All-or-None
response- in order for the sarcolemma to depolarize, and thus result in a
contraction a certain stimulus strength (threshold stimulus) must be
achieved. If the stimulus strength
falls short of this threshold, the chain-reaction will not occur and Ca++
will not be released from the cisternae of the sarcoplasmic reticulum. If the stimulus strength is much greater
than the threshold, the muscle fiber response (strength and speed of
contraction) will be the same as if the threshold was just barely met.
Think
of it like a light switch. It doesn’t
matter how strong or fast you turn on a light switch, the light from the bulb
is the same. If you want to have more
light in the room you have to turn on more bulbs.
·
The
same with the strength of muscle contraction.
If you want more strength you must activate a greater number of muscle
fibers. That means more motor units
must be recruited, and thus more muscle fibers will be involved in the
contraction.
Latent
Period- time lag between application of stimulus (membrane depolarization) and
when the contraction occurs. This time
lag can be attributed to the amount of time it takes for Ca++ to diffuse out of
the sarcoplasmic reticulum, bind to troponin, for troponin to remove
tropomyosin from the active sites on Actin, and for cross-bridges to form.
Period
of Contraction- time period during which muscle fiber contracts
Period
of Relaxation- time after muscle fiber contracts.
Refractory
Period- the membrane depolarization that causes Ca++ to diffuse out
of the sarcoplasmic reticulum is set up by a concentration gradiant of Na+
on the outside of the cell and K+ on the inside of the cell.
·
When
depolarized, membrane proteins open channels that cause Na and K to diffuse
down their chemical gradients, until the membrane proteins close the channels
again.
·
Then
it takes time for the active transport pumps to bring Na back into the cell and
K out of the cell. Until this happens,
no further muscle contraction can be induced and this is called the refractory
period.
Tetany-
When Ca++ remain at high concentrations with the sarcoplasm for extended
periods, the muscle fiber will not relax, but maintains a sustained
contraction. This can occur normally,
when the depolarization stimulus of the sarcolemma occurs at high rates, and is
in fact faster than the Ca++ active transport pumps can sequester the Ca++ back
into the sarcoplasmic reticulum. Thus,
more and more Ca++ can build up in the sarcoplasm and result in maximal muscle-fiber
contraction (called summation).
Even
while at “rest”, a muscle fiber undergoes low levels of contraction resulting
in muscle tone. Period signal from the
nervous system maintain muscle tone and these are especially important in
postural muscles.
Note: if the innervation to muscle fibers is cut
or lost, muscle tone will be lost and eventually the muscle will atrophy from
loss of use. The body does not maintain
structures that are not being used.
Types
of Muscle Fibers
Slow-twitch fibers (Type I)- oxidative
metabolism, resistant to fatigue, high amounts in postural muscles (back), deep
red in appearance due to high concentrations of myoglobin bound with
Oxygen. Lots of mitochondria are
present for generation of ATP via Citric Acid cycle and Electron Transport
Chain.
Fast-twitch
glycolytic fibers (Type IIa)- white in appearance because of lesser amounts of
myoglobin, and poorer blood supply.
Reduced respiratory capacity.
Found in high concentrations in muscles of rapid movement, hands and oculomotor
muscles. Faster ATPase activity and
higher concentrations of store Ca++ (steeper chemical gradient gives
faster diffusion rate and faster contraction speed). These muscles do fatigue
Fast-twitch
fatique resistant fibers (Type II b)- intermediate properties between Type I
and II a.
All
muscles consist of combinations of these fiber types, with varying
concentrations of the different types depending on the function.
Smooth
Muscle-
Shorter
than skeletal muscle and have centrally located nucleus
Not
a syncytium like skeletal muscle
Lack
Transverse tubules and extensive Sarcoplasmic reticulum.
Fewer
myofibrils, and not all arranged in parallel (non-striated)
Multiunit
smooth muscle- not well organized and each cell is independent of the others (has
independent innervation). Found in iris
of eye and in walls of blood vessels
Visceral
smooth muscle- sheets of spindle-shaped cells interconnected via gap junctions
(recall this permits the communication of cytoplasm)
Cells
respond as single unit, that is they coordinate contraction. When one cell is stimulated its neighbors
quickly thereafter become stimulated.
Found in tubular organs like Gastrointestinal tract.
Peristalsis-
wavelike muscle contraction within tubular organs that move contents along.
Contraction-
·
Calmodulin
used in place of troponin. Ca++ comes
from extracellular fluid rather than within the sarcoplasmic reticulum.
·
Neurotransmitters
include acetylcholine and norepinephrine.
These stimulate or inhibit contraction by binding to membrane
post-synaptic membrane proteins.
·
Hormones
also exhibit a stimulatory or inhibitory effect. Stretching the muscle can also stimulate contraction.
·
Slower
contraction than skeletal muscle, but more sustained contraction for a given
amount of ATP
Cardiac
Muscle- branched, striated cells attached end to end. (Intercalated disks mark the boundaries between
cells). More similar to skeletal
muscle, except the cisternae of sarcoplasmic reticulum are less well developed
and store less Ca++. Ca++
comes from extracellular fluid via deeper penetration of the transverse
tubules.
Intercalated
disks are specialized gap junctions that allow ions to diffuse between cells,
and cause membrane depolarization to travel and branch to other cells. This results in a coordinated contraction
and relaxation of cardiac muscle.
There
are specialized areas of cardiac muscle called the Sinoatrial node and the
Atrioventricular node that are made of cardiac muscle with leaky membranes to
Ca++. This causes a self-exciting and rhythmic
stimulus for contraction, and explains why a heart can continue to beat in the
absence of innervation.