BIOL-4000 LECTURE NOTES #6
MUSCLE - POWERPOINT
Muscle tissue is responsible for most
types of body movement.
I. Types of muscle:
A. Striated muscle - skeletal musculature, striated in appearance
under microscope. Cells are unbranched and are multinucleate syncytia. These
muscles are capable of voluntary, quick, forceful contractions.
B. Cardiac muscle - heart muscle, striated, cells may be
branched. Cardiac muscle cells are either mono- or binucleate cells. They are
connected with one and other by specialized junctional complexes called
intercalated disks. Cardiac
muscle is capable of involuntary, strong, rhythmic contractions
C. Smooth muscle - this muscle is not striated and is found in
the walls of the visceral organs. Mononucleate cells. In addition to
contraction, smooth muscle cells synthesize collagen, elastin, and
proteoglycans (like fibroblasts). These muscle cells are capable of relatively
slow contractions as compared to those of striated or cardiac muscle.
II. Basic units of muscle cell
A. sarcolemma - plasmalemma of muscle cells. External to this
cell membrane is a well developed basement membrane.
B. sarcoplasm - cytoplasm of muscle cells excluding the
C. sarcoplasmic reticulum - smooth endoplasmic reticulum of muscle cells.
D. epimysium - thick layer of collagenous connective tissue
that separates large bundles of muscle.
E. perimysium - collagenous connective tissue that separates
smaller bundles of muscle cells called fascicles.
F. fascicle - bundle of muscle cells bounded by perimysium.
G. endomysium - thinner layer of connective tissue that
separates individual muscle cells.
III. Skeletal muscle
A. Connective tissue in the form of epimysium,
perimysium, and endomysium surrounds the components of striated muscle as
B. Skeletal muscle is generally connected to
bone via a piece of connective tissue called a tendon.
C. As mentioned above, individual muscle cells
are syncytial (That is to
say, each striated muscle cell contains multiple nuclei (multinucleate).
D. Each individual muscle cell is called a muscle
fiber. Within the sarcoplasm of these cells are myofibrils composed of repeating sarcomere units (see below). These sarcomeres are the
actual contractile apparatus.
E. The myofibrils are linear arrays of
structures known as sarcomeres
that are arranged in an end to end repeating pattern. The sarcomeres contain
filaments of actin and myosin
that interact to cause
contraction of the muscle cells. See below for sarcomere structure.
F. If we look at the membranous surface of a skeletal,
muscle cell we find multiple synapses from motor neurons in the central nervous system. These synapses
are called myoneural junctions.
Within the swollen ends of the axons that contact the muscle cell are small
vesicles called synaptic vesicles
that contain acetylcholine.
When a nervous impulse (action potential) reaches the synapse, the contents of these vesicles are released into
the gap between the axon and the muscle cell and are responsible for initiating
an electric action potential
in the sarcolemma that causes the muscle to contract.
IV. The Basic Sarcomere Unit of
A. The basic unit of striated muscle, the
sarcomere is diagramed above. It is composed of thin actin and thick myosin
filaments (as well as a number of other molecules) arranged in an
interdigitated linear array. These molecules form the sarcomere unit, that
A-band (Anisotropic band) - area containing overlapping actin and
myosin filaments accept in H-band region.
H-band - area of myosin filaments where there is
no over lap of actin filaments.
I-band (Isotropic band) - thin actin filaments +
Z-line - where actin filaments of adjacent
sarcomeres are anchored.
M-line - formed by connections between the
central portions of adjacent myosin filaments.
B. Let's consider at the muscle cell for a
moment and consider how it contracts.
C. In skeletal muscle, contraction is under
voluntary control. I decide to raise my arm for instance. Nervous stimuli in
the form of electric action potentials are transmitted along motor nerves to
the muscles concerned with raising the arm. These impulses cause the release of
acetylcholine in synaptic vesicles at the myoneural junctions on the muscle
cells. This causes an action potential in the sarcolemma that is transmitted
along the surface of the muscle cell.
D. The skeletal muscle cell surface contains
many tubular invaginations that extend through the cell sarcoplasm and around
the individual myofibrils. These structures effectively carry the electrical
impulse into the interior of the cell. In this way, the action potential
reaches the sarcoplasmic reticulum of the cell much more quickly than it would
in the electrical impulse had to pass from the surface of the cell into the
cytoplasm to reach the sarcoplasmic reticulum.
E. The action potential causes the sarcoplasmic
reticulum to release Ca+2 ions.
