Muscles and Muscle Tissue

LAB 6

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

MARTINIPG 133

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

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

• 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

Muscle Classification: Functional Groups

• Prime movers – provide the major force for producing a specific movement

• Antagonists – oppose or reverse a particular movement

• Synergists– Add force to a movement– Reduce undesirable or unnecessary movement

• Fixators – synergists that immobilize a bone or muscle’s origin

Naming Skeletal Muscles

• Location of muscle – bone or body region associated with the muscle

• Shape of muscle – e.g., the deltoid muscle (deltoid = triangle)

• Relative size – e.g., maximus (largest), minimus (smallest), longus (long)

• Direction of fibers – e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis)

Naming Skeletal Muscles

• Number of origins – e.g., biceps (two origins) and triceps (three origins)

• Location of attachments – named according to point of origin or insertion

• Action – e.g., flexor or extensor, as in the names of muscles that flex or extend, respectively

Bone-Muscle Relationships: Lever Systems

• Lever – a rigid bar that moves on a fulcrum, or fixed point

• Effort – force applied to a lever

• Load – resistance moved by the effort

Bone-Muscle Relationships: Lever Systems

Figure 10.2a

Bone-Muscle Relationships: Lever Systems

Figure 10.2b

Lever Systems: Classes

• First class – the fulcrum is between the load and the effort

• Second class – the load is between the fulcrum and the effort

• Third class – the effort is applied between the fulcrum and the load

Lever Systems: First Class

Figure 10.3a

Lever Systems: Second Class

Figure 10.3b

Lever Systems: Third Class

Figure 10.3c

Major Skeletal Muscles: Anterior View

The 40 superficial muscles here are divided into 10 regional areas of the body:

• 1.- Facial• 2.- Neck• 3.-Thorax• 4.- Shoulder• 5.- Arm• 6.- Forearm• 7.- Abdomen• 8.- Pelvis• 9.- Thigh• 10.- Leg Figure 10.4b

Major Skeletal Muscles: Posterior View

The 27 superficial muscles here are divided into seven regional areas of the body:

1.- Neck

2.- Shoulder

3.-Arm

4.- Forearm

5.- Hip

6.-Thigh

7.- LegFigure 10.5b

Muscles of the Face

• 11 muscles are involved in lifting the eyebrows, flaring the nostrils, opening and closing the eyes and mouth, and smiling

• All are innervated by cranial nerve VII (facial nerve)

• Usually insert in skin (rather than bone), and adjacent muscles often fuse

Muscles of the Face

Figure 10.6

Muscles of Mastication

• There are four pairs of muscles involved in mastication– Prime movers – temporalis and masseter– Grinding movements – pterygoids and

buccinators

• All are innervated by cranial nerve V (trigeminal nerve)

Muscles of Mastication

Figure 10.7a

Muscles of Mastication

Figure 10.7b

Extrinsic Tongue Muscles

• Three major muscles that anchor and move the tongue

• All are innervated by cranial nerve XII (hypoglossal nerve)

Extrinsic Tongue Muscles

Figure 10.7c

Homeostatic Imbalance• Many toxins, drugs and diseases interfere

with events at the neuromuscular junction

Ex: Myastenia gravis: Characterize by:

1.- Drooping of the upper eyelids

2.- Difficulty of swallowing and talking

3.- Muscle weakness

4.- Serum antibodies against acetilcholine (Ach)

receptor

Developmental Aspects: Male and Female

• There is a biological basis for greater strength in men than in women

• Women’s skeletal muscle makes up 36% of their body mass

• Men’s skeletal muscle makes up 42% of their body mass

• The outside (extracellular) face is positive, while the inside face is negative

• This difference in charge is the resting membrane potential

Figure 9.8 (a)

Action Potential: Electrical Conditions of a Polarized Sarcolemma

• The predominant extracellular ion is Na+

• The predominant intracellular ion is K+

• The sarcolemma is relatively impermeable to both ions

Figure 9.8 (a)

Action Potential: Electrical Conditions of a Polarized Sarcolemma

• An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open)

Figure 9.8 (b)

Action Potential: Depolarization and Generation of the Action Potential

• Na+ enters the cell, and the resting potential is decreased (depolarization occurs)

• If the stimulus is strong enough, an action potential is initiated

Figure 9.8 (b)

Action Potential: Depolarization and Generation of the Action Potential

• Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch

• Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize

Figure 9.8 (c)

Action Potential: Propagation of the Action Potential

• Thus, the action potential travels rapidly along the sarcolemma

• Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

Figure 9.8 (c)

Action Potential: Propagation of the Action Potential

Action Potential: Repolarization• Immediately after the

depolarization wave passes, the sarcolemma permeability changes

• Na+ channels close and K+ channels open

• K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma

Figure 9.8 (d)

Action Potential: Repolarization• Repolarization

occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)

• The ionic concentration of the resting state is restored by the Na+-K+ pump

Figure 9.8 (d)

Excitation-Contraction Coupling

• Once generated, the action potential:– Is propagated along the sarcolemma– Travels down the T tubules– Triggers Ca2+ release from terminal cisternae

• Ca2+ binds to troponin and causes: – The blocking action of tropomyosin to cease– Actin active binding sites to be exposed

Excitation-Contraction Coupling

• Myosin cross bridges alternately attach and detach

• Thin filaments move toward the center of the sarcomere

• Hydrolysis of ATP powers this cycling process

• Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes

Excitation-Contraction Coupling

Figure 9.9

• At low intracellular Ca2+ concentration:– Tropomyosin blocks the

binding sites on actin– Myosin cross bridges

cannot attach to binding sites on actin

– The relaxed state of the muscle is enforced

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

Figure 9.10 (a)

Figure 9.10 (b)

• At higher intracellular Ca2+ concentrations:– Additional calcium binds

to troponin (inactive troponin binds two Ca2+)

– Calcium-activated troponin binds an additional two Ca2+ at a separate regulatory site

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

• Calcium-activated troponin undergoes a conformational change

• This change moves tropomyosin away from actin’s binding sites

Figure 9.10 (c)

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

• Myosin head can now bind and cycle

• This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin

Figure 9.10 (d)

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

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

Myosin cross bridge attaches to the actin myofilament

1

2

3

4 Working stroke—the myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line

As new ATP attaches to the myosin head, the cross bridge detaches

As ATP is split into ADP and Pi, cocking of the myosin head occurs

Myosin head (high-energy

configuration)

Thick filament

Myosin head (low-energy configuration)

ADP and Pi (inorganic phosphate) released

Sequential Events of Contraction

Figure 9.11

Thin filament

Motor Unit: The Nerve-Muscle Functional Unit

• Large weight-bearing muscles (thighs, hips) have large motor units

• Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Motor Unit: The Nerve-Muscle Functional Unit

• A motor unit is a motor neuron and all the muscle fibers it supplies

• The number of muscle fibers per motor unit can vary from four to several hundred

• Muscles that control fine movements (fingers, eyes) have small motor units

Motor Unit: The Nerve-Muscle Functional Unit

Figure 9.12 (a)