KN 230 Exam IV (Chapters 10, 11, 14)

Functions of skeletal muscle

• Move the body

• Maintain posture

• Protect and support

• Regulate elimination of materials

• Produce heat

Sphincter

Muscle that encircles a duct, tube, or orifice such that its contraction constricts the lumen or orifice and restricts movement through the tube

Orifice

Aperture or opening

Characteristics of skeletal muscle

• Excitability

• Conductivity

• Contractility

• Extensibility

• Elasticity

Excitability

The ability of a cell to respond to a stimulus

Conductivity

The ability to propagate an action potential along the plasma membrane

Contractility

• When contractile proteins within skeletal muscle cells slide past one another

• What enables muscle cells to cause body movement and to perform the other functions of muscles

Extensibility

The lengthening of a muscle cell

Elasticity

• The ability of a muscle cell to return to its original length following either shortening or lengthening of the muscle

• Dependent upon the release of tension in the springlike connectin protein associated with contractile proteins

Skeletal muscle fibers

• The term used to refer to skeletal muscle cells

• Also called muscle fibers or myofibers

Skeletal muscle

• Location: Generally attached to the skeleton (or to subcutaneous tissue); voluntary movement of skeleton, skin, or other select body components

• Associated connective tissue: Epimysium, perimysium, endomysium

• Cell appearance and shape: Long, cylindrical fibers with multiple peripheral nuclei; striated; T-tubules; large diameter (10-500 μm); long length (100 μm- 30 cm)

• Calcium source: Well-developed sarcoplasmic reticulum

• Contractile unit and Ca²⁺ binding: Sarcomere; Ca²⁺ binds to troponin

• Stimulation: Nervous control is voluntary (regulated by somatic nervous system at neuromuscular junction); excitatory

• Rate of response and primary energy source: Slow oxidative (SO) [Slow; aerobic production of ATP]; Fast oxidative (FO) [Rapid and powerful; aerobic production of ATP]; Fast glycolytic (FG) [Rapid and powerful; glycolytic production of ATP]

• Tissue repair ability: Limited

Fascicle

Bundle of muscle fibers or axons

Connective tissue layers of muscles

• Layers that provide protection and support, a means of attachment of the muscle to the skeleton or other structures within the body, and sites for distribution of blood vessels and nerves

• Collectively extend past the muscle fibers to form either a tendon or an aponeurosis

  • Epimysium

  • Perimysium

  • Endomysium

Epimysium

• A layer of dense irregular connective tissue that surrounds the whole skeletal muscle

• Ensheathes the entire skeletal muscle to protect and support it like a tough leather sleeve

Perimysium

• A layer of dense irregular connective tissue around each fascicle

• Provides protection and support to each bundle of muscle fibers

Endomysium

• A layer of areolar connective tissue that surrounds each muscle fiber

• More delicate coverings that function to electrically insulate the muscle fibers

Tendon

A thick, cordlike structure composed of dense regular connective tissue that connects muscle to bone

Aponeurosis

A thin, flattened sheet of dense regular connective tissue

Deep fascia

• An additional, expansive sheet of dense irregular connective tissue that is external to the epimysium, but often blends with it

• Separates individual muscles

• Binds together muscles with similar functions

• Contains nerves, blood vessels, and lymph vessels

• Fills spaces between muscles

• Also called visceral fascia or muscular fascia

Superficial fascia

• The layer that the deep fascia is internal to or deep to

• Composed of areolar connective tissue and adipose connective tissue that separates muscle from skin

• Also called the subcutaneous layer

Somatic motor neurons

• Neurons that extend from the brain and spinal cord to skeletal muscle fibers

• Each has an axon that branches extensively at its terminal end

Axon

• The process of a neuron that propagates nerve signals away from the cell body

• Extends through all three connective tissue layers to almost make contact with an individual muscle fiber

