muscle extreme

Skeletal muscle

• Appearance: Striated (striped) ​

• Nuclei: Many nuclei per cell (multinucleated), nuclei at periphery ​

• Location: Attached to bones ​

• Special features: ​

• Long, cylindrical fibers ​

• Voluntary control ​

• No branching ​

Cardiac muscle

• Appearance: Striated ​

• Nuclei: Usually 1 (sometimes 2), central nucleus ​

• Location: Heart only ​

• Special features: ​

• Branched cells ​

• Intercalated discs (gap junctions + desmosomes) ​

• Involuntary, rhythmic contraction​

Smooth muscle

• Appearance: Non-striated (smooth) ​

• Nuclei: Single, central nucleus ​

• Location: Walls of organs (intestines, blood vessels, bladder) ​

• Special features: ​

• Spindle-shaped cells ​

• Involuntary ​

• No branching or intercalated discs ​

levels of muscle organization

1. Muscle (organ level) → made of bundles of fascicles ​

2. Fascicle → bundle of muscle fibers (cells) ​

3. Muscle fiber / muscle cell → a single long muscle cell ​

4. Myofibril → thread-like structures inside a muscle fiber; made of repeating units ​

5. Sarcomere → basic contractile unit of a myofibril (repeats along the myofibril) ​

6. Myofilament → protein filaments inside sarcomeres (actin + myosin)

Why Striations Exist

Striations come from the repeating pattern of sarcomeres:​

• A bands (dark): thick + thin overlap ​

• I bands (light): thin only ​

• Z lines: boundaries of sarcomeres ​

• 👉 The aligned arrangement of actin and myosin creates visible stripes.​

Thick vs Thin Filaments​

Thick filament (myosin)​

• Made of myosin protein ​

• Has: ​

• Head: binds actin + uses ATP ​

• Tail: structural support ​

• Function: ​

• Pulls actin during contraction

Thin filament​

• Made of: ​

• Actin (F-actin): binding site for myosin ​

• Tropomyosin: blocks binding sites ​

• Troponin: calcium-binding switch protein ​

• Function: ​

• Regulates contraction + provides binding sites

Steps of Muscle Contraction (from neuron to relaxation)​

1. Action potential reaches motor neuron terminal ​

2. Acetylcholine (ACh) released ​

3. ACh binds receptors on muscle fiber ​

4. Muscle membrane depolarizes → muscle action potential ​

5. Signal travels along T-tubules ​

6. Sarcoplasmic reticulum releases Ca²⁺ ​

7. Ca²⁺ binds troponin → moves tropomyosin ​

8. Myosin binds actin (cross-bridge forms) ​

9. Power stroke pulls actin → contraction ​

10. ATP binds myosin → detaches actin ​

11. Calcium is pumped back into SR ​

12. Muscle relaxes

role of calcium in contraction of a skeletal muscle.​

• Calcium is the ON switch for contraction:​

• Stored in sarcoplasmic reticulum ​

• Released when muscle is stimulated ​

• Binds troponin ​

• Causes tropomyosin to move ​

• Exposes actin binding sites ​

• 👉 Without Ca²⁺ → no contraction

Role of ATP

ATP is required for:​

• Detaching myosin from actin ​

• ATP binds myosin → breaks cross-bridge ​

• Powering the myosin head ​

• ATP hydrolysis “cocks” myosin head ​

• Calcium removal ​

• ATP pumps Ca²⁺ back into SR ​

• 👉 Without ATP → muscle stays locked (rigor)

Effects of Sarcomere Contraction

During event:​

• I band gets smaller ​

• H zone disappears ​

• Z lines move closer together ​

• A band stays the same length ​

• 👉 Filaments do NOT shorten— they slide past each other (sliding filament theory)​

Identify the phases of a muscle twitch and match them to the events of contraction.

• 1. Latent period​

• Action potential spreads ​

• Ca²⁺ released ​

• No visible contraction yet

2. Contraction phase​

• Cross-bridges form ​

• ​Myosin pulls actin ​

• Muscle shortens

3. Relaxation phase​

Ca²⁺ pumped back into SR ​

Cross-bridges stop forming ​

Muscle returns to resting length​

​

Wave (twitch) summation

• Repeated stimuli before full relaxation ​

• Ca²⁺ stays elevated → stronger contraction​

👉 Force increases with each stimulus ​

Tetanus

• Rapid, continuous stimulation ​

• No relaxation between twitches​
  👉 Results in sustained, maximal contraction

Recruitment

• (multiple motor unit summation)​

• Increasing number of motor units activated​
  👉 More fibers contracting = stronger force

