Muscle Tissue Review Questions

What is a motor unit, and how are motor units involved in regulating delicacy and strength of movement?

A motor unit is one neuron and all the myofibers that it innervates (1 to 500). All myofibers in the unit contract at once. Delicate movement muscles have fewer myofibers/unit, and more units. An example is eye movements. Not all units contract at once, allowing smooth movement. Stronger contraction of a muscle requires recruitment of more motor units.

What are some unique features of cardiac muscle with respect to the signal for contraction, metabolism, regenerative ability, and endocrine function? What is the role of the nervous system in controlling heart beat?

Cardiac muscle has aerobic metabolism only; mainly from fatty acids. Anaerobiosis is myofiber death in five minutes. It has no regenerative ability (it lacks satellite cells), dead cells are replaced by scar tissue. Sliding filament mechanism of contraction is similar to that of skeletal muscle. Atrial myofibers secrete a hormone (atrial natriuretic factor) from granules to cause loss of Na+ and H20 from kidney to reduce blood pressure. Autonomic nerves function only to speed up and slow down heartbeat.

What is the relationship between skeletal myofibers and capillaries?

Each myofiber is in contact with one or more capillaries, which supply nutrients and oxygen and remove wastes

What are the initial sources of energy for muscle cell contraction? What type of metabolism is used for prolonged (e.g., a long jog) vs. short burst (e.g., a sprint to the top of Lone Mountain) activity?

For muscle cell contraction, the initial sources of energy (ATP) are intracellular ATP (4-6 seconds), creatine phosphate (another 10-15 seconds), aerobic metabolism (fatty acids -rest or light exertion, and glucose), and anaerobic metabolism (1 to 2 minutes). Muscle fatigue is due to low ATP and increased lactic acid. Slow red oxidative fibers (type 1) are used for postural muscles of the neck/back. there is increased myoglobin, mitochondria, capillaries; contract slowly, resistant to fatigue, use aerobic metabolism of fatty acids. For short bursts of activty fast intermediate oxidative glycolytic fibers are involves (lla) which use aerobic and anaerobic metabolism and the major muscles of the leg are used.

What is the origin and function of skeletal muscle satellite cells?

Satellite cells are immature myoblasts and are below the external lamina. They give limited ability for regeneration and possibly growth.

How do single unit and multi-unit smooth muscles differ with respect to innervation and location?

Single unit smooth muscle neuron may innervate only one fiber. signal is passed to adjacent fibers via gap junctions (allows flow of Ca2+) and it is the most common type (ex. in walls of small blood vessels, gut, bladder, uterus). Multi-unit smooth muscles are like skeletal muscles and allows more fine-tuned control and example is in walls of large arteries and the large airway of the lung.

How is skeletal muscle attached to bone?

Skeletal muscle is attached to bone by a tendon at the myotendinous junction.

How does the appearance of the sarcomere change when a muscle cell contracts?

The A band does not change its length, but the I bands and H bands shorten which brings the Z bands closer together. (*double check this*)

What is the embryonic origin of skeletal muscle cells? What are the two mechanisms by which cells can become multinucleate, and which mechanism applies here?

The embyronic origin of skeletal muscle cells is the mesoderm. (*double check this*) The two mechanisms by which cells can become multinucleate by nuclear division without cytokinesis or by fusion of preexisting cells. Skeletal muscle cells are multinucleate because of fusion of embyronic myoblasts.

What connective tissue layers are found in cardiac muscle? What structure joins adjacent cardiac myocytes, and what types of cell junctions occur in this structure? How is the signal to contract passed from one myocyte to the next?

The endomysium and perimysium are the connective tissue layers found in cardiac muscle. Intercalated discs join adjacent cardiac myocytes (*double check*) and adhering junctions occur on transverse sfc; attachment points for actin filaments of last sarcomere of myofibrils, desmosomes are on transverse sfc, and gap junctions are on the longitudinal surface. The signal to contract is passed from one myocyte to the next through gap junctions. (*describe how?)

