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Chapt. 9 Muscles and Muscle Tissue

Anatomy & Physiology I Dr. L. Bacha

Chapter Outline (Marieb & Hoehn 2013) I

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TYPES OF MUSCLE TISSUE (see Fig. 4.9 on pages 138 to 139 and Table 9.3 on p. 310)

list the three types of muscle tissue:

list the three prefixes that are used to refer to muscle:

skeletal muscle tissue ◦ it is called “skeletal” because it forms muscles that attach to the skeleton

◦ skeletal muscle cells have striations; what are striations?

◦ why is it called voluntary muscle?

u go back to p. 138 in Chapt. 4 and observe skeletal muscle tissue in Fig. 4.9 (a) - the skeletal muscle cells are long, cylindrical cells - each cell is multinucleated, with the nuclei just deep to the cell membrane

cardiac muscle tissue ◦ it forms the middle, muscular layer of the heart wall called the myocardium

◦ are the cells striated?

◦ is it voluntary or involuntary

u go back to p. 138 in Chapt. 4; read about cardiac muscle and observe Fig. 4.9 (b)

- are cardiac muscle cells branched?

- does each cell have one nucleus or many?

- the ends of the cells fit together tightly at unique junctions called what?

some terms pertaining to muscle cells: sarcoplasm = the cytoplasm of a muscle cell sarcolemma = the cell membrane of a muscle cell

because muscle cells are elongated, they are sometimes referred to as muscle fibers


the intercalated discs are important because they: 1. strengthen the tissue and hold the cells together during contractions 2. allow conduction of muscle action potentials (impulses) to spread quickly throughout the

heart

smooth muscle tissue

◦ where is smooth muscle tissue found?

◦ is it striated?

◦ is it voluntary or involuntary?

◦ contraction of smooth muscle tissue is regulated by hormones and the autonomic division of the nervous system

u now back to p. 138 in Chapt. 4 to read about smooth muscle tissue and observe smooth muscle tissue in Fig. 4.9(c)

- describe the shape of a smooth muscle cell and the nucleus:

SPECIAL CHARACTERISTICS OF MUSCLE TISSUE

list and briefly describe the 4 special characteristics of muscle tissue:

MUSCLE FUNCTIONS

list (and read about!) the four important functions of muscle:


SSKKEELLEETTAALL MMUUSSCCLLEE

GROSS ANATOMY OF A SKELETAL MUSCLE

- a skeletal muscle (e.g., biceps brachii; triceps brachii, etc.) is an organ because it is formed by two or more types of tissue (e.g., skeletal muscle tissue, areolar CT, dense CT, nervous tissue, etc.)

Nerve and Blood Supply in general, each muscle is served by what?

so, skeletal muscles are said to be well-innervated and well-vascularized Connective Tissue Sheaths - associated with skeletal muscles:

superficial fascia (subcutis; hypodermis) is loose CT that separates muscles from skin; it provides a pathway for vessels and nerves to and from muscles

deep fascia is dense irregular CT that fills spaces between muscles and holds muscles with similar functions together; it allows free movement of muscles and carries vessels and nerves

There are three layers of connective tissue that extend from the deep fascia to protect and strengthen skeletal muscle (locate these connective tissues in Fig. 9.1):

1. epimysium - dense CT that encircles the entire skeletal muscle

2. perimysium - dense CT that surrounds groups of 10 to 100 muscle cells, grouping them into bundles called fascicles

3. endomysium - areolar CT within each fascicle that surrounds individual muscle cells

Attachments • review the definitions of origin and insertion

• describe a direct (fleshy) muscle attachment:

• describe indirect muscle attachments:

tendon - cordlike; formed of dense regular CT aponeurosis – a broad flat sheet of dense regular CT


MICROSCOPE ANATOMY OF A SKELETAL MUSCLE FIBER

Examine Fig. 9.2 as you study this information!

sarcolemma = the cell membrane of a muscle cell

skeletal muscle cells are huge; what is their range in diameter?

