BIO 201 Exam 3

3 kinds of muscle tissue

• skeletal

• cardiac

• smooth

Major purpose of muscle

• converting the chemical energy in ATP into mechanical energy of motion

functions of muscles

• movement of whole body, body parts, organ contents

• stability - maintain posture and prevent movement

• communication - speech, facial expressions and writing

• controls openings and passageways

  • sphincters

• body heat production

Connective tissues of a muscle - fascia

• separates neighboring muscles or muscle groups from each other and the subcutaneous tissue

connective tissues of a muscle - epimysium

• fibrous sheath surrounding the entire muscle

• outer surface grades into the fascia

connective tissues of a muscle - perimysium

surrounds fascicles

• carry larger nerves and blood vessels, and stretch receptors

connective tissues of a muscle - endomysium

• thin sleeve of loose connective tissue surrounding each muscle fiber

• allows room for capillaries and nerve fibers to reach each muscle fiber

connective tissue elements - tendons

• attachments between muscle and bone

• epimysium surrounding the entire muscle is continuous with collagen fibers of tendons

  • dense-regular connective tissue composed of collagen fibers

  • collagen is somewhat extensible and elastic

    • stretches slightly under tension and recoils when released

    • resists excessive stretching and protects muscle from injury

    • returns muscle to its resting length

    • contribute to power output and muscle efficiency

skeletal muscle shapes

• fusiform - thick in middle and tapered at ends

  • ex. biceps brachii and gastrocnemius

• triangular (convergent) - broad at origin and tapering to a narrow insertion

  • ex. pectoralis major, temporalis

• parallel muscles - parallel fascicles

  • ex. rectus abdominus, zygomaticus major

• circular muscles

  • act as sphincters

  • ex. orbicularis oris, urethral and anal sphincters

• pennate muscles

  • fascicles insert obliquely on a tendon (feather shaped)

  • ex. palmar interosseus, rectus femoris, and deltoid

indirect attachment to bone

• tendons

  • collagen fibers of endo-, peri-, and epimysium continue into the tendon

  • tendon merges into periosteum of bone

  • very strong structural continuity from muscle to bone

  • aponeurosis - tendon is a broad, flat sheet

direct attachment to bone

• little separation between muscle and bone

• muscle seems to immerge directly from bone

• margins of brachialis, lateral head of triceps brachii

origin

bony attachment at stationary end of muscle

insertion

bony attachment to mobile end of muscle

belly

thicker, middle region of muscle between origin and insertion

prime mover (agonist)

muscle that produces most of the force during a joint action

ex. brachialis - prime mover for elbow flexion

synergist

muscle that aids the prime mover

• stabilizes the nearby joint

• modifies the direction of movement

• ex. biceps brachii - synergist for elbow flexion

antagonist

opposes the prime mover

• relaxes to give prime mover control over an action

• preventing excessive movement and injury

• antagonistic pairs - muscles that act on opposing sides of a joint

• ex. triceps brachii - antagonist for elbow flexion

fixator

muscle that prevents movement of bone

ex. rhomboideus holds scapula firmly in place

intrinsic muscles

contained within a region

• both its origin and insertion occur there

extrinsic muscles

act on a designated region, but has origins elsewhere

• ex. fingers

characteristics of muscles

• responsiveness (excitability) - to chemical signals, stretch and electrical changes across the plasma membrane

• conductivity - local electrical change triggers a wave of excitation that travels along the muscle fiber

