Chapter 9 - Muscles and Muscle Tissue
- Skeletal muscle is associated with the bony skeleton, and consists of large cells that bear striations and are controlled voluntarily.
- Cardiac muscle occurs only in the heart, and consists of small cells that are striated and under involuntary control.
- Smooth muscle is found in the walls of hollow organs, and consists of small elongated cells that are not striated and are under involuntary control.
Functions of Muscles
- Muscles produce movement by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems.
- Muscles aid in maintaining posture by adjusting the position of the body with respect to gravity.
- Muscles stabilize joints by exerting tension around the joint.
- Muscles generate heat as a function of their cellular metabolic processes.
- Functional Characteristics of Muscle Tissue
- Excitability, or irritability, is the ability to receive and respond to a stimulus.
- Contractility is the ability to contract forcibly when stimulated.
- Extensibility is the ability to be stretched.
- Elasticity is the ability to resume the cells' original length once stretched.
Skeletal Muscle Anatomy - Gross Anatomy of Muscle Organ
- Origin, belly and insertion
- Made up of thousands of muscle fibers bundled in
- connective tissue coverings which contains
- many blood vessels
- and a motor nerve ending for each muscle fiber
- Connective Tissue Wrappings (deep fascia)
- Deep fascia surrounds and penetrates the muscle
- epimysium surrounds the muscle organ
- perimysiumsurrounds the fascicles
- endomysium surrounds each muscle fiber
- The deep fascia is interconnected to the subcutaneous fascia and the subserous fascia
- The deep fascia connects to bones via tendons & aponeuroses
- Fasica surrounds groups of muscles forming compartments
- Muscles attach:
- Directly - epimysium of the muscle is fused to the periosteum of a bone
- Indirectly - connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis
- Muscle Classification: Functional Groups
- Skeletal muscles work together or in opposition
- Muscles only pull (never push)
- As muscles shorten, the insertion generally moves toward the origin
- Whatever a muscle (or group of muscles) does, another muscle (or group) "undoes"
- Prime movers - provide the major force for producing a specific movement
- Antagonists - oppose or reverse a particular movement
- Synergists - add force to a movement or reduce undesirable or unnecessary movement
- Fixators - synergists that immobilize a bone or muscle's origin
- Arrangement of Fascicles
- Parallel - fascicles run parallel to the long axis of the muscle
- Fusiform - spindle-shaped muscles
- Pennate - short fascicles that attach obliquely to a central tendon running the length of the muscle
- Convergent - fascicles converge from a broad origin to a single tendon insertion
- Circular - fascicles are arranged in concentric rings
- 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)
- 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
- Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma (plasma membrane - supported by dystrophin)
- Sarcoplasma - specialties
- glycogen
- myoglobin
- mitochondria
- Myofibrils - 80% of the cell
- Surrounded by sarcoplasmic reticulum
- Contains transverse tubules which surface at the sarcolemma
- Stores Calcium ions
- Made up of myofilaments
- Made up sarcomeres -smallest contractile unit
- Striations - caused by arrangement of myosin and actin
- I band- light
- A band - dark
- Z line to Z line (one sarcomere)
- Molecular composition of myofilaments
- myosin -cross bridges
- cross bridges (heads contain ATPases)
- elongated tail proteins
- actin -double stranded helix
- titin...another
- Review Matching
- Sliding filament model
- Which filament moves?
- In what direction?
- Role of Ca++ in contraction
- Where is the binding site for the cross bridge?
- How is covered up during relaxation ?
- How is it exposed for contraction?
- When does myosin bind to actin?
- Excitation Contraction Coupling
- Binding sites exposed (Ca++ present)
- Crossbridges form when myosin heads attach using ATP energy
- Myosin crossbridges bends pulling on actin using energy from ATP
- Crossbridges detach when ATPase hydrolyses new ATP
- Relaxation: Ca++ are pumped back into _______.
- 1. At the axon ending a nerve impulse causes the release of ____________ from the synaptic _________.
(Acetylcholine is a _______________)
2. Acetylcholine crosses the _______ _____ and binds to ___ _________ of the _____ ___ _____ on the sarcolemma.
3. This opens _____ channels and ____ enter the muscle cell.
4. Na+ enters the cell and the membrane becomes __________ generating an
______ _________
5. The __- _______ carries the action potential
through out the cell causing the release of ______from the _________ ___________.
6. Which then stimulates the _______ _______ action.
Meanwhile back at the junction.............................
7. The membrane is repolarized to accept another stimulus (refractory period)
- K+ leaves the cel lfor a quick repoarlization.
- Cholinesterase decomposes ________and removes it from _________
- Na+-K+ pump
returns ions to _______ membrane potential conditions.
