chapter 10 - muscular system

muscle characteristics

1. excitable — respond to stimuli and produce APs

2. contractile — can shorten and lengthen

3. extensible — stretch when pulled

4. elastic — return to its original shape after contraction/extension

muscle functions

1. movement

   • ex. walking, breathing, running

   • pull on bone across joints to create movement

2. posture and facial expression

   • ex. sitting (posture) — muscles are constantly working to keep the body upright

   • facial muscles are connected to skin, not bones — provides non-verbal communication

3. heat production

   • generate heat when contracting

   • cold → muscles shiver to crank up the heat

4. protection of viscera

   • where no thick bones shield

   • tense up and absorb impact

   • ex. rectus/transverse abdominus, oblique

motor unit

a single motor neuron and all the muscle fibres it innervates

presynaptic cell

a motor neuron with ACh (nt) vesicles in its axon terminal

postsynaptic cell membrane

a muscle sarcolemma

motor end plate

a specialized region on a sarcomlemma with many ACh receptors that detect signals and trigger contractions

steps of AP on a muscle sarcolemma

1. AP reaches the axon terminal and synaptic end bulb of a motor neuron

2. Ca2+ enters via voltage gates, causing exocytosis of ACh

3. ACh binds to ACh receptors on the motor end plate

4. chemical gates open and Na+ enters, causing an end plate potential (EPP)

5. EPP causes opening of Na+ voltage gates on adjacent sarcolemma; AP then propagates along the sarcolemma and down the T-tubules

myofibril

a structure in a muscle fiber that shortens when the muscle contracts

sarcomere

basic structural unit of contraction

myosin

thick filament in a muscle fiber

actin

thin filament in a muscle fiber

steps of muscle fiber contraction

1. excitation of muscle fiber

   i. sarcolemma depolarized → EPP → AP

   • ACh binds to motor end plate

   ii. AP propagates down the T-tubules to deep within the fiber

   • brings the electrical signal closer to SR (stores Ca2+)

2. excitation-contraction coupling

   i. AP in T-tubules cause the release of Ca2+ from terminal cisternae of the sarcoplasmic reticulum (SR) via mechanically gated channels

   • “coupling agent” — links electrical (AP) to mechanical (contractions) event

   ii. Ca2+ binds to troponin

   • high concentration inside the SR to low in the cytosol

   • troponin responds to Ca2+ levels

     • changes shape when Ca2+ binds, and shifts tropomyosin

   iii. troponin-tropomyosin complex moves, exposing myosin binding sites on actin

3. contraction

   i. activated myosin heads attach to binding sites on actin — “cross bridge” formation

   ii. energy stored in myosin head is released → myosin head pivots (”power stroke”), ADP + Pi released → actin slides myosin toward center of sarcomere (M line)

   • thin filament pulled inward

   iii. ATP attaches to myosin head → released from actin and unpivots (”recovery stroke”)

   • myosin head gets back to its position

   iv. myosin head reactivates — ATP → ADP + Pi

   v. if Ca2+ in cytosol remains high, these steps repeat

power stroke

the movement of the myosin head pulling actin filaments during muscle contraction

recovery stroke

the process of the myosin head reattaching to actin after a movementduring muscle contraction

sliding filament mechanism

1. sarcomeres shorten

   • H zone and I band shorten

   • A band is same length

2. myofibrils shorten → muscle shortens

3. thin (actin) and thick (myosin) myofilaments remain the same length

steps of muscle fiber relaxation

1. ACh broken down by AChE on the motor end plate (facing cleft)

2. SR actively takes up Ca2+ (using Ca2+ ATPase)

3. ATP binds to and releases myosin heads

4. tropomyosin moves back to cover myosin binding sites on actin

things that ATP is necessary for

1. cross bridge release — not broken down

2. activation of myosin (ATP → ADP + Pi)

3. pump Ca2+ into SR

4. fiber Na+/K+ ATPase activity

botulism

a rare but potentially fatal illness caused by botulin toxin produced by the bacterium Clostridium botulinum, due to improper canning

• prevents exocytosis of ACh, causing flaccid paralysis

• medical application: used to treat uncontrolled blinking and crossed eyes

• cosmetic application: used to reduce wrinkles and sweating

rigor mortis

a condition that occurs after someone passes where muscles stiffen up

• intracellular Ca++ increase from ECF and SR (leakage) → binding sites exposed (cross bridges form) → myosin heads not released from actin (no new ATP produced)

• starts ~3 hrs after death, max ~12 hrs

• gradually subsides over days as cells break down

myasthenia gravis

an autoimmune disorder that leads to flaccid paralysis

• decrease in ACh receptors (blocked/destroyed by antibodies) → ACh can’t bind effectively

• treatment: AChE inhibitors → increase ACh binding to remaining receptors

curare poisoning

a condition caused by the use of curare, a plant-based toxin, that results in flaccid paralysis

