Muscle Physiology Quiz

Nebulin

• Long, non-elastic protein
• Wraps around the entire length of each thin filament

Actin Binding

• Actin's active sites (myosin-binding sites) are blocked by tropomyosin molecules at rest

Tropomyosin and Troponin

• Tropomyosin is held in place by troponin at rest
• Troponin is a globular molecule with three subunits:
  • Subunit 1 binds to tropomyosin
  • Subunit 2 binds to G-actin
  • Subunit 3 binds to calcium ions

Sliding Filament Mechanism

• Z lines (Z disks) move closer together during contraction
• The H zones and I bands get smaller during contraction
• The width of the A band remains constant during contraction
• The zone of overlap gets larger during contraction (more overlap between thin & thick filaments)

Contraction Initiation

• Muscle fiber contraction starts when calcium ions are released from the terminal cistern of the sarcoplasmic reticulum
• Thick filaments pivot toward the M line when cross-bridges bind to the active site of actin thin filaments

Active Site Exposure

• Active sites on the actin thin filament become available (exposed) when calcium ions bind to troponin subunit
• Troponin moves tropomyosin away from the myosin-binding sites of the thin filament once calcium ions bind

Contraction Cycle

• Step 1: Myosin head hydrolyzes ATP and becomes energized and oriented
• Step 2: Myosin head binds to actin, forming the cross-bridge
• Step 3: Myosin head pivots, pulling the thin filament past the thick filament toward the center of the sarcomere (power stroke)
• Step 4: Another ATP binds to the cross-bridge, causing the myosin head to detach from the actin. Then, the cycle repeats

Cross Bridges

• Are portions of the thick filament (myosin head)
• Act as ATPase enzymes during the contraction cycle of muscle
• Generate force

Cross-Bridge Formation Requirements

• ATP must be broken down by the enzyme ATPase
• Calcium ions must be present in the sarcoplasm
• Myosin binding sites (actin active site) must be exposed

Calcium Concentration in Muscle Cells

• When a muscle fiber is relaxed, the concentration of Ca2+ in the sarcoplasm is very low, but the concentration of Ca2+ in the sarcoplasmic reticulum (SR) is very high.
• The concentration of Ca2+ in the sarcoplasm rises rapidly during muscle contraction.

Energy Sources for Muscle Contraction

• Creatine phosphate acts as an energy reserve in muscle tissue
• The combined amounts of creatine phosphate and ATP provide enough energy for the muscle to contract maximally for approximately 15 seconds
• Anaerobic glycolysis supplies enough ATP for muscles for two minutes of maximal activity
• Aerobic respiration supplies enough ATP for muscles during periods of rest or moderate exercises (running, jogging, swimming, and walking)
• At peak levels of exertion, mitochondria can provide only one-third (33%) of the ATP needed. The remainder comes from glycolysis.

Muscle Fatigue

• Inadequate release of calcium ions from the SR
• Depletion of creatine phosphate
• Insufficient oxygen
• Depletion of glycogen and other nutrients
• Buildup of lactic acid
• Failure of action potentials in the motor neurons to release enough Ach
• Muscle fatigue occurs when energy reserves in a muscle are exhausted and lactic acid levels increase

Motor Unit

• A somatic motor neuron and all the skeletal muscle fibers it stimulates

Twitch

• A brief contraction of all muscle fibers in a motor unit in response to a single action potential moving down the somatic motor neuron

Myogram

• A record of muscle contraction

Myogram of Twitch Contraction

• Latent period
• Contraction period
• Relaxation period

Refractory Period

• A period that the muscle fiber loses its excitability and cannot respond to a second stimulus
• Additional oxygen is required to metabolize the lactic acid produced during exercise during the refractory period

Treppe

• Increased strength of a contraction that occurs when a second stimulus arrives after the muscle fiber has relaxed completely following the previous stimulus

Wave Summation

• Increased strength of a contraction that occurs when a second stimulus arrives before the muscle fiber has relaxed completely

Tetanus

• Complete tetanus (fused tetanus): a muscle that is stimulated so frequently that the relaxation phase is completely eliminated
• Incomplete tetanus (unfused tetanus): a muscle producing peak tension with rapid cycles of contraction and relaxation
• Tetanus is due to a toxin that makes the muscle cell membrane more permeable to calcium ions (Ca2+), causing powerful tetanic contraction.