8. In this process where are the 3 places ATP is necessary?
9. Review Matching
Resting Membrane Potential
- Why does the nerve impulse cross the synapse?
- Outside of the cell membrane the electrical charge is positive
- Inside the the cell the membrane has a negative charge.
- The predominant extracellular ion is Na+
- The predominant intracellular ion is K+
- The sarcolemma is relatively impermeable to both ions
- All the above facts establish a - mV resting membrane potential
- Initially, this is a local electrical event called end plate potential (synapse)
- Later, it ignites an action potential that spreads in all directions across the sarcolemma and down the T-Tubules
Motor Unit: 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
- 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
- Muscles that control fine movements (fingers, eyes) have small motor units
- Large weight-bearing muscles (thighs, hips) have large motor units
- A muscle twitch is the response of a muscle to a single, brief threshold stimulus
- The three phases of a muscle twitch are:
- Latent period - first few milliseconds after stimulation when excitation-contraction coupling is taking place
- Period of contraction - cross bridges actively form and the muscle shortens
- Period of relaxation - Ca++ is reabsorbed into the SR, and muscle tension goes to zero
- Graded muscle responses are:
- Variations in the degree of muscle contraction
- Required for proper control of skeletal movement
- Responses are graded by:
- Changing the frequency of stimulation
- More rapidly delivered stimuli result in incomplete tetanus
- If stimuli are given quickly enough, complete tetanus results
- Changing the strength of the stimulus
- Threshold stimulus - the stimulus strength at which the first observable muscle contraction occurs
- Beyond threshold, muscle contracts more vigorously as stimulus strength is increased
- Force of contraction is precisely controlled by multiple motor unit summation
- This phenomenon, called recruitment, brings more and more muscle fibers into play
- Starting length of the muscle - optimal length-tension relationship
- The relative size of the muscle - the bulkier the muscle, the greater its strength
Contraction of Skeletal Muscle (Organ Level)
- The two types of muscle contractions are:
- Isometric contraction - Tension increases to the muscle's capacity, but the muscle neither shortens nor lengthens
- Isotonic contraction - the muscle changes in length (decreasing the angle of the joint) and moves the load
- Muscle tone:
- Is the constant, slightly contracted state of all muscles, which does not produce active movements
- Keeps the muscles firm, healthy, and ready to respond to stimulus
- Spinal reflexes account for muscle tone by:
- Activating one motor unit and then another
- Responding to activation of stretch receptors in muscles and tendons
- Treppe: The Staircase Effect
- Increased contraction in response to multiple stimuli of the same strength
- Contractions increase because:
- There is increasing availability of Ca2+ in the sarcoplasm
- Muscle enzyme systems become more efficient because heat is increased as muscle contracts
- The partial catabolism of glucose to generate ATP occurs in anaerobic cellular respiration. This system can provide enough energy for about 30-40 seconds of maximal muscle activity (e.g., 300-meter race).
- Muscular activity lasting more than 30 seconds depends increasingly on aerobic cellular respiration (reactions requiring oxygen). This system of ATP production involves the complete oxidation of glucose via cellular respiration (biological oxidation).
- Heat Production
- Only 40% of the energy released in muscle activity is useful as work
- The remaining 60% is given off as heat
- Dangerous heat levels are prevented by radiation of heat from the skin and sweating
- Creatine Phosphate
- Excess ATP within resting muscle used to form creatine phosphate
- Creatine phosphate 3-6 times more plentiful than ATP within muscle
- Its quick breakdown provides energy for creation of ATP
- Sustains maximal contraction for 15 sec (used for 100 meter dash).
- Athletes tried creatine supplementation gain muscle mass but shut down bodies own synthesis (safety?)