F. These bind to a site on the troponin
molecules of the thin actin filaments. This causes the troponin molecules to
change shape and expose a site on the actin that can bind the side chains of
heavy meromysin on the thick myosin filaments. This binding catalyses a
reaction that breaks down ATP that is bound to the myosin side chains, thus
releasing energy that causes the heavy meromysin side chain to bend. As the
numerous meromysin side chains bind to the actin filament and bend together in
the same direction (the net bending after binding to actin is toward the M
line), the actin and myosin filaments slide over each other. This movement
lines up other actin and heavy meromyosin molecules which can bind and bend to
move the filaments further. As the activity rapidly repeats itself, the overlap
of filaments becomes complete and the I-bands and H-bands of the sarcomeres become
thinner until the muscle relaxes.
G. This sort of action in thousands of repeating
sarcomeres of each myofibril in the cell causes considerable decrease in the
muscle cells length.
H. The ATP for all this activity comes from
numerous mitochondria (called sarcosomes) that are associated with the
myofibrils. During contraction, energy is initially derived from ATP and
phosphocreatine from stores present in the sarcoplasm. As activity continues,
the mitochondria metabolise glucose derived from glycogen molecules that are
stored in the sarcoplasm in order to provide more ATP.
I. Oxygen is obviously necessary for
mitochondrial metabolism of glucose. In order to provide enough oxygen, the
muscle cells also contain myoglobulin which is an oxygen binding protein
similar to hemoglobulin and has a high affinity for oxygen. This makes it
possible for the muscle cells to have a ready store of oxygen on hand for their
activity. It also allows them to pull oxygen out of the blood at a high rate so
that the high metabolic rates can be maintained.
V. Cardiac muscle
A. Unlike skeletal muscle, cardiac muscle cells
are not a syncytium for the most part, though some cells may have two nuclei.
B. The structure of cardiac muscle cells is
similar to that of striated skeletal muscle in that myofibrils and sarcomeres
are present with activity mediated by release of Ca+2 from sarcoplasmic
C. The major differences between these two
muscle types are that the cardiac muscle cells are branched and are held
together by intercalated disks. They are also shorter in length than skeletal muscle
1. Intercalated disks are specialized junctional complexes that bind
cardiac muscle cells together. These so-called disks are interdigitating
regions of the plasmalemma of adjacent cardiac muscle cells that hold the cells
together. The intercalated disks form the irregular, jagged, dark lines that
are characteristic seen in appropriately stained cardiac muscle sections.
2. This junctional complex is composed of a
number of structures that are organized along adjacent muscle cell plasmalemmas
in a repeating array,
a. desmosomes (macula adherens) - previously described
structures that hold cells together. Located between adjacent myofilaments.
b. fascia (zonula) adherens - where the myofilaments of the sarcomeres at
the ends of myofibrils adhere to the sarcolemma. Located where myofilaments end
at the muscle cell plasmalemma.
c. gap junctions are present - connectiong between muscle cells
that allow transfer of ions between them. This allows the cells to coordinate
their activities. Action potentials can spread quickly between the sarcoplasmic
reticulum of cardiac muscle cells via gap junctions. Thus, cardiac muscle cells
can coordinate their movements. The gap junctions are located along the
plasmalemmas of adjacent muscle cells in the regions between myofilaments.
D. Cardiac muscle cells also have a transverse
tubule system present, but it
is not as regular as the sarcolemma system of skeletal muscle and there are
VI. Smooth muscle
A. This sort of muscle consists of long, overlapping,
spindle shaped cells that have some characteristics that are similar to those
B. Smooth muscle cells similarity to fibroblasts
is evident in that they are able to synthesize collagen, elastin, and
C. There are no sarcomere structures, but filaments
of actin and a type of myosin are present. Thus, contraction is much less
organized and occurs more slowly than it does in striated or cardiac muscle.
Another reason for this slower contraction is that smooth muscle cells do not
contain a transverse tubule system. (Also see G. below)
D. Since the actin and myosin filaments are not
constrained by a sarcomere/myofilament arrangement, the actin and myosin
filaments are able to achieve a greater degree of overlap when they contract
resulting in a greater degree of contraction.
E. While smooth muscle cells are slow to
contract, they have the ability to remain contracted for long periods of time.
F. The bundles of smooth muscle are organized as
fascicles similar to what is seen in striated and cardiac muscle, Thus, a
perimysium with endomysium between cells and epimysium deliniating bundles of
fascicles can be identified. However, in sectioned tissue, this arrangement is often
not very evident, presumably because the regions of connective tissue are much
thinner than those of cardiac and skeletal muscle.
G. The contaction of smooth muscle cells is
involuntary and the neuromuscular junctions controlling contractile rhythms may
be on the surrounding epimysium rather than directly on muscle cells. As a
result, neurotransmitters have to diffuse across this connective tissue layer
and onto the plasmalemma of the smooth muscle cells in order to initiate the
action potential that causes contraction. This is another reason for the slower
contraction of smooth muscle cells.
H. Smooth muscles exhibit spontaneous
contractile activity (doesn't require nervous stimulation). Thus, the
innervation that is present acts to modify the contractile activity rather than