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All muscle cells have the following characteristics:
R(E)CCEE
- 1. Responsiveness (excitability)
- 2. Conductivity
- 3. Contractility
- 4. Extensibility
- 5. Elasticity
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skeletal muscle
-- may be defined as voluntary striated muscle that is usually attached to one or more bones
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- a skeletal muscle exhibits alternating light and dark transverse bands -
-- or striations, that result from an overlapping arrangement of their internal contractile proteins
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because of their extraordinary length, skeletal muscle cells are usually called:
-- muscle fibers or myofibers
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the plasma membrane of a muscle fiber is called:
-- the sarcolemma
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the cytoplasm of a muscle fiber is called:
the sarcoplasm.
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the sarcoplasm is occupied by long protein bundles called:
myofibrils
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the sarcoplasm also contains an abundance of glycogen and myoglobin.
the sarcoplasm also contains an abundance of glycogen and myoglobin.
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glycogen
- a starch like carbohydrate that provides energy for the cell during heightened levels of exercise
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myoglobin
-- the red pigment which stores oxygen until needed for muscular activity
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muscle fibers have multiple flattened or sausage-shaped nuclei pressed against the inside of the sarcolemma.
Their unusual multinuclear condition results from their embryonic development - several stem cells called myoblasts fuse to produce each muscle fiber.
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some myoblasts remain as unspecialized satellite cells between the muscle fiber and endomysium.
When a muscle is injured, satellite cells can multiply and produce new muscle fibers to some degree.
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- most other organelles of the cell, such as mitochondria, are packed into the spaces between the myofibrils.
- most other organelles of the cell, such as mitochondria, are packed into the spaces between the myofibrils.
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the smooth endoplasmic reticulum (SR), forms a network around each myofibril
It periodically exhibits dilated end-sacs called terminal cisternae, which cross the muscle fiber from one side to the other.
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The sarcolemma has tubular infoldings called transverse (T) tubules, which penetrate through the cell and emerge on the other side.
- a T tubule and the two terminal cisternae associated with it constitute a triad.
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the SR is a reservoir of calcium ions
the T tubule signals the SR when to release these calcium bursts
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each myofibril is a bundle of parallel protein microfilaments called:
myofilaments
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there are three kinds of Myofilaments:
- 1. Thick filaments
- 2. Thin filaments
- 3. Elastic filaments
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Thick filaments
- each is made of several hundred molecules of a protein called myosin
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Thin filaments
composed primarily of two intertwined strands of a protein called fibrous (F) actin
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Elastic filaments
- made of huge springy protein called titin (connectin)
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myosin and actin are called:
-contractile proteins because the do the work of shortening the muscle fiber.
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tropomyosin and troponin are called:
-regulatory proteins because they act like a switch to determine when the fiber can contract and when it cannot
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dystrophin
- - an enormous protein located between the sarcolemma and the outermost myofilaments; - it links actin filaments to a peripheral protein on the inner face of the sarcolemma
- - it is a key element transferring the forces of the myofilament movement to the connective tissues of the muscle as a whole
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striated muscle has dark A bands alternating with lighter I bands
in the middle of the A band, there is a lighter region called the H band, into which the thin filaments do not reach
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the thick filaments originate at a dark M line in the middle of the H band.
each light I band is bisected by a dark narrow Z disc (Z line), which provides anchorage for the thin filaments and elastic filaments.
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each segment of a myofibril from one Z disc to the next is called a sarcomere
-- the functional contractile unit of the muscle fiber.
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skeletal muscles are served by nerve cells called somatic motor neurons,
- whose cell bodies are in the brainstem and spinal cord.
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their axons, called ???, lead to the skeletal muscles
somatic motor fibers
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each nerve fiber branches out to a number of muscle fibers, but each muscle fiber is supplied by only one motor neuron
one nerve fiber and all the muscle fibers innervated by it are called a motor unit
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the point where a nerve fiber meets its target cell is called a:
--synapse.
