Module 6 - BIO160

Lesson 6A: Overview of muscle tissues

Cardiac muscle – striated muscle found in the heart; joined to one another at intercalated discs under the regulation of pacemaker cells, which contract as one unit to pump blood through the circulatory system; this muscle is under involuntary control

Contractility – ability to shorten forcibly

Elasticity – ability to stretch and rebound

Excitability – ability to undergo neural stimulation

Extensibility – ability to lengthen

Skeletal muscle – striated, multinucleated muscle that requires signaling from the nervous system to trigger contraction; most are referred to as voluntary muscles that move bones and produce movement

Smooth muscle – nonstriated, mononucleated muscle in the skin that is associated with hair follicles and assists in moving materials in the walls of internal organs, blood vessels, and internal passageways; this muscle is under involuntary control

Name the three muscle tissue types.

Cardiac, skeletal, and smooth

Which of the muscles tissue types is/are striated?

Skeletal and cardiac

Which of the muscle types is under strict voluntary control?

Skeletal muscle

Critical thinking: Why is elasticity an important quality of muscle tissue?

Lesson 6B: Skeletal Muscle

Acetylcholine (ACh) – neurotransmitter that binds at a motor end-plate to trigger depolarization

Actin – protein that makes up most of the thin myofilaments in a sarcomere muscle fiber

Action potential – change in voltage of a cell membrane in response to stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers

Aponeurosis – broad, tendon-like sheet of connective tissue that attaches a skeletal muscle to another skeletal muscle or to a bone

Depolarize – to reduce the voltage difference between the inside and outside of a cell’s plasma membrane (the sarcolemma for muscle fiber), making the inside less negative than at rest

Endomysium – loose, and well-hydrated connective tissue covering each muscle fiber in a skeletal muscle

Epimysium – outer layer of connective tissue around a skeletal muscle

Excitation-contraction coupling – sequence of events from motor neuron signaling to a skeletal muscle fiber to contraction of the fiber’s sarcomeres

Fascicle – bundle of muscle fibers within a skeletal muscle

Motor end-plate – sarcolemma of muscle fiber at the neuromuscular junction, with receptors for the neurotransmitter acetylcholine

Myofibril – long, cylindrical organelle that runs parallel with the muscle fiber and contains the sarcomeres

Myosin – protein that makes up most of the thick cylindrical myofilament within a sarcomere muscle fiber

Neuromuscular junction (NMJ) – synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle fiber with receptors for the acetylcholine released by the terminal

Neurotransmitter – signaling chemical released by nerve terminals that bind to and activate receptors on target cells

Perimysium – connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle; it surrounds each fascicle

Sarcomere – longitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associated proteins involved in contraction

Sarcolemma – plasma membrane of a skeletal muscle fiber

Sarcoplasm – cytoplasm of a muscle cell

Sarcoplasmic reticulum (SR) – specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca⁺⁺

Synaptic cleft – space between a nerve (axon) terminal and a motor end-plate

T-tubule – projection of the sarcolemma into the interior of the cell

Thick filament – the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs

Thin filament – thin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere

Triad – the grouping of one T-tubule and two terminal cisternae

Troponin – regulatory protein that binds to actin, tropomyosin, and calcium

Tropomyosin – regulatory protein that covers myosin-binding sites to prevent actin from binging to myosin

Voltage-gated sodium channels – membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na⁺ enters through the channel

What is the name of the connective tissue layer surrounding an individual fascicle? Perimysium
What is the largest unit of organization in skeletal muscle tissue? Fascicle
What is the smallest unit of organization in skeletal muscle tissue? myofilament (or filament)
What is the difference between a muscle fiber and a muscle cell? there is no difference; they mean the same thing
Which is larger, a myofibril or a myofilament? a myofibril contains many myofilaments; thus, the myofibril is bigger; recall, also, that a muscle fiber (cell) contains many myofibrils, and thus a muscle fiber is bigger than a myofibril
Critical thinking: Describe how tendons facilitate body movement. When a muscle contracts, the force of movement is transmitted through the tendon, which pulls on the bone to produce skeletal movement.
Critical thinking: What are the five primary functions of skeletal muscle? Produce movement of the skeleton, maintain posture and body position, support soft tissues, encircle openings of the digestive, urinary, and other tracts, and maintain body temperature. Note that the openings of some organs are controlled by skeletal muscle, but the walls of these organs have smooth muscle under involuntary control, which moves substances through the organs.

