Skeletal Muscle Tissue chapter 9 Flashcards
Muscle Tissue
is one of the four primary tissue types
- converts the chemical energy of ATP into mechanical energy
- muscle contraction
Important functions of skeletal muscle include
-movement
- contractions pull on tendons to move the bones of the skeleton
-maintain posture
- constant tension maintains body position
-guard openings to the digestive and urinary systems
- support soft tissues
- protect and support visceral organs
- thermoregulation
- heat generation from muscle contraction
Skeletal muscles are comprised of layers muscle fibers an connective tissue:
epimysium is an exterior collagen layer covering muscle
- blends between muscles and other tissue
perimysium is a dividing layer of connective tissue surrounding bundles of cells called a fascicle
- allows for blood vessels and nerves to penetrate muscle tissue
endomysium is a thin areolar tissue layer around each muscle fiber
- contains capillaries, terminal axons, and myosatellite cells
Endomysium, perimysium, and epimysium form connective tissue attachments to bone
form tendons/aponeuroses that merges into periosteum as perforating fibers
- tendons attach at points; aponeuroses attach broad areas
Stress will break a bone before
pulling the tendon/aponeurosis loose
connective tissue
Skeletal muscle
are long, striated muscles attached to bones containing multiple nuclei that develop via fusion of myoblasts
Muscle Fibers
sarcolemma
cell membrane of muscle fiber that holds sarcoplasm
stores glycogen and myoglobin
myofibrils
subdivision of muscle fibers responsible for contraction
exhibits alternating light and dark striations due to overlapping arrangement of myofilaments (actin and myosin)
sarcoplasmic reticulum
surrounds myofibril and forms terminal cisternae (to store and concentrate Ca2+)
transverse tubules (T tubules)
transmits action potential allowing whole fiber to contract simultaneously
triad is 1 T tubule and 2 terminal cisternae
sarcomere
Skeletal Muscle Fibers
Sarcomeres
are the basic functional, contractile units of muscle
- comprised of myofilaments (-fibrils) that form striations with muscle fibers
3 types of myofilaments
myosin (thick)
arranged in a bundle with heads directed outward in a spiral
interact with actin to form cross-bridges that pivot to produce motion
actin (thin)
intertwined strands of (G) actin with an active site
Ca2+ binds to actin receptors causing a shape change in troponin-tropomyosin complex that exposes active sites
active sites bind to myosin
Muscle contraction is caused by the interactions of
myosin and actin filaments
Sarcomeres and Striations
Actin and myosin are abundant and highly organized in sarcomere of muscle tissue
Lines and Bands of actin and myosin
M line and Z line
M line is center of A band (midline of sarcomere)
Z disc is center of I band (ends of sarcomere)
A band
dark region consisting of thick filaments
I band
light region consisting of thin filaments
H band (zone)
area around M line that has only thick filaments
Sliding filament theory
thin filaments of sarcomere slide towards M line in between thick filaments
- width of A band stays same; H and I bands get smaller
In muscle contractions, sarcomeres are pulled towards center; that is,
Z lines shorten the I band to produce tension
Neuromuscular junction
is the location of neural stimulation; functional connection between nerve fiber and muscle cell
3 parts of neromusclar junctions
synaptic knob
swollen end of nerve fiber (contains acetylcholine - ACh)
motor end plate and junctional folds (sarcolemma)
increases surface area for ACh receptors
contains acetylcholinesterase (AChE) that breaks down ACh and causes relaxation
synaptic cleft
gap between nerve and muscle cell
Skeletal muscle must be stimulated by a nerve or
it will not contract
Neuromuscular Junction: Neural Stimulation steps
- Neural action potential reaches synaptic knob
- Synaptic terminal releases acetylcholine (Ach) into the cleft
- Acetylcholine (Ach) :binds to receptors on junctional folds of sarcolemma to propagate action potential
- Ach is removed by acetylcholinesterase (AChE)Action potential causes Na+ (in extracellular fluid) to travel to T tubule
-Ca2+ from terminal cisternae is released causing actin-myosin interaction
Muscle function is a repeating cycle of
contraction and relaxation
Activation encompasses
excitation
neural stimulation leads to action potentials in muscle fiber
excitation-contraction coupling
action potentials on the sarcolemma activate myofilaments
contraction
shortening of muscle fiber
relaxation
return to resting length
Muscle contraction is ____; muscle relaxation is____
active; passive
Excitation
is the process leading to an action potential in the muscle fiber
Steps in excitation:
-nerve stimulus arrives at synaptic knob
causes Ca2+ to allow release of Ach
-Ach diffuses across cleft and binds to receptors on sarcolemma
