Skeletal Muscle Tissue and Muscle Organization

Chapter 9 The Muscular System Skeletal Muscle Tissue and Muscle Organization

Lecture Presentation by Steven Bassett Southeast Community College

• Humans rely on muscles for: • Many of our physiological processes • Virtually all our dynamic interactions with the environment • Skeletal muscles consist of: • Elongated cells called fibers (muscle fibers) • These fibers contract along their longitudinal axis

• There are three types of muscle tissue • Skeletal muscle • Pulls on skeletal bones • Voluntary contraction • Cardiac muscle • Pushes blood through arteries and veins • Rhythmic contractions • Smooth muscle • Pushes fluids and solids along the digestive tract, for example • Involuntary contraction

• Muscle tissues share four basic properties • Excitability • The ability to respond to stimuli • Contractility • The ability to shorten and exert a pull or tension • Extensibility • The ability to continue to contract over a range of resting lengths • Elasticity • The ability to rebound toward its original length

• Skeletal muscles perform the following functions: • Produce skeletal movement • Pull on tendons to move the bones • Maintain posture and body position • Stabilize the joints to aid in posture • Support soft tissue • Support the weight of the visceral organs

• Skeletal muscles perform the following functions (continued): • Regulate entering and exiting of material • Voluntary control over swallowing, defecation, and urination • Maintain body temperature • Some of the energy used for contraction is converted to heat

• Gross anatomy is the study of: • Overall organization of muscles • Connective tissue associated with muscles • Nerves associated with muscles • Blood vessels associated with muscles • Microscopic anatomy is the study of: • Myofibrils • Myofilaments • Sarcomeres

• Gross Anatomy • Connective tissue of muscle • Epimysium: dense tissue that surrounds the entire muscle • Perimysium: dense tissue that divides the muscle into parallel compartments of fascicles • Endomysium: dense tissue that surrounds individual muscle fibers

Epimysium Nerve Muscle fascicle Muscle fibers Endomysium Blood vessels Perimysium

SKELETAL MUSCLE (organ) Perimysium

Muscle fiber Endomysium

Epimysium Blood vessels and nerves MUSCLE FASCICLE (bundle of cells)

Capillary Mitochondria Endomysium Endomysium Sarcolemma Myosatellite Tendon Myofibril cell

Perimysium Axon Sarcoplasm Nucleus

MUSCLE FIBER (cell)

• Connective Tissue of Muscle • Tendons and aponeuroses • Epimysium, perimysium, and endomysium converge to form tendons • Tendons connect a muscle to a bone • Aponeuroses connect a muscle to a muscle

• Gross Anatomy • Nerves and blood vessels • Nerves innervate the muscle by penetrating the epimysium • There is a chemical communication between a nerve and a muscle • The chemical is released into the neuromuscular synapse (neuromuscular junction)

Neuromuscular synapse

Skeletal muscle fiber

Axon

Nerve

LM x 230 SEM x 400 a A neuromuscular synapse as seen b Colorized SEM of a neuromuscular on a muscle fiber of this fascicle synapse

• Gross Anatomy • Nerves and blood vessels (continued) • Blood vessels often parallel the nerves that innervate the muscle • They then branch to form coiled networks to accommodate flexion and extension of the muscle

• Microanatomy of Skeletal Muscle Fibers • Sarcolemma • Membrane that surrounds the muscle cell • Sarcoplasm • The cytosol of the muscle cell • Muscle fiber (same thing as a muscle cell) • Can be 30–40 cm in length • Multinucleate (each muscle cell has hundreds of nuclei) • Nuclei are located just deep to the sarcolemma

Muscle fibers develop through the fusion of mesodermal cells Myoblasts called myoblasts.

a Development of a skeletal muscle fiber.

Myosatellite cell

Nuclei

Immature muscle fiber

b External appearance and histological view.

• Myofibrils and Myofilaments • The sarcoplasm contains myofibrils • Myofibrils are responsible for the contraction of muscles • Myofibrils are attached to the sarcolemma at each end of the muscle cell • Surrounding each myofibril is the sarcoplasmic reticulum

• Myofibrils and Myofilaments • Myofibrils are made of myofilaments • Actin • Thin protein filaments • Myosin • Thick protein filaments

b External appearance and histological view.

Myofibril

Sarcolemma Nuclei

c The external organization Sarcoplasm MUSCLE FIBER of a muscle fiber.

