Knowledge Check 2: Skeletal muscle structure and APs

What factors influence the diversity of skeletal muscle?

shape, size, # fiber/area, length, angle of pennation

What is the pennation angle?

Angle of orientation between muscle fibers and tendons

What does angle of pennation do?

Angle of orientation between muscle fibers and tendons and limits length change during contraction (#biomechanical advantage)

What are the characteristics of skeletal muscle?

Excitable (controlled voluntarily by NS) and contractile

What are the functions of skeletal muscle?

movement, support, protection, energy stores, heat generation

What is the organization of skeletal muscle (macro to micro)?

entire muscle organ, fascicles, muscle fibers, and myofibrils

What are fascicles?

group of muscle fibers

What are muscle fibers?

groups of myofibrils

What are myofibrils?

cylindrical organelles that contain sarcomeres (thick and thin filaments)

What are the components of myofibrils?

sarcolemma, T tubules and terminal cisternae, sarcoplasm, sarcomeres, nuclei and mitochondria

What is the sarcolemma?

plasma membrane; lipid bilayer surrounding muscle fiber

What is the function of the sarcolemma?

regulating [ion], receiving and transmitting of AP, scaffolding

How does the sarcolemma receive and transmit AP?

includes NMJ and T-tubule; propagates them own along the muscle fiber

What does the sarcolemma provide scaffolding for?

neighboring muscle cells attachment via extracellular connections and fiber regeneration

What are Transverse Tubules (T-Tubules)?

extensions of the sarcolemma (muscle cell membrane) that run deep into the muscle fiber

What is the function of T-tubules

Path for nerve impulses (i.e., carry AP deep into muscle fibers)

What is the terminal cisternae?

Part of the sarcoplasmic reticulum that stores calcium ions

What is a triad in muscle fibers?

A triad consists of a T-tubule and two terminal cisternae of the sarcoplasmic reticulum

What structures are T-tubules associated wit?

terminal cisternae (ends of the SR)

Where are T-tubules positioned?

over the z-line; sarcomere is sitting under the sarcoplasm

What is the sarcoplasmic reticulum (SR)?

Membrane-bound structure that functions like an endoplasmic reticulum in other cells

What is the function of the SR?

store calcium (Ca2+)

Where does the SR run?

parallel ot muscle fiber

What is the sarcoplasm?

cytosol/cytoplasm

What is the function of the sarcoplasm?

Storage site for glycogen, myoglobin, and other proteins, minerals, fats, organelles, etc.

What is a sarcomere?

contractile unit of a myofibril

What is the structure of a sarcomere?

repeating structure w/in the myofibrils (z-line to z-line_

What are the contractile proteins within the sarcomere?

thick (myosin) and thin (actin) filaments

How do the contractile proteins affect muscle apperance?

causes striation (overlap = dark band; white = just actin)

What % of skeletal muscle proteins are myosin?

66%

What are the structural components of myosin?

head and tail

What does the head of myosin contain?

binding site for actin and ATPase

What. makes up the thick filaments?

Multiple myosin proteins group together

What makes up the thin filaments?

actin (primarily), tropomyosin, and troponin

What is actin?

globular proteins that twist together in a helical configuration to form chains; contain myosin binding sites

What is tropomyosin?

twists around actin strand and its function is to blocks myosin binding sites on the actin

What is troponin?

bound to acting at regular intervals. Function is to lock tropomyosin in place

What is the structure of sarcomeres?

contain proteins that are highly arranged and thick and thin filaments are involved in muscle contraction through cross-bridging

What is cross-bridging?

mechanism for generating force at the molecular level (Actin and Myosin)

What is the function of the nuclei?

control gene expression and mediate the replication of DNA during the cell cycle

What is the function of mitochondria (mitoc.)?

Produce ATP (i.e., phosphorylation of ADP) through oxidative metabolism

What do the mitoc. use to make energy for cells?

oxygen and water

What are the central reactions involved in mitoc. creating energy?

krebs cycle, electron transport chain, oxidative phosphorylation

Why do we need mitoc.?

to generate force, muscle cells need to manage and supply ATP

What are the 3 CTS we are concerned with for skeletal tissue?

epimysium, perimysium, and endomysium

What is the epimysium?

surrounds entire muscle

What is the perimysium?

surround the fascicles

What does the perimysium contain?

blood vessels and nerves

What is the endomysium?

thin layer that surround the individual muscle fibers

What does the endomysium contain?

tiny capillaries and individual neurons

What is the function of skeletal CT?