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when the target cell is a muscle fiber, the synapse is called a:
- neuromuscular junction (NMJ), or motor end plate
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at each synapse, the nerve fiber ends in a bulbous swelling called a:
synaptic knob
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the knob doesn’t directly touch the muscle fiber but is seperated from it by a narrow space called the:
synaptic cleft
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Schwann cell
-- envelops the entire junction and isolates it from the surrounding tissue fluid
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contraction
- the step in which the muscle fiber develops tension and may shorten (muscles often “contract” or develop tension, without shortening)
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- sliding filament theory
-- holds that the myofilaments do not become any shorter during contraction; rather, the thin filaments slide over the thick ones and pull the Z discs behind them, causing each sarcomere as a whole to shorten
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rigor mortis
- the hardening of the muscles and stiffening of the body that begins 3 to 4 hours after death; the thick and thin filaments remain rigidly cross-linked until the myofilaments begin to decay; it peaks about 12 hours after death and then diminishes over the next 48-60 hours
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length-tension relationship
- the amount of tension generated by a muscle, and therefore the force of its contraction, depends on how stretched or contracted it was before it was stimulated
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the central nervous system continually monitors and adjusts the length of the resting muscles, maintaining a state of partial contraction called muscle tone
- this maintains optimum length and makes the muscles ideally ready for action
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the elastic filaments of the sarcomere also help to maintain enough myofilament overlap to ensure an effective contraction when the muscle is called into action
a stimulus thus causes a quick cycle of contraction and relaxation called a twitch
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Isometric contraction
(never move the load) contraction without a change in length
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Isotonic contraction
(move the load) contraction with a change in length but no change in tension
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all muscle contractions depend on ATP; no other energy source can serve in its place.
the supply of ATP depends, in turn, on the availability of oxygen and organic energy sources such as glucose and fatty acids.
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two main pathways of ATP synthesis:
anaerobic fermentation & aerobic respiration
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anaerobic fermentation
- enables a cell to produce ATP in the absence of oxygen, but the ATP yield is very limited and the process produces a toxic end product, lactic acid, which is a major factor in muscle fatigue
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aerobic respiration
produces far more ATP and less toxic end products (carbon dioxide and water), but it requires a continual supply of oxygen.
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muscle fatigue
- the progressive weakness and loss of contractility that results from prolonged use of the muscles
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oxygen debt
-- the difference between the resting rate of oxygen consumption and the elevated rate following an exercise
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Physiological Classes of Muscle Fibers
- 1. Slow oxidative (SO), slow-twitch, red, or type I fibers - high endurance, nonfatigable, ex: marathon runner
- 2. Fast glycolytic (FG), fast-twitch, white, or type II fibers - low endurance, fatigable, ex: sprinter
- 3. Intermediate fibers - in-between red and white fibers, ex: football/baseball players (start and stop)
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any of the three types of muscle cells can be called myocytes
this term is preferable to muscle fiber for smooth and cardiac muscle because these two types of cells do not have the long fibrous shape of skeletal muscle cells
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cardiac myocytes are called
cardiocytes
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Properties of Cardiac Muscles
- 1. It must contract with a regular rhythm
- 2. The muscle cells of a given heart chamber must contract in unison so that the chamber can effectively expel blood
- 3. Each contraction must last long enough to expel blood from the chamber
- 4. It must function in sleep and wakefulness, without fail and without need of conscious attention
- 5. It must be highly resistant to fatigue
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each myocytes is joined to several others at its ends through linkages called:
intercalated discs - appear as thick dark lines in stained tissue sections
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an intercalated disc has electrical gap junctions that allow each myocyte to directly stimulate its neighbors, and mechanical junctions that keep the myocytes from pulling apart when the heart contracts
unlike skeletal muscle, cardiac muscle can contract without the need of nervous stimulation
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cardiac muscle contains a built in pacemaker that rhythmically sets off a wave of electrical excitation; this wave travels through the muscle & triggers the contraction of the heart chambers
cardiac muscle is said to be autorhythmic - ability to contract rhythmically and dependently
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multiunit smooth muscle
- occurs in some of the largest arteries and pulmonary air passages, in the pilorector muscles of hair follicles, and in the iris of the eye
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single-unit smooth muscle
- more widespread, occurs in most blood vessels and in the digestive, respiratory, urinary, and reproductive tracts - also called visceral muscle
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