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Anatomy and Physiology
Muscles – Chapter 6 Part 1
* Muscles make up nearly half of our body mass. There are over 600 muscles in your body.
* Muscles are the “machines” of the body: muscles are responsible for essentially all body
movement.
* Function of the muscle is CONTRACTION or shortening.
THREE MUSCLE TYPES
They differ in the structure
of their cells, body location,
function, means by which they
are activated to contract.
1. Skeletal or Striated
2. Cardiac
3. Smooth or Visceral
MUSCLE FIBER = muscle cell, elongated; contraction due to movement of microfilaments
All muscles share similar terminology
Prefix myo refers to muscle
Prefix mys refers to muscle
Prefix sarco refers to flesh
1. Skeletal muscle tissue (striated) – make up skeletal muscle, attached to and cover bony
skeleton.
Usually longest muscle fibers
Contain bands or “striations”
Controlled voluntarily
Only type made to work by conscious control
Each muscle fiber is multinucleated
Most are attached by tendons to bones
SYNONYMOUS: Skeletal – Striated – Voluntary
Skeletal muscles can contract rapidly but tire easily and must rest short periods of time after
activity. They can also exert tremendous power.
2. Cardiac muscle tissue – found in heart making up most of its mass.
Striated
Involuntary
SYNONYMOUS: Cardiac – Striated – Involuntary
Cardiac muscle contracts at a fairly steady rate set by “pacemaker”. Neural control can shift
heart muscle into “higher gear” for brief periods of time, like when racing – etc.
3. Smooth muscle tissue (visceral) – found in walls of visceral organs like stomach, urinary
bladder, bronchi of lungs.
No striations
Involuntary
SYNONYMOUS: Smooth – Visceral – Involuntary – Nonstriated
Contractions of visceral muscle tissue is steady, slow, and sustained.
MUSCLE FUNTIONS:
1. MOVEMENT
2. MAINTAIN POSTURE OR STABILITY
3. GENERATE HEAT
4. COMMUNICATION
5. CONTROL OF BODY OPENINGS AND PASSAGES.
1. MOVEMENT – almost all movements of body are controlled my muscle contractions.
Skeletal muscles responsible for all locomotion and manipulation allowing you to respond
quickly to changes in external environment. Muscle contractions of respiratory, circulatory,
digestive, defecation, urination and childbirth muscles may or may not be under conscious
control but are still all controlled by muscle contraction in moving someone or something.
2. MAINTAIN POSTURE OR STABILITY – muscles of skeletal system continuously making one
or more tiny adjustments enabling us to maintain an erect or seated posture despite the
always present downward pull of gravity.
3. GENERATE HEAT – by-product of muscle metabolism and contractile activity. In making
ATP for muscle consumption, ¾ of the energy escapes as heat which helps to keep a normal
body temperature. The skeletal muscles produce as much as 85% of our body heat, which
helps in the function of enzymes and therefore to all of our metabolism.
4. COMMUNICATION – muscles are used for facial expressions, other body language,
writing, speech
5. CONTROL OF BODY OPENINGS AND PASSAGES – ring-like “sphincter muscles”
around eyelids, pupils, mouth control admission of light, food, drink into body; others
that encircle urethral and anal orifices control elimination of waste; and other sphincters
control movement of food, bile, and other materials through the body.
FUNCTIONAL CHARACTERISTICS OF MUSCLES
1. EXCITABILITY
2. CONTRACTILITY
3. EXTENSIBILITY
4. ELASTICITY
1. EXCITABILITY – ability to receive and respond to a stimulus present either internally or
externally. In muscles, stimulus is usually a chemical. Ex. Neurotransmitter released by a
nerve cell to send a message throughout the body.
2. CONTRACTILITY – ability to shorten (forcibly) when an adequate stimulus is received.
This is what sets muscles apart from all other tissues
3. EXTENSIBILITY – ability to be stretched or extended. Muscle fibers shorten when
contracted and extend when relaxed.
4. ELASTICITY – ability of a muscle fiber to resume its resting length after it has
contracted or has been stretched.
THE REMAINDER OF NOTES WILL DEAL WITH SKELETAL MUSCLES ONLY.
LEVEL OF ORGANIZATION - SKELETAL MUSCLES
LARGEST****Muscle (organ) – Fascicle (a portion of the muscle) – Muscle fiber
(cell) – Myofibril or fibril (complex organelle composed of bundles of myofilaments)
– Sarcomere (a segment of a myofibril) – Myofilament or filament (extended
macromolecular structure)****SMALLEST
GROSS ANATOMY OF A SKELETAL MUSCLE
Depending on size, a muscle is made up of hundreds to thousands of muscle fibers.
Muscles are made up mostly of muscle fibers but also connective tissue, blood vessels,
nerve fibers.
Muscles fibers are about 10 to 100 nanometers in diameter and up to 30 cm long.
It is surrounded by a layer of connective tissue called ENDOMYSIUM which allows room
for blood capillaries and nerve fibers to reach each muscle fiber.
Muscle fibers are grouped in bundles called FASCICLES which are visible to the naked
eye as parallel strands. These are the “grain” in a cut of meat; tender meat is easily
pulled apart along its fascicles.
Each fascicle is separated from neighboring ones by a thicker connective tissue sheath
called the PERIMYSIUM.
The muscle as a whole is surrounded by still another connective tissue layer, the
EPIMYSIUM. The epimysium are “overcoats” of dense fibrous connective tissue that
binds several fascicles together into FASCIA. Deep fascia (stringy) is found between
adjacent muscles and superficial fascia (sheets) between the muscles and skin. A lot of
fat is found with superficial fascia in areas such as the buttocks and abdomen. The
epimysia blend into the strong, cordlike tendons, or into sheet like APORNEUROSES
which attach muscles indirectly to bones, cartilages, or connective tissue coverings of
each other.