Voluntary muscle

• A muscle whose fibers can be consciously controlled by the nervous system

• Ex: Skeletal muscle

Sarcoplasm

The cytoplasm of a muscle cell

Myoblast

• An undifferentiated muscle cell with the potential of becoming a muscle fiber

• Groups of the embryonic muscle cells fuse to form single skeletal muscle fibers during development

• Each of their nuclei contributes to the eventual total number of nuclei in the muscle fiber (multinucleated)

Multinucleated

Having numerous nuclei

Satellite cells

• Adult stem cells

• Myoblasts that do not fuse with muscle fibers during development and instead remain in adult skeletal muscle tissue

• Some may be stimulated to differentiate and then fuse with a damaged skeletal muscle fiber to assist to a limited extent in its repair and regeneration

Sarcolemma

The plasma membrane of a skeletal muscle fiber

T-tubules

• Deep invaginations of the sarcolemma that extend into the skeletal muscle fiber as a network of narrow, membranous tubules to the sarcoplasmic reticulum (ER of the muscle)

• Also called transverse tubules

Myofibril

• Bundles of myofilaments that are enclosed in portions of the membranous sarcoplasmic reticulum within skeletal and cardiac muscle cells

• Composes approximately 80% of the volume of a skeletal muscle fiber

• Each extends the entire length of the skeletal muscle fiber and is about 1 to 2 micrometers in diameter

Sarcoplasmic reticulum (SR)

• An internal membrane complex found in muscle cells that is similar to the membranous smooth endoplasmic reticulum of other cells

• Stores calcium ions required for contraction

• Segments of it fit around each myofibril like a sleeve of membrane netting

Terminal cisternae

• Blind sacs at either end of individual sections of the sarcoplasmic reticulum

• Immediately adjacent to each T-tubule

Triad

A structure formed by two terminal cisternae and a centrally located T-tubule

Voltage-sensitive Ca²⁺ channels

Dihydropyridine receptors that are responsive to electrical signals

Ca²⁺ release channels

Ryanodine receptors at the terminal cisternae membrane of the sarcoplasmic reticulum

Myofilaments

• Contractile protein filaments that are bundled within myofibrils in both skeletal and cardiac muscle cells

• Not as long as a myrofibril and takes many successive units to extend the entire length of the myofibril

• Two types: Thick filaments and thin filaments

Thick filaments

• Type of myofilament that is assembled from bundles of 200 to 500 myosin protein molecules

• Also called thick myofilaments

Thin filaments

• Type of myofilament that is primarily composed of two strands of actin protein twisted around each other to form a helical shape

• Approximately half of the diameter of thick filaments (about 5 to 6 nanometers)

• Also called thin myofilaments

Troponin-tropomyosin complex

The formation of tropomyosin and troponin

Tropomyosin

• A short, thin, twisted filament that is a stringlike protein

• Regulatory protein associated with thin filaments

• Consecutive molecules cover small regions of the actin strands, including the myosin binding sites in a noncontracting muscle

Troponin

• A globular, or ball-like, protein attached to tropomyosin

• Regulatory protein associated with thin filaments

• Contains the binding site for Ca²⁺

Sarcomere

• The functional unit of skeletal muscle and cardiac muscle

• Each is composed of overlapping thick filaments and thin filaments

Z discs

• Delineate each sarcomere at both ends

• Composed of specialized proteins that are positioned perpendicular to the myofilaments and serve as anchors for the thin filaments

I band

• Region of the sarcomere that contains only thin filaments and is bisected by the Z disc

• Appears light when viewed with a microscope

• The bands disappear when the thin filaments are pulled parallel along the thick filaments at maximal muscle shortening

A band

• The central region of a sarcomere that contains the entire thick filament

• Thin filaments partially overlap the thick filament on each end of the band

• Appears dark when viewed with a microscope

• Does not change in length during muscle contraction

H zone

• The most central portion of the A band in a resting sarcomere

• Does not have thin filament overlap (only thick filaments are present)

• Disappears when the thin filaments are pulled past thick filaments during maximal muscle shortening

