PHYSIOLOGY DEN 1003 ANP 1001 Prepared by: Dr. D. Boyd.
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Transcript of PHYSIOLOGY DEN 1003 ANP 1001 Prepared by: Dr. D. Boyd.
PHYSIOLOGY
DEN 1003 ANP 1001
Prepared by:
Dr. D. Boyd
MUSCULOSKELETAL PHYSIOLOGY
• Muscle cells are specialized for contraction• Action potential transmitted along the
sarcolemma (muscle cell membrane) activates contractile mechanism
• 3 Types of Muscles:– Skeletal (striated; striped under
microscope)• Under voluntary control• Rapid acting
MUSCULOSKELETAL PHYSIOLOGY
• 3 Types of Muscles: (cont)– Smooth (blood vessel walls & internal
organs)• Involuntary (inherent rhythmic
contraction)• Slow acting
– Cardiac: striated, features of both skeletal, & smooth muscle
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• Composed of many parallel muscle fibers
(myofibers)• Myofibers:
– Run length of muscle – Terminate in tendons that attach fiber to
Skeletal system.
– Multi-nucleated structure
– Surrounded by sarcolemma
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• A. Structural Changes During Contraction• 1. Shortening:
– Result from increase in extent of thin-thick filament overlap
– Thin filament (Actin) slide over Thick filament (Myosin)
center of sarcomere (unit of myofiber)– “sliding filament mechanism”– Sarcomere length decreases– Length of filaments do not change
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE
Sarcomere
Actin
Myosin
H band Z line
I band (light)
A band (dark)
Relaxed State Contracted StateContraction of Sarcomere
Contraction
I & H bands shorten
Z line
MUSCULOSKELETAL PHYSIOLOGY
Actin
Myosin
ADP-Pi
Pi
ADP
ATP
ATP
Molecular Aspects of Contraction
ATP
THE CONTRACTILE CYCLE (mechanochemical coupling)
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• 2. Molecular Aspects of Contraction• Upon stimulation of myofiber, myosin heads
with (ADP + Pi) connect with actin filaments• Actin-myosin complex form (ADP + Pi released)
• Actin-myosin bond broken (ATP added) • Cycle repeated• CLINICAL CORRELATION• No ATP, myosin heads can NOT release, leads to
stable actin-myosin complex rigor mortis
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• B. Excitation-Contraction Coupling • Action Potential generated by motor neuron
initiates mechanical contraction• AP transmitted along muscle membrane, down T
tubule to sarcoplasmic reticulum• Cytoplasmic Ca++ released • Ca++ contact myosin & actin filaments • Site on actin expose that binds to myosin • Ca++-ATPase (sarcoplasmic reticulum) depletes
Ca • Filaments return to inactivated state
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• C. Summary of Contraction Sequence• AP get to end of axon • ACh released at neuromuscular junction • ACh diffuse across gap • Nicotinic ACh receptor at end-plates
react with ACh• Muscle cell membrane depolarized • AP travel along muscle cell membrane • T tubule depolarization, travel to sarcoplasmic
reticulum Ca++ release into cytoplasm
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• C. Summary of Contraction Sequence (cont)• Ca++ binds to troponin-tropomyosin • Myosin heads bind to actin • Myosin-ATPase activated • Cross-bridges attach & detach • Myosin & actin slide past each other• Sarcoplasmic reticulum pumps Ca++
back into lumen• Ca++ removed from tropin-tropomyosin `
complex & actin-myosin interaction inhihited
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• Single Action Potential muscle twitch,
brief contraction followed by relaxation• Twitch starts 2 msec after depolarization of
membrane begins, i.e. during re-polarization.
Action Potential
Muscle Twitch
Contraction Time
Relative Timing of AP & Muscle Contraction
Start of Electrical Response
Peak of Contraction
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• CLINICAL CORRELATION:• Ca++ re-uptake mechanism of the sarcoplasmic
reticulum = Ryanodine Receptor
• In some people this receptor is blocked by general anesthetic with succinylcholine.
• Ca++ is NOT taken up quickly enough and muscles “overcontract”, generating enormous amounts of heat (malignant hyperthermia), which can be fatal if not treated with dantroline.