receptors change shape and allow Na+ and K+ to cross plasma membrane
-Na+/K+ movements alter resting membrane potential (-90mV) and create an action potential
muscle fiber is now excited
Excitation-Contraction Coupling
refers to the activation of the myofilaments
Steps of excitation-contraction coupling:
-action potential spreads to T tubules
- terminal cisternae of SR release stored Ca2+ into sarcoplasm
- Ca2+ binds to troponin-tropomyosin molecules of actin causing a shape change
-active sites on actin are exposed
- actin-myosin cross bridges can now form
Contraction
refers to the development of tension in the muscle fiber
Steps in contraction:
- myosin head, using ATP, activates and “cocks” into extended position
- myosin binds to actin and a cross bridge is formed
- myosin head flexes, pulling actin filament towards H zone
- referred to as power stroke
- myosin binds to new ATP and process repeats
Myosin heads contract sequentially so as to not allow actin to
slide back to the resting position
Relaxation
is the process of a muscle “passively” returning to resting length
Steps in relaxation:
-nerve stimulation stops
ACh is no longer released and is broken down by AChE
-Ca2+ is actively transported back into cisternae
Ca2+ also dissociates from troponin-tropomyosin causing a shape change
-active sites on actin are blocked
myosin can no longer bind to actin
Rigor mortis
is a stiffening of the body beginning 3 to 6 hours after death
- deteriorating sarcoplasmic reticulum releases Ca2+
- activates actin-myosin cross bridges so muscle contracts but cannot relax
- relaxation requires ATP and ATP production is no longer produced after death
Fibers remain contracted until
myofilaments decay
Rigor mortis is a temporary condition lasting about
24-48 hrs
Tension
generated during contraction depends on length of muscle before it was stimulated
- i.e. thick filaments are overly contracted and too close to Z discs; can’t slide far
- weak contraction results
- i.e. little overlap of thin and thick is too stretched; does not allow for many cross bridges to form
- weak contraction results
Muscle tone
Muscle tone (firmness of muscle at rest due to partial contraction) is maintained by the CNS ensuring optimal resting length
- produces greatest force when muscle contracts
- increasing muscle tone increases metabolic energy used, even at rest
Weak stimuli do not cause
muscle contraction
muscle tissue has a threshold
level which the strength or frequency of stimuli will cause a contraction
After threshold is reached, a latent period of no contraction follows:
- time required for excitation, excitation-contraction coupling, and internal tension to build
- i.e., the contraction cycle has not begun
Contraction (external tension)
followed by relaxation occurs
-Ca2+ is released (cross-bridges form) and tension peaks but Ca2+ is quickly reabsorbed (cross-bridges detach)
- contraction phase is shorter than relaxation phase
Twitch
is a single stimulus (latency)–contraction-relaxation sequence:
twitch is a single stimulus (latency)–contraction-relaxation sequence:
- treppe (staircase phenomenon) - faster stimuli produce stronger twitches
- rare for skeletal muscles
- wave summation - higher frequency stimuli arrives before muscle relaxes and recovers gradually producing more tension
- incomplete tetanus (sustained, fluttering peak tension) is a form of wave summation
- complete tetanus is when twitches fuse into a prolonged, continuous contraction
- maximum frequency stimulation gives muscles no time to relax
- rarely occurs in the body
Motor unit
is a motor neuron and all the muscle fibers it innervates; dispersed within muscle causing weak contractions over wide area
2 motor units
- small motor units
- fine control units and may contain as few as 20 muscle fibers per nerve fiber
- eye and hand muscles
- large motor units
- strength control units and may contain 200 or more muscle fibers per nerve fiber
- gastrocnemius muscle (1000 fibers per nerve fiber)
Muscles contain multiple motor units to sustain long term contraction; motor units alternate
to prevent fatigue (asynchronus motor unit summation)
Motor Units
Isotonic and Isometric Contractions
Muscle contraction does not always change the muscle length but tension will always develop
Types of tension development:
isotonic muscle contraction develops tension while changing muscle length
- tension while shortening is concentric (isotonic) contraction
- muscle tension > resistance*
- tension while lengthening is eccentric (isotonic) contraction
- muscle tension < resistance*
isometric muscle contraction develops tension without changing length
- important in postural muscle function and antagonistic muscle joint stabilization
Isometric and Isotonic Contractions
ATP, CP and Pyruvic Acid ATP
Sustained muscle contraction uses a lot of ATP (adenosine triphosphate)
-muscles store enough energy via CP (creatine phosphate) to start contraction