Isotonic contraction

muscle changes length but tentnus is constant (normal)

Concentric

• Muscle shortens ​

• Example: lifting a weight ​

(Isotonic)

Eccentric

• Muscle lengthens while contracting ​

• Example: lowering a weight slowly (Isotonic)

Isometric contraction

• Muscle does NOT change length ​

• Tension increases but no movement ​

• Example: holding a weight still ​

​

Wave summation​

tapping a muscle repeatedly → stronger contractions ​

Tetanus

holding a heavy object steady (continuous contraction) ​

Concentric isotonic

lifting a dumbbell

Eccentric isotonic

lowering a dumbbell ​

Isometric

• plank, wall sit, holding a book still ​

​

Isometric

plank, wall sit, holding a book still

Motor Unit​

One motor neuron + all the muscle fibers it controls

Gradation of Muscle Tension

• You control strength in 2 main ways:​

• 1. Recruitment​

• Activate more motor units → stronger force ​

• 2. Frequency of stimulation​

• Increase impulse rate → summation → tetanus ​

• 👉 Light tap = few motor units + low frequency​
  👉 Strong hit = many motor units + high frequency​

Direct phosphorylation

• Very fast ​

• Uses creatine phosphate (CP) ​

• Lasts ~10–15 seconds ​

• No oxygen needed ​

(Energy Production)

Anaerobic glycolysis

• Breaks down glucose → lactic acid ​

• No oxygen required ​​

• Produces small ATP ​

• Supports ~30–40 seconds of activity ​

(Energy Production)

Aerobic respiration

• Uses oxygen ​

• Occurs in mitochondria ​

• Produces large ATP ​

• Supports long-duration activity

(Energy Production)

Causes of Muscle Fatigue​

• ATP depletion ​

• Lactic acid buildup → ↓ pH ​

• Ion imbalances (Na⁺, K⁺, Ca²⁺) ​

• Decreased Ca²⁺ release ​

• Oxygen debt ​

• Nervous system fatigue ​

• 👉 Fatigue = reduced ability to contract

(Skeletal Muscle Fibers​)

Slow Oxidative (Type I)

Feature​	?????​
Speed​	Slow​
Fatigue resistance​	High​
Mitochondria​	Many​
Blood vessels​	Many​
Myoglobin​	High​
Diameter​	Small​
Color​	Red​
Metabolism​	Aerobic​
Function​	Endurance (marathon)

(Skeletal Muscle Fibers​)

Fast Glycolytic (Type IIb)

Feature​

Speed​

Fatigue resistance​

Mitochondria​

Blood vessels​

Myoglobin​

Diameter​

Color​

Metabolism​

Function

??? (Type IIa)​

Fast​

Moderate​

Moderate​

Moderate​

Moderate​

Medium​

Pink​

Aerobic + anaerobic​

Mixed activity

(Skeletal Muscle Fibers​)

Fast Glycolytic (Type IIb)

Feature​

Speed​

Fatigue resistance​

Mitochondria​

Blood vessels​

Myoglobin​

Diameter​

Color​

Metabolism​

Function

Fast Glycolytic (Type IIb)​

Very fast​

Low​

Few​

Few​

Low​

Large​

White​

Anaerobic​

Power (sprinting, lifting)

(Skeletal Muscle Fibers​)

Fast Glycolytic (Type IIb)

Feature​

Speed​

Fatigue resistance​

Mitochondria​

Blood vessels​

Myoglobin​

Diameter​

Color​

Metabolism​

Function

Fast Glycolytic (Type IIb)​

Very fast​

Low​

Few​

Few​

Low​

Large​

White​

Anaerobic​

Power (sprinting, lifting)

Skeletal Muscle

Feature​	???​
Structure​	Long, cylindrical, striated, multinucleated​
Location​	Attached to bones​
Energy production​	All 3 (CP, anaerobic, aerobic)​
Innervation​	Somatic nervous system​
Initiation of contraction​	Motor neuron releases ACh​
Role of Ca²⁺​	Binds troponin​
Presence of T-tubules​	Present, well-developed​
Sarcoplasmic reticulum (SR)​	Highly developed​
Source of Ca²⁺​	SR only​
Mechanism of Ca²⁺ action​	Ca²⁺ → troponin → moves tropomyosin → exposes actin sites​
Means of gradation (force control)​	Recruitment + frequency (summation)

Cardiac Muscle

Feature​

Structure​

Location​

Energy production​

Innervation​

Initiation of contraction​

Role of Ca²⁺​

Presence of T-tubules​

Sarcoplasmic reticulum (SR)​

Source of Ca²⁺​

Mechanism of Ca²⁺ action​

Means of gradation (force control)