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• D. Muscle Mechanics• Most physical activity include both isometric
& isotonic contractions• 1. Definitions:
– Isometric contraction:• Both ends of muscle are fixed• No change in length during contraction• Tension (force) increases• E.g. pushing against a wall
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• D. Muscle Mechanics• 1. Definitions: (cont)
– Isotonic contraction• Muscle shorten during contraction • Tension (force) remains constant
–E.g. lifting weights
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• D. Muscle Mechanics• 1. Definitions: (cont)
– Dynamic contraction• Muscle length & force change during
contraction• Muscle may shorten = concentric
contraction• Muscle may be pulled out by load =
eccentric contraction
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• D. Muscle Mechanics• 2. Length-Tension Relationship• Tension developed in an isometric contraction
varies with the initial length of the muscle fiber.
• There an optimal length at which a muscle is able to develop maximum tension.
Passive Tension
Active Tension
Lopt Length of Muscle
Tmax
Tension
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• NOTE:• Sarcomeres (in series) of the same myofibril
do NOT generate additive force
• To generate more force, more fibers must be recruited (in parallel)
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• Isotonic Contraction• The velocity at which the muscle contracts
varies inversely with the load it lifts.• At 0 load there is rapid but finite velocity of
shortening• With increasing load the velocity approaches
0.
• At 0 velocity contraction becomes iosometric.
• This point = maximum active force of muscle
MUSCULOSKELETAL PHYSIOLOGY
• SKELETAL MUSCLE• Isotonic Contraction
Force
Initial Velocity of Contraction
Force-Velocity Relationship in Skeletal Muscle
Isometric Contraction Maximum Force
0
MUSCULOSKELETAL PHYSIOLOGY
• E. Types of Skeletal Muscle Fibers• Property Fast Twitch Slow Twitch• & Type• Color White Red• SR & T tubules Many Few• Myosin ATPase High Low• Mitrochondria Few (short,
Many (sustained rapid mov’ts contractions)
• SR = sarcoplasmic reticulum
MUSCULOSKELETAL PHYSIOLOGY
• E. Types of Skeletal Muscle Fibers (cont)• 1. Fast Twitch (Type 11)
• White Few fibers per motor unit• Large diameter No myoglobin• Use glycolysis to generate energy, function
under anaerobic conditions• Adapted for rapid contraction
• Enable fine, careful movements (e.g. contraction of extraocular muscles of Eye, & superior head of Temporalis muscle)
MUSCULOSKELETAL PHYSIOLOGY
• E. Types of Skeletal Muscle Fibers (cont)• 2. Slow-twitch(Type 1)(muscles of
mastication)• Red due presence of myoglobin• Small diameter fibers• Less sarcoplasmic reticulum & Ttubules• Smaller motor end plates• Slow to contract, adapted for long,
sustained contraction• Oxidative metabolism used for energy• Large number of mitrochondria & more
blood supply
MUSCULOSKELETAL PHYSIOLOGY
• IN SUMMARY• Fast-twitch (Type 11) vs Slow-twitch (Type 1)• Think of chicken:
– White meat (white muscle) (Type 11) found in breast, used for intermittent flapping
of wings.
– Dark meat (red muscle) (Type 1)found in thighs, used for stained maintenance
of posture.
MUSCULOSKELETAL PHYSIOLOGY
• F. Motor Units• Consists of all the muscles innervated by a
single alpha motor nerve axon• Excitation of motor neuron result in
contraction of all fibers in the motor unit
• Each muscle fibers of a given motor unit is of the same muscle type
• If motor nerve is destroyed , all muscle fibers innervate by that neuron atrophy
(e.g. spinal injury)
MUSCULOSKELETAL PHYSIOLOGY
• Twitch & Tetanus• 1. Single Twitch• Elastic elements (tendons, connective tissue)
within muscle & between the muscles & its attachments represent “slack” that must be stretched before the active tension generated by the muscle can be exerted.– This time delay for elastic stretch is
enough for the active twitch to decline.
– Therefore peak tension is never exerted by a single twitch.
