Chapter 9 The Muscular System Skeletal Muscle Tissue and Muscle Organization Lecture Presentation by Steven Bassett Southeast Community College
Introduction 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
Introduction 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
Introduction 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
Functions of Skeletal Muscles 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
Functions of Skeletal Muscles 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
Anatomy of Skeletal Muscles 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
Anatomy of Skeletal Muscles 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
Figure 9.1 Structural Organization of Skeletal Muscle Epimysium Muscle fascicle Endomysium Perimysium Nerve Muscle fibers Blood vessels SKELETAL MUSCLE (organ) Perimysium Muscle fiber Endomysium Epimysium Blood vessels and nerves MUSCLE FASCICLE (bundle of cells) Tendon Endomysium Mitochondria Sarcolemma Myofibril Capillary Endomysium Myosatellite cell Perimysium Axon Sarcoplasm Nucleus MUSCLE FIBER (cell)
Anatomy of Skeletal Muscles 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
Anatomy of Skeletal Muscles 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)
Figure 9.2 Skeletal Muscle Innervation Neuromuscular synapse Skeletal muscle fiber Axon Nerve a A neuromuscular synapse as seen on a muscle fiber of this fascicle LM x 230 SEM x 400 b Colorized SEM of a neuromuscular synapse
Anatomy of Skeletal Muscles 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
Anatomy of Skeletal Muscles 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
Figure 9.3ab The Formation and Structure of a Skeletal Muscle Fiber Myoblasts Muscle fibers develop through the fusion of mesodermal cells called myoblasts. a Development of a skeletal muscle fiber. Myosatellite cell Nuclei Immature muscle fiber b External appearance and histological view.
Anatomy of Skeletal Muscles 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
Anatomy of Skeletal Muscles Myofibrils and Myofilaments Myofibrils are made of myofilaments Actin Thin protein filaments Myosin Thick protein filaments
Figure 9.3b-d The Formation and Structure of a Skeletal Muscle Fiber b External appearance and histological view. Myofibril Sarcolemma Nuclei c The external organization of a muscle fiber. Sarcoplasm MUSCLE FIBER Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament d Internal organization of a muscle fiber. Note the relationships among myofibrils, sarcoplasmic reticulum, mitochondria, triads, and thick and thin filaments. Triad Sarcoplasmic reticulum T tubules
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
Anatomy of Skeletal Muscles 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
Anatomy of Skeletal Muscles Sarcomere Organization Each sarcomere consists of: Z line (Z disc) I band A band (overlapping A bands create striations) H band M line
Figure 9.4b Sarcomere Structure I band A band H band Z line Titin Zone of overlap M line Sarcomere Thin filament Thick filament I band A band H band Z line b A corresponding view of a sarcomere in a myofibril in the gastrocnemius muscle of the calf and a diagram showing the various components of this sarcomere Z line Zone of overlap M line Sarcomere TEM x 64,000
Anatomy of Skeletal Muscles 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
Figure 9.5 Levels of Functional Organization in a Skeletal Muscle Fiber 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 Z line M line H band Titin Z line
Anatomy of Skeletal Muscles 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
Figure 9.6ab Thin and Thick Filaments Actinin Z line Titin Sarcomere H band a The attachment of thin filaments to the Z line Troponin Active site Nebulin Tropomyosin G actin molecules Myofibril b The detailed structure of a thin filament showing the organization of G actin, troponin, and tropomyosin F actin strand Z line M line
Anatomy of Skeletal Muscles 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
Figure 9.6cd Thin and Thick Filaments Sarcomere H band Myofibril Z line M line Titin c The structure of thick filaments M line Myosin head Myosin tail Hinge d A single myosin molecule detailing the structure and movement of the myosin head after cross-bridge binding occurs
Muscle Contraction 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
Muscle Contraction 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
Figure 9.7 Sliding Filament Theory (1 of 11) Resting Sarcomere A resting sarcomere showing the locations of the I band, A band, H band, M, and Z lines. Contracted Sarcomere After repeated cycles of bind, pivot, detach, and reactivate 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
Muscle Contraction 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
Figure 9.8 The Neuromuscular Synapse Arriving action potential Synaptic cleft ACh receptor site Sarcolemma of motor end plate Synaptic vesicles ACh Motor neuron AChE molecules Junctional fold Glial cell Axon Path of action potential b Detailed view of a terminal, synaptic cleft, and motor end plate. See also Figure 9.2. Synaptic terminal Myofibril Muscle Fiber Motor end plate Myofibril Sarcolemma a A diagrammatic view of a neuromuscular synapse. Mitochondrion
Muscle Contraction 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
Figure 9.7 Sliding Filament Theory (2 of 11) 1 Contraction Cycle Begins The contraction cycle involves a series of interrelated steps. The cycle begins with electrical events in the sarcolemma that trigger the release of calcium from the terminal cisternae of the sarcoplasmic reticulum (SR). These calcium ions enter the zone of overlap. Ca 2+ Actin 2 Active-Site Exposure Calcium ions bind to troponin in the troponin tropomyosin complex. The tropomyosin molecule then rolls away from the active sites on the actin molecules of the thin filaments. Tropomyosin Active site Ca 2+
Muscle Contraction 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
Figure 9.7 Sliding Filament Theory (3 of 11) 3 Cross-Bridge Formation Once the active sites are exposed, the myosin heads of adjacent thick filaments bind to them, forming cross-bridges. Cross-bridge Myosin head 4 Myosin Head Pivoting After cross-bridge formation, energy is released as the myosin heads pivot toward the M line.