Cardiac Muscle​

Branched, striated, 1–2 nuclei, intercalated discs​

Heart​

Mostly aerobic​

Autonomic nervous system​

Pacemaker cells + autonomic input​

Binds troponin​

Present, less developed​

Moderately developed​

SR + extracellular fluid​

Same as skeletal​

Frequency of Ca²⁺ entry + hormonal control

Smooth Muscle (Single Unit)

Smooth Muscle (Single Unit)​

Non-striated, spindle-shaped, single nucleus, gap junctions​

Walls of organs (intestines, uterus)​

Mostly aerobic​

Autonomic + self-excitable​

Stretch, hormones, pacemaker activity​

Binds calmodulin​

Absent​

Poorly developed​

Mostly extracellular fluid​

Ca²⁺ → calmodulin → activates MLCK → phosphorylates myosin​

Degree of stretch + Ca²⁺ levels + hormones

Feature​

Structure​

Location​

Energy production​

Innervation​

Initiation of contraction​

Role of Ca²⁺​

Presence of T-tubules​

Sarcoplasmic reticulum (SR)​

Source of Ca²⁺​

Mechanism of Ca²⁺ action​

Means of gradation (force control)

function of the muscular system

1. Movement of the body​

2. Maintenance of posture​

3. Respiration​

4. Production of body heat​

5. Communication​

6. Constriction of organs and vessels​

7. Contraction of the heart​

General Properties of Muscle

• Contractility​

• Elasticity: ​

• Excitability: ​

• Conductivity: ​

• Extensibility:

Contractility:

ability of muscle to recoil to original resting length after stretched​

Excitability

capacity of muscle to respond to a stimulus (from our nerves)​

Conductivity:

ability to propogate

Extensibility

muscle can be stretched to its normal resting length and beyond to a limited degree​

Skeletal Muscle Structure​

• highly vascular​

• well innervated​

• Fascia (hypodermis)

• around individual muscle = epimysium​

• holds blood vessels and nerves​

Skeletal Muscle Structure

Composed of muscle cells (fibers), connective tissue, _______, and _______.​

Fibers are long, cylindrical, multinucleated​

Striated appearance​

Sarcomeres

highly ordered repeating units of myofilaments​

Actin (Thin) Myofilaments​

attached to the Z-discs on each end. Fibrous (F) actin forms a double helix attached at sarcomere.

​​Tropomyosin

Troponin

Actin (Thin) Myofilaments

• forms a double helix attached at sarcomere.​

• Composed of G actin monomers each of which has an active site ​

• Actin site can bind myosin during muscle contraction. ​

Tropomyosin

winds along the groove of the F actin. ​

Troponin

• three subunits: ​

• one binds to actin​

• second that binds to tropomyosin​

• third that binds to calcium ions. ​

• The tropomyosin/troponin regulates the interaction actin and myosin.​

Tropomyosin

winds along the groove of the F actin.

Myosin (Thick) Myofilament​

Myosin heads​

1. Can form cross-bridges. ​

2. Attached to the rod portion by a hinge region. ​

3. Are ATPase enzymes: Part of the energy from breaking down ATP is used to bend the hinge region of the myosin.​

Sarcomere

The fundamental, repeating contractile unit of striated muscle fibers, bordered by Z-lines.

• generates force through the sliding of actin (thin) and myosin (thick) filaments

Z disk

filamentous network of protein. Serves as attachment for actin​

I bands

from Z disks to start of thick filaments​

A bands:

length of thick filaments​

H zone

region in A band where actin and myosin do not overlap​

M line

middle of H zone; delicate filaments holding myosin in place​

Motor neurons

• stimulate muscle fibers to contract. ​

• Axons branch so that each muscle fiber is innervated​

• Contact is neuromuscular junction​

motor end plate

specialized area, part of neuromuscular junction​

transverse (T) tubules

tube-like invaginations of plasma membrane that penetrate to deep part of fiber

• conduct impulse rapidly through cell​

Capillary beds

surround muscle fibers​

Neuromuscular junction (NMJ)

axon terminal resting in an invagination of the sarcolemma​

Calcium and Muscle Contraction (Skeletal Muscle)

1. Sarcoplasmic reticulum stores calcium​

The sarcoplasmic reticulum (SR) stores a large amount of Ca²⁺ (calcium) inside the muscle cell. ​

2. Calcium is released​

When the muscle receives a signal, Ca²⁺ floods out of the sarcoplasmic reticulum into the cytoplasm of the muscle cell. ​

