BIO 220 Chapter 10: Muscle Tissue

Properties of muscles:

1. Excitability (responsibility)
2. Contractility (ability of cells to shorten)
3. Extensibility (stretching)
4. Elasticity (recoil)

Skeletal muscle

striated, voluntary, multinucleated

Cardiac muscle

Involuntary muscle tissue found only in the heart. Branched

Do skeletal muscles push or pull?

Pull

Functions of skeletal muscle:

1. Producing movement - act on bones
2. Maintaining posture and body position
3. Supporting soft tissues
4. Guarding body entrances and exits - like the mouth.
5. Maintaining body temperature - contract and give off heat.
6. Storing nutrients

What do skeletal muscles contain?

Skeletal muscle tissue (primarily)
Connective tissues
Blood vessels
Nerves

Three layers of connective tissues:

Epimysium, perimysium, endomysium

Epimysium

Layer of collagen that surrounds muscle. Seperates muscle from surrounding tissues

Perimysium

Surrounds muscle fiber bundles (fasicles)
Contains collagen fibers, elastic fibers, blood vessels, nerve

Endomysium

Surrounds individual muscle cells (fibers)
Contains capillary networks, myosatellite cells (stem cells) and nerve fibers

Are skeletal muscles vascularized?

Yes, highly. Deliver oxygen, nutrients. Remove metabolic wastes.

Diaphragm

skeletal muscle, but works subconciously

Skeletal muscle fibers

Enormous compared to other cells
Hundreds of nuclei (multinucleate)
Made of myofibrils.
Striated muscle - made of actin and myosin

Sarcolemma

Outer covering of muscle fiber. Inside the endomysium.
Plasma membrane of a muscle fiber.
Surrounds sarcoplasm (cytoplasm of muscle fiber)

Transverse tubules (T tubules)

Tubes that extend from surface of muscle fibers deep into sarcoplasm.
Transmit action potentials from sarcolemma into cell interior.

Sarcoplasmic reticulum

Holds calcium. Stores and releases calcium ions
Network surrounding each myofibril.
Forms chambers (terminal cisternae) that attach to T tubules
Triad is two terminal cisternae (where it bulges) and a T tubule.

Myofibrils

Inside the muscle fiber. Made of bundles of protein filaments (myofilaments)
Two types of myofilaments: thin (actin) and thick (myosin)

Functional unit for muscle contraction:

Sarcomere
On one myofibril you can have 10,000 sacromeres.
Interactions between filaments produce contractions.
Arrangement of filaments account for striated pattern of myofibrils.
Sarcomeres have dark bands (A bands) and light bands (I bands)

The A band

Has both actin and myosin.
- M line: center of A band
- H band: has thick filament, no thin
- Zone of overlap: dark region where thick and thin filaments overlap

The I band

Only thin filaments (actin)
- Z lines: only actin. bisect I bands
- Titin: elastic protein

One sarcomere is from _____ line to _____ line.

Z to Z

When sarcomere shortens:

I band shortens
M line stays the same
Zone of overlap gets larger
H band gets shorter
A and M line stay the same

Thin filaments contain:

1. Filamentous actin (F-actin): twisted strand of two G-actin molecules
2. Tropomyosin: covers active sites of G-actin
3. Troponin: controlled by calcium; initiates contraction

What protein initiates contraction?

troponin

Thick filaments consist of:

each contains about 300 myosin molecules
Consists of a tail and a head. Tail binds to other myosin molecules, head projects towards nearest thin filament
Titin core: recoils after stretching

Initiating contraction

Ca2+ binds to receptor on troponin molecule
Troponin-tropomyosin complex changes
Exposes active site of F-actin

During contraction:

myosin heads interact with actin filaments and form cross-bridges, which produces motion

sliding filament theory

theory that actin filaments slide toward each other during muscle contraction, while the myosin filaments are still.
H and I bands narrow, Zones of overlap widen, Z lines move closer together, A bands and M line stay the same.

Depolarization and repolarization produce _______

action potentials

Sodium concentrates ____ the neuron, potassium concentrates ______ inside the cell

outside; inside.
Sodium will come inside the cell, potassium leaves the cell

neuron is around _____ mV at rest

-70

Neuromuscular junction (NMJ)

synapse between a neuron and a skeletal muscle fiber.
axon terminal releases neurotransmitter acetylcholine

What does acetylcholine (ACh) do?

open chemically gated Na+ channel on the muscle fiber; Na+ enters cell and depolarizes motor end plate
action potential is generated
acetylcholine will bind to sarcolemma

What enzyme breaks down acetylcholine?