Mitochondria

Terminal cisterna

Sarcolemma Sarcolemma

Sarcoplasm

Myofibril Myofibrils

Thin filament

Thick filament

Triad Sarcoplasmic T tubules d Internal organization of a muscle fiber. reticulum Note the relationships among myofibrils, sarcoplasmic reticulum, mitochondria, triads, and thick and thin filaments. © 2015 Pearson Education, Inc. Anatomy of Skeletal Muscles

• Sarcomere Organization • Myosin (thick filament) • Actin (thin filament) • Both are arranged in repeating units called sarcomeres • All the myofilaments are arranged parallel to the long axis of the cell

• Sarcomere Organization • Sarcomere • Main functioning unit of muscle fibers • Approximately 10,000 per myofibril • Consists of overlapping actin and myosin • This overlapping creates the striations that give the skeletal muscle its identifiable characteristic

• Sarcomere Organization • Each sarcomere consists of: • Z line (Z disc) • I band • A band (overlapping A bands create striations) • H band • M line

I band A band

H band Z line Titin

Zone of overlap M line Thin Thick filament filament Sarcomere

I band A band

H band Z line

TEM x 64,000 Z line Zone of overlap M line

b A corresponding view of a sarcomere in a myofibril in Sarcomere the gastrocnemius muscle of the calf and a diagram showing the various components of this sarcomere

• Sarcomere Organization • Skeletal muscles consist of muscle fascicles • Muscle fascicles consist of muscle fibers • Muscle fibers consist of myofibrils • Myofibrils consist of sarcomeres • Sarcomeres consist of myofilaments • Myofilaments are made of actin and myosin

SKELETAL MUSCLE Surrounded by: Epimysium Contains: Muscle fascicles

MUSCLE FASCICLE

Surrounded by: Perimysium Contains: Muscle fibers

MUSCLE FIBER

Surrounded by: Endomysium Contains: Myofibrils

MYOFIBRIL

Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line)

SARCOMERE I band A band

Contains: Thick filaments Thin filaments

• Thin Filaments (Actin) • Consists of: • Twisted filaments of : • F actin strands • G actin globular molecules • G actin molecules consist of an active site (binding site) • Tropomyosin: A protein that covers the binding sites when the muscle is relaxed • Troponin: Holds tropomyosin in position

Actinin Z line Titin

a The attachment Sarcomere of thin filaments to the Z line H band Troponin Active site Nebulin Tropomyosin G actin molecules

F actin strand Myofibril b The detailed structure of a thin filament showing the organization of G actin, troponin, and tropomyosin

M line Z line

• Thick Filaments (Myosin) • Myosin filaments consist of an elongated tail and a globular head (cross-bridges) • Myosin is a stationary molecule. It is held in place by: • Protein forming the M line • A core of titin connecting to the Z lines • Myosin heads project toward the actin filaments

Sarcomere

H band

Myofibril

M line Z line Titin

c The structure of thick filaments Myosin M line head

Myosin tail Hinge

d A single myosin molecule detailing the structure and movement of the myosin head after cross-bridge binding occurs

• A contracting muscle shortens in length • Contraction is caused by interactions between thick and thin filaments within the sarcomere • Contraction is triggered by the presence of calcium ions • Muscle contraction requires the presence of ATP • When a muscle contracts, actin filaments slide toward each other • This sliding action is called the sliding filament theory

• The Sliding Filament Theory • Upon contraction: • The H band and I band get smaller • The zone of overlap gets larger • The Z lines move closer together • The width of the A band remains constant throughout the contraction

Resting Sarcomere Contracted Sarcomere A resting sarcomere showing the locations of the After repeated cycles of “bind, pivot, detach, and reactivate” I band, A band, H band, M, and Z lines. the entire muscle completes its contraction.

I band A band M line

Contracted myofibril

I band A band M line

Z line H band Z line

Resting myofibril Z line H band Z line

In a contracting sarcomere the A band stays the same width, but the Z lines move closer together and the H band and the I bands get smaller

• The Neural Control of Muscle Fiber Contraction • An impulse travels down the axon of a nerve • Acetylcholine is released from the end of the axon into the neuromuscular synapse • This ultimately causes the sarcoplasmic reticulum to release its stored calcium ions • This begins the actual contraction of the muscle

Arriving action potential Synaptic Synaptic vesicles cleft ACh ACh receptor site Sarcolemma of Motor motor end plate neuron AChE molecules Glial cell Axon Path of action Junctional fold potential

Synaptic b Detailed view of a terminal, synaptic cleft, terminal and motor end plate. See also Figure 9.2.

Muscle Fiber Myofibril

Motor end plate

Myofibril

Sarcolemma Mitochondrion a A diagrammatic view of a neuromuscular synapse.