• prevents ACh from binding to receptors

• was used in surgery to temporarily paralyze a patient during procedure

nicotine

a drug that mimics ACh effects and binds to receptors, producing muscle spasms/twitching

• stronger, longer-lasting contractions

• chronic use → too much ACh in body → body compensates → less ACh + receptors (due to too much stimulation) → rely on nicotine → addiction

  • quit → reverse but takes time → withdrawals

black widow spider venome

a substance that triggers a massive release of ACh causing muscles to continuously contract andcant relax, may lead to a person to stop breathing

• causes spasmic paralysis

muscle tension

the force exerted by a muscle or muscle fiber

• determined by the number of crossbridges formed — more cross bridges = higher force/tension

wave summation

the process by which muscle contractions can increase strength by utilizing successive stimuli to enhance tension

incomplete tetanus

a state of partial muscle contraction resulting from rapid successive stimuli that do not allow the muscle to completely relax, leading to a sustained but not maximal contraction

complete tetanus

a condition where a muscle remains fully contracted due to a high frequency of stimulation, resulting in maximum tension without relaxation

factors that affect muscle tension in a fiber

1. frequency of stimulation

   a. single stimulus

   b. 2nd stimulus arrives before relaxation from 1st → wave summation

   c. rapid sequence of stimuli → incomplete tetanus

   d. high frequency of stimulu → complete tetanus

2. fiber length

   • resting fiber length is optimal

   • tension decreases if shorter/longer

3. size of fiber

   • thicker = more myofibrils/fibers = more crossbridges form = more tension

4. fatigue

   • reduced max tension

fast fibers

a type of muscle fiber that contracts and relaxes quickly

• appear white, has little myoglobin

• great for speed, quick and powerful bursts — ex. sprinting, weight lifting

• ex. white meat in chicken — breasts and wings

slow fibers

a type of muscle fiber that contracts and relaxes slowly, sustaining contractions longer

• appear red, have a lot of myoglobin

• ex. postural muscles

• great for endurance activities — ex. marathon running

• ex. dark meat in chicken — legs

factors that affect muscle tension in a whole muscle

1. number of fibers contracting

   • more active motor neurons = more tension

2. number of fibers per motor unit

   • more units/fiber = more tension

3. muscle size

   • larger = more fibers = more tnesion

4. fatigue

size principle

a principle that states that muscle fibers are recruited from smallest to largest based on the force needed

muscle tone

low level of tension in a few fibers that develops as different groups of motor units are alternately stimulated over time

• gives firmness to muscles

• small amt of tension even when muscle is relaxed

isotonic contraction

a type of whole muscle contraction in which the muscle changes length while generating force, maintaining constant tension

• trension exceeds the resistance of a load lifted

• uses ATP

isometric contraction

a type of whole muscle contraction where the muscle generates tension without changing length, maintaining an equal force against a load

• when a load doesn't budge because the force isn't enough to move it

• there isn’t enough strength to overcome resistance

• still uses ATP because crossbiedges are still actively working

muscle metabolism during resting conditions

1. fatty acids used to produce ATP — aerobic

2. storage of:

   • glycogen

   • creatine phosphate (C~P)

     • ATP + creatine → ADP + C~P

   • little ATP

muscle metabolism during short-term exercise

primarily anaerobic

1. use available ATP

   • last 4-6 secs

2. creatine phosphate (system) used to produce ATP

   • C~P + ADP → ATP + creatine

   • lasts ~15 secs

   • C~P is a high-energy molecule

3. muscle glycogen → glucose → pyruvid acid → anaerobic pathway → lactic acid

   • lasts ~30 secs

muscle metabolism during long-term exercise

• ATP — from aerobic pathway

• glucose — from liver

• fatty acids — more as exercise continues

• O2 sources: blood hemoglobin + muscle myoglobin

• but sometimes anaerobic

physiological muscle fatigue

a type in muscle fatigue that results from the inability to maintain tension

• lowers ATP use

factors of physiological muscle fatigue

1. depletion of energy supplies — ex. glycogen

2. build-up of end products

   a. H+ from lactic acid

   • muscle contraction compresses blood vessels → ↓ O2 to muscle

   • ATP production is anaerobic for periods, even in long-term exercise

   • can inhibit hydrolysis (no ATP)

   b. Pi (from ATP → ADP + Pi) binds to Ca2+

   • less binds to troponin

   • slows release of Pi from myosin → slows cross bridge release from actin

   • weakens contraction

   • everything still work but not as efficient

3. failure of APs (in muscle fiber)

   • increase [K+] (build up) in small spaces of T-tubules during rapid stimuli → disturbs MP → stops Ca2+ release from SR

     • membrane doesn't repol properly, voltage channels wont behave properly → no contractions even if NS sending signal

   • long term: neuron runs out of ACh

psychological fatigue

a type of psycholocial0n hiwch