• Also called the H band

M line

• A thin transverse protein meshwork structure in the center of the H zone

• Serves as an attachment site for the thick filaments are keeps the thick filaments aligned during contraction and relaxation events

Striations

• The repeating light and dark bands of the overlapping myofilaments that form unique striped patterns within a skeletal muscle fiber

• Due to both the size and density differences between thin filaments and thick filaments

Anisotropic

A band whose area appears dark when viewed under polarized light

Isotropic

A band whose area appears light when viewed under polarized light

Connectin

• A cable-like protein that extends from the Z discs to the M line through the core of each thick filament

• Stabilized the position of the thick filament and maintains thick filament alignment within a sarcomere

• Portions of its molecules are coiled and springlike so that during sarcomere shortening, they are compressed to produce passive tension

• Contributes to skeletal muscle fiber elasticity

• Also called titin

Dystrophin

• A protein that is part of a protein complex that anchors myofibrils that are adjacent to the sarcolemma to proteins within the sarcolemma

• Extend to the connective tissue of the endomysium that encloses the muscle fiber

• Links internal myofilament proteins of a muscle fiber to proteins within the plasma membrane, which are linked to extracellular proteins

Nebulin

• An actin-binding protein of the sarcomere that is anchored in the Z-disc and extends the length of the thin filament

• Helps determine thin filament length during sarcomere assembly, has structural and mechanical properties, and plays a role during skeletal muscle contraction in cross-bridge cycling

Glycogen

• Polysaccharide formed from glucose monomers

• Stored in granules called glycosomes in muscle fibers for use as an immediately fuel molecule

Myoglobin

• A molecule that binds oxygen when the muscle is at rest and releases it for use during muscular contraction

• A reddish, globular protein that is somewhat similar to hemoglobin

Creatine phosphate

• A type of molecule that is contained in skeletal muscle fibers

• Provides muscle fibers with a very rapid means of supplying ATP

• Has a high-energy chemical bond between creatine and P_i

• Present in tissues with both large and fluctuating energy needs

Motor unit

A single motor neuron and the skeletal muscle fibers it controls

Neuromuscular junction

• The specific location, usually in the mid-region of the skeletal muscle fiber, where it is innervated by a motor neuron

• Has the following parts: Synaptic knob, motor end plate, and synaptic cleft

Synaptic knob

• An expanded tip of an axon of a motor neuron

• Is enlarged and flat to cover a relatively large surface area of the sarcolemma

• Houses numerous synaptic vesicles filled with molecules of the neurotransmitter acetylcholine

• Ca²⁺ pumps and voltage-gated Ca²⁺ channels are embedded within its plasma membrane

Synaptic vesicle

A package of membrane enclosing neurotransmitter molecules in the synaptic knob

Acetylcholine (ACh)

• Neurotransmitter produced by the central and peripheral nervous systems

• Stimulates skeletal muscle

Motor end plate

• A specialized region of the sarcolemma of a skeletal muscle fiber

• Has numerous folds and indentations (junction folds) to increase the membrane surface area covered by the synaptic knob

• Has a vast numbers of ACh receptors

ACh receptors

Plasma membrane protein channels at the motor end plate that bind to ACh to allow Na⁺ entry into the muscle fiber and K⁺ to exit

Synaptic cleft

An extremely narrow (30 nanometers), fluid-filled space separating the synaptic knob and the motor end plate

Acetylcholinesterase (AChE)

An enzyme that resides within the synaptic cleft and quickly breaks down ACh molecules following their release into the synaptic cleft

Resting membrane potential (RMP)

• The electrical difference at the plasma membrane when a cell is at rest

• About -90 mV in skeletal muscle fibers

Muscle fiber excitation

• The first physiologic event of skeletal muscle contraction

• An event that occurs at the neuromuscular junction and results in the release of ACh and its subsequent binding to ACh receptors