- some are up to how long?

define sarcoplasm:

the sarcoplasm contains glycosomes; what are glycosomes?

it also contains myoglobin; what is myoglobin?

Myofibrils

myofibrils = tiny rod like structures that are packed in the cytoplasm and run the length of the cell

how many are in a single muscle cell? Striations, Sarcomeres, and Myofilaments

describe striations:

myofibrils contain thick filaments and thin filaments (myofilaments)

thick and thin filaments are arranged in units called sarcomeres

sarcomere = the unit from one Z disc to the next Z disc

Z disks = proteins to which thin filaments attach A band = a region that extends the entire length of the thick filaments and it contains thick

filaments and overlapping thin filaments I band = area on each side of an A band that contains the rest of the thin filaments but no thick

filaments H zone = an area in the center of the A band that contains only thick filaments

Molecular Composition of Myofilaments - the two main contractile proteins in muscle that form the myofilaments are actin and myosin

THICK FILAMENTS formed by the protein MYOSIN each myosin molecule is shaped like a golf club with two heads called globular heads

in Fig. 9.3 observe one myosin molecule, then note how many myosin molecules are arranged to form a single thick filament, with the heads facing outward at each end


the myosin heads have actin-binding sites and binding sites for ATP; They also contain ATPase enzymes that split what?

THIN FILAMENTS formed mainly by the protein ACTIN

actin has polypeptide subunits, called globular actin (G actin), that are what shape?

the G actin molecules are polymerized into what?

thus, the backbone of each thin filament is formed by two intertwined F actin filaments that look like what?

each G actin of the actin filament has a myosin-binding site to which the myosin heads attach

during contraction in relaxed muscle cells, the myosin-binding sites are covered by a protein called tropomyosin,

which is held in place by another protein called troponin troponin has binding sites for Ca+2

Sarcoplasmic Reticulum and T Tubules

List the two sets of intracellular tubules that help to regulate muscle contraction in skeletal muscle cells:

Sarcoplasmic Reticulum

• formed of membrane-enclosed interconnecting tubules that surround what?

terminal cisterns = large, perpendicular channels of sarcoplasmic reticula on each side of a T tubule

what does the sarcoplasmic reticulum store (in a relaxed muscle cell)?

T Tubules describe what forms a T tubule (transverse tubule): see Fig. 9.5 and note that the T tubules and sarcoplasmic reticula alternate with each other and wrap

around the myofibrils the lumen of each T tubule is continuous with what?


along its length, each T tubule runs between the paired terminal cisterns of the sarcoplasmic reticulum, forming what?

- triads are successive groupings of what three membranous structures?

because T tubules are continuations of the sarcolemma, they conduct what? - what do the impulses signal? SLIDING FILAMENT MODEL OF CONTRACTION

to physiologists, what does the term contraction refer to? when does shortening occur?

the sliding filament model of contraction: During muscle contraction, shortening is achieved when

myosin heads of the thick filaments bind to the actin and pivot, causing the thin filaments to slide past the thick filaments toward the center of a sarcomere. The thin filaments may overlap. The Z discs are pulled toward the middle of the sarcomere and the sarcomere shortens. (Read the 4 statements marked with red squares on p.285 that summarizes the sliding filament mechanism.)

- overall, as a muscle cell shortens; do the I bands shorten?

- does the distance between successive Z discs shorten?

- do the H zones disappear?

- what happens to the A bands within a sarcomere?

- do you think that the lengths of the individual thick and thin filaments change?


PHYSIOLOGY OF SKELETAL MUSCLE FIBERS (MUSCLE CELLS)

describe the four steps that must occur for a skeletal muscle cell to contract:

- where do steps 1 and 2 occur?

- together steps 2 and 3 link what and are called what?