What is the relationship between the sarcolemma, T-tubules, and the sarcoplasmic reticulum (SR)? What is a triad? What is the function of the SR in skeletal muscle?

The sarcolemma sends closed ended tubules into the sarcoplasm (T-tubules) that associate with terminal cisternae of sarcoplasmic reticulum (smooth ER, a site of Ca2+ storage) forming triads of A-I borders. A triad is T-tubules with a pair of terminal cisternae. The function of SR in skeletal muscle is The SR regualtes intracellular levels of calcium in skeletal muscle which in turn regulates contraction/relaxation of muscles.

What are the 2 general mechanisms by which an organ can increase in size, and which mechanism applies to enlarged skeletal muscles? Which component actually increases?

The two general mechanism by which an organ can increase in size is hypertrophy or hyperplasia. Hypertrophy applies to enlarged skeletal muscles. There is an increased number of myofibrils per cell. This can occur through resistance exercises.

What are unique features of smooth muscle cells with respect to stretching, mitotic activity, signal for contraction, speed of contraction, ECM synthetic capability, and energy metabolism

Unique features of smooth muscle cells are that it stretches wihtout developing tension which is important for the bladder and the stomach. it can divide by hyperplasia (ex. the uterus during pregnancy), cells usually contract spontaneously, but can be stimulated or inhibited by the autonomic NS. contraction is slow and resistant to fatigue (uses less than 1% of ATP of skeletal muscle). Synthesizes extracellular matrix, like fibroblasts. the cells are capable of mitotic division for repair, unlike skeltal and cardiac myofibers. Mostly anaerobic metabolism is used.

Describe the sliding filament mechanism in smooth muscle cells, pointing out the differences with skeletal muscle contraction, especially with respect to the role of calcium.

depolarization of sarcolemma causes increased cytoplasmic ca2+ from SR (via GPCR) and extracellular fluid (via voltage-gated channel). Ca2+ binds calmodulin (increased ca2+ binds troponin in skeletal muscle contraction), .and activates myosin light chain kinase.( which displaces tropomyosin, thus exposing myosin binding site in skeletal muscle contraction) myosin light chain kinase catalyzes transfer of phosphate from ATP to regulatory light chain of myosin, allowing the head to bind actin, and myosin binds actin - sliding filaments. (similar to how in skeletal muscle myosin heads bind to actin, initiating the sliding filament mechanism)

What connective tissue layers are associated with smooth muscle? Why do same-sized smooth muscle cells have different diameters when cut in cross section?

Endomysium and Perimysium are the connective layers associated with smooth muscle. Smooth muscle cells have different diameters when cut in cross section because of their spindle-shape. It depends on where they are sectioned. (*double check*

Describe the complete sequence of events, beginning with an action potential in a somatic motor neuron, resulting in skeletal muscle contraction. What actually gets shorter in the muscle cell?

In a relaxed cell, myosin is in a high energy state (cocked) but is unable to bind actin due to tropomyosin covering the myosin binding site. Ca2+ is required to dislodge tropomyosin. Ach from motor neuron is released at the NMJ , binds to LGICs on motor end plate; removed by acetylcholinesterase. Action potential in sarcolemma, then T tubules . release of Ca2+ from sarcoplasmic reticulum. increased ca2+ binds troponin, which displaces tropomyosin, thus exposing myosin binding site. myosin heads bind to actin, initiating the sliding filament mechanism. cycle continues as long as Ca2+ is present. the result is that sarcomeres shorten, myofibrils contract and myofiber contracts. Sarcomeres contract but myofilaments do not. Cell contraction is an all or nothing phenomenon. (refer to diagram*)

What sensory structures are associated with skeletal muscles, and what are their functions?

Sensory neurons are involved in sensing pain and pressure. Tension is sensed when relay info from muscle spindles and the golgi tendon organs to the CNS. A change in length of extrafusal muscle fibers to allow CNS to control posture, reflexes, opposing muscle groups, and a change in tension on collagen fibers to allow CNS to inhibit motor neurons to prevent injury.

