muscle is stimulated, the impulse spreads to other fibers of the network and the whole network contracts as a unit (self-excitation & rhythmicity)

• Cardiac muscle is self-exciting & rhythmic so that it does not require stimulation outside the heart to produce regular periods of contraction & relaxation.


VIII. Skeletal Muscle ActionsA. Muscle Attachments

• Each muscle has at least two points of attachment to bone: the origin and the insertion.

• The origin is the end of a skeletal muscle that is immovable (or relatively immovable)

• The insertion is the end of a skeletal muscle that movable end.

• When a muscle contracts, its insertion is pulled toward its origin.


VIII. Skeletal Muscle ActionsB. Muscle Movement

• The prime mover (aka agonist) is the muscle that provides most of the movement for a particular body movement

• A synergist is a muscle that contracts and assists the prime mover, making its action more effective.

• An antagonist is a muscle that can resist a prime mover’s action and/or cause movement in the opposite direction.



• If an agonist and its antagonist contract simultaneously, the part they act upon remains rigid.

• Smooth body movements depend on antagonists relaxing and giving way to prime movers whenever the prime mover contracts.



Flexion & Extension are terms that describe changes in the angle where two bones meet:

Flexion decreases the angle where 2 bones meet

Extension increases the angle where 2 bones meet


Muscle Functions1. producing movement- result of contraction2. maintaining posture- via skeletal muscles

– overcoming gravity effects while sitting or standing

3. stabilizing joints- pull of skeletal muscles on bones

4. generating heat- by-product of muscle activity– 75% of ATP energy creates heat (only 25% used to

contract muscle)P. Wilson Human Anatomy & Physiology 38

Interesting Tidbits

• Rigor mortis• Botulinus toxin• Motor unit• Muscle strain• Tendinitis• Muscle pull

• Does it take less energy (ATP) to smile or to frown? Explain!

• What is TMJ? What causes it?


Do you use more muscles to smile than to frown?

"It takes one more muscle to smile than to frown, according to plastic surgeon David H. Song, MD, FACS, assistant professor at the University of Chicago Hospitals. Only Cecil's "The Straight Dope" got an expert (Dr. Song) to go through the motions. A genuine smile takes two muscles to crinkle the eyes, two to pull up the lip corners and nose, two to elevate the mouth angle, and two to pull the mouth corners sideways. Total smile: 12.

On the other hand, a frown needs two muscles to pull down the lips and wrinkles in the lower face, three to furrow the brow, one to purse the lips, one to depress the lower lip, and two to pull the mouth corners down. Total frown: 11.

A fake smile, however, only takes two muscles. We detect the fake because "the eyes aren't smiling."


Muscle Trivia

• Smallest muscle in the human body (name & location).

• Largest muscle in the human body (name & location).

• Longest muscle in the human body (name & location).


Muscle Names• The name of a muscle often describes it:

may indicated size &/or location: pectoralis major is located in the pectoral (chest) region and is of large size;

deltiod is shaped like the Greek letter delta (a triangle);

biceps brachii has two heads (points of origin) and is located in the arm;

external oblique is located near the outside with fibers that run obliquely (at an angle)


the muscular system chapter 8 p. wilson human anatomy & physiology1

Documents

striations of skeletal

skeletal musclec

structure function

structure of myofibril

muscle results

types of muscle

skeletal aka

skeletal musclefascicles