- must manufacture more ATP as needed
- mitochondria are responsible for the production of ATP
Glucose is metabolized into
ATP
Different mechanisms synthesize ATP depending on
exercise duration
Cells produce ATP in 2 ways:
glycolysis
aerobic metabolism
glycolysis
- breaks down glucose from glycogen stored in skeletal muscles when stored ATP is exhausted
- produces 2 ATP and 2 pyruvate molecules per glucose molecule; a lactic acid byproduct is also produced
>>>>lactic acid is the “feel the burn” of exercise
aerobic metabolism
- mitochondria utilize pyruvate, O2, ADP to enzymatically synthesize ATP
- produces net 34 ATP per glucose molecule (17 per 1 pyruvate molecule created by glycolysis)
>>>primary source of ATP for resting muscles
Active muscles utilize glycolysis (approximately 10 mins) until oxygen consumption allows
aerobic respiration to resume
Citric acid cycle
to remove and deliver H+ from organic molecules to the ETS (via coenzymes NAD/FAD)
- 2-carbon molecule (acetate) is attached to coenzyme A, forming acetyl-CoA
- acetyl group is removed and attached to a 4-carbon molecule forming citric acid
- 1 ATP is produced for each processed acetyl group
ETS function
to transfer (e-) from H+ (oxidation) creating a concentration gradient which results in the production of ATP
- cytochromes pass (e-) to generate energy that pumps H+ out of mitochondrial interior
- diffusion of H+ powers attachment of high-energy phosphate to ADP (phosphorylation) using ATP synthase
-(e-) transferred to oxygen, eventually forming water
ATP Production
Fatigue
causes muscles to become progressively weaker; causes include:
- ATP synthesis declines as glycogen is consumed
- lactic acid inhibits enzyme function
- lowers pH causing decreased Ca2+ troponin binding
After exercise, the body needs more O2 than
After exercise, the body needs more O2 than usual to normalize metabolic activities resulting in
oxygen debt
oxygen debt
- replenishes O2 reserves
- replenishes ATP and CP resting levels
- reoxides lactic acid into pyruvic acid
- pyruvic acid is converted back to glucose to be stored
- serves metabolic rate
- active muscles produce heat and raise body temperature and consume extra oxygen
Muscle Fiber types
Slow-twitch Fibers
Fast Twitch Fibers
Slow-twitch fibers
have more mitochondria, myoglobin and capillaries (aerobic respiration)
- small diameter; slow contractions and are resistant to fatigue
- soleus and postural muscles of the back
Fast-twitch fibers
Fast-twitch fibers have enzymes for glycogen-lactic acid systems (anaerobic fermentation)
- large diameter; quicker/more forceful contractions; not resistant to fatigue
- extraocular eye muscles, gastrocnemius and biceps brachii
Intermediate fibers
are similar to fast-twitch fibers but more have capillaries; more fatigue resistant
All muscles contain all fiber types;
proportions are genetically determined
Slow vs. Fast Fibers
Muscle Performance
is a measure of strength and conditioning
strength depends on:
- muscle size and fascicle arrangement
-length of muscle at start of contraction
>>length-tension relationship
- # of motor units utilized
conditioning depends on:
- resistance training (weight lifting)
stimulates cell enlargement due to synthesis of more myofilaments
- endurance training (aerobic exercise)
produces an increase in mitochondria, glycogen and density of capillaries
Muscle performance ultimately depends on the types of
muscle fibers and to lesser extent physical conditioning
Hypertrophy:
muscle growth from heavy training
- increases diameter of muscle fibers and number of myofibrils
- increases mitochondria and glycogen reserves
Atrophy:
-muscle shrinkage from lack of exercise
- reduction in diameter of muscle and number of myofibrils
- nominal decrease in mitochondria
Muscles become flaccid when inactive for days or weeks;
what you don’t use, you lose
Cardiac Muscle Cells
are involuntary, thick with a single large nucleus and notched ends
-have less developed SR with no cisternae but have wide T tubules
-use aerobic respiration
resistant to fatigue but very vulnerable to interruptions in oxygen supply
-intercalated discs
link heart cells mechanically (desomosomes), chemically and electrically (gap junctions)
heart functions as a single fused mass of cells
-demonstrate automaticity
Structure of Cardiac Cells
Smooth Muscle
are involuntary, spindle-shaped cells with a single central nucleus
- no striations, sarcomeres or Z discs
- SR is scanty and there are no T tubules
- Ca2+ for contraction comes from extracellular fluid
- disorderly arrangement of actin and myosin filaments
- dense bodies transmit contraction from cell to cell
- nerve supply is autonomic (if present)
- releases either ACh or norepinephrine
Myofilaments are scattered so resting length is not related to tension development; functions
over a wide range of lengths (plasticity)
Smooth Muscle