• Steps

  • Calcium entry at synaptic knob

  • Release of ACh from synaptic knob

  • Binding of ACh at motor end plate

Nerve signal

• An electrical signal that is propagated down an axon

• Sent along a motor neuron of the somatic nervous system

• Triggers the opening of voltage-gated Ca²⁺ channels within the synaptic knob plasma membrane

Excitation-contraction coupling

• The second physiologic event of muscle contraction

• An event that involves the sarcolemma, T-tubules, and sarcoplasmic reticulum of a skeletal muscle fiber

• Links the events of skeletal muscle stimulation (excitation) to the events of contraction

• Steps

  • Development of an end-plate potential at the motor end plate

  • Initiation and propagation of an electrical signal along the sarcolemma and T-tubules

  • Release of calcium from the sarcoplasmic reticulum

End-plate potential (EPP)

The minimum voltage change (or threshold) in the motor end plate that can trigger opening of voltage-gated channels in the sarcolemma to initiate an action potential

Action potential

Self-propagating change in membrane potential occurring in excitable cells (e.g., neurons, muscle cells)

Depolarization

Change in membrane potential or voltage to a more positive value

Repolarization

Change in membrane potential from a depolarized value back to the resting value

Absolute refractory period

Time period when an excitable cell cannot be restimulated to respond

Calcium binding to troponin

What initiates crossbridge cycling

Crossbridge cycling

A four step process that is repeated

• Crossbridge formation (attaching of myosin head to actin)

• Power stroke (pulling thin filament by movement of myosin head)

• Release of myosin head from actin

• Resetting of myosin head

Sliding filament theory

A description of the repetitive movement of thin filaments sliding past thick filaments

Glycolysis

• A metabolic pathway that involves the breakdown of glucose into two pyruvate molecules, producing a net of 2 ATP molecules

• Occurs within the cytosol

• Can function in the presence of oxygen, but oxygen is not required

Aerobic cellular respiration

• The breakdown of nutrients (e.g., pyruvate, fatty acids) to produce ATP, water, and carbon dioxide

• Occurs within mitochondria and requires oxygen

• Involves three stages: Intermediate step, citric acid cycle, electron transport system

Lactate formation

• Occurs from pyruvate under conditions of low oxygen availability (ex: during intense exercise when skeletal muscle’s oxygen demands for aerobic cellular respiration cannot be met)

• Pyruvate molecules are converted to lactate molecules, an enzymatic reaction catalyzed by lactate dehydrogenase

• Lactate can either enter a mitochondrion within a skeletal muscle fiber, where it is converted back to pyruvate and oxidized into carbon dioxide through aerobic cellular respiration, or leave the skeletal muscle fiber and enter the blood

Lactic acid cycle

• The cycling of lactate to the liver, where it is converted to glucose, and the subsequent blood transport of glucose from the liver to the muscle

• Also called the Cori cycle

Oxygen debt

• The amount of additional oxygen that is consumed following exercise to restore pre-exercise conditions

• This additional oxygen is required primarily by skeletal muscle fibers and liver cells

Classification of skeletal muscle fibers

• The type of contraction generated

• The primary means used for supplying ATP

Power

Related to the diameter of a muscle fiber

• Large muscle fibers have a larger number of myofibrils in parallel, allowing them to produce a more powerful contraction

Speed

Based on whether the skeletal muscle fiber expresses the relatively slow or fast genetic variant of myosin ATPase, the enzyme that splits ATP

Fast-twitch fibers

• Skeletal muscle fibers with a fast variant of myosin ATPase

• Typically produce a strong contraction, initiate a contraction more quickly following stimulation, and produce a contraction of shorter duration

• Exhibits more power and speed in comparison to slow-twitch fibers

Slow-twitch fibers

• Skeletal muscle fibers with the slow variant of myosin ATPase

• Exhibits less power and speed in comparison to fast-twitch fibers

Oxidative fibers

• Skeletal muscle fibers that specialize in providing ATP through aerobic cellular respiration and have several features that support these processes (extensive capillary network, large numbers of mitochondria, large supply of the red pigment myoglobin)