• contractility - shortens when stimulated

• extensibility - capable of being stretched between contractions

• elasticity - returns to its original resting length after being stretched

skeletal muscle

• voluntary, striated muscle attached to one or more bones

  • made up of muscle cells called muscle fibers or myofibers

myofiber

• composed of myofibrils

• also known as muscle cell or muscle fiber

myofibril

long protein bundles that occupies the main portion of the interior of a muscle fiber

myofilaments

• make up myofibrils

• a protein microfilament responsible for muscle cell contraction

• composed of myosin or actin proteins

sacrolemma

plasma membrane of a muscle fiber

sarcoplasm

cytoplasm of a muscle fiber

sarcoplasmic reticulum (SR)

smooth ER that forms a network around each myofibril - calcium reservoir

• calcium activates the muscle contraction process

terminal cisternae

• dilated end-sacs of SR which cross muscle fiber from one side to the other

T tubules

tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side

Triad

a T tubule and two terminal cisterns

glycogen function in myofibers

stored in abundance to provide energy with heightened exercist

myoglobin functions in myofibers

red pigment that stores oxygen needed for muscle activity

Muscle growth and repair

• myoblasts - stem cells fuse to form each muscle fiber

• satellite cells - unspecialized myoblasts remaining between the muscle fiber and endomysium

  • may multiply and produce new muscle fibers to some degree

• repair by fibrosis

3 kinds of myofilaments found in a myofibril

1. thick filaments - myosin proteins

2. thin filaments - primarily actin proteins

3. elastic filaments - titin (connectin) proteins

thick myofilaments

• made of several hundred myosin molecules

• shaped like a gold club

• heads directed outward in a helical array around the bundle

  • bare zone has no heads in the middle

thin myofilaments

• fibrous (F) actin - 2 intertwined strands

• string of globular (G) actin subunits each with an active site that can bind to head of myosin molecule

• tropomyosin - each blocking 6 or 7 active sites on G actin subunits

• troponin complex - small, calcium-binding protein on each tropomyosin molecule

elastic myofilaments

• titin (connectin) - huge springy protein

  • flank each thick filament and anchor it to the Z disc

  • help the cell recoil to its resting length (elasticity)

  • keeps thick and thin filaments aligned

  • prevents over stretching

regulatory proteins

tropomyosin and troponin

• like a switch that starts and stops contraction

• contraction activated by release of calcium into sarcoplasm and its binding to troponin

• troponin changes shape and moves tropomyosin off the active sites on actin

contractile proteins

myosin and actin

• do the work of contracting the muscle

accessory proteins

• at least 7 other accessory proteins in or associated with thick or thin filaments

  • anchor the myofilaments

  • regulate length of myofilaments

  • alignment of myofilaments for optimal effectiveness

dystrophin

• links actin in outermost myofilaments to transmembrane proteins and eventually to fibrous endomysium surrounding the entire muscle cell

  • transfers forces of muscle contraction to connective tissue around myofiber

  • genetic defects in dystrophin produce muscular dystrophy

  • normal allele makes dystrophin

    • absence of it leads to torn cell membranes

Explain the effects of a mutation in Myostatin

removes the body's primary "brake" on muscle growth. This disruption causes significant skeletal muscle hyperplasia (increased number of muscle fibers) and hypertrophy (increased size of fibers), resulting in exceptional muscularity

Describe the arrangement of myofilaments in sarcomeres and how that results in

striations

• A band - dark, anisotropic

  • part of A band where thick and thin filaments overlap is especially dark

• H band - in middle of A band - just thick filaments

• M line - middle of H band

• I band - alternating lighter band - I stands for isotropic

• Z disc - provides anchorage for thin filaments and elastic filaments

  • bisects I band at edge of sarcomere

sarcomere

the segment of the myofibril from one z disc to the next

• functional contractile unit of the muscle fiber

• muscle shortens because individual sarcomeres shorten

denervation atrophy

shrinkage of paralyzed muscle when connection not restored

somatic motor neuron

stimulate skeletal muscle

• cell bodies are located in the brainstem and spinal cord

somatic motor fibers

axons of somatic motor neurons

• lead to the skeletal muscle

• each nerve fiber branches out to a number of muscle fibers

  • 200 myofibers on avg are controlled by a single somatic motor neuron

• each myofiber is supplied by only one motor neuron

motor unit

one nerve fiber and all the muscle fibers innervated by it

• myofibers of one motor unit

  • dispersed throughout the muscle

  • contract in unison

  • produce weak contraction over wide area

  • provides ability to sustain long-term contraction as motor units take turns contracting

