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Chapter 8

BIOL 2404 Chapter Notes - Chapter 8: Resting Potential, Neuromuscular Junction, Skeletal Muscle


Department
Biology
Course Code
BIOL 2404
Professor
Henderson
Chapter
8

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Chapter 8: Histology and Physiology of Muscles
Functions of the Muscular System
1. Body movement (Skeletal Muscle)
2. Maintenance of posture (Skeletal Muscle)
3. Respiration (Skeletal Muscle)
4. Production of body heat (Skeletal Muscle)
5. Communication (Skeletal Muscle)
6. Constriction of organs and vessels (Smooth Muscle)
7. Heartbeat (Cardiac Muscle)
Functional Characteristics of Muscle
1. Contractility: the ability to shorten forcibly
2. Excitability: the ability to receive and respond to stimuli
3. Extensibility: the ability to be stretched or extended
4. Elasticity: the ability to recoil and resume the original resting length
Types of Muscle Tissue
The three types of muscle tissue are skeletal, smooth, and cardiac
These types differ in
o Structure
o Location
o Function
o Means of activation
Each muscle is a discrete organ composed of muscle tissue, blood vessels,
nerve fibers, and connective tissue
Skeletal muscles are responsible for most body movements
o Maintain posture, stabilize joints, and generate heat
Smooth muscle is found in the walls of hollow organs and tubes, and moves
substances through them
o Helps maintain blood pressure
o Squeezes or propels substances (i.e., food, feces) through organs
Cardiac muscle is found in the heart and pumps blood throughout the body
Skeletal Muscle Structure
Skeletal muscle cells are elongated and are often called skeletal muscle fibers
Each skeletal muscle cell contains several nuclei located around the
periphery of the fiber near the plasma membrane
Fibers appear striated due to the actin and myosin myofilaments
A single fiber can extend from one end of a muscle to the other
Contracts rapidly but tires easily
Is controlled voluntarily (i.e., by conscious control)
Fascia is a general term for connective tissue sheets
The three muscular fascia, which separate and compartmentalize individual
muscles or groups of muscles are:
o Epimysium: an overcoat of dense collagenous connective tissue that
surrounds the entire muscle
o Perimysium: fibrous connective tissue that surrounds groups of
muscle fibers called fascicles (bundles)
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o Endomysium: fine sheath of connective tissue composed of reticular
fibers surrounding each muscle fiber
The connective tissue of muscle provides a pathway for blood vessels and
nerves to reach muscle fibers
The connective tissue of muscle blends with other connective tissue based
structures, such as tendons, which connect muscle to bone
Muscle Fibers
o Terminology
Sarcolemma: muscle cell plasma membrane
Sarcoplasm: cytoplasm of a muscle cell
Myo, mys, and sarco: prefixes used to refer to muscle
o Muscle contraction depends on two kinds of myofilaments: actin and
myosin
Myofibrils are densely packed, rod-like contractile elements
They make up most of the muscle volume
Actin (thin) myofilaments consist of two helical polymer strands of F actin
(composed of G actin), tropomyosin, and troponin
The G actin contains the active sites to which myosin heads attach during
contraction
Tropomyosin and troponin are regulatory subunits bound to actin
Myosin (thick) myofilaments consist of myosin molecules
Each myosin molecule has
o A head with an ATPase, which breaks down ATP
o A hinge region, which enables the head to move
o A rod
A cross-bridge is formed when a myosin head binds to the active site on G
actin
Sarcomeres
o The smallest contractile unit of a muscle
o Sarcomeres are bound by Z disks that hold actin myofilaments
o Six actin myofilaments surround a myosin myofilament
o Myofibrils appear striated because of A bands and I bands
Thick filaments: extend the entire length of an A band
Thin filaments: extend across the I band and partway into the A band
Z-disc: a coin-shaped sheet of proteins (connectins) that anchors the thin
filaments and connects myofibrils to one another
Thin filaments do not overlap thick filaments in the lighter H zone
M lines appear darker due to the presence of the protein desmin
The arrangement of myofibrils within a fiber is so organized a perfectly
aligned repeating series of dark A bands and light I bands is evident
Sliding Filament Model
Actin and myosin myofilaments do not change in length during contraction
Thin filaments slide past the thick ones so that the actin and myosin
filaments overlap to a greater degree
o Upon stimulation, myosin heads bind to actin and sliding begins
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o Each myosin head binds and detaches several times during
contraction (acting like a ratchet to generate tension and propel the
thin filaments to the center of the sarcomere)
In the relaxed state, thin and thick filaments overlap only slightly
As this event occurs throughout the sarcomeres, the muscle shortens
The I band and H zones become narrower during contraction, and the A band
remains constant in length
Actin and myosin myofilaments in a relaxed muscle (below) and a contracted
muscle are the same length. Myofilaments do not change length during
muscle contraction
During contraction, actin myofilaments at each end of the sarcomere slide
past the myosin myofilaments toward each other. As a result, the Z disks are
brought closer together, and the sarcomere shortens
As the actin myofilaments slide over the myosin myofilaments, the H zones
(yellow) and the I bands (blue) narrow. The A bands, which are equal to the
length of the myosin myofilaments, do not narrow because the length of the
myosin myofilaments does not change
In a fully contracted muscle, the ends of the actin myofilaments overlap at the
center of the sarcomere and the H zone disappears
Physiology of Skeletal Muscle Fibers
Membrane Potentials
o The nervous system stimulates muscles to contract through electric
signals called action potentials
o Plasma membranes are polarized, which means there is a charge
difference (resting membrane potential) across the plasma membrane
o The inside of the plasma membrane is negative as compared to the
outside in a resting cell
o An action potential is a reversal of the resting membrane potential so
that the inside of the plasma membrane becomes positive
Ion Channels
o Assist with the production of action potentials
Ligand-gated channels
Voltage-gated channels
Action Potentials
o Depolarization results from an increase in the permeability of the
plasma membrane to Na+
o If depolarization reaches threshold, an action potential is produced
o The depolarization phase of the action potential results from the
opening of many Na+ channels
o The repolarization phase of the action potential occurs when the Na+
channels close and K+ channels open briefly
o Occur in an all-or-none fashion
A stimulus below threshold produces no action potential
A stimulus at threshold or stronger will produce an action
potential
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