• Sometimes called red fibers due to the presence of both mitochondria and myoglobin giving these skeletal muscle fibers a red appearance

Fatigue-resistant

Oxidative fibers that continue contracting for extended periods of time without tiring due to the higher levels of ATP generated providing energy

Glycolytic fibers

• Skeletal muscle fibers that specialize in providing ATP more rapidly through glycolysis

• Generally have fewer structures needed for aerobic cellular respiration (less extensive capillary networks, fewer mitochondria, and smaller aounts of myoglobin)

• Sometimes called white fibers due to the relatively small amount of mitochondria and myoglobin giving these skeletal muscle fibers a white appearance

Fatigable

Glycolytic fibers that generally tire easily after a short time of sustained muscular activity

Subtypes of skeletal muscle fibers

• Slow oxidative (SO) fibers

• Fast oxidative (FO) fibers

• Fast glycolytic (FG) fibers

Slow oxidative (SO) fibers

• Type I fibers

• Primary fiber function: Endurance (e.g., maintaining posture, marathon running)

• Muscles with a large abundance of fiber type: Trunk and lower limbs

• Power of contraction: Relatively less powerful

  • Fiber diameter: Smallest

  • Relative number of myofibrils per muscle fiber: Smallest

• Speed (contraction velocity): Slow

  • Myosin ATPase: Slow

  • Speed of propagation and Ca²⁺ regulation: Slow

• Capacity to make ATP: High

• Concentration of capillaries: Extensive

• Number of mitochondria: Many

• Amount of myoglobin: Large

• Color fibers: Red

• Resistance to fatigue: Highest

Fast oxidative (FO) fibers

• Type IIa fibers

• Primary fiber function: Medium duration, moderate movement (e.g., walking, biking)

• Muscles with a large abundance of fiber type: Lower limb muscles

• Power of contraction: Relatively more powerful

  • Fiber diameter: Intermediate

  • Relative number of myofibrils per muscle fiber: Intermediate

• Speed (contraction velocity): Fast

  • Myosin ATPase: Fast

  • Speed of propagation and Ca²⁺ regulation: Fast

• Capacity to make ATP: Moderate

• Concentration of capillaries: Moderately extensive

• Number of mitochondria: Many

• Amount of myoglobin: Medium

• Color fibers: Lighter red

• Resistance to fatigue: High

Fast glycolytic (FG) fibers

• Type IIx fibers

• Primary fiber function: Short duration, intense movement (e.g., sprinting, lifting weights)

• Muscles with a large abundance of fiber type: Upper limb muscles

• Power of contraction: Relatively more powerful

  • Fiber diameter: Largest

  • Relative number of myofibrils per muscle fiber: Largest

• Speed (contraction velocity): Fast

  • Myosin ATPase: Fast

  • Speed of propagation and Ca²⁺ regulation: Fast

• Capacity to make ATP: Limited

• Concentration of capillaries: Sparse

• Number of mitochondria: Few

• Amount of myoglobin: Small

• Color fibers: White (pale)

• Resistance to fatigue: Low

Muscle tension

The force generated when a skeletal muscle is stimulated to contract

Myogram

A graphic recording of changes in muscle tension when it is stimulated

Muscle twitch

• A single, brief contraction period and then a relaxation period of a skeletal muscle in response to a single stimulation

• Three periods: A latent period, a contraction period, and a relaxation period

Threshold

The minimum voltage needed to stimulate the skeletal muscle to generate a twitch

Latent period

• The phase of a muscle twitch that occurs after the stimulus is applied and before the contraction of the skeletal muscle fiber begins

• No change in fiber length during this period due to the time necessary for all of the events in excitation-contraction coupling, Ca²⁺ release from the sarcoplasmic reticulum into the cytosol, and the beginning of tension generation within the skeletal muscle fiber

• Also called the lag period