The Nerve Stimulus and Events at the Neuromuscular Junction

• name the nerve cells that activate skeletal muscle cells:

Skeletal muscle cells must be stimulated by a somatic motor neuron to contract. So, how does a muscle cell receive a signal from a neuron to initiate a muscle action potential?! First, you should understand the neuromuscular junction! • read the information about the neuromuscular junction on page 286.

• The neuromuscular junction is the synapse between the axon terminal of a motor neuron and the sarcolemma of a skeletal muscle cell

Electrical currents called action potentials (impulses) are conducted along axons of motor neurons to the neuromuscular junction.

Review the components of a neuromuscular junction in Fig. 9.8:

The neuromuscular junction is formed by three main parts:

1. axon terminal = the end of the axon of the motor neuron

- describe synaptic vesicles and indicate what they contain:


2. synaptic cleft = the space filled with gel-like extracellular substance between an axon terminal and the sarcolemma of the skeletal muscle cell

3. junctional folds of the sarcolemma = the folded part of the sarcolemma that is associated with the axon terminal

∙ name the receptors associated within junctional folds of the sarcolemma:

You should read through pages 286 to 293, but here is my summary of the important information: Events at the neuromuscular junction that lead to the transmission of an impulse from the motor neuron to the skeletal muscle cell:

1. Release of acetylcholine (ACh): A nerve impulse along the axon of the motor neuron reaches the axon terminal. This triggers the release of the neurotransmitter acetylcholine (ACh) from the synaptic vesicles by exocytosis into the synaptic cleft.

2. Activation of ACh receptors: Acetylcholine diffuses across the synaptic cleft and binds to ACh receptors on the junctional folds of the sarcolemma. This opens ion channels in the sarcolemma (cell membrane of the muscle cell).

3. Production of a muscle action potential: The inflow of ions generates a muscle action potential (impulse) in the junctional folds of the sarcolemma of the muscle cell.

4. Termination of ACh activity: Acetylcholine that is bound to ACh receptors of the junctional folds is then rapidly broken down in the synaptic cleft by acetylcholinesterase (an enzyme in the synaptic cleft near the sarcolemma of the junctional folds). The destruction of ACh prevents continued contraction of the muscle cell.

Now for the events that lead to the contraction cycle of the skeletal muscle cell:

1. A muscle action potential at the junctional folds (from #3 above) spreads along the sarcolemma and then down the T tubules.

2. The action potential causes calcium ion channels of the sarcoplasmic reticulum to open.

3. Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm (cytoplasm of the muscle cell).

4. Calcium ions bind to troponin in the thin filaments; this causes the tropomyosin to move away from actin, which exposes the binding sites for myosin heads on the actin.


Now that the myosin-binding sites are uncovered, the contraction cycle begins:

1. ATP hydrolysis (splitting ATP): ATP binds to the myosin heads and is broken down by the enzyme ATPase (stored in the heads). As a result, energy is released and is used for:

(a.) the formation and movement of the crossbridges during the power stroke (b.) to release the myosin heads, so that they move back to their resting positions during the

recovery stroke (c.) the active transport of calcium ions back into the sarcoplasmic reticulum

- in addition to the above, some energy is also released as heat

2. Attachment of myosin to actin to form crossbridges: The myosin heads bind to the now exposed myosin-binding sites on the actin, forming crossbridges.

3. Power stroke: Energy stored in the myosin heads is used to move the heads. (The myosin heads "swing" or pivot toward the center of a sarcomere.) This movement of the myosin heads is called the power stroke, and it causes the thin filaments to slide past the thick filaments toward the center of a sarcomere.

4. Detachment of myosin from actin: At the end of the power stroke, energy is used to detach the myosin heads from the actin and to cause the myosin heads to swing back to their resting positions. This movement is called the recovery stroke.

The myosin heads are now ready to bind again with the actin. The cycle of power strokes and recovery strokes repeats many times during a muscle contraction, so that the myosin heads repeatedly attach to the actin, pivot and move the actin toward the center of the sarcomere, and then detach from the actin!