Acetylcholinesterase (AChE)

Excitation-contraction coupling (ECC)

action potentials travel down t-tubules to triads
Ca2+ is released from terminal cisternae of sarcoplasmic reticulum
Ca2+ binds to troponin and changes its shape
troponin-tropomyosin complex changes position; exposes active sites of thin filaments

Cross bridge cycling (CBC)

1. contraction cycle begins (when calcium comes in)
2. active site exposure
3. cross bridge formation (myosin binds to actin)
4. myosin head pivoting (power stroke); pulls actin towards the M line
5. cross bridge detachment
6. myosin reactivation

When muscle cells contract, they produce ______

tension
Tension must overcome resistance to produce movement.
Entire muscle shortens at the same rate. Speed depends on cycling rate (number of power strokes per second)

Duration of contraction depends on:

1. Duration of neural stimulus (how long ACh is bound to the Na+ / K+ channel)
2. Presence of free calcium ions in the sarcoplasm.
3. Availability of ATP (powers myosin head)

'resetting' muscles

Acetylcholine must be broken down
Ca2+ is pumped back to SR
1. Ca2+ detaches from troponin
2. troponin returns to original position
3. tropomyosin re-covers the active sites

Rigor mortis

fixed muscle contraction after death
ATP runs out; ion pumps cease to function
calcium ions build up in sarcoplasm

ATP needed for:

moving myosin heads
moving calcium ions back to SR
resetting polarization of cell

Amount of tension produced depends on:

1. number of power strokes performed
2. fiber's resting length at time of stimulation
3. frequency of stimulation (acetylcholine)

A muscle twitch has 3 phases:

Latent period: action potential moves across sarcolemma, SA releases Ca2+ (ECC)
Contraction phase: calcium ions bind to troponin, cross bridges form (CBC)
Relaxation phase: Ca2+ levels fall, cross-bridges form

Treppe

A stair-step increase in tension caused by repeated stimulations immediately after relaxation phase
Stimulus frequency

Wave summation

Increasing tension due to summation of twitches caused by repeated stimulations before the end of relaxation phase
Stimulus frequency >50/second

Incomplete tetanus

Muscle produces near-maximum tension

Complete tetanus

Higher stimulation eliminates relaxation phase
Muscle in continuous contraction

What is a motor unit?

Motor neuron and all the muscle fibers it controls
May contain few or thousands
All fibers will contract at the same time

Recruitment

Increase in number of active motor units
To sustain contraction, you want fibers to exchange

Muscle tone

Normal tension and firmness of a muscle at rest.
Without causing movement, motor units actively stabilize positions of bones and joints, maintain balance.

The only energy source used directly for muscle contraction:

ATP
More ATP must be generated to sustain a contraction.

ATP is generated by:

1. direct phosphorylation of ADP by creatine phosphate
2. anaerobic respiration (glycolosis)
Aerobic respiration (citric acid cycle and ETC)

Muscle metabolism

Skeletal muscles at rest metabolize fatty acids and store glycogen and CP
Moderate activity: muscles generate ATP through aerobic respiration
At peak activity, pyruvate produced via glycolysis is converted to lactate

Recovery period

Time required after exertion for muscles to return to normal

What is oxygen debt?

Excess Postexercise Oxygen Consumption
Body needs more oxygen than normal to normalize metabolic activity.
Breathing rate and depth increased

Muscles produce ______

Heat

Hormones that increase metabolic activities in skeletal muscles:

Growth hormone (GH)
Testosterone
Thyroid Hormone (TH)
Epinephrine (EPI)

Endurance

The amount of time an activity can be sustained

Force

The max amount of tension produced

Three types of skeletal muscle fibers:

Fast fibers, slow fibers, intermediate fibers

Fast fibers

Large diameter, contract quickly
Lots of glycogen in reserve
Use lots of ATP in anaerobic respiration
Fatigue quickly

Intermediate fibers

Mid-sized
Has Myoglobin-helps store oxygen
Uses anaerobic

Slow fibers

Slow to contract
Lots of mitochondria; aerobic respiration
High oxygen supply
Contain high amount of myoglobin (red pigment that binds to oxygen)

Red meat and white meat:

Red meat is slow fibers because it contains myoglobin and it is a darker pigment
White meat is fast fibers

Muscle Hypertrophy

Muscle growth from heavy training that causes increase in:
Diameter of muscle fibers, number of myofibrils, number of mitochondria, glycogen reserves

Muscle atrophy

Reduction of muscle size, tone, and power due to lack of activity

Importance of exercise

What you don't use, you lose
Muscle tone indicates base activity in motor units of skeletal muscles
Muscles become flaccid when inactive for days or weeks
Muscle fibers break down proteins, become smaller and weaker
With prolonged inactivity, fibrous tissue may replace muscle fibers

When muscles are fatigued"

there is a depletion of metabolic reserve, damage to sarcolemme and SR, decline in pH

Cardiac Muscle

only in the heart, striated, has excitable membranes

Structural characteristics of cardiac muscle

small, branched with a single nucleus
can only do AEROBIC respiration
lots of myoglobin and mitochondria

Intercalated disks

Join sarcolemmas of adjacent cardiac muscle by gap junctions and desmosomes
Functional Syncytium (allowing ions to move from one cell to another, allowing heart to beat in rhythm)

Pacemaker cells

Contraction without neural stimulation
Never want tetanic contraction for the heart

Structural characteristics of smooth muscle

single, central nucleus, spindle-like
nonstriated, no t-tubules, myofibrils, or sarcomeres
scattered thick filaments with many myosin heads
thin filaments attached to dense bodies

excitation-contraction coupling in smooth

No troponin
Calcium binds to Calmodulin; activates myosin light chain kinase (allows myosin heads to attach to actin, no troponin)

Smooth muscle has plasticity

can change a wide range of lengths

Three types of skeletal muscle fibers:

Fast fibers, slow fibers, intermediate fibers