What connects skeletal muscles to bones?
Tendons
Muscle Body
generates force and contains bundles (fascicles) of muscle cells
Sarcolemma
plasma membrane of the muscle cell
Muscle Cells
muscle fibers", multinucleate
Myofibrils
rod-like fibers that are the contractile component of the muscle cell, contain overlapping thick and thin filaments (myosin and actin), composed repeating units called sarcomeres
Sarcoplasmic Reticulum
The smooth ER that surrounds the myofibrils
Transverse Tubules
inward extensions of the sarcolemma
Striations
the striped appearance of skeletal muscle due to an orderly arrangement of thick and thin filaments that run parallel to the long axis of the fiber
Sarcomere
Bordered by Z lines,
M lines in the center,
A Band is the dark band that is the length of the thick filament,
H Zone is the light region in the center of the A Band,
I Band is the light band where only think filaments are located
Actin
Thin filaments (globular, and linked to form helical strands)
Myosin
Thick Filaments (myosin molecules are composed of a head and tail, and the thick filament consists of pairs of myosin molecules, attached by their tails)
Myosin Head
Has a site that binds to actin (to form cross-bridges) and an ATPase site that hydrolyzes ATP
Cross-bridges
How actin and myosin are attached
What are the two regulatory proteins associated with actin?
Tropomyosin (covers binding sites on actin) & Troponin (uncovers binding sites when bound to Ca)
Titin
an elastic protein that extends the length of each thick filament, from M line to Z line, and gives the sarcomere its elasticity.
Sliding Filament Theory
A model that explains muscle contraction by the sliding of thick and thin filaments over one another.
Cross-Bridge Cycle
1. Binding of Myosin to Actin.
2. Power Stroke.
3. Rigor.
4. Unbinding of Myosin and Actin.
5. Cocking of Myosin Head.
Step One: Binding of Myosin to Actin
The myosin head is in its high-energy conformation, bound to ADP and Pi, and binds to an actin subunit in the adjacent thin filament.
Step Two: Power Stroke
Binding causes release of ADP and Pi and energy is released as the myosin head pivots toward the middle of the sarcomere and pulls the attached actin filament with it.
Step Three: Rigor
Myosin head is in its low energy form and remains bound to actin until the myosin head binds to ATP.
Step Four: Unbinding of Myosin and Actin
After ATP binds to the myosin head a conformational change occurs that causes the myosin head to attach from actin
Step Five: Cocking of the Myosin Head
The ATP is soon hydrolyzed bt the ATP associated with the myosin head and the energy that is releases forces the myosin head back into its high-energy conformation from where the cycle can repeat itself
Muscles contract smoothly because
crossbridges going through the cycle are at different phases in the cycle.
The corssbridges on either side of the M line bend ________ each other causing the actin to slide toward the __________ which results in the sarcomere getting _______________.
towards, middle, shorter
Excitation Contraction Coupling Steps
1. a. ACh is released from the axon terminal of a motor neuron and binds to the receptors in the motor end plate.
b. The binding elicits an end-plate potential.
c. the end-plate potential triggers an AP in the muscle cell.
2. AP propagates along the sarco
How many motor neurons is each muscle fiber innervated by?
One
End plate potential is ALWAYS followed by
an action potential
Why is there a high concentration of Ca++ in the SR?
because of the presence of Ca++ pumps
After the Ca++ leaves the SR where does it go?
cytosol
Protein Receptors in the SR are called
Ryanodine receptors (or foot structures)
Protein Receptors in the T-tubule are called
Dihydropyradine receptors (DHP)
What causes the DHP recpetor to open the Ca++ channel of the ryanodine receptor?
Depolarization of the T-tubule
The Ca++ then acts as a ___________.
Ligand, binds to ligand-gated Ca++ channels un the SR to cause the release of more Ca++
What causes calcium to bind to the troponin complex?
An increase in cytosolic calcium
Under high exertion ATP in the muscle may be rapidly depleted. What is the body's way of meeting this demand temporarily? And, what catalyzes the reaction?
Creatine/Creatine Phosphate system (creatine phosphate + ADP <--> creatine + ATP)
creatine kinase
Continuous muscle contraction at moderate rates is sustained by the ATP produced by _____________ ______________.
oxidative phsophorylation
What happens after glycogen reserves are used up (within the first few seconds)?
glucose and fatty acids are supplied by the blood stream
What is the primary energy source after 20-30 minutes?
fatty acids
During heavy exercise ATP is produced at a rate that ________ builds up too rapidly to undergo phsophorylation, so it is converted into ____________. What does this cause?
pyruvate, lactic acid, burning/soreness of the muscle
Muscle twitch
the mechanical response of an individual muscle fiber
Phases of a muscle twitch
Latent Period (delay between action potential and start of contraction),
Contraction Phase (end of the latent phase to peak of muscle tension),
Relaxation Phase (peak of tension to return of tension to zero)
Reproducibility
Repetitive stimulation produces twtches of the same magnitude and shape
Isometric Twitch
Muscle fiber DOES NOT shorten because the load is greater than the force of the contraction, graphing this results in a "bell-shaped curve
Load
force opposing contraction
Isotonic Twitch
Muscle fiber DOES shorten because the force of the contraction is at least equal to the load, graphing this show a plateau during which force or tension is constant
Factors that affect the force generation of individual muscle fibers
1. Frequency of Stimulation
2. Fiber Diameter
3. Changes in Fiber Length
Frequency of Stimulation
~("treppe") - peak in tension rises in step-wise fashion bc muscle is stimulated so frequently. Ca++ release exceeds Ca++ reuptake,
~another explanation is muscle fiber is "warmed up" and enzymatic rate increases
~summation - muscle twitch is slower than
tetanus
when summation results in levels of tension reaching a plateau, can be incomplete/unfused or complete/fused
maximum tetanic tension
when a muscle is at a maximum sustained tension
Fiber Diameter
The number of crossbridges in each sarcomere, and the geometrical arrangement (number parallel) of sarcomeres, can affect the amount of force generating capacity. The greater the cross-sectional area of a fiber, the more force it can generate.