Thus, the I bands and the H bands decrease in width, and the sarcomere shortens. Each sarcomere along the length of each myofibril of a muscle cell shortens, so that the myofibrils, and therefore the muscle cell itself, shortens, resulting in shortening of the whole skeletal muscle.

Review Figs. 9.11 and 9.12, which summarize the events of skeletal muscle cell contraction

Relaxation

When the motor neuron stops transmitting an impulse to the muscle cell, relaxation of the muscle cell begins:

Remember, the acetylcholine that is bound to ACh receptors of the junctional folds is rapidly broken down in the synaptic cleft by acetylcholinesterase. The destruction of ACh prevents continued contraction of the muscle cell.

Calcium ions diffuse away from the troponin as they are actively transported from the sarcoplasm into the sarcoplasmic reticulum. The troponin and tropomyosin reestablish their position, so that tropomyosin again blocks the binding sites on the actin for myosin heads.

Because the binding sites on the actin are now blocked, crossbridges cannot reform once they have been released.


The thin filaments slip back to their resting position, so that the sarcomeres lengthen. Therefore, the myofibrils and the muscle cell overall lengthen, and the muscle returns to its original length. Some force is required for each sarcomere to lengthen back to its original length (i.e., the pull of gravity or contraction of an antagonistic muscle.)

Contraction of a Skeletal Muscle (page 293)

read the statements marked with the red squares on page 293

The Motor Unit

• define motor unit:

• in response to stimulation by a motor neuron, do all of the muscle cells in one motor unit contract?

• the number of muscle cells per motor unit may vary from what to what? • skeletal muscles that exert fine control have small motor units

- give two examples:

• large, weight-bearing skeletal muscles that are responsible for powerful, less precise movements

have large motor units

- give an example:

• are the muscle cells in a single motor unit spread throughout the muscle?

- as a result, stimulation of a single motor unit causes what?

• not all motor units of one entire skeletal muscle contract at once

• motor units allow graded force of contraction; a stronger contraction is not due to a stronger stimulus by motor neurons, but by an increase in the number of motor units contracting at once; so, the more motor units that are recruited, the greater the force of contraction of the muscle

• motor units help prevent muscle fatigue while allowing prolonged contraction of a muscle (because not all motor units need to contract at the same time to cause the whole muscle to contract; they can alternate between contracting and relaxing)


Isotonic and Isometric Contractions (page 297)

• indicate what happens to muscle length, the load, and tension in isotonic contractions (on p. 297):

• indicate what happens to muscle length, the load, and tension in isometric contractions (on p. 298):

Muscle Tone

define muscle tone:

what is muscle tone due to?

muscle tone does not produce active movements, but it does what?

first look back on p. 293 and

define muscle tension: define load:


Muscle Metabolism Providing Energy for Contraction ATP is present in muscle cells, but only provides enough energy for how many seconds of contraction?

because ATP is the only source of energy, it must be regenerated using energy released from the breakdown of:

- creatine phosphate - in skeletal muscle cells; write the equation from p. 298:

- glucose - from the blood and glycogen stored in skeletal muscle cells; glucose can be used to

produce ATP by aerobic and anaerobic cellular respiration

(read about Muscle Fatigue, Oxygen Deficit, and Heat Production During Muscle Activity on p. 300 to p. 301)

Force of Muscle Contraction

The force of muscle contraction depends on the number of myosin crossbridges that are attached. This is turn is affected by what four factors?

Number of Muscle Fibers Recruited what is the relationship between the number of motor units that are recruited and muscle force?

Size of Muscle Fibers what is the relationship between the size of muscle cells (fibers) and muscle force?


how does regular exercise increase muscle force?