Recruitment

• Increase in muscle tension that is produced by increasing the number of active motor units

Muscle Tone

• Small amount of tension in the muscle at rest due to weak, involuntary contractions of its motor units

Hypotonia

• Decreased or lost muscle tone

Flaccid

• A state of limpness in which muscle tone is lost

Hypertonia

• Increased muscle tone

Spasticity

• Increased muscle tone associated with an increase in tendon reflexes

Rigidity

• Increased muscle tone in which tendon reflexes are not affected (tetanus)

Isometric Contraction

• Type of contraction in which the muscle fibers produce tension but do not shorten nor lengthen (object cannot be moved)

Isotonic Contraction

• Type of contraction in which the muscle fibers produce tension while muscle length is changed

Concentric Isotonic Contraction

• Muscle fibers shorten (object is lifted)

Eccentric Isotonic Contraction

• Muscle fibers lengthen (object is dropped)

Cori Cycle

• Cycle in the liver in which lactic acid is converted to pyruvic acid and glucose

Fast Fibers

• Fast glycolytic fibers (FG)
  • Large diameter
  • Produces powerful contraction
  • Less resistant to fatigue
  • Glycolytic fibers
    • Have fewer mitochondria and large glycogen reserves
    • Use anaerobic respiration
  • Low concentration of myoglobin (White meat)

Slow Fibers

• Slow oxidative fiber (SO)
  • Small diameter
  • Most resistant to fatigue
  • Oxidative fibers
    • Have many mitochondria
    • Use aerobic respiration
  • High concentration of myoglobin (Red meat)
  • Produce slow contraction

Aerobic Endurance Activities

• Most of the muscle's energy is produced in mitochondria
• Krebs cycle is a main source of ATP (36 ATPs)
• No oxygen debt
• Oxygen is required
• Dependent on slow oxidative fibers
• Fatigue resistant (sustained contraction for hours)

Anaerobic Endurance Activities

• Most of the muscle’s energy is produced in the cytoplasm by glycolysis
• Glycolysis is a main source of ATP (2 ATPs)
• Oxygen debts are common
• Dependent on fast glycolytic fibers
• Oxygen is NOT required
• Muscle fatigue quickly (not able to have sustained contraction for hours)

Rigor Mortis

• A condition in which muscles are in a state of rigidity 3-4 hours after death and last about 24 hours
• The mechanisms of rigor mortis is due to calcium ions leaking out of the SR (allowing myosin heads to bind to actin) and muscle fibers running out of ATP (causing the cross-bridges cannot detach from actin)
• Rigor mortis disappears as proteolytic enzymes from lysosomes digest the cross-bridges

Post-Exercise Oxygen Consumption

• After prolonged strenuous exercise has stopped, heavy breathing will often continue for several minutes in order to provide the oxygen needed to:
  • Convert the lactic acid produced during exercise back into glycogen
  • Resynthesize creatine phosphate
  • Replace oxygen displaced from muscle myoglobin

Types of Smooth Muscle

• Single-unit visceral smooth muscle
• Multi-unit smooth muscle

Single-Unit Smooth Muscle

• Action potentials are initiated in response to neurotransmitters, hormones, or an auto-rhythmic signal
• The action potential spreads throughout the tissue by moving through gap junctions that connect all the muscle cells together within the tissue.
• Single-unit smooth muscle is found in the skin and in tubular arrangements that form part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder

Multi-Unit Smooth Muscle

• Muscle cells have few gap junctions with neighboring cells and thus must be excited by their own motor neuron terminal
• Found in the walls of large arteries, in airways to the lungs, in the arrector pili muscles that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye.

Smooth Muscle Cell Characteristics

• Uninucleate
• Lack sarcomeres
• Thin filaments are attached to dense bodies
• Transmitting the contractile forces from cell to cells throughout the muscle tissue
• Involuntary control (pacesetter cells)

Smooth Muscle Functions

• Altering the diameter of the respiratory passageways
• Moving food materials along the digestive tract
• Moving sperms in the male reproductive tract and oocytes in the uterine tract
• Expelling the fetus by contraction of the wall of the uterus
• Acting as a sphincter

Smooth Muscle Contraction

• Smooth muscle contracts when calcium ions Ca2+ interact with calmodulin, which activates the enzyme myosin light chain kinase, enabling myosin heads to attach to actin.