Lesson 6C: Muscle Fiber Contraction and Relaxation

Aerobic Respiration – production of ATP in the presence of oxygen

ATPase – enzyme that hydrolyzes ATP to ADP

Atrophy – decrease in mass or bulk of a skeletal muscle, resulting from a decrease in the number of myofibrils and sarcomeres (but not the number of muscle fibers)

Creatine phosphate – phosphagen used to store energy from ATP and transfer it to a muscle

Glycolysis – anaerobic breakdown of glucose to ATP

Hypertrophy – increase in mass and bulk of a skeletal muscle, resulting from an increase in the number of myofibrils and sarcomeres (but not the number of muscle fibers)

Lactic acid – product of anaerobic glycolysis

Muscular dystrophy – inherited disorder caused by an abnormal X chromosome leading to cellular damage and muscle fiber degradation

Oxygen debt – amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction

Power stroke – action of myosin pulling actin inward (toward the M line)

Pyruvic acid – product of glycolysis that can be used in aerobic respiration or converted to lactic acid

Which molecule is released by a neuron at the neuromuscular junction (NMJ)? acetylcholine (ACh)
What causes the depolarization of the muscle cell membrane? influx of positively charged sodium (Na⁺) ions into the muscle cell via voltage-gated channels on the muscle cell membrane
At rest, what molecule blocks the myosin-binding sites on actin? Tropomyosin
What causes the exposure of the myosin-binding sites on actin? binding of calcium (Ca⁺⁺) to troponin moves the tropomyosin such that the binding sites become exposed
What must attach to the myosin heads in order for them to be able to detach from the actin, thus breaking the cross-bridges for the onset of muscle relaxation? for muscle relaxation to occur, ATP must attach to the myosin head; a lack of ATP in the body, such as occurs upon death, leads to an extended contracted state called rigor mortis
Critical thinking: What causes the striated appearance of skeletal muscle tissue? Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell cause the entire cell to appear striated.
Critical thinking: How would muscle contractions be affected if ATP was completely depleted in a muscle fiber? Without ATP, the myosin heads cannot detach from their binding sites on actin. All of the “stuck” cross-bridges result in muscle stiffness. In a live person, this can cause a condition like “writer’s cramps.” In a recently dead person, it results in rigor mortis.

Lesson 6D: Types of Muscle Fibers

Fast glycolytic (FG) – muscle fiber that primarily uses anaerobic glycolysis; fatigues the quickest of the three fiber types; used for quick, powerful movements such as a weightlifter or sprinter might perform

Fast oxidative (FO) – Intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers

Slow oxidative (SO) – muscle fiber that primarily uses aerobic respiration; fatigues the slowest of the three fiber types; used to maintain posture, stabilize joints, and perform muscle movements with small, but sustained force

Name the three types of skeletal muscle fibers and give their two letter abbreviations.

slow oxidative (SO), slow glycolytic (SG), and fast glycolytic (FG)

Which fatigues the fastest?

fast glycolytic

Which fatigues the slowest?

slow oxidative

Critical thinking: Why do muscle cells use creatine phosphate instead of glycolysis to supply ATP for the first few seconds of muscle contraction?

Creatine phosphate is used because creatine phosphate and ADP are converted very quickly into ATP by creatine kinase. Glycolysis cannot generate ATP as quickly as creatine phosphate.

Lesson 6E: Exercise and Muscle Performance

Hypertrophy – an increase in muscle mass due to the addition of structural proteins

Atrophy – the loss of muscle mass due to a breakdown of structural proteins

Angiogenesis – formation of blood capillary networks

Sarcopenia – age-related muscle atrophy

Critical thinking: What changes occur at the cellular level in response to endurance training?

Endurance training modifies slow fibers to make them more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell and formation of more extensive capillary networks around the fiber.

Critical thinking: What changes occur at the cellular level in response to resistance training?

Resistance exercises affect muscles by causing the formation of more actin and myosin, thus increasing the number of myofibrils and sarcomeres in the muscle fibers. This increases the thickness of muscle fibers and the overall bulk of a muscle.

Lesson 6F: Cardiac and Smooth Muscle Tissues

Auto-rhythmicity - heart’s ability to control its own contractions

dense body - sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments

desmosome - cell structure that anchors the ends of cardiac muscle fibers to allow contraction to occur

intercalated disc - part of the sarcolemma that connects cardiac tissue, and contains gap junctions and desmosomes

visceral muscle - smooth muscle found in the walls of visceral organs

Skeletal muscle characteristics:

striated
multinucleated
under the control of the voluntary (somatic) nervous system
contains myofibrils composed of sarcomeres (gives it the striated appearance)
contains T tubules
no gap junctions
well-developed sarcoplasmic reticulum
fast contracting
every fiber is controlled by a nerve