• Muscle Contraction: A Summary • The nerve impulse ultimately causes the release of a neurotransmitter (ACh), which comes in contact with the sarcoplasmic reticulum • This neurotransmitter causes the sarcoplasmic reticulum to release its stored calcium ions • Calcium ions bind to troponin

© 2015 Pearson Education, Inc. Figure 9.7 Sliding Filament Theory (2 of 11) 1 Contraction Cycle Begins The contraction cycle involves a series of Ca2+ interrelated steps. The cycle begins with electrical events in the sarcolemma that trigger the release of calcium from the Actin terminal cisternae of the sarcoplasmic reticulum (SR). These calcium ions enter the zone of overlap.

2 Active-Site Exposure Ca2+ Tropomyosin Calcium ions bind to troponin in the troponin– tropomyosin complex. The tropomyosin molecule then rolls away Active from the active sites on the actin site molecules of the thin filaments.

• Muscle Contraction: A Summary (continued) • The bound calcium ions cause the tropomyosin molecule to roll so that it exposes the active sites on actin • The myosin heads now extend and bind to the exposed active sites on actin • Once the myosin heads bind to the active sites, they pivot in the direction of the M line

3 Cross-Bridge Formation Myosin head Cross-bridge Once the active sites are exposed, the myosin heads of adjacent thick filaments bind to them, forming cross-bridges.

4 Myosin Head Pivoting

After cross-bridge formation, energy is released as the myosin heads pivot toward the M line.

• Muscle Contraction: A Summary (continued) • Upon pivoting of the myosin heads, the actin filament slides toward the M line • ATP binds to the myosin heads causing them to release their attachment and return to their original position to start over again

5 Cross-Bridge Detachment ATP ATP then binds to the myosin heads, breaking the cross-bridges between the myosin heads and the actin molecules. ATP

6 Myosin Reactivation ATP provides the energy to reactivate the myosin heads and return them to their original positions. Now the entire cycle can be repeated as long as calcium ion concentrations remain elevated and ATP reserves are sufficient.

• Muscle Contraction: A Summary (continued) • Upon contraction: • I bands get smaller • H bands get smaller • Z lines get closer together

STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION

Synaptic Motor terminal end plate T tubule Sarcolemma

2 Action 1 ACh released, binding potential 6 ACh removed by AChE to receptors reaches T tubule 3 Sarcoplasmic 7 Sarcoplasmic reticulum reticulum releases Ca2+ recaptures Ca2+ Ca2+ 4 Active-site Actin 8 exposure, Active sites cross-bridge covered, no formation Myosin cross-bridge interaction

9 Contraction ends 5 Contraction begins 10 Relaxation occurs, passive return to resting length

• Motor Units (Motor Neurons Controlling Muscle Fibers) • Precise control • A motor neuron controlling two or three muscle fibers • Example: the control over the eye muscles • Less precise control • A motor neuron controlling perhaps 2000 muscle fibers • Example: the control over the leg muscles

Axons of motor neurons

Motor nerve

Muscle fibers

• Muscle tension depends on: • The frequency of stimulation • A typical example is a muscle twitch • The number of motor units involved • Complete contraction or no contraction at all (all or none principle) • The amount of force of contraction depends on the number of motor units activated

• Muscle Tone • The tension of a muscle when it is relaxed • Stabilizes the position of bones and joints • Example: the amount of muscle involvement that results in normal body posture • Muscle Spindles • These are specialized muscle cells that are monitored by sensory nerves to control muscle tone

• Muscle Hypertrophy • Enlargement of the muscle • Exercise causes: • An increase in the number of mitochondria • An increase in the activity of muscle spindles • An increase in the concentration of glycolytic enzymes • An increase in the glycogen reserves • An increase in the number of myofibrils • The net effect is an enlargement of the muscle

• Muscle Atrophy • Discontinued use of a muscle • Disuse causes: • A decrease in muscle size • A decrease in muscle tone • Physical therapy helps to reduce the effects of atrophy

• Three Major Types of Muscle Fibers • Fast fibers (white fibers) • Associated with eye muscles (fast contractions) • Intermediate fibers (pink fibers) • Slow fibers (red fibers) • Associated with leg muscles (slow contractions)

Slow fibers Smaller diameter, darker color due to myoglobin; fatigue resistant

LM x 170

Fast fibers Larger diameter, paler color; easily fatigued

LM x 170

a Note the difference in the size of slow muscle fibers (above) and fast muscle fibers (below).