Why do skeletal muscle cells appear striated? What is a sarcomere, and what are the major proteins found in the sarcomere? What are the functions of troponin and tropomyosin?

Skeletal muscle cells appear striated due to regular arrangements of two types of protein filaments in sarcomeres. A sarcomere is a regular arrangements of thick and thin filaments; it is the distance from one Z disc to another. The major proteins found in a sarcomere are F-actin and myosin (*double check if these are it?*). The function of troponin is to bind Ca2+, it is attached to tropomyosin. Tropomyosin is two polypeptides covering the myosin-binding site.

What structures or molecules occur in the following regions: I band, A band, H zone, M line? Know the difference between a myofilament, myofibril, and myofiber. Which is largest and which is smallest? What is the structural relationship between thick and thin filaments in a myofibril cut in cross section?

The I band is the region on either side of Z disc composed of thin filaments only. The A band is the region in teh center of the sarcomere where thick and thin filaments overlap. The H zone is a region in teh center of the A band composed of thick filaments only with the M line in the middle (visible only with TEM). A myofiber is made up of many myofibrils. A myofibril is made up of many myofilaments. Myofilaments are smallest and myofiber is largest. Thick and thin filaments assemble into cylindrical myofibrils with each thick filament surrounded by six thin filaments. they run the entire length of the cell. The alignment of sarcomeres within the myofibril and then alighnment of myofibrils within the cell, cause the entire cell to appear striated.

What are the layers of connective tissue associated with muscle skeletal muscle tissue? Where does the external lamina occur? What histochemical stain could be used to demonstrate it?

The layers of connective tissue associated with muscle skeletal muscle tissue are the fascia, epimysium, perimysium, and endomysium. The external lamina occurs in endomysium. (*could not find answer to third question?)

What are the major functions of muscle tissue, and what are the characteristics of muscle cells that allow these functions? From what embryonic tissue(s) does muscle tissue develop?

The major functions of muscle tissue are movement: body, parts, blood, and food; maintenance of posture (skeletal muscle); joint stability: shoulder/knee (skeletal); heat production-85% (skeletal). The characteristics that allow these functions are contractility which is unique to muscle and excitability which is responding to sitmuli by generating an action potential

What are the 3 types of muscle tissue, and how do they differ in location, cell shape, striations, number of nuclei, and control?

The three types of muscle tissue are skeletal, cardiac, and smooth. Skeletal muscle is attached to bones, has cylindrical cells, is striated, multinucleate, and voluntary control. Cardiac is the heart, branched cells, striated, uni or dinucleate, and involuntary. Smooth muscle is in walls of hollow organs, spindle-shaped cells, nonstriated, uninucleate, and involuntary.

What are the 3 types of skeletal muscle fiber, and how do they differ with respect to myoglobin content, energy metabolism, and function?

The three types of skeletal muscle fibers are slow red oxidative fibers, fast intermediate oxidative-glycolytic fibers and fast white glycolytic fibers. Slow red oxidative fibers (type 1) have high myoglobin content, have mitochondria and capillaries, contract slowly and are for resistance to fatigue. Uses aerobic metabolism of fatty acids (decreased glycogen content) and is in postural muscles of neck and back. Fast intermediate oxidative glycolytic fibers (lla) are used for short bursts of activity . aerobic and anaerobic metabolism is used, and they are in major muscles of the leg. Fast white glycolytic fibers (llb) have low myoglobin, mitochondria and capilalries. they contract rapidly, fatique quickly, use anaerobic (increased glycogen content) metabolism. and are found in muscles that move the eyes and digits.

How do smooth muscle cells differ from striated cells with respect to the role of T-tubules, tropomyosin, and attachment of thin fibers?

There are not tubules, the sarcoplasmic reticulum is in direct contact with sarcolemma. There are no sarcomeres. no tropomyosin on thin filaments so the myosin binding site is always exposed. Thin filaments attach to dense bodies on sarcollemma or in the cytoplasm instead of Z discs.