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Muscles and Muscle Tissue. 8. Part A. Muscle Overview. Muscle tissue makes up nearly half the body mass. The most distinguishing functional characteristic of muscles is their ability to transform chemical energy ATP into directed mechanical energy

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Muscle overview
Muscle Overview

Muscle tissue makes up nearly half the body mass.

The most distinguishing functional characteristic of muscles is their ability to transform chemical energy ATP into directed mechanical energy

The three types of muscle tissue are skeletal, cardiac, and smooth

These types differ in structure, location, function, and means of activation


Muscle similarities
Muscle Similarities

Skeletal and smooth muscle cells are elongated and are called muscle fibers

Muscle contraction depends on two kinds of myofilaments – actin and myosin

Muscle terminology is similar

Sarcolemma – muscle plasma membrane

Sarcoplasm – cytoplasm of a muscle cell

Prefixes – myo, mys, and sarco all refer to muscle


Skeletal muscle tissue
Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton

Has obvious stripes called striations

Is controlled voluntarily (i.e., by conscious control)

Contracts rapidly but tires easily

Is responsible for overall body motility

Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

Key words: skeletal, striated and voluntary


Cardiac muscle tissue
Cardiac Muscle Tissue

Occurs only in the heart (“blood pump”)

Is striated like skeletal muscle but is not voluntary

Contracts at a fairly steady rate set by the heart’s pacemaker

Neural controls allow the heart to respond to changes in bodily needs

Key words: cardiac, striated and involuntary


Smooth muscle tissue
Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages

Forces food and other substances through internal body channels

It is not striated and is involuntary

Key words: visceral, non striated and involuntary


Functional characteristics of muscle tissue
Functional Characteristics of Muscle Tissue

Excitability, or irritability – the ability to receive and respond to stimuli

Contractility – the ability to shorten forcibly

Extensibility – the ability to be stretched or extended

Elasticity – the ability to recoil and resume the original resting length


Muscle function
Muscle Function

Skeletal muscles are responsible for all locomotion

Cardiac muscle is responsible for coursing the blood through the body

Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs

Muscles also maintain posture, stabilize joints, and generate heat


Skeletal muscle gross anatomy
Skeletal Muscle Gross Anatomy

Tissues:

Blood vessels

Nerves – branches to each fiber

Connective Tissue

Endomysium –wraps each fiber

Perimysium –wraps fibers into fascicles

Epimysium –wraps fascicles into a muscle

All are continuous with each other and the tendons.



Skeletal muscle nerve and blood supply
Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins

Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins


Structural organization of skeletal muscle
Structural Organization of Skeletal Muscle

Muscle (Organ)

Fascicles (Bundles of fibers)

Fiber (Cell)

Myofibrils (densely packed contractile elements)

Myofilaments (contractile proteins)

Actin (thin filaments)

Myosin (thick filaments)


Myofibrils
Myofibrils

Figure 9.3 (b)


Sarcomeres
Sarcomeres

The smallest contractile unit of a muscle

The region of a myofibril between two successive Z discs

Composed of myofilaments made up of contractile proteins

Myofilaments are of two types – thick and thin


Sarcomeres1
Sarcomeres

Figure 9.3 (c)


Myofilaments banding pattern
Myofilaments: Banding Pattern

Thick filaments (composed by myosin) – extend the entire length of an A band

Thin filaments (composed by actin) – extend across the I band and partway into the A band

Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another



Ultrastructure of myofilaments thick filaments
Ultrastructure of Myofilaments: Thick Filaments

Thick filaments are composed of the protein myosin

Each myosin molecule has a rodlike tail and two globular heads

Tails – two interwoven, heavy polypeptide chains

Heads – two smaller, light polypeptide chains called cross bridges



Ultrastructure of myofilaments thin filaments
Ultrastructure of Myofilaments: Thin Filaments

Thin filaments are chiefly composed of the protein actin

Each actin molecule is a helical polymer of globular subunits called G actin

The subunits contain the active sites to which myosin heads attach during contraction

Tropomyosin and troponin are regulatory subunits bound to actin



Arrangement of the filaments in a sarcomere
Arrangement of the Filaments in a Sarcomere

Longitudinal section within one sarcomere

Figure 9.4 (d)


Sliding filament model of contraction
Sliding Filament Model of Contraction

Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

In the relaxed state, thin and thick filaments overlap only slightly

Upon stimulation, myosin heads bind to actin and sliding begins


Sliding filament model of contraction1
Sliding Filament Model of Contraction

Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

As this event occurs throughout the sarcomeres, the muscle shortens


Skeletal muscle contraction
Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:

Be stimulated by a nerve ending

Propagate an electrical current, or action potential, along its sarcolemma

Have a rise in intracellular Ca2+ levels, the final trigger for contraction


Brain spinal cord motor neuron muscle
Brain  Spinal Cord  Motor Neuron  Muscle

Motor = movement

Neuron = nerve cell


Motor neuron muscle fiber
Motor Neuron  Muscle Fiber

Motor neuron releases neurotransmitter called acetylcholine (ACh).

ACh causes muscle fiber to produce an electrical signal.

Electrical signal causes actin & myosin to move, and this causes muscle to move.


Destruction of acetylcholine
Destruction of Acetylcholine

ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase

This destruction prevents continued muscle fiber contraction in the absence of additional stimuli



A myofibril at rest1
A Myofibril at Rest

At rest” = fiber has not received message to move

Actin (thin) attaches to Z-line.

Myosin (thick) does not attach to actin or the Z-line.

Myosin heads are bent back (cocked).


Contraction
Contraction

Let’s compare length of sarcomeres:

Relaxed:

Contracted:


Contraction in the myofibril
Contraction in the Myofibril

After contraction, muscle relaxes (back to original length).

Myosin heads pick up ATP & break it apart.

This cocks the head, detaching it from actin.

Now ready to contract again.

ATP

Head is Cocked


Sequential events of contraction
Sequential Events of Contraction

Cross bridge formation – myosin cross bridge attaches to actin filament

Working (power) stroke – myosin head pivots and pulls actin filament toward M line

Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches

“Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state



Contraction of skeletal muscle organ level
Contraction of Skeletal Muscle (Organ Level)

Contraction of muscle fibers (cells) and muscles (organs) is similar

The two types of muscle contractions are:

Isometric contraction – increasing muscle tension (muscle does not shorten during contraction)

Isotonic contraction – decreasing muscle length (muscle shortens during contraction)


Types of muscular contraction
Types of muscular contraction

Isometric

Muscular contraction where the tension developed occurs with no change in length

Otherwise known as static contraction or position.

Improves muscular strength at fixed joint angle

Does not develop aerobic fitness

Can be done anywhere

Examples; rugby scrum, tug of war.


Types of muscle contraction
Types of muscle contraction

Isotonic

Muscles contact at speed controlled by the performer

Motor unit recruitment is at the speed required for the specific sports activity.

Develops aerobic and anaerobic fitness

Most physical activities are isotonic

Can occur in two ways: concentric and eccentric.


Contraction2
Contraction

Concentric contraction

Muscle shortens under tension

Insertion moves towards origin

Occurs in agonist muscle

e.g. Chin-ups – use of bicep brachii in upward phase


Contraction3
Contraction

Eccentric contraction

Muscle lengthens under tension

Insertion moves away from origin

Occurs in antagonist muscle

Only occurs if the antagonist is acting as a brake to help control the joint movement

E.g. Chin-ups – use of biceps in downward phase.


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