  • effective contraction requires the contraction of several motor units at once

number of muscle fibers per neuron in an average, small and large motor units

• average motor unit - 200 muscle fibers per neuron

• small motor unit - 3-6 muscle fibers per neuron

  • fine degree of control

  • eye and hand muscles

• large motor units - 1000 myofibers per neuron

  • ex. gastrocnemius

  • powerful contractions supplied by large motor units

neuromuscular junction

when target cell is a muscle fiber

• each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber

• one nerve fiber stimulates the muscle fiber at several points within the NMJ

synapse

point where a nerve fiber meets its target cell

Components of neuromuscular junction

• Synaptic knob - swollen end of nerve fiber

  • contains synaptic vesicles filled with acetylcholine

  • synaptic vesicles undergo exocytosis to release ACh into synaptic cleft

• synaptic cleft - tiny gap between synaptic knob and muscle sarcolemma

• Schwann cell - envelopes and isolates all of the NMJ from surrounding tissue fluid

• 50 million ACh receptors - proteins incorporated into muscle cell plasma membrane

• Basal lamina - think layer of collagen and glycoprotein separates Schwann cell and entire muscle cell from surrounding tissue

  • contains acetylcholinesterase (AChe) that breaks down ACh after contraction

How can neuromuscular toxins disrupt muscle contraction?

• interfere with synaptic function

• some pesticides contain cholinesterase inhibitors

  • bind to AChe and prevent it from degrading ACh

  • spastic paralysis - a state of continual contraction of the muscles

• tetanus (lockjaw) is a form of spastic paralysis caused by toxin Clostridium tetani

  • glycine in spinal cord normally stops motor neurons from producing unwanted muscle contractions

  • tetanus toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles

flaccid paralysis

a state in which the muscles are limp and cannot contract

• compete with ACh for receptor sites, but do not stimulate the muscles

What makes muscle fibers and neurons electrically excitable cells?

• their plasma membrane exhibits voltage changes in response to stimulation

Unstimulated (resting) cell

• there are more anions (negative ions) on the inside of the plasma membrane than on the outside

• plasma membrane is electrically polarized (charged)

• there are excess sodium (Na+) ions in the ECF

• there are excess potassium (K+) ions in the ICF

Resting membrane potential of a myofiber is -90 mV

Voltage (electrical) potential

a difference in electrical charge from one point to another

Stimulated (active) muscle fiber or nerve cell

• ion gates open in the plasma membrane

• Na+ instantly diffuses down its concentration gradient into the cell

• these cations override the negative charges in the ICF

• depolarized occurs as inside of plasma membrane becomes briefly positive

• Na+ gates close and K+ gates open

• K+ rushes out of cell

• repolarization as loss of positive K+ ions turns membrane negative again

action potential

quick up and down voltage shift from the negative RMP to a positive value, and back to the negative value again

• a quickly fluctuating voltage seen in an active stimulated cell

• an action potential at one point on a plasma membrane causes another one to happen immediately in front of it, which triggers another one a little farther along and so forth

4 Major Phases of Contraction and Relaxation

• excitation

  • the process in which nerve action potentials lead to muscle action potentials

• excitation-contraction coupling

  • events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract

• contraction

  • step in which muscle fiber develops tension and may shorten

• relaxation

  • when work is done, a muscle relaxes and returns to its resting length

Order of muscle contraction and relaxation

1. nerve fiber releases ACh

2. ACh binds to receptors on the muscle fiber, triggering an action potential

3. Calcium is released from the SR of the muscle fiber

4. Calcium binds to troponin

5. Tropomyosin moves to expose active sites on the thin filament

6. Myosin binds to actin and pulls creating a power stroke that shortens the sarcomere

7. muscle contracts

8. AChe breaks down ACh

9. Calcium is removed from troponin and returns to the SR

10. Muscle relaxes

rigor mortis

hardening of muscles and stiffening of body beginning 3-4 hours after death

• deteriorating SR releases calcium

• deteriorating sarcolemma allows calcium to enter cytosol

• calcium activates myosin-actin cross-bridging

• muscle contracts, but cannot relax

Muscle relaxation requires ATP, but NO ATP after death

Rigor mortis peaks at 12 hours AD, then diminishes over next 48-60 hours

Length-tension relationship

the amount of tension generated by a muscle and the force of contraction depends on how stretched or contracted it was BEFORE being stimulated

