Contractility
The ability of muscle to shorten forcefully
Functions of the muscular system
Movement, maintenance of posture, respiration, body heat, communication, constriction of organs and vessels, contraction of the heart
Excitability
The capacity of muscle to respond to a stimulus
Extensibility
A muscle can be stretched beyond its normal resting length and still be able to contract
Elasticity
Ability of muscle to recoil to its original resting length after it has been stretched.
Types of muscle
Skeletal, smooth, cardiac
Skeletal muscle
Associated with connective tissue. About 40% of body weight. Locomotion, facial expressions, posture, respiratory etc
Smooth muscle
Most widely distributed throughout body. Walls of hollow organs and tubes, interior of eye, walls of blood vessels,
Cardiac muscle
Only in heart. Many smooth muscles that contract rhythmically. Controlled by nervous system and endocrine system
Skeletal muscle fibers
Skeletal muscle cells
Fasciculi
Visible bundles of muscle
Perimyseum
Connective tissue layer surrounding fasciculi
Epimysium
Surrounds entire muscle. Dense collagenous connective tissue.
Fascia
General term for connective tissue sheets within the body
Muscular fascia
Superficial to epimysium, separates and compartmentalizes individual muscle groups
Motor neurons
Specialized nerve cells that stimulate muscles to contract
Myoblasts
Muscle fibers develop from less mature, multinucleated cells called myoblasts
Hypertrophy
Enlargement
Sarcolemma
Plasma membrane of a muscle fiber. Two layers
External lamina
Deeper thinner layer of sarcolemma. Cannot be distinguished under light microscope
Endomysium
Second layer of sarcolemma. Thicker layer
Transverse (t) tubules
Tube like invanginations of sarcolemma. Connects extra cellular environment with interior of muscle fiber
Sarcoplasmic reticulum
Endoplasmic reticulum in muscle cells
Sarcoplasm
The cytoplasm of muscle cells. Contains energy-storing glycogen granules and mitochondria
Myofibrils
Within sarcoplasm. Bundles of protein filaments I
Myofilaments
Actin and myosin
Sarcomeres
Actin and myosin myofilaments highly ordered units
Actin Myofilaments are composed of
Two strands fibrous actin (f actin) tropomyosin molecules and troponin molecules
Globular actin
G actin. Active site where myosin molecules bind during contraction
Myosin molecules are
Myosin heavy chains to make a rid portion and myosin heads
Cross-bridges
Myosin heads bind to active sites on actin to form cross bridges
Myosin heads
ATPase enzymes which break down ATP and release energy.
ATP
Adenosine triphosphate
Z disk
Network of protein forming a dislike structure for the attachment of actin myofilaments
I band
Isotropic band. Includes Z disk, and extends from one side to the other. Only actin filaments
A band
Anisotropic band. Extends the length of myosin filaments within a sacromere
H zone
Actin and myosin filaments do not overlap and only myosin is present
M line
Middle of H zone. Chain in the middle that holds myosin in place
Titin
Largest protein. Attaches to z disk to m line
Ligand-gated ion channels
Ion channels that open as a result of a neurotransmitter
Mv
Millivolts. Measures resting membrane potential
Depolarization phase
Brief period during which further depolarization occurs and inside of cell becomes positively charged
Repolarization phase
Return to resting value
All or nothing principle
If depolarization reaches threshold, permeable changes proceed without stopping
Propagate
Action potentials can travel across plasma membrane to adjacent location
Neuromuscular junction
Synapse
Presynaptic
Axon terminal
Synaptic cleft
Space between presynaptic terminal and muscle fiber
Postsynaptic membrane
Motor end plate. Muscle area of junction
Synaptic vesicles
Spherical sacs on presynaptic terminal. Contain acetylcholine
Acetylcholine
ACh. Neurotransmitter that alters post synaptic cell
Acetylcholinesterase
Breaks down acetylcholine and keeps it from accumulating in the synaptic cleft.