• Features of Fast Fibers • Large in diameter • Large glycogen reserves • Relatively few mitochondria • Muscles contract using anaerobic metabolism • Fatigue easily • Can contract in 0.01 second or less after stimulation • Produce powerful contractions

• Features of Slow Fibers • Half the diameter of fast fibers • Take three times longer to contract after stimulation • Can contract for extended periods of time • Contain abundant myoglobin (creates the red color) • Muscles contract using aerobic metabolism • Have a large network of capillaries

• Features of Intermediate Fibers • Similar to fast fibers • Have low myoglobin content • Have high glycolytic enzyme concentration • Contract using anaerobic metabolism • Similar to slow fibers • Have lots of mitochondria • Have a greater capillary supply • Resist fatigue

• Distribution of Fast, Slow, and Intermediate Fibers • Fast fibers • High density associated with eye and hand muscles • Sprinters have a high concentration of fast fibers • Repeated intense workouts increase the fast fibers

• Distribution of Fast, Slow, and Intermediate Fibers (continued) • Slow and intermediate fibers • None are associated with the eyes or hands • Found in high density in the back and leg muscles • Marathon runners have a high amount • Training for long distance running increases the proportion of intermediate fibers

• Muscles can be classified based on shape or by the arrangement of the fibers • Parallel muscle fibers • Convergent muscle fibers • Pennate muscle fibers • Unipennate muscle fibers • Bipennate muscle fibers • Multipennate muscle fibers • Circular muscle fibers

• Parallel Muscle Fibers • Muscle fascicles are parallel to the longitudinal axis • Examples: biceps brachii and rectus abdominis

Parallel Muscles (h) a Parallel muscle b Parallel muscle with (d) (Biceps brachii muscle) tendinous bands (g) (Rectus abdominis muscle) (a) (b) (e) (c)

Fascicle (f)

Body (belly)

Cross section

• Convergent Muscle Fibers • Muscle fibers form a broad area but come together at a common point • Example: pectoralis major

Convergent Muscles (h) (d) d Convergent muscle (Pectoralis muscles) (g)

(a) (b) Tendon (e) (c) Base of muscle

(f)

Cross section

• Pennate Muscle Fibers (Unipennate) • Muscle fibers form an oblique angle to the tendon of the muscle • An example is unipennate • All the muscle fibers are on the same side of the tendon • Example: extensor digitorum

Pennate Muscles (h) e Unipennate (d) muscle (Extensor (g) digitorum muscle) (a) (b) (e) (c)

(f)

Extended tendon

• Pennate Muscle Fibers (Bipennate) • Muscle fibers form an oblique angle to the tendon of the muscle • An example is bipennate • Muscle fibers are on both sides of the tendon • Example: rectus femoris

Pennate Muscles (h) (d) f Bipennate (g) muscle (Rectus femoris (a) muscle) (b) (e) (c)

(f)

• Pennate Muscle Fibers (Multipennate) • Muscle fibers form an oblique angle to the tendon of the muscle • An example is multipennate • The tendon branches within the muscle • Example: deltoid muscle

Pennate Muscles (h) (d) g Multipennate muscle (g) (Deltoid muscle)

(a) (b) (e) (c)

(f) Tendons

Cross section

• Circular Muscle Fibers • Muscle fibers form concentric rings • Also known as sphincter muscles • Examples: orbicularis oris and orbicularis oculi

Circular Muscles (h) (d) h Circular muscle (g) (Orbicularis oris muscle)

(a)

(b) (e) Contracted (c)

(f)

Relaxed

• Origins and Insertions • Origin • Point of muscle attachment that remains stationary • Insertion • Point of muscle attachment that is movable • Actions • The function of the muscle upon contraction

• There are two methods of describing muscle actions • The first makes reference to the bone region the muscle is associated with • The biceps brachii muscle causes “flexion of the forearm” • The second makes reference to a specific joint the muscle is associated with • The biceps brachii muscle causes “flexion at the elbow”

• Muscles can be grouped according to their primary actions into four types • Prime movers (agonists) • Responsible for producing a particular movement • Antagonists • Actions oppose the action of the agonist • Synergists • Assist the prime mover in performing an action • Fixators • Agonist and antagonist muscles contracting at the same time to stabilize a joint

• Prime Movers example: • Biceps brachii – flexes the lower arm • Antagonists example: • Triceps brachii – extends the lower arm • Synergists example: • Latissimus dorsi and teres major – contract to move the arm medially over the posterior body • Fixators example: • Flexor and extensor muscles contract at the same time to stabilize an outstretched hand

• Most muscle names provide clues to their identification or location • Muscles can be named for: • Specific body regions or location • Shape of the muscle • Orientation of the muscle fibers • Specific or unusual features • Its origin and insertion points • Primary function • References to occupational or habitual action