• overly contracted at rest - a weak contraction occurs

• too stretched before stimulated - a weak contraction occurs

optimum resting length

• produces greatest force when muscle contracts

• CNS continually monitors and adjusts length of the resting muscle

  • maintains a state of partial contraction - muscle tone (tonus)

threshold

the minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction

Twitch

a quick cycle of contraction when stimulus is at threshold or higher

• phases of a twitch contraction:

  • latent period

  • contraction phase

  • relaxation phase

latent period of twitch contraction

2 msec delay between the onset of stimulus and onset of twitch response

• time required for excitation, excitation-contraction coupling and tensing of elastic components of the muscle

• internal tension - force generated during latent period and no shortening of muscle occurs

contraction phase of twitch contraction

phase in which filaments slide and the muscle shortens

• once elastic components are taut, muscle begins to produce external tension - in muscle that moves a load

• short-lived phase

relaxation phase of twitch contraction

SR quickly reabsorbs calcium, myosin releases the thin filaments and tension declines

• muscle returns to resting length

• entire twitch lasts from 7-100 msec

factors affecting twitch strength

• stimulus frequency

  • stimuli arriving closer together produce stronger twitches

• concentration of calcium in sarcoplasm

• how stretched the muscle was before it was stimulated

• temperature of the muscles

  • warmed up muscle contracts more strongly

  • enzymes work more quickly

• lower than normal pH of sarcoplasm weakens the contraction

• state of hydration of muscle

  • affects overlap of thick and thin filaments

What happens when you stimulate the nerve with higher and higher voltages?

it produces stronger contractions

• higher voltages excite more and more nerve fibers in the motor nerve which stimulates more and more motor units to contract

recruitment or multiple motor unit (MMU) summation

• the process of bringing more motor units into play

What happens when stimulus intensity (voltage) remains constant but stimulus

frequency varies?

• Up to 10 stimuli per second:

  • each stimulus produces identical twitches and full recovery between twitches

• 10-20 stimuli per second:

  • produces treppe (staircase) phenomenon

  • muscle still recovers fully between twitches, but each twitch develops more tension than the one before

  • stimuli arrive so rapidly that SR doesn’t have time between stimuli to completely absorb all calcium released

  • Calcium conc. in cytosol rises higher and higher with each stimulus causing subsequent twitches to be stronger

  • heat released by each twitch causes muscle enzymes like myosin ATPase to work more efficiently and produce stronger twitches as muscle warms up

• 20-40 stimuli per second:

  • produces incomplete tetanus

  • each stimulus arrives before the previous twitch is over

  • new twitch rides piggy back on previous one, generating higher tension

  • temporal summation - 2 stimuli arrive close together

  • wave summation - results from one wave of contraction added to another

  • each twitch reaches a higher level of tension than the one before

  • muscle relaxes only partially between stimuli

  • produces a state of sustained fluttering contraction (incomplete tetanus)

• 40-50 stimuli per second:

  • muscle has no time to relax between stimuli

  • twitches fuse to a smooth, prolonged contraction called complete tetanus

  • muscle in complete tetanus produces 4x the tension as a single twitch

isometric muscle contraction

• muscle is producing internal tension which an external resistance causes it to stay the same length or become longer

• can be a prelude to movement when tension is absorbed by elastic component of muscle

• important in postural muscle function and antagonistic muscle joint stabilization

isotonic muscle contraction

• muscle changes in length with no change in tension

concentric contraction

muscle shortens while maintains tension

• a type of isotonic contraction

eccentric contraction

muscle lengthens as it maintains tension

• a type of eccentric contraction

2 main pathways of ATP synthesis

1. anaerobic fermentation

   1. enables cells to produce ATP in absence of oxygen

   2. yields little ATP

   3. produces lactic acid, a major factor in muscle fatigue

2. aerobic respiration

   1. produces far more ATP

   2. less toxic end produces (CO2 and water)

   3. requires a continual supply of oxygen

What 2 enzyme systems control phosphate transfers for immediate energy needs?