Plasticity

• Ability of smooth muscle to function over a wide range of lengths

Stress-Relaxation Response

• Ability of single-unit smooth muscle fibers to maintain a constant tension in the muscle despite changes in length

Muscle Growth

• Individuals lifting weights build larger muscles because skeletal muscles increase the number of myofibrils but not the number of cells

Mitochondrial Activity

• Mitochondrial activity in skeletal muscle cells is usually efficient, but can have limited ATP production if there is limited O2 availability

Myasthenia Gravis

• Autoimmune disorder that targets the ACh receptors at the NMJ and ultimately reduces the number of available receptors

Muscle Structure

• Sarcolemma is the muscle cell membrane.
• Sarcoplasm is the cytoplasm of a muscle cell and stores glycogen and myoglobin.
• Sarcoplasmic reticulum (SR) is a network of channels around each myofibril and stores calcium ions (Ca2+).
• Transverse tubules (T tubules) are tunnels from the surface to the center of a muscle fiber.
• Terminal cisterns are expanded ends of the SR, forming a triad with a T tubule.
• Myofibrils are the contractile organelles of muscle cells.
• Microfilaments are smaller proteins within myofibrils, including thin and thick filaments.
• Sarcomere is the basic functional unit of a myofibril, containing both thin and thick filaments.
• Z line is a plate-shaped region separating sarcomeres.
• M line links thick and thin filaments laterally.
• A band is the darker middle area of a sarcomere, containing both thick and thin filaments.
• I band is the lighter area containing only thin filaments.
• H zone is the narrow central area of the A band containing only thick filaments.
• Zone of overlap is the dark region where thin and thick filaments overlap.

Muscle Proteins

• Myosin is the main protein in thick filaments and functions as a motor protein.
• Actin is the main protein in thin filaments, arranged in a helix.
• Tropomyosin and troponin regulate muscle contraction.
• Titin is a large protein extending from the Z disk to the M line, contributing to elasticity.
• Myomesin is part of the M line.
• Nebulin is a non-elastic protein wrapped around thin filaments.
• Dystrophin reinforces the sarcolemma and transmits tension to tendons.

Muscle Contraction

• Active sites on actin are blocked by tropomyosin at rest.
• Calcium ions bind to troponin, causing it to move tropomyosin away from active sites.
• Myosin head hydrolyzes ATP and becomes energized, binding to actin.
• Power stroke: the myosin head pivots, pulling thin filaments toward the center of the sarcomere.
• Another ATP binds to the myosin head, causing it to detach from actin, and the cycle repeats.

Muscle Fiber Types

• Fast fibers (FG): Large diameter, produce powerful contractions, fatigue quickly, use anaerobic respiration, low myoglobin (white meat).
• Slow fibers (SO): Small diameter, highly resistant to fatigue, use aerobic respiration, high myoglobin (red meat).
• Intermediate fibers: Have characteristics of both fast and slow fibers.

Muscle Tension

• Treppe: gradual increase in tension after repeated stimuli.
• Wave summation: successive stimuli increase the tension before relaxation is complete.
• Incomplete tetanus: rapid cycles of contraction and relaxation, producing peak tension.
• Complete tetanus: sustained contraction with no relaxation.
• Recruitment: increasing the number of active motor units to increase muscle tension.
• Muscle tone: slight tension in resting muscles due to involuntary contractions.
• Hypotonia: Decreased or lost muscle tone.
• Flaccidity: Limpness with loss of muscle tone.
• Hypertonia: Increased muscle tone.
• Spasticity: Increased muscle tone with increased tendon reflexes.
• Rigidity: Increased muscle tone with unaffected tendon reflexes.

Muscle Contractions

• Isometric contraction: Muscle tension increases, but length remains constant (no movement).
• Isotonic contraction: Muscle tension remains constant, but length changes.
• Concentric isotonic contraction: Muscle shortens (lifting).
• Eccentric isotonic contraction: Muscle lengthens (lowering).
• Cori cycle: Lactic acid is converted to pyruvic acid and glucose in the liver.

Smooth Muscle

• Single-unit smooth muscle: Cells are connected by gap junctions and contract as a unit, found in walls of hollow organs.
• Multi-unit smooth muscle: Cells are less connected and contract independently, found in large arteries and airways.
• Characteristics of smooth muscle: Uninucleate, lack sarcomeres, thin filaments attach to dense bodies, involuntary control.
• Functions of smooth muscle: Regulating airway diameter, moving food through the digestive tract, moving reproductive fluids, acting as sphincters.
• Smooth muscle contraction:: Calcium ions bind to calmodulin, activating myosin light chain kinase.

Additional Points

• Muscle tissue can undergo stress-relaxation response: Allowing smooth muscle to function over a wide range of lengths.
• Weightlifting increases myofibrils: Not the number of cells.
• Mitochondrial activity in skeletal muscle: Typically efficient, but limited ATP production with limited oxygen availability.
• Myasthenia Gravis: Autoimmune disorder affecting ACh receptors at the neuromuscular junction, reducing the number of available receptors.