3. Calcium binds troponin​

The Ca²⁺ binds to a protein called troponin. ​

4. Troponin moves tropomyosin​

When calcium binds troponin changes shape. ​

This pulls tropomyosin away from the binding sites on actin. ​

5. Actin binding sites are exposed​

Normally tropomyosin blocks the actin binding sites. ​

When it moves away, the myosin-binding sites on actin become exposed. ​

6. Cross-bridge formation​

Myosin heads attach to the exposed actin sites. ​

This attachment is called a cross-bridge. ​

Cross-bridges allow the muscle to contract. ​

Cross-Bridge Movement

• The myosin head has ATP attached, which is split into ADP + a phosphate group (Pi). ​

• The phosphate group is released, which activates the myosin head and causes it to bend. ​

• This bending creates the power stroke, where the myosin pulls on the actin filament. ​

• After the power stroke, ATP binds to the myosin head. ​

• The binding of ATP causes myosin to detach from actin. ​

• The ATP is then broken down again, which re-cocks (straightens) the myosin head so it is ready for another cycle. ​

• This cycle repeats as long as calcium is present and ATP is available​

do filaments chnage length?

Filaments do NOT change length — they slide past each other. ​

This causes the sarcomere to shorten, which contracts the muscle.

Sarcomere shortens during when

contraction.

what happnes to Z disks during contraction

move closer together.

what happnes to Z disks during contraction

they get closer together

what happnes to i band during contraction

I band shortens.

what happnes to H band during contraction

shortens or disappears.

What Happens to a bands during contraction

stays the same length.

Muscle Relaxation​

• Ca2+ moves back into sarcoplasmic reticulum by active transport. Requires atp

• Ca2+ moves away from troponin-tropomyosin complex ​

• Complex re-establishes its position and blocks binding sites. ​

Motor unit

a single motor neuron and all muscle fibers innervated by it​

Motor Unit numbers

• Large muscles have motor units with many muscle fibers.​

• Small muscles that make delicate movements contain motor units with few muscle fibers​

Regulation of Smooth Muscle

• Innervated by autonomic nervous system

• ​Neurotransmitters are acetylcholine and norepinephrine​

• Hormones are epinephrine and oxytocin​

• Receptors present on plasma membrane

Excitation–Contraction Coupling

Excitation–Contraction Coupling is the process that links a nerve signal to muscle contraction.

Muscle Twitch

muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers​

Phases of Twitch

Lag or latent: Ca2+ must be released from SR​

Contraction: contraction occurs​

Relaxation: Ca2+ is taken up into SR​

Electrical Properties of Smooth Muscle

• Slow waves of depolarization and repolarization transferred from cell to cell​

• Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell​

• Does not follow the all-or-none law​

• May have pacemaker cells​

• Contraction regulated by nervous system and by hormones​

Regulation of Smooth Muscle​

• Innervated by autonomic nervous system​

• Neurotransmitters are acetylcholine and norepinephrine​

• Hormones are epinephrine and oxytocin​

• Receptors present on plasma membrane

All-or-none

law for muscle fibers​;Contraction of equal force in response to each action potential

Sub-threshold stimulus

no action potential; no contraction​

Multiple motor unit summation

strength of contraction depends upon recruitment of motor units. A muscle has many motor units

• ​Submaximal stimuli​

• Maximal stimulus​

• Supramaximal stimuli

Strength of contraction is graded

ranges from weak to strong depending on stimulus strength​

Treppe

• Graded response​

• Occurs in rested muscle​

• Each subsequent contraction is stronger than previous until all equal after few stimuli​

• More and more Ca2+ remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum​

Incomplete tetanus

muscle fibers partially relax between contraction​

Complete tetanus

no relaxation between contractions​

Multiple-wave summation

muscle tension increases as contraction frequencies increase​

Maxium tension

produces maxium tension in reponse to maxium stimulus at sacomores optional length

lever systems

Muscles and their tendons and bones act together,to move either parts of the body or the whole body. ​

Lever

rigid shaft or bone​

Fulcrum:

pivot point or joint​

Weight or resistance

(force of gravity either in the form of the weight of the body parts or the weight of an object being lifted, pulled, or pushed)​

Active tension

force applied to an object to be lifted when a muscle contracts​

Stretched muscle

not enough cross-bridging​

Crumpled muscle

myofilaments crumpled, cross-bridges can't contract​

Passive tension

tension applied to load when a muscle is stretched but not stimulated​

Total tension

active plus passive​

Muscle Tone

Alternating contractions in motor units stimulated by spinal reflexes

• gives firmness to relaxed muscle without movement​

• keeps muscle healthy, ready to respond​

• important to posture​​