MUSCULOSKELETAL PHYSIOLOGY
Tetanus
Unfused Tetanus (clonus)
Response to 2 stimuliSingle Twitch
0.5 1.0 1.5 2
Peak Tension (%) 25
50
Time
Tetanus
Twitch Amplitude & Relative Timing & Amplitude for Force Generated100
MUSCULOSKELETAL PHYSIOLOGY
• Tetanus• Summation (fusion) of Contractions• Result from high frequency neural
stimulation over short period of time
• Cause partly because elastic elements have been fully stretched from early contractions hence maximum tension develop wit no time for relaxation of fibers
• Caused by increased Ca++ availability over repeated contractions
MUSCULOSKELETAL PHYSIOLOGY
• G. Skeletal Muscle Muscle Receptors:• Two types:
– Muscle spindle (embedded within group of
fibers)
– Golgi Tendon Organs (arranged in tendon in series with
myofibers)
MUSCULOSKELETAL PHYSIOLOGY
• G. Skeletal Muscle Receptors• 1. Muscle Spindle
Gamma Efferent
Primary 1a Afferent11 Afferent
Nuclear Chain Fiber
Nuclear Bag Fiber
Intrafusal Fiber & Innervation
Poles
MUSCULOSKELETAL PHYSIOLOGY
• G. Skeletal Muscle Receptors• 1. Muscle Spindle (cont)• Intrafusal fibers = small muscle fibers
innervated by small gamma motor neurons
• Primary (annulospiral) type 1a Afferent fibers, rapid conducting, innervate center of both the Nuclear Bag & Nuclear Chain
• Secondary (flower-spray) type 11 Afferent fibers, slow conducting, innervate Nuclear
Chain only
MUSCULOSKELETAL PHYSIOLOGY
1. Muscle Spindle (cont)• Motor innervation of Intrafusal fibers = small
slow conducting gamma Efferent fibers• Stretching muscle causes stretching &
deformation of Muscle Spindle, which result in volley of impulses in Primary Afferent neurons, that synapse directly on alpha motor neurons innervating extrafusal fibers of the muscle in which the Spindle is embedded. E.g. contraction of the Quadriceps muscle is elicited when the Patella Tendon is tapped leading to the “ knee-jerk reflex”
MUSCULOSKELETAL PHYSIOLOGY
1. Muscle Spindle (cont)Primary Afferent type 1a neurons discharge rapidly during the lengthening of the muscle, therefore respond to length as well a velocity of stretch of the muscle.
• Secondary type 11 neurons discharge rapidly during the entire period of stretch of the muscle, therefore respond mainly to length.
MUSCULOSKELETAL PHYSIOLOGY
1. Muscle Spindle (cont)• SUMMARY• Muscle Spindle (intrafusal fibers) contain:
– A contractile element innervated by gamma motor neurons
– A non-contractile element enveloped by stretch-sensitive afferent neurons
• Muscle stretch causes an increase rate of firing from Spindle afferents, resulting in increased firing of alpha motor neurons to cause muscle contraction
MUSCULOSKELETAL PHYSIOLOGY
• G. Skeletal Muscle Receptors (cont)• 2. Golgi Tendon Organs• Arranged in series with a discrete number of
skeletal muscle fibers
• When skeletal contracts, the tendon in which the muscle inserts lengthens & stretches the nerve ending of the afferent fibers, causing them to fire.
MUSCULOSKELETAL PHYSIOLOGY
• G. Skeletal Muscle Receptors (cont)• 2. Golgi Tendon Organs (cont)• Supplied by 1b afferent fibers, which
synapse on inhibitory inter-neurons, which synapse with alpha motor neurons, which inhibit contraction which is protective.
• SUMMARY• Muscle Spindle sense muscle length• Golgi tendon Organs sense muscle tension
MUSCULOSKELETAL PHYSIOLOGY
• SMOOTH MUSCLE• Regulate internal environment of the body• Smaller in size & uni-nucleated• Have fewer myofibrils & less organized• Dense bodies on cell membrane & inside
cytoplasm act as site of actin filament insertion
• Have much less myosin• Have no T tubules & little sarcoplasmic
reticulum. • Ca++ enter from extracellular fluid
MUSCULOSKELETAL PHYSIOLOGY
• SMOOTH MUSCLE• Contraction & Relaxation:• Occur slowly • Involve overlap of actin & myosin• Thin filaments inserted into Dense bodies are
pulled closer together by bridging myosin units.
• Dense bodies on cell surface are pulled together so that cell is deformed contraction.