• myokinase - transfers Pi from one ADP to another converting the later to ATP

• creatine kinase - obtains Pi from a phosphate-storage molecule creatine phosphate (CP)

  • fast-acting system that helps maintain the ATP level while other ATP-generating mechanisms are being activated

phosphagen system

ATP and CP collectively

• provides nearly all energy used for short bursts of intense activity

• one minute of brisk walking

• 6 seconds of sprinting or fast swimming

• important in activities requiring brief, but maximum effort

  • football, baseball, and weight lifting

short term energy needs

• glycogen-lactic acid system

  • the pathway from glycogen to lactic acid

  • produces enough ATP for 30-40 seconds of maximum activity

long-term energy needs

• aerobic respiration

• theoretically produces 36-38 ATP per glucose

• efficient means of meeting the ATP demands of prolonged exercise

• one’s rate of oxygen consumption rises from 2-3 min and levels off to a steady state in which aerobic ATP production keeps pace with demand

• little lactic acid accumulates under steady state conditions

causes of muscle fatigue

• ATP synthesis declines as glycogen is consumed

• ATP shortage slows down Na+K+ pumps

  • compromises ability to maintain the resting membrane potential and excitability of the muscle fibers

• lactic acid lowers pH of sarcoplasm

  • inhibits enzymes involved in contraction, ATP synthesis, and other aspects of muscle function

• release of K+ with each action potential causes the accumulation of extracellular K+

  • hyperpolarizes the cell and makes the muscle fiber less excitable

• motor nerve fibers use up their ACh

• CNS fatigues by unknown processes

endurance

the ability to maintain high-intensity exercise for more than 4-5 minutes

• determined in large part by one’s maximum oxygen uptake (VO2max)

maximum oxygen uptake (VO2max)

the point at which the rate of oxygen consumption reaches a plateau and does not increase further with an added workload

• proportional to body size

• peaks at around age 20

• usually greater in males than females

• can be twice as great in trained endurance athletes as in untrained person

  • results in twice the ATP production

oxygen debt

• heavy breathing continues after strenuous exercise

  • excess post-exercise oxygen consumption (EPOC) - difference between the resting rate of oxygen consumption and the elevated rate following exercise

  • typically about 11 liters extra is needed after strenuous exercise

  • repaying the oxygen debt

• oxygen debt is needed to:

  • replace oxygen reserves that were depleted in the 1st minute of exercise

  • replenish the phosphagen system

  • oxidize lactic acid

  • serve the elevated metabolic rate

oral creatine supplement

increases level of creatine phosphate in muscle tissue and increases speed of ATP regeneration

• useful in burst type exercises like weight-lifting

• risks are not well known

  • muscle cramping, electrolyte imbalances, dehydration, water retention, gastro issues, weight gain, seizures, strokes

  • kidney disease from overloading kidney with metabolite creatine

  • body can stop producing natural creatine

carbohydrate loading

dietary regimen

• packs extra glycogen into muscles cells

• extra glycogen is hydrophilic and adds 2.7 g water/g glycogen

slow oxidative (SO), slow twitch, red, or type I fibers

• dark meat

• abundant mitochondria, myoglobin, and capillaries - deep red color

• adapted for aerobic respiration and fatigue resistance

  • relative long twitch lasting about 100 msec

  • soleus of calf and postural muscles of the back

fast glycolytic (FG), fast-twitch, white, or type II fibers

• white meat

• fibers are well adapted for quick responses, but not for fatigue resistance

• rich in enzymes of phosphagen and glycogen-lactic acid systems

• less mitochondria, myoglobin, and blood capillaries which gives paler appearance

