Muscles

Focus: Skeletal Muscles CH 10

Functions

  • Produce skeletal movement
  • Maintain posture and body position
  • Support soft tissues
  • Guard entrances and exits
  • Maintain body temperature
  • Store nutrient reserves

There are over 600 muscles!

Smooth
Cardiac
Skeletal
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Skeletal Muscles

Are attached to the skeletal system

Allow us to move the muscular system

Includes only skeletal muscles

Functions of the skeletal muscle tissue

Produce skeletal movement

Maintain posture and body position

Support soft tissues

Guard entrances and exits

Maintains body temperature

Store nutrient reserves

What makes up muscle?

Muscle tissue (muscle cells or fibers)

Nerves

Blood vessels

Connective tissues:


Epimysium

  • Exterior collagen layer

  • Connected to deep fascia
  • Separates muscle from surrounding tissues

Perimysium
  • Surrounds muscle fiber bundles (fascicles)
  • Contains blood vessel and nerve supply to fascicles


Endomysium

  • Surrounds individual muscle cells (muscle fibers)
  • Contains capillaries and nerve fibers contacting muscle cells
  • Contains myosatellite cells (stem cells) that repair damage
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Tendons are formed from connective tissues.

Muscle Attachments

Endomysium, perimysium, and epimysium come together at the ends of muscles to form connective tissue attachment to bone matrix

i.e., tendon (bundle) or aponeurosis (sheet)

Blood vessels and nerves

Muscles have extensive vascular systems that:

Supply large amounts of oxygen

Supply nutrients

Carry away wastes

Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)

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Skeletal muscle cells (muscle fibers)

Are very long and develop through fusion of mesodermal cells (myoblasts)

These become very large and contain hundreds of nuclei

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Characteristics of skeletal muscle fibers

The Sarcolemma and Transverse Tubules


The sarcolemma = the cell membrane of a muscle fiber (cell)

Surrounds the sarcoplasm (cytoplasm of muscle fiber)

A change in transmembrane potential begins contractions


Transverse tubules (T tubules) = tubules that transmit action potential through cell

Allow entire muscle fiber to contract simultaneously

Have same properties as sarcolemma

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Myofibrils

Lengthwise subdivisions within muscle fiber

Made up of bundles of protein filaments (myofilaments)

Myofilaments are responsible for muscle contraction

Types of myofilaments:

Thin filaments

Made of the protein actin

Thick filaments

Made of the protein myosin

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The Sarcoplasmic Reticulum (SR) (Endoplasmic reticulum)

A membranous structure surrounding each myofibril

Helps transmit action potential to myofibril

Similar in structure to smooth endoplasmic reticulum

Forms chambers (terminal cisternae) attached to T tubules


Triad of the sarcoplasmic reticulum

Is formed by one T tubule and two terminal cisternae

Cisternae

Concentrate Ca2+ (via ion pumps)

Release Ca2+ into sarcomeres to begin muscle contraction

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Sarcomeres

The contractile units of muscle

Structural units of myofibrils

Form visible patterns within myofibrils

A striped or striated pattern within myofibrils

Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

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Sarcomere components

The A Band contains:

M line - located in the center of the A band located at the midline of the sarcomere

The H Band - in the area around the M line, it has thick filaments but no thin filaments

Zone of overlap - The densest, darkest area on a light micrograph, where thick and thin filaments overlap


The I Band contains:

Z lines - located in the center of the I band at the ends of the sarcomere

Titin - strands of protein that reach from tips of thick filaments to the Z line, these stabilize the filaments

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Thin filaments

F-actin (filamentous actin)

Is two twisted rows of globular G-actin

The active sites on G-actin strands bind to myosin


Nebulin

Holds F-actin strands together


Tropomyosin

Is a double strand

Prevents actin–myosin interaction


Troponin

A globular protein

Binds tropomyosin to G-actin

Controlled by Ca2+

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Thick filaments

Contain about 300 twisted myosin subunits

Contain titin strands that recoil after stretching the mysosin molecule

Tail - Binds to other myosin molecules

Head - Made of two globular protein subunits

Reaches the nearest thin filament

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Initiating Contraction

Ca2+ binds to receptor on troponin molecule

Troponin–tropomyosin complex changes

Exposes active site of F-actin


Myosin Action

During contraction, myosin heads:

Interact with actin filaments, forming cross-bridges

Pivot, producing motion

Sliding filament theory

Thin filaments of sarcomere slide toward M line, alongside thick filaments

The width of A zone stays the same

Z lines move closer together

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Muscle contractions: Putting it all together

A nerve impulse is sent to the muscle fiber, which undergoes a cascade of events resulting in the contraction of the muscle fiber.
Ach is released from the axon terminal into the synaptic cleft
ACh binds to ACh receptors located in the muscle fiber causing cation channels to open
Sodium rushes in and stimulates an action potential down the sarcolemma
ACH is immediately broken down by the enzyme acetylcholinesterase
As the action potential moves down the sarcolemma of the muscle fiber, CA2+ ions are released from the sarcoplasmic reticulum
Ca2+ ions bind to tbe troponin changing its configuration so that tropomyosin moves off of the myosin-binding site on actin allowing myosind to bind to actin and beginning the contraction

Contraction occurs as follows:

The myosin heads hydrolyze ATP so that they reorient and become energized.
The myosin heads bind to the exposed actin sites creating crossbridges
The myosin crossbridges rotate toward the middle of the sarcomere
The myosin heads, while still attached to the actin bind with ATP, which allows the myosin heads to detach from the actin

This continues as long as CA2+ and ATP are available
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Contraction duration

Depends on:

Duration of neural stimulus

Number of free calcium ions in sarcoplasm

Availability of ATP

Muscle relaxation

Requires that:
  • The motor neuron cease firing and the neurotransmitter ACh is removed from the synapse
  • The calcium release channels of the sarcoplasmic reticulum close and pumps bring back the level of CA2+ ions in the SR to levels prior to muscle contraction
  • The tropomyosin returns to its configuration of blocking myosin binding sites in actin

Rigor mortis

A fixed muscular contraction after death

Caused when: Ion pumps cease to function; ran out of ATP

Calcium builds up in the sarcoplasm

Tension and Contraction

As a whole, a muscle fiber is either contracted or relaxed

Depends on:

The number of pivoting cross-bridges

The fiber’s resting length at the time of stimulation

The frequency of stimulation



Length–Tension Relationships

Number of pivoting cross-bridges depends on:

Amount of overlap between thick and thin fibers

Optimum overlap produces greatest amount of tension

Too much or too little reduces efficiency

Normal resting sarcomere length is 75 to 130 percent of optimal length

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The Frequency of Stimulation

A single neural stimulation produces:

A single contraction or twitch which lasts about 7–100 msec.

Sustained muscular contractions require many repeated stimuli

Twitches

Latent period - The action potential moves through sarcolemma causing Ca2+ release

Contraction phase - Calcium ions bind and tension builds to peak

Relaxation phase- Ca2+ levels fall and active sites are covered and tension falls to resting levels

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Treppe

A stair-step increase in twitch tension

Repeated stimulations immediately after relaxation phase

Stimulus frequency <50/second

Causes a series of contractions with increasing tension

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Wave summation

Wave summation

Increasing tension or summation of twitches

Repeated stimulations before the end of relaxation phase

Stimulus frequency >50/second

Causes increasing tension or summation of twitches

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Incomplete tetanus

Twitches reach maximum tension

If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension

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Complete tentanus

If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

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