muscular system pa 481 c anatomy & physiology tony serino, ph.d
TRANSCRIPT
Muscular SystemMuscular System
PA 481 CPA 481 C
Anatomy & Physiology Anatomy & Physiology
Tony Serino, Ph.D.Tony Serino, Ph.D.
Muscular System
• Functions: Movement –generation of
force and/or shortening Maintenance of posture Joint stabilization Heat Generation
Attributes: contractility, irritability,
extensibility, and elasticity
Types of Muscle Cells
Skeletal Muscle –voluntary, striated
Cardiac Muscle –involuntary, striated
Smooth Muscle –involuntary, no striations
Muscles wrapped with CT, that is continuous with tendon and periosteum
The elasticity of the CT sheaths, tendon and the muscle cells =
the Series Elastic Component
Antagonistic Muscle Arrangement
This arrangement plus the series elastic component allows the muscle to return to its original length.
Skeletal Muscle Cells• Long, cylindrical, non-branching, multinucleated• 10-100 mcm wide and up to 35 cm long• Voluntary, no spontaneous depolarization normally• Contractile proteins (myosin & actin) arranged in
bundles called myofibrils
Develop as a fusion of myoblasts, which accounts for multinucleated cells, extra myoblasts remain as satellite cells.
Unique Muscle Cell Structures
Sarcomere
Each skeletal muscle cell must be innervated by a
motor neuron to begin contracting.
Neuronal AP triggers release of ACh at
neuromuscular junction (motor end plate).
Neuromuscular Junction(Motor End Plate)
ACh is released and diffuses across gap
ACh bind to the nicotinic receptor and triggers a MEPP
The MEPP triggers an AP that races along the sarcolemma and down the T-tubules.The depolarization affects the SR cisternae which releases Ca++ into the cytoplasm.
The rise of intracellular Ca++ triggers the mechanical events of contraction.
Muscle Cell Contraction (Excitation-Contraction Coupling)
• A motor neuron is stimulated to fire an AP• AP reaches synaptic terminal triggering an influx
of Ca++
• The Ca++ stimulates the release of ACh• ACh diffuses across cleft and binds to nicotinic
receptors in motor end plate• This causes Na+ channels to open; causing the
generation of a MEPP• The MEPP triggers an AP along sarcolemma
and into T-tubules• This deplorarizes the SR cisternae which
releases stored Ca++ into the cytoplasm
Each myofibril consists of overlapping thick and thin filaments arranged in units called sarcomeres.
Muscle Contraction: Mechanical Events (Sliding Filaments)
• Calcium ions from SR flood the myofibrils
• This causes the thick and thin filaments to bind to each other (generates tension) and may cause them to slide past each other
• This causes the sarcomere to shorten
H Band
M Line Z Line
Myofibril Anatomy
Myofibril Structure (cross section)
Cross sections:
H Band M Line
Overlapping Tick and Thin Filaments
Thick Filament Structure
Thin Filament Structure: Twisted bead chain of actin proteins
Thin Filament: Actin, Tropomyosin and Troponin
Calcium is trigger
Detachment
Contraction Events
Detachment
Reset: energize myosin head
Detachment
Reset
Attachment
Detachment
Reset
Attachment
Power Stroke
Muscle Contraction Review
Muscles are arranged as Motor UnitsMotor Unit = 1 motor neuron + all the muscle fibers it controls (innervates)
The size of the motor unit depends on the degreeof control needed in that particular whole muscle.
Biomechanics of Force Production
• Tension = force exerted on an object by a muscle
• Load = force exerted on muscle by the weight of an object
• Twitch = the mechanical response of a muscle to an AP
• Types of Contractions:• Isometric = muscle increases
tension without shortening• Isotonic = muscle shortens with no
further increase in tension Load
TensionBicep
Fulcrum(pivot point) Weight of arm + object
Single Muscle Twitch
Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction• Increasing load decreases velocity
Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in
strength, size, ATP splitting rate, and resistance to fatigue
Load Effect on Degree andDuration of Contraction
Load vs. Velocity of Contraction
Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction• Increasing load decreases velocity
Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in
strength, size, ATP splitting rate, and resistance to fatigue
Mechanical (Wave) Summation
Increase frequency of stimulation allows tension to add to previous contraction’s tension
Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction• Increasing load decreases velocity
Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in
strength, size, ATP splitting rate, and resistance to fatigue
Initial Length of Muscle Fiber: affects the maximum tension that can be developed due to degree of overlap between thick and thin filaments
Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction• Increasing load decreases velocity
Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in
strength, size, ATP splitting rate, and resistance to fatigue
Types of Muscle Fiber: each motor unit consists of only one type of muscle fiber
• Slow twitch, red (oxidative) fibers (SO) –small diameter, weakest, slow ATPase, much myoglobin and mitochondria, abundant blood supply, fatigue resistant
• Fast twitch, red (oxidative) fibers (FO) –medium diameter, moderate strength, fast ATPase, abundant mitochondria and myoglobin, good blood supply, moderate fatigue resistance
• Fast twitch, white (glycolytic) fibers (FG) –largest diameter, great strength, fast ATPase, low amount of myoglobin or mitochondria, decreased blood supply, high in glycolytic enzymes, tire quickly
Control of Whole Muscle Tensiondependent on:
• Tension developed by each fiber– Dependent on fiber type, initial length and
degree of wave summation
• Amount of fibers stimulated to contract– The number of motor units responding is
directly related to amount of tension produced– If the body needs more power, it recruits more
motor units to respond– Known as recruitment (motor unit summation)
Energy Use: stored ATP in muscle used quickly so re-supply is crucial to function
1. Creatine Phosphate –quick re-supply, allowing time for aerobic respiration to gear up
2. Aerobic Respiration –oxidative phosphorylation dependent on adequate blood supply of oxygen, uses different sources for energy:
a) Stored glycogen
b) Glucose and fatty acids from blood
c) Fatty acids from blood
3. Anaerobic Respiration-becomes dominant as need for oxygen exceeds ability of blood to transport it into muscles
After exercise, energy continues to be consumed at increased levels to re-build reserves, etc., this is the oxygen debt incurred during the exercise
Fatigue –inability to maintain contraction tension even while being stimulated. Two kinds:
• Primary Fatigue –due to accumulation of lactic acid in sarcoplasm, this changes the cytoplasm pH and begins to change protein configurations which ends contraction.
• Secondary Fatigue –related to the loss of energy reserves in the body, as seen in day after soreness. Why this triggers a low intensity pain signal (a dull ache) is unknown.
Cardiac MuscleStriated, single nucleus,branched cells, connectedtogether by intercalateddiscs (with many gap junctions)
Spontaneously contracts, needs no innervation,involuntary
Smooth MuscleNo sarcomeres, therefore, no striations, single nucleated, small spindle shaped cells
Spontaneously contracts,involuntary control, can remaincontracted for long periods oftime without fatiguing
Two types:Visceral (single unit)
–united by gap junctionsMulti-unit –needs innervations, behaves like
skeletal muscle (Ex. Iris)
Smooth Muscle Cell
Visceral Smooth Muscle