Excitation-contraction coupling
Action potential causes contraction
Terminal cisternae
Near t tubules, the sarcoplasmic reticulum is enlarged to form terminal cisternae
Triad
A t tubule and two adjacent terminal cisternae together
Cross bridge cycling
During contraction, myosin molecules undergo cross bridge formation and return of position many times
Power stroke
Movement of myosin molecule while cross bridge is attached
Recovery stroke
Return of myosin head to its original position after cross bridge
Muscle twitch
Single, brief contraction. Does not last long enough to perform any work
Lag phase
Time between the application of stimulus to motor neuron and muscle contraction
Contraction phase
Time when contraction occurs
Relaxation phase
When relaxation occurs
Motor unit
Single
Motor neuron and all the muscle fibers it innervates
Graded
Strength of muscle contractions varies from weak to strong. Wholesales respond.
Summation
Involves increasing the force of contraction of the muscle fibers
Recruitment
Increasing number of muscle fibers contracting
Treppe
A muscle fiber, when stimulated in rapid succession, contracts with greater force with each subsequent stimulus
Multiple motor unit summation
Relationship between increasing stimulus strength and increased number of contracting motor units. More motor units with more force of contraction
Subthreshold stimulus
Not strong enough to cause an action potential or contraction
Threshold stimulus
Strong enough to produce an action potential in a single motor unit axon
Submaximal stimuli
Produce action potentials in axons of additional motor units
Maximal stimulus
Produces action potentials in the axons of all the motor units of that muscle
Tetanus
Frequency of action potentials in skeletal muscle increases the frequency of contraction also increases until a period of sustained contraction
Incomplete tetanus
Muscle fibers partially relax between the contractions
Complete tetanus
No relaxation between contractions
Multiple wave summation
When the frequency of contractions increases, the tension produced is called multiple wave summation
Active tension
Force applied to an object to be lifted when a muscle contracts
Active tension curve
Muscle length plotted against the tension produced by the muscle in response to maximal stimuli
Passive tension
Tension applied to the load when a muscle stretches but is not stimulated
Total tension
The sum of active and passive tension
Isometric contractions
The length of the muscle does not change but the amount of tension increases during contraction
Isotonic contractions
The amount of tension produced by the muscle is constant during contraction but the length of the muscle changes
Concentric contractions
Isotonic contractions in which tensions in the muscle is great enough to overcome the opposing resistance and the muscle shortens
Eccentric contractions
Isotonic contractions in which tension is maintained in a muscle to increase in length. Lowers heavy weight
Muscle tone
Constant tension produced by muscles for long periods of time
Fatigue
decreased capacity to do work and the reduced efficiency of performance that normally follows a period of activity
Psychological fatigue
Involved central nervous system, individual perceives that no more work is possible
Muscular fatigue
Calcium ion imbalances as ATP levels drop. Cross bridges cannot function properly.
Synaptic fatigue
Neuromuscular junction. High frequency of acetylcholine without enough synthesizing.
Physiological contracture
In cases of extreme fatigue. Muscles are not capable of contraction or relaxing. No ATP
Rigor Mortis
Development of ridged muscles several hours after death, similar to physiological contracture
Creative phosphate
Accumulates in muscle fibers where it stores energy that can be used to synthesize ATP
Anaerobic respiration
Breakdown of glucose to yield ATP and lactic acid
Glycolysis
Glucose molecule is broken down into two molecules of pyruvic acid. Lactic acid diffuses into bloodstream
Aerobic respiration
Requires oxygen and breaks down glucose to produce ATP carbon dioxide and water.
Oxygen deficit
Insufficient oxygen consumption relative to increased activity.
Recovery oxygen consumption
Elevated oxygen levels after exercise has ended
Slow twitch oxidative muscle fibers
Contract more slowly, better blood supply, more mitochondria and more fatigue resistant
Myosin ATPase
Enzymes on the myosin heads responsible for the breakdown of ATP
Myoglobin
Dark pigment similar to hemoglobin in RBC. Binds oxygen and stores it
Fast twitch muscle fibers
Respond rapidly to nervous stimulation. Myosin head have fast enzymes to break down ATP
Calmodulin
Cal modulus and calcium bind to activate myosin kinase
Myosin phosphatase
Causes Relaxation of smooth muscle
Visceral smooth muscle
Sheets of muscle in digestive, reproductive and urinary tracts
Multi unit smooth muscle
Sheets in blood vessels, small bundles in arrest or pili muscles, iris and single cells in spleen
Cardiac muscle
Heart only, branching fibers, intercalated disks
Sarcopenia
Aging, muscle atrophy