CONNECTIVE TISSUE WRAPPINGS OF SKELETAL MUSCLES
MICROSCOPIC ANATOMY OF SKELETAL MUSCLE CELLS (MUSCLE FIBERS)
Muscle fibers are multinucleated with the nuclei appearing right below the plasma
membrane.
SARCOLEMMA – plasma membrane of the muscle fiber (means “muscle husk”)
MYOFIBRILS – bundle of protein microfilaments (myofilaments) that push the nuclei up to
right below the sarcolemma. They nearly, completely fill the cytoplasm of the muscle fiber.
LIGHT (I) BANDS & DARK (A) BANDS - Both of these bands alternate in appearance along
the length of the myofibrils. This gives the muscle cell its striped appearance.
Z DISC – darker area of the myofibril that represents a midline break of the I –bands
H ZONE – lighter central area of the myofibril’s A-band
SARCOMERES – chains of tiny contractile units of the muscle cell aligned end to
end like a trains boxcars along the length of the myofibrils.
It is the smaller units within the sarcomeres that actually produce the banding patterns of
muscle fibers.
2 types of myofilaments in sarcomeres:
a. THICK FILAMENTS (myosin) – made of bundled molecules of protein called myosin.
ATPase (enzymes) found here to split ATP to generate the power for muscle contraction.
b. THIN FILAMENTS (actin) – made of contractile protein called actin. They are anchored to
the Z-disc
BARE ZONE – break in
the thin filaments for
contraction of muscle
fiber
SARCOPLASMIC RETICULUM – specialized ER surrounding each myofibril - like a sleeve. It
stores calcium and releases it on demand when the muscle fiber is stimulated to contract.
Calcium provides the final “go” signal for contraction.
PROPERTIES OF SKELETAL MUSCLE
ACTIVITY
·Irritability – ability to receive and
respond to a stimulus
·Contractility – ability to shorten when
an adequate stimulus is received
NERVE STIMULUS TO MUSCLE
Skeletal muscles must be stimulated
by nerve impulses to contract
MOTOR UNIT – One neuron (nerve
cell) and all the muscle fibers it
stimulates.
NEUROMUSCULAR JUNCTIONS –
association site of nerve and muscle
SYNAPTIC CLEFT – gap between nerve and muscle
Nerve and muscle do not make contact
Area between nerve and muscle is filled with interstitial fluid
TRANSMISSION OF NERVE IMPULSE TO MUSCLE
NEUROTRANSMITTER – chemical released
by nerve upon arrival of nerve impulse. The
neurotransmitter for skeletal muscle is
acetylcholine.
Neurotransmitter attaches to receptors on
the sarcolemma.
Sarcolemma becomes permeable to sodium
(Na+).
Sodium rushing into the cell generates an
action potential.
Once started, muscle contraction cannot be
stopped.
THE SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION
Activation by nerve causes myosin heads to attach to binding sites on the thin filament
Myosin heads then bind to the next site of the thin filament.
This continued action causes a sliding of the
myosin along the actin.
The result is that the muscle is shortened
(contracted).
CONTRACTION OF A SKELETAL MUSCLE
-Muscle fiber contraction is “all or none”
-Within a skeletal muscle, not all fibers may be stimulated during the same interval
-Different combinations of muscle fiber contractions may give differing responses
MUSCLE RESPONSE TO STRONG STIMULI
-Muscle force depends upon the number of fibers stimulated
-More fibers contracting results in greater muscle tension
-Muscles can continue to contract unless they run out of energy
ENERGY FOR MUSCLE CONTRACTION
·Initially, muscles used stored ATP for energy
·Bonds of ATP are broken to release energy
·Only 4-6 seconds worth of ATP is stored by muscles
·After this initial time, other pathways must be utilized to produce ATP
MUSCLE FATIGUE AND OXYGEN DEBT
-When a muscle is fatigued, it is unable to contract
-The common reason for muscle fatigue is oxygen debt
-Oxygen must be “repaid” to tissue to remove oxygen debt
-Oxygen is required to get rid of accumulated lactic acid
-Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less
TYPE OF MUSCLE CONTRACTIONS: ISOTONIC AND ISOMETRIC
ISOTONIC CONTRACTIONS – “same tension”– muscle shortens, movement occurs. Ex.
bending the knee, rotating the arms, and smiling
ISOMETRIC CONTRACTIONS –
“same measurement” – tension in
the muscle keeps increasing.
Muscle is pitted against immovable
object (more or less). This occurs
when a muscle attempts to move a
load that is greater than the force
that the muscle is able to develop.
Ex. attempting to lift a car singlehandedly, your arm muscles are
contracting isometrically (without
causing movement).
MUSCLE TONE is the result of different motor units, which are scattered through the
muscle, being stimulated by the nervous system in a systematic way.
·Some fibers are contracted even in a relaxed muscle
·Different fibers contract at different times to provide muscle tone
·The process of stimulating various fibers is under involuntary control
If the nerve supply to a muscle is destroyed, the muscle is no longer stimulated in this
manner. It loses tone and becomes paralyzed. Soon after it becomes soft and flabby
and begins to ATROPHY – waste away. Decreases at the rate of 5% per day of
immobilization.
EFFECTS OF EXERCISE
Results of increased muscle use
·Increase in muscle size
·Increase in muscle strength
·Increase in muscle efficiency
·Muscle becomes more fatigue resistant