• Examples of muscle names related to: • Specific body regions or locations • Brachialis: associated with the brachium of the arm • Tibialis anterior: associated with the anterior tibia • Shape of the muscle • Trapezius: trapezoid shape • Deltoid: triangular shape

• Examples of muscle names related to: • Orientation of the muscle fibers • Rectus femoris: straight muscle of the leg • External oblique: muscle on outside that is oriented with the fibers at an angle • Specific or unusual features • Biceps brachii: two origins • Teres major: long, big, rounded muscle

• Examples of muscle names related to: • Origin and insertion points • Sternocleidomastoid: points of attachment are sternum, clavicle, and mastoid process • Genioglossus: points of attachment are chin and tongue • Primary functions • Flexor carpi radialis: a muscle that is near the radius and flexes the wrist • Adductor longus: a long muscle that adducts the leg

• Examples of muscle names related to: • References to occupational or habitual actions • Buccinator (means “trumpet player”): the buccinator area moves when playing a trumpet • Sartorius: derived from the Latin term (sartor), which is in reference to “tailors.” Tailors used to cross their legs to form a table when sewing material

• Most of the time, upon contraction, a muscle causes action • This action is applied to a lever (a bone) • This lever moves on a fixed point called the fulcrum (joint) • The action of the lever is opposed by a force acting in the opposite direction

© 2015 Pearson Education, Inc. Levers and Pulleys: A Systems Design for Movement • There are three classes of levers • First class, second class, third class • First class • The fulcrum (joint) lies between the applied force and the resistance force (opposed force) • Example: tilting the head forward and backward

© 2015 Pearson Education, Inc. Figure 9.13 Levers and Pulleys (2 of 6) First-Class Lever In a first-class lever, the applied force and the resistance are on opposite sides of the fulcrum. This lever can change the amount of force transmitted to the resistance and alter the direction and speed of movement. There are very few first-class levers in the human body.

Resistance Fulcrum Applied force

R F AF

Movement completed

© 2015 Pearson Education, Inc. Levers and Pulleys: A Systems Design for Movement • Classes of Levers • Second class • The resistance is located between the applied force and the fulcrum (joint) • Example: standing on your tiptoes

© 2015 Pearson Education, Inc. Figure 9.13 Levers and Pulleys (3 of 6) Second-Class Lever In a second-class lever, the resistance lies between the applied force and the fulcrum. This arrangement magnifies force at the expense of distance and speed; the direction of movement remains unchanged. There are very few second-class AF levers in the body.

AF R F

Movement completed

© 2015 Pearson Education, Inc. Levers and Pulleys: A Systems Design for Movement • Classes of Levers • Third class • The force is applied between the resistance and fulcrum (joint) • Example: flexing the lower arm

© 2015 Pearson Education, Inc. Figure 9.13 Levers and Pulleys (4 of 6) Third-Class Lever In a third-class lever, which is the most common lever in the body, the applied force is between the resistance and the fulcrum. This arrangement increases speed and distance moved but requires a larger applied force.

AF R

Movement completed

• Sometimes, a tendon may loop around a bony projection • This bony projection could be called a pulley • Example: lateral malleolus and trochlea of the eye

The Lateral Malleolus as an Anatomical Pulley The lateral malleolus of the fibula is an example of an anatomical pulley. The tendon of insertion for the fibularis longus Fibularis muscle does not follow a direct path. longus Instead, it curves around the posterior margin of the lateral malleolus of the fibula. This redirection of the contractile force is essential to the normal function of the fibularis longus in producing plantar flexion at the ankle.

Lateral malleolus

Pulley

Plantar flexion of the foot

Quadriceps muscles The Patella as an Quadriceps tendon

Anatomical Patella

Pulley Patellar The patella is another ligament example of an anatomical pulley. The quadriceps femoris is a group of four muscles that form the anterior musculature of the thigh. These four muscles attach to the patella by the quadriceps tendon. The patella is, in turn, attached to the tibial tuberosity by the patellar ligament. The quadriceps femoris muscles produce extension at the knee by this two-link system. The quadriceps tendon pulls on the patella in one direction throughout the movement, but Extension the direction of force applied to the tibia by the patellar of the leg ligament changes constantly as the movement proceeds.

• Changes occur in muscles as we age • Skeletal muscle fibers become smaller in diameter • Due to a decrease in the number of myofibrils • Contain less glycogen reserves • Contain less myoglobin • All of the above results in a decrease in strength and endurance • Muscles fatigue rapidly

• Changes occur in muscles as we age (continued) • There is a decrease in myosatellite cells • There is an increase in fibrous connective tissue • Due to the process of fibrosis • The ability to recover from muscular injuries decreases