Elasticity - ability to return to original shape and length
Muscle
tissue composes 40-50% of total body weight. There are three types of
muscle: smooth muscle found in the walls of the viscera, cardiac muscle found
only in the heart, and skeletal muscle associated with bones.
3 types of muscles:
Non-striated
muscle
1.Smooth muscle - controlled by the autonomic nervous system;
may either be generally inactive and then respond to neural stimulation or
hormones or may be rhythmic.
Occurs in small groups
(ex. arrector pili) or sheets of overlapping cells tightly bound together (ex.
digestive tube, uterus, bladder, respiratory tract, vessels), can regenerate.
Striated
muscle
2.Cardiac muscle - found in the heart, acts like rhythmic smooth
muscle, modulated by neural activity and hormones
Branching
network of cells forming layers that wind in overlapping spirals to form heart.
Cells connected by intercalated discs containing desmosomes and gap junctions.
3.Skeletal muscle - move us around and responsible for most of
our behavior; most attached to bones at each end via tendons
Bundles of
cylindrical fibers (cells) run in parallel (fascicle); in various arrangements
such as spindles, bands, or sheets; typically between 2 bones & across a
joint
Motor unit -
functional unit of muscle, consists of a motor neuron and all muscle fibers it
innervates. Degree of muscle contraction depends on number of active motor
unit or recruitment
movement:
flexion - contraction
of flexor muscles, drawing in of a limb
extension - opposite of
flexion, produced by contraction of extensor muscles (antigravity)
Motor Units
·Motor unit: all of the individual muscle fibers innervated by a
single motor nerve fiber
small muscles that
require fine control have many motor units, with few fibers per unit (2-3
possible)
large muscles that do
not require fine control typically have fewer motor units, with many
fibers per unit
·To get larger force production, more motor units are recruited,
more individual fibers contract
The muscle that you can see is composed
of subunits called fascicles. Fascicles are
bundles of individual muscle fibers. Each fiber
is one elongated cell that may extend for the length of the muscle. Each muscle
fiber cell has several nucleii (unlike most cells, which have only one), and is
segmented into distinct sectional bands. Within each muscle cell are numerous myofibrils,
which also extend for the length of the muscle cell. Sarcomeres
are the basic contractile subunit of myofibrils.
Actin and myosin
are the two principal muscle proteins, and they are found in myofibrils. They
are arranged in a ring-like structure, usually with six (thin) actin strands
surrounding a (thicker) myosin fibril. Again, they run parallel and lengthwise.
The myosin fibril has numerous small protrusions called crossbridges. The actin
strand is actually intertwined with an even thinner, ribbon-like protein called
tropomyosin, and a smaller molecule, called troponin, associates with
tropomyosin in this structure.
When a nerve impulse signals the muscle to 'do something' the activity causes
channels in the sarcoplasmic reticulae to open their gates and release calcium
into the cytoplasm. In skeletal muscle cells the sarcoplasmic reticulae (SR) is
an extensive intracellular network which serves as the storage area for calcium.
Usually, the inside of the cell cytoplasm has a very low calcium concentration.
When it rises, some of the calcium diffuses over to the muscle protein fibers
and causes a conformational change.
Ca++ ions will bind to troponin and cause it to rotate slightly. This is
enough so that the tropomyosin moves, and actin now is exposed to the myosin
below. The myosin crossbridge then jumps up and binds to the exposed actin. The
myosin crossbridge drags along the actin fiber like a ratchet, completing a
"power stroke".
When all the crossbridges in a sarcomere do this at the same time, the
sarcomere contracts. After the nerve impulse ends, the SR has mechanisms to
reabsorb the free calcium and put it back into storage. As calcium disassociates
from the troponin, ATP binds to the crossbridge to 'disconnect' the bridge from
the actin. The actin fibers change back to their previous positions and the
sarcomere relaxes.
One Sarcomere
contracting and relaxing
A muscle cell does not necessarily go back to complete relaxation right away.
It can remain contracted through a series of stimulations. This process, called
summation, increases the total force of muscular contraction. When the stimulus
is great enough, many sarcomeres in many fibers are "recruited" and
the muscle as a whole contracts. This is why we can lift or push varying amounts
of resistance . . . more or less cells are recruited, and to a greater or lesser
extent. Muscle failure occurs at the point where the maximum number of fibers
are being stressed to their limits.
Muscle organization
Muscles are made up of many fascicles
Fascicles are made up of many fibers
Fibers are made up of many myofibrils
Myofibrils are made up of many sarcomeres
endomysium
surrounds the individual muscle fiber
perimysium
surrounds the fascicles
epimysium
surrounds the whole muscle
Click here (Muscles
Tutorial) to find help learning the muscles of the
human body!
If your muscles don't look like the picture above here are
some exercises that may help.
Muscles produce movement by contraction. Muscles can only
pull on a bone in one direction. To allow for movement in two directions two
muscles are required. Muscles that cause movement must be paired. When one
muscle contracts (pulls) the opposite muscle must extend (relax). Most muscle
injuries are due to muscles not being properly relaxed (stretched out &
warmed up).
Muscular coordination is the ability of the various 400
body muscles to respond in harmony with the nerve impulses.
response to exercise
increased muscle strength & endurance
increased size of muscle fibers
higher myoglobin content
more efficient mitochondrial energy system
more capillaries / muscle fiber
more efficient oxygen extraction
exercise training
high intensity, low repetition
muscular strength
greatest increase in muscle fiber size
most effective for developing type II fibers
high repetition, low intensity
muscular endurance
most effective for developing type I fibers
Muscles
contracting and relaxing