  • SR releases and reabsorbs Ca quickly, so contractions are quicker

  • extrinsic eye muscles, gastrocnemius, and biceps brachii

factors affecting muscle strength

• muscle size

• fascicle arrangement

  • pennate stronger than parallel, and parallel stronger than circular

• size of motor units

  • larger the motor unit, the stronger the contraction

• multiple motor unit summation = recruitment

  • when stronger contraction is required, the nervous system activates more motor units

• temporal summation

  • nerve impulses usually arrive at a muscle in a series of closely spaced action potentials

  • the greater the stimulation frequency, the more strongly a muscle contracts

• length-tension relationship

• fatigue

resistance training (weight lifting)

• contraction of a muscle against a load that resist movement

• a few minutes of resistance exercise a few times a week is enough to stimulate muscle growth

• growth is from cellular enlargement

• muscle fibers synthesize more myofilaments and myofibrils and grow thicker

endurance training (aerobic exercise)

• improves fatigue resistant muscles

• slow twitch fibers produce more mitochondria, glycogen, and acquire a greater density of blood capillaries

• improves skeletal strength

• increases the red blood cell count and oxygen transport capacity of the blood

• enhances the function of the cardiovascular, respiratory, and nervous systems

cardiac muscle

• limited to the heart where it functions to pump blood

• required properties of cardiac muscle

  • contraction with regular rhythm

  • muscle cells of each chamber must contract in unison

  • contractions must last long enough to expel blood

  • must work in sleep or wakefulness, without fail and without conscious attention

  • must be highly resistant to fatigue

• striated

• intercalated discs joins myocytes

• electrical gap junctions allow each myocyte to directly stimulate its neighbors

• SR less developed, but T tubules are larger

• damaged cardiac muscle cells repair by fibrosis

• very slow twitches

smooth muscle

• composed of myocytes with fusiform shape

• one nucleus located near the middle of cell

• no visible striations

• z discs are absent and replaced by dense bodies

• cytoplasm has extensive cytoskeleton of intermediate filaments

• No T tubules

• capable of mitosis and hyperplasia

• some lack nerve supply, others receive autonomic fibers, not somatic motor fibers as in skeletal muscle

2 types of smooth muscle

• multiunit smooth muscle

  • occurs in some of the largest arteries and pulmonary air passages, in piloerector muscle of hair follicle, and in iris of eye

  • autonomic innervation similar to skeletal muscle

    • terminal branches of a nerve fiber synapse with individual myocytes and form a motor unit

    • each motor unit contracts independently of the others

• single unit smooth muscle

  • more widespread

  • found in most blood vessels, digestive, respiratory, urinary, and reproductive tracts (visceral muscle)

  • 2 layers

  • gap junction

  • large number of cells contract together as a single unit

stimuli of smooth muscle

• chemical stimuli

  • hormones, carbon dioxide, low pH, oxygen deficiency

  • in response to stretch

  • single unit smooth muscle in stomach and intestines has pacemaker cells that set off waves of contraction throughout entire layer of muscle

varicosities

in a single unit smooth muscle, each autonomic nerve fibers have up to 20,000 beadlike swellings known as varicosities

• each contains synaptic vesicles and a few mitochondria

• nerve fiber passes amid several myocytes and stimulates all of them at once when it releases its neurotransmitter

contraction and relaxation of smooth muscle

• contraction and relaxation are very slow compared to skeletal muscle

• latent period is 50-100 msec compared to skeletal muscle’s 2 msec

• tension peaks at about 500 msec

• declines over a period of 1-2 seconds

• slows myosin ATPase enzyme and slow pumps that remove Ca+2

• Ca+2 bonds to calmodulin instead of troponin

  • activates kinases and ATPases that hydrolyze ATP