Muscle Contraction 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
Figure 9.7 Sliding Filament Theory (4 of 11) 5 Cross-Bridge Detachment ATP then binds to the myosin heads, breaking the cross-bridges between the myosin heads and the actin molecules. ATP 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 Muscle Contraction: A Summary (continued) Upon contraction: I bands get smaller H bands get smaller Z lines get closer together
Figure 9.7 Sliding Filament Theory
Figure 9.9 The Events in Muscle Contraction STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION Synaptic terminal Motor end plate T tubule Sarcolemma 1 3 ACh released, binding to receptors Sarcoplasmic reticulum releases Ca 2+ Ca 2+ 2 Action potential reaches T tubule 7 6 Sarcoplasmic reticulum recaptures Ca 2+ ACh removed by AChE 4 Active-site exposure, cross-bridge formation Actin Myosin 8 Active sites covered, no cross-bridge interaction 5 Contraction begins 9 10 Contraction ends Relaxation occurs, passive return to resting length
Motor Units and Muscle Control 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
Figure 9.10 The Arrangement of Motor Units in a Skeletal Muscle Axons of motor neurons Motor nerve Muscle fibers
Motor Units and Muscle Control 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
Motor Units and Muscle Control 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
Motor Units and Muscle Control 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
Motor Units and Muscle Control 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
Types of Skeletal Muscle Fibers 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)
Figure 9.11a Types of Skeletal Muscle Fibers 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).
Types of Skeletal Muscle Fibers 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
Types of Skeletal Muscle Fibers 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
Types of Skeletal Muscle Fibers 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
Table 9.1 Properties of Skeletal Muscle Fiber Types
Types of Skeletal Muscle Fibers 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
Types of Skeletal Muscle 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
Organization of Skeletal Muscle 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
Organization of Skeletal Muscle Fibers Parallel Muscle Fibers Muscle fascicles are parallel to the longitudinal axis Examples: biceps brachii and rectus abdominis
Figure 9.12ab Skeletal Muscle Fiber Organization (h) (d) (g) (a) (b) (e) (c) Parallel Muscles a Parallel muscle (Biceps brachii muscle) b Parallel muscle with tendinous bands (Rectus abdominis muscle) (f) Fascicle Body (belly) Cross section
Organization of Skeletal Muscle Fibers Convergent Muscle Fibers Muscle fibers form a broad area but come together at a common point Example: pectoralis major
Figure 9.12d Skeletal Muscle Fiber Organization (h) (d) (g) Convergent Muscles d Convergent muscle (Pectoralis muscles) (a) (b) (e) (c) Base of muscle Tendon (f) Cross section
Organization of Skeletal Muscle Fibers 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
Figure 9.12e Skeletal Muscle Fiber Organization (h) (d) (g) Pennate Muscles e Unipennate muscle (Extensor digitorum muscle) (a) (b) (e) (c) (f) Extended tendon
Organization of Skeletal Muscle Fibers 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
Figure 9.12f Skeletal Muscle Fiber Organization (h) (d) (g) (a) Pennate Muscles f Bipennate muscle (Rectus femoris muscle) (b) (e) (c) (f)
Organization of Skeletal Muscle Fibers 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
Figure 9.12g Skeletal Muscle Fiber Organization (h) (d) (g) Pennate Muscles g Multipennate muscle (Deltoid muscle) (a) (b) (e) (c) (f) Tendons Cross section
Organization of Skeletal Muscle Fibers Circular Muscle Fibers Muscle fibers form concentric rings Also known as sphincter muscles Examples: orbicularis oris and orbicularis oculi
Figure 9.12h Skeletal Muscle Fiber Organization (h) (d) (g) Circular Muscles h Circular muscle (Orbicularis oris muscle) (a) (b) (e) (c) Contracted (f) Relaxed
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Muscle Terminology 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
Levers and Pulleys: A Systems Design for Movement 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
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
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. R F AF Resistance Fulcrum Applied force R F AF Movement completed
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
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. R F R AF F Movement completed
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
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. R F AF R F Movement completed
Levers and Pulleys: A Systems Design for Movement 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
Figure 9.13 Levers and Pulleys (5 of 6) Fibularis longus 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 muscle does not follow a direct path. 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
Figure 9.13 Levers and Pulleys (6 of 6) Pulley The Patella as an Anatomical Pulley The patella is another 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 the direction of force applied to the tibia by the patellar ligament changes constantly as the movement proceeds. Quadriceps muscles Quadriceps tendon Patella Patellar ligament Extension of the leg
Aging and the Muscular System 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
Aging and the Muscular System 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