Cardiac muscle characteristics:

striated and branched
uni- or binucleate
contains intercalated discs
under the control of the involuntary (autonomic) nervous system or ANS
contains myofibrils composed of sarcomeres (gives it the striated appearance)
contains T tubules
presence of gap junctions at the intercalated discs
less developed sarcoplasmic reticulum than skeletal muscle
found in the wall of the heart only
not every fiber is controlled by a nerve; can contract without input from the nervous system

Smooth muscle characteristics:

un-striated and with a spindle shape
uninucleate
under the control of the involuntary (autonomic) nervous system or ANS
does not contain myofibrils composed of sarcomeres
does not contain T tubules
presence of gap junctions
less developed sarcoplasmic reticulum than skeletal muscle
found in the walls of hollow organs
not every fiber is controlled by a nerve; can contract without input from the nervous system

How many nuclei are typically found in each muscle type?
skeletal = multinucleated; cardiac = uni- or binucleate (one or two); smooth = uninucleate
Which of the muscle types is/are under involuntary control?
cardiac and smooth
Which of the muscle types has/have gap junctions?
cardiac and smooth
Which muscle type has/have intercalated discs?
cardiac
Which muscle type does/do not contain myofibrils and T tubules?
smooth

Lesson 6G: Naming Skeletal Muscles

abductor - moves the bone away from the midline

adductor - moves the bone toward the midline

agonist (also, prime mover) - muscle whose contraction is responsible for producing a particular motion

antagonist - muscle that opposes the action of an agonist

bi - two

brevis - short

extensor - muscle that increases the angle at the joint

fixator - synergist that assists an agonist by preventing or reducing movement at another joint, thereby stabilizing the origin of the agonist

flexion - movement that decreases the angle of a joint

flexor - muscle that decreases the angle at the joint

insertion - end of a skeletal muscle that is attached to the structure (usually a bone) that is moved when the muscle contracts

lateralis - to the outside

longus - long

maximus - largest

medialis - to the inside

medius - medium

minimus - smallest

oblique - at an angle

origin - end of a skeletal muscle that is attached to another structure (usually a bone) in a fixed position

prime mover (also, agonist) - principal muscle involved in an action

rectus - straight

synergist - muscle whose contraction helps a prime mover in an action

tri – three

Critical thinking: When a muscle contracts it gets shorter. If you know the points of attachment (e.g., origin and insertion) of a particular muscle, you can predict its action (e.g., flexion, extension, adduction, abduction, etc.). Consider the deltoid, which has origins in the scapula and clavicle and inserts into the humerus. What type of movement do you think it will do to the arm when contracted?
The deltoid contracts and gets shorter, it performs abduction of the arm. In other words, it moves the arm away from the body. Try standing and then lifting your arms away from your body in opposite, lateral directions, while holding them completely straight (don't bend your elbows). Stop when your arms are parallel to the ground. You will feel a tightness in your deltoids. If you perform lateral dumbell raises at the gym, you are developing your deltoid muscles.
Critical thinking: When you perform chin ups, the latissimus dorsi pull your elbows toward your body. What term is used to describe that movement of your arms?
You are performing adduction of the arms. In other words, you are moving the arms toward the midline of the body. Wide-grip chin ups are a good way to develop your "lats!"
Critical thinking: Would you expect the adductor longus to spread the legs apart or bring them together?
Adduction involves moving a bone toward the midline, so adductor muscles, such as the adductor longus, bring the legs together.
Critical thinking: Consider the biceps brachii and triceps brachii. Which performs flexion of the forearm and which performs extension of the forearm?
The biceps brachii performs flexion of the forearm, whereas the triceps bracii extends it.
Name three of the four muscles of the quadriceps femoris.
rectus femoris, vastus medialis, vastus lateralis.
Name two of the hamstring muscles.
biceps femoris and semitendinosus
Critical thinking: Consider the quadriceps and hamstring muscles. Which flexes the leg and which extends the leg?
The quadriceps extend the leg, whereas the hamstrings flex the leg. Try sitting in a chair and then extending a leg (remember the "leg" is just the lower half of the lower extremity). You should feel tension in your quadriceps. Now, stand up and bend one leg (flex it), trying to touch your heel to your buttocks. You should now feel the tension in your hamstrings.
Critical thinking: Dorsiflexion involves the flexion of the foot (pointing your toe toward your shin), whereas plantar flexion involves pointing the toes away from the body (picture a ballet dancer standing on her toes). Which leg muscle would you expect to be involved in dorsiflexion of the foot? Which would be involved in plantar flexion?
The tibialis anterior does dorsiflexion of the foot, whereas the gastrocnemius does plantar flexion of the foot. If you consider the locations of each muscle, and then picture them getting shorter, this should make total sense!