Frequency of Stimulation the more rapidly a muscle is stimulated, the greater the force it exerts Degree of Muscle Stretch in the body, skeletal muscles are maintained near their optimal length to generate maximum force by

the way they are attached to bones joints normally prevent bone movements that would stretch attached muscles beyond optimal length Muscle Cell Type see Table 9.2 on p.303 to fill out the following table to contrast the three main types of skeletal muscle cells:

Slow Oxidative Fibers

Fast Oxidative Fibers

Fast Glycolytic Fibers

speed of contraction

primary pathway for ATP synthesis

myoglobin content

rate of fatigue

activities best suited for

color

Adaptations to Exercise

(read about Adaptations to Exercise on p.304)


SSMMOOOOTTHH MMUUSSCCLLEE

MICROSCOPIC STRUCTURE OF SMOOTH MUSCLE CELLS

read the information on p.305 to p.306 about the microscopic structure of smooth muscle

what is the typical diameter of smooth muscle cells?

how long are smooth muscle cells?

how does the diameter and length of smooth muscle cells contrast with skeletal muscle cells?

most smooth muscle is organized into what?

- where do these sheets occur?

- in most cases, how many sheets of smooth muscle are present?

- describe the longitudinal layer and what happens when the cells in this layer contract?

- describe the circular layer and what happens when the cells in this layer contract?

- what is the effect of the alternating contraction and relaxation of these opposing layers?

- peristalsis = wave like smooth muscle contractions that propel the contents of hollow organs in

one direction

do smooth muscle cells have highly structured neuromuscular junctions?

- instead, they are innervated by nerve fibers of the autonomic nervous system, which end in numerous bulbous swellings called what?


- what do the varicosities release?

are striations present in smooth muscle?

do smooth muscle cells contain interdigitating thick and thin myofilaments?

- is the organization and numbers of myofilaments the same as or different than in skeletal muscle?

- thus, myofilaments are not arranged into orderly sarcomeres, so the cells do not have striations

in addition, smooth muscle tissue has dense bodies and intermediate filaments; see Fig. 9.27

the sliding filament mechanism involving actin and myosin generates tension on the intermediate filaments that pull on the dense bodies, so the cell shortens and twists when it contracts

CONTRACTION OF SMOOTH MUSCLE

Mechanism of Contraction in most cases, adjacent smooth muscle cells exhibit what type of contractions?

this synchronization reflects what?:

in what three ways is contraction in smooth muscle like contraction in skeletal muscle?

Regulation of Contraction the contraction of smooth muscle can be regulated by what three things?

Neural Regulation autonomic nerves (parasympathetic and sympathetic) supply the smooth muscle of visceral organs;

neurotransmitters may excite or inhibit smooth muscle cells

Hormones and Local Chemical Factors

do all smooth muscles have a nerve supply?


some depolarize spontaneously or in response to chemical stimuli; list the chemical factors that cause smooth muscle contraction or relaxation without an action potential:

TYPES OF SMOOTH MUSCLE

Unitary Smooth Muscle

why is it also called visceral muscle?

unitary smooth muscle has electrically coupled fibers that contract synchronously as a unit and

often spontaneously Multi Unit Smooth Muscle

list the examples of multi unit smooth muscle:

the cells lack gap junctions and are structurally independent of one another multi unit smooth muscle is innervated by the autonomic nervous system and is responsive to hormones

REGENERATION OF MUSCULAR TISSUE

∙ muscle hypertrophy = increase in size of muscle cells

∙ muscle hyperplasia = increase in number of muscle cells

∙ after birth, skeletal and cardiac muscle tissue lose the ability to undergo cell division and growth of a muscle is due to hypertrophy of the cells; smooth muscle in some cases, such as the uterus, retains the ability to undergo cell division and can grow by hyperplasia

in general, skeletal and cardiac muscle tissue has very limited ability to regenerate; smooth muscle tissue can regenerate throughout life

AGING AND MUSCULAR TISSUE ∙ in general, as we age, what happens to the amount of connective tissue in our skeletal muscles and the number of muscle fibers (see p.315)?

The End

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