Force Generating Capacity
Maximum tetanic tension a muscle fiber can generate in an isometric twitch
Changes in Fiber Length
The optimum length of a muscle fiber is when it allows for the greatest number of active crossbridges, but it decrease as fibers are stretched to far or shorten too much. (Thin filaments begin to overlap and interfere with each others movements, and thick
Factors Regulating Force Generated by Whole Muscles
Recruitment
Size Principle
Velocity of Shortening
Recruitment
The whole muscle can generate greater force by increasing the number of individual fibers that contract, due to varying the number of active motor units
Size Principle
When a muscle is called upon to generate small forces only small motor units are stimulated, but when larger forces are needed larger motor units are recruited.
Velocity of Shortening
When a muscle contracts isotonically under increasing loads the following occur:
1. latent period increases
2. duration of shortening decreases
3. velocity of shortening decreases
Factors of Types of Fibers
Speed of Contraction - slow-twitch (ex. soleus) or fast-twitch (ex. extraocular) myosin
Primary Mode of ATP Production - Glycolytic or Oxidative
Glycolytic Fibers
high cytosolic concentration of glycolytic enzymes, few mitochondria, bigger, fewer capillaries, lighter in color, can cause lactic acid build up
Oxidative Fibers
rich in mitochondria, high capacity for oxidative phsophorylation but are slower in producing ATP, smaller, darker in color, more capilaries, more resistant to fatigue, contain myoglobin
Very High intensity exercise may cause neuromuscular fatigue due to
depletion of ACh
Muscle growth
increase in fiber diameter due to increase in myofibrils
Two types of receptors for communicating info to CNS for coordinated activity
- muscle spindles
-Golgi tendon organs
Muscle Spindles
stretch receptors", 2-12 intrafusal fibers enclosed in a sheath of connective tissue surrounded regular skeletal muscle fibers (extrafusal fibers)
Central Bag Region
Expanded central region of muscle spindle fibers where the nuclei of these fibers are located
Two types of Sensory Receptors associated with intrafusal receptors
Annulospiral endings (end of a type Ia afferent fiber)
flower spray endings (branching end of type II afferent fibers of these neurons are located on either side of the central bag region)
Gamma Motor Neurons
innervate contractile fibers and adjust the sensitivity of stretch receptors, when they fire it increases tension in the intrafusal fibers
What happens when a voluntary muscle contraction occurs?
Both alpha (going to extrafusal fibers) and gamma (going to intrafusal fibers) contract together
Golgi Tendon Organs
capsules of connective tissue intertwined with the collagen fibers of the tendons, innervated by type Ib fibers that detect the degree of stretch imposed on the tendon
Smooth Muscle
Has thick and thin filaments but they are not arranged in myofibrils. Filaments are parallel but bundles are arranged obliquely in various directions and are connected to the cells exterior by dense bodies
Excitation Contraction Coupling in Smooth Muscle
1. Smooth Muscle contracts when voltage-gated Ca++ channels cause calcium to enter the cytosol from the SR and from outside the cell.
2. Calcium binds reversibly with calmodulin
3. Calcium-Calmodulin complex activates myosin light chain kinase.
4. MLCK ca
How does the contraction of skeletal muscle compare to smooth muscle contraction?
Smooth muscle contraction takes longer to initiate and terminate.
Smooth muscles are innervated by:
-Autonomic Neurons
-Can be Symp, P-Symp, or both
-Can be excitatory or inhibitory
-May cause relaxation or contraction depending on differences in the NT receptors in different locations
Autonomic nuerons form synapses with
groups of cells
Varicosities
The swellings found along the length of the axon where NT is released for smooth muscles, which cause a neighboring group of cells to contract.
Why do smooth muscle contract in groups?
Gap junctions between cells allow electrical signals to spread from one cell to another
Most smooth muscle cells respond to neuron stimulation in a
graded fashion.
Some smooth muscle cells display a resting degree of
tension or tone
Smooth muscle may also respond to the presence of
hormones and mechanical stretch
Multi-Unit Smooth Muscle
An organization where smooth muscle cells are mostly seperate and richly supplied with neurons. They are found in places where fine control of contraction is needed (ex. respiratory airways and large arteries).
Single-Unit Smooth Muscle
Smooth muscles are connected by gap junctions and there are fewer neurons (ex. walls of GI tract and uterus), may serve as pacemakers by depolarizing on a regular basis to produce pacemaker potentials
Cardiac Muscle
Has a sarcomere structure and a troponin/tropomyosin system for regulating contractions.
Extensively connected by gap junctions that allow APs to spread rapidly
Cardiac APs are broad and last for hundreds of milliseconds, which prevents summation and allo
Some Cardiac Cells in SA and AV nodes show
pacemaker activity, which allows the heart to beat by itself without neural input.