- Anaerobic Respiration
- ATP produced from glucose breakdown into pyruvic acid during glycolysis
- if no O2 present
- pyruvic converted to lactic acid which diffuses into the blood
- Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race)
- Aerobic Respiration
- ATP for any activity lasting over 30 seconds
- if sufficient oxygen is available, pyruvic acid enters the mitochondria to generate ATP, water and heat
- fatty acids and amino acids can also be used by the mitochondria
- Provides 90% of ATP energy if activity lasts more than 10 minutes
- Muscle Fatigue - Inability to contract after prolonged activity
- Factors that contribute to fatigue
- central fatigue is feeling of tiredness and a desire to stop (protective mechanism)
- insufficient release of acetylcholine from motor neurons
- depletion of creatine phosphate
- decline of Ca+2 within the sarcoplasm
- insufficient oxygen or glycogen
- buildup of lactic acid and ADP
- Oxygen Debt - the extra amount of O2 needed for a muscle to return to a resting state:
- Oxygen reserves must be replenished
- Lactic acid must be converted to pyruvic acid
- Glycogen stores must be replaced
Muscle Fatigue and Oxygen Debt
- Muscle fatigue - the muscle is in a state of physiological inability to contract
- Muscle fatigue occurs when:
- ATP production fails to keep pace with ATP use
- There is a relative deficit of ATP, causing contractures
- Lactic acid accumulates in the muscle
- Ionic imbalances are present - Na+-K+ pumps cannot restore ionic balances quickly enough
- SR is damaged and Ca2+ regulation is disrupted
- Intense exercise produces rapid muscle fatigue (with rapid recovery)
- Low-intensity exercise produces slow-developing fatigue
- Oxygen Debt - the extra amount of O2 needed for a muscle to return to a resting state:
- Oxygen reserves must be replenished
- Lactic acid must be converted to pyruvic acid
- Glycogen stores must be replaced
- Determined by the two following factors:
- Contraction rate depends on speed in which ATPases split ATP
- Anaerobic (Glycolytic) or Aerobic (oxidative)
- Slow Twitch Oxidative Fibers
- slow acting myosin ATPases
- slow contraction
- always oxidative (aerobic)
- resistant to fatigue
- red fibers
- Fast-twitch oxidative fibers
- Fast acting myosin ATPases
- Fast contraction
- moderate resistance to fatigue
- pink to red in color
- Fast-twitch glycolytic fibers
- ATPases fast or slow?
- contraction rate?
- aerobic or anaerobic?
- white fibers (myoglobin?)
- blood supply?
- fatigable?
Smooth Muscle
- Composed of spindle-shaped fibers
- Lack coarse connective tissue sheaths of skeletal muscle, but have endomysium
- Organized into two layers (longitudinal and circular) of closely apposed fibers
- Found in walls of hollow organs (except the heart)
- When longitudinal layer contracts, the organ dilates & contracts
- When the circular layer contracts, the organ elongates
- Peristalsis - alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs
- Innervation of Smooth Muscle
- Smooth muscle lacks neuromuscular junctions
- Innervating nerves have bulbous swellings called varicosities
- Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions
- Whole sheets of smooth muscle exhibit slow, synchronized contraction
- They contract in unison, reflecting their electrical coupling with gap junctions
- Action potentials are transmitted from cell to cell
- Characteristics of Smooth Muscle
- Unique characteristics of smooth muscle include:
- Smooth muscle tone
- Slow, prolonged contractile activity
- Low energy requirements
- Response to stretch
- Smooth muscle exhibits stress-relaxation response in which:
- Smooth muscle responds to stretch only briefly, and then adapts to its new length
- The new length, however, retains its ability to contract
- This enables organs such as the stomach and bladder to temporarily store contents
- Smooth muscle has good regenerative ability
- This is shown by estrogen's effect on the uterus
- At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
- During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus
- Autonomic nervous system and endocrine systems are major controls
- Neurotransmitters and hormones?
Conditions, Aging and Use
- Abnormal Contractions
- Spasm = involuntary contraction of single muscle
- Cramp = a painful spasm
- Tic = involuntary twitching of muscles normally under voluntary control--eyelid or facial muscles
- Tremor = rhythmic, involuntary contraction of opposing muscle groups
- Fasciculation = involuntary, brief twitch of a motor unit visible under the skin
- Atrophy
- wasting away of muscles
- caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy)
- the transition to connective tissue can not be reversed
- Hypertrophy
- increase in the diameter of muscle fibers
- resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria
- Exercise-Induced Muscle Damage
- Intense exercise can cause muscle damage
- electron micrographs reveal torn sarcolemmas, damaged myofibrils an disrupted Z discs
- increased blood levels of myoglobin & creatine phosphate found only inside muscle cells
- Delayed onset muscle soreness
- 12 to 48 Hours after strenuous exercise
- stiffness, tenderness and swelling due to microscopic cell damage
- Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours
- After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin
- Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.
- Regeneration of Muscle
- Skeletal muscle fibers cannot divide after 1st year
- growth is enlargement of existing cells
- repair
- satellite cells & bone marrow produce some new cells
- if not enough numbers---fibrosis occurs most often
- Cardiac muscle fibers cannot divide or regenerate
- all healing is done by fibrosis (scar formation)
- Smooth muscle fibers (regeneration is possible)
- cells can grow in size (hypertrophy)
- some cells (uterus) can divide (hyperplasia)
- new fibers can form from stem cells in BV walls
- Aging and Muscle Tissue
- Skeletal muscle starts to be replaced by fat beginning at 30
- "use it or lose it"
- Slowing of reflexes & decrease in maximal strength
Helpful Activities to do after you finish reading this chapter
Forward comments to RM Chute .....
Home Page
.... Updated: