8 Muscle

What are the 4 Special Characteristics

Excitability
Contractility
Extensibility
Elasticity

Excitability

(responsiveness or irritability): ability to receive and
respond to stimuli

Contractility

ability to shorten when stimulated

Extensibility

Ability to be stretched

Elasticity

Ability to recoil to resting length

What are the 4 basic muscle function

Movement - of bones or fluids
Maintaining - posture and body position
Stabilizing - joints
Heat generating (esp. skeletal muscle)

What are the 3 muscle tissue types?

Skeletal Muscle
Cardiac Muscle
Smooth Muscle

Attaches to bone and skin
Striated and Voluntary
Long, multinucleated

Skeletal Muscle

Striated and involuntary
Branching, intercalated discs

Cardiac Muscle

Walls of hallow organs
Not striated, involuntary
Short, spindle shape

Smooth muscle

Skeletal muscle surrounded by Fascia

thin fibrous C.T. sheets for reinforcement, passage for nerves, blood vessels and attachement

What are the connective tissue sheaths of skeletal muscle

epimysium, perimysium, endomysium

Epimysium

surrounds entire muscle

Perimysium

surrounds fascicles (groups of muscle fibers)

Endomysium

surrounds each muscle fiber (cell)

Muscles attach either

Directly or Indirectly

Directly means

epimysium fused to periosteum or perichondrium

Indirectly

Connective tissue extend beyond muscle as tendon or aponeurosis

What is the structural and organization level of muscle?

Muscle
Fascicle
Muscle fiber

Muscle

100-1000s of cells wrapped in C.T. with blood and nerves
- Epimysium surrounds muscle

Fascicle

portion of the muscle
Bundle of cells wrapped in C.T. sheath
Perimysium surrounds fasicle...

Muscle Fiber (cell)

- Long, multinucleate with mitochondria
- Striated appearance
- Endomysium surrounds each cell...
- Sarcolemma = Cell Membrane
- Sarcoplasmic Reticulum = calcium

Sarcolemma

muscle cell membrane

Sarcoplasmic Reticulum is a Smooth ER modified for

Calcium Storage

Microscopic Structure of Muscle cells

Muscle Cell
Myofibril
Sacromere

Many myofibrils, mitochondrion, wrapped in sarcolemma (membrane)

Muscle cell

Striations evident

Myofibril

'muscle segment'
› Smallest contractile (functional) unit
› Composed of myofilaments (actin & myosin proteins)
› Bands have names (e.g. A, H, I)
› Z-discs = edges of sarcomere

Sarcomere

Order of Muscle

Epimysium
Muscle
Perimysium
Fascicle
Endomysium
Sarcolemma
Muscle Fiber
Myofibril
Sarcomere
Filaments

Myosin is

- Composed myosin protein
- Myosin heads form 'cross bridges'
- Contain binding sites for actin & binding sites for ATP
- ATPase enzymes

Actin

• Composed of F (fibrous) actin protein, consisting of G (globular) actin subunits
• G actin bears active sites for myosin head binding
• Tropomyosin and troponin: regulatory proteins

In actin and myosin which one is a thick or thin filament?

Myosin = Thick
Actin = Thin

What is tropomyosin?

It is a long, fibrous protein that winds around the actin polymer, blocking all the myosin-binding sites.

What is troponin?

a regulatory protein that moves tropomyosin aside & exposes myosin binding sites when Ca+ is released during muscle contraction

What allows muscles to relax?

ATP

What causes Muscle cramps?

lack of ATP

Membrane Potential created by

separation of charge
- Na+/K+ pump and gradients
- Potential energy - measured in volts (mV)

Action Potential in muscles

AP is conducted on the membrane to areas containing voltage dependent Calcium channels. When these channels are opened, Ca activates to contraction mechanism.

Depolarization

Na+ channels open

Repolarization

K+ channels open, allowing efflux of K+, and cell becomes more negative

Voluntary Muscle Contractions Steps

- Motor Command
- Action Potential
- Electrical Signal
- Synapse = The Gap
- Chemical Signal = neurotransmitter = acetylcholine

Understanding the Sarcomere Strucure

• Myosin heads - ATPase enzymes that break 3rd phosphate off to change conformation of myosin head, and thus allow the myosin head to create cross-bridges with actin active sites.
• G (globular) actin subunits - the infamous blueberries or pearls components of actin that bind together to form the F (fibrous) actin.
• Troponin - the site where calcium ions can bind allowing troponin to change its grip on tropomyosin which moves tropomyosin to uncover the actin active sites.

Sarcoplasmic Reticulum

- Smooth Er surrounds each myofibril
- Paired terminal cisternae
- Functios in the regulation of intracellular Ca2+ levels

What is the terminal sisternae?

Enlarged portion of the sarcoplasmic reticulum; part
of Sarcoplasmic reticulum.

T-Tubules

- Continuous with sarcolemma (contains ECF)
- Penetrates cells interior at A band-I band junction
- Form 'triads'

What is a Triad?

a T tubule and two adjacent terminal cisternae

How does acetylcholine enter the synapse (type of membrane transport)?

Vesicular transport

What stimulates vesicles to release neurotransmitter (acetylcholine)?

Voltage-Sensative Calcium Channels open and Ca2+ enters

What controls the action of acetylcholine for causing further contractions?

Enzyme - acetylcholinesterase, and diffusion or reuptake

Excitation-Contraction (E-C) Coupling

AP transmission along sarcolemma leading to sliding of the myofilaments takes time
Meaning - Nerve impulse does not directly cause muscle contraction
- It causes increase in Calcium-> allowing filaments to slide

excitation-contraction coupling

sequence of events from motor neuron signaling to a skeletal muscle fiber to contraction of the fiber's sarcomeres

Steps in EC coupling

1. Action potential is propagated along the sarcolemma and down the T- Tubules
2. Calcium ions are released
3. Calcium binds to troponin and removes the blocking actin of tropomyosin
4. Contraction begins

Cross Bridge Cycle

1. cross bridge formation: myosin heads attach to actin
2. power stroke: myosin pulls actin 90 to 45 degrees
3. cross bridge detachment: myosin head detaches
4. recovery: myosin head swivels back to 90
*sarcomeres shorten

What is accruing in patients with Rigor Mortis and Muscle Cramps?

• ATP separates actin & myosin
• But no new ATP is available, so Ca2+ remains and/or leaks from the SR
• Cross-bridges remain intact or form →'rigor mortis' - in Latin 'Stiffness of death'
• 15-24 hours...until proteins breakdown by enzymes...influenced by heat