Resists excessive passive stretching of the muscle and distributes forces to minimize damage to the muscle fibers

What is the sarcolemma?

cell/plasma membrane (lipid bilayer)surrounding each muscle fiber; not CT

What is the function of the skeletal CT?

scaffolding, provides conduit for blood vessels and nerves, and resists excessive passive stretching and distributes force

What is the importance of scaffolding?

Once muscle development is complete, the CT holds the muscle fiber together and largely determines the gross structure of the muscle belly

What are the other CTs within skeletal muscle?

tendon

What is the tendon?

Tough, fibrous, cord-like tissue that is an extension of the 3 CTs

What is the function of tendon?

Physically connects muscle to bone (or another structure) and wraps around and within the muscle to transmit force

What is the Myotendinous junction (MTJ)?

the site of connection between tendon and muscle

What is the MTJ function?

force transmission and contractile elements of the muscle fiber proteins

What is the path of transmission from brain to muscle?

nerve cells (brain) to brain stem to spinal cord to muscle

What is the motor unit?

consists of the motor neuron and all the scattered muscle fibers which it innervates

What is the neuromuscular junction (NMJ)?

the synapse that allows communication between the motor neuron and ALL the muscle fiber it innervates

What is the NT for the NMJ?

acetylcholine (Ach)

What are the NMJ receptors?

nicotinic acetylcholine receptor

What is the motor end plate?

pocket formed around motor neuron by sarcolemma

Where are the nicotinic acetylcholine receptors?

on the motor end plate

What is membrane potential (MP)?

electrical (voltage) potential difference, or voltage, between the intracellular and extracellular spaces

What is excitation-contraction coupling?

Describes the rapid communication between electrical events occurring at the sarcolemma of skeletal muscle fibers

What is excitation?

When an NT inc. the (+) charge of the PS neuron. Inc. the likelihood that the neuron will fire and pass on the electrical impulse

What is the 1st step of excitation?

NT molecules are synthesized and packaged in vesicles

What is the 2nd step of excitation?

AP arrives at the pre- terminal

What is the 3rd step of excitation?

V-G Ca2+ channels open Ca2+ enters

What is the 4th step of excitation?

A rise in Ca2+ triggers fusion of synaptic vesicles w/ pre- membrane

What is the 5th step of excitation?

Ach molecules diffuse across the synaptic cleft and bind to specific receptors on the post- cell

What is the 6th step of excitation?

Binding of Ach to the receptor open the ligand-gated Na+ Channel

How does the binding of Ach to the receptor open the ligand-gated Na Channel?

Influx of Na changes the membrane potential

What is the 7th step of excitation?

Ach esterase (break down), is taken up by the pre- terminal or other cells, or diffusion away from synapse

What is the skeletal muscle RMP?

-85 mV (harder to depol at NMJ)

What is the threshold potential of NMJ?

-55mV

NMJ characteristics?

sum of excitatory and inhibitory, propagate wave of sarcolemma depol, travels at a constant V

How does AP get "into" fibers?

T-tubules interfaces with an intracellular network, the SR (the triad)

How do t-tubule networks and the sarcolemma affect the AP?

Allows for depolarization "deep" into fibers (not just on surface)

What is the 1st step of E-C coupling?

Initiation and propagation of the AP along the sarcolemma

What is the 2nd step in E-C coupling?

Radial spread of the potential along the T-tubule system

What is the 3rd step in E-C coupling?

DHPR (Ca2+channel) detect a change in membrane potential and interacts with the RyR

What is the 4th step in E-C coupling?

RyR opens and release Ca2+from the SR

What is the 5th step in E-C coupling?

Release of Ca2+ from the SR and transient increase of [ Ca 2+] in the myoplasm

What is the 6th step in E-C coupling?

muscle contraction begins

What is the 7th step in E-C coupling?

Ca2+ reuptake by SR via SERCA, leads to relaxation of actomyosin (and muscle fibers)

What is Ca2+ is re-sequestered (or Ca2+ reuptake)?

into the sarcoplasmic reticulum by the sarco-endoplasmic reticulum ATPase (SERCA)

What occurs during Ca2+ re-sequestered?

Muscle relaxation

What are the two major players in E-C coupling?

Dihydropyridine receptors (DHPR) and Ryanodine receptors (RyR)

What is Dihydropyridine receptors (DHPR)?

a voltage-dependent calcium channels functions in skeletal muscle essentially as a voltage sensor interacts with the Ryanodine receptors (RyR)

What are Ryanodine receptors (RyR)?

are huge ion channels that are responsible for the release of Ca2+ from the SR

What is the clinical use of E-C coupling?

Being able to know what a normal AP should be to be able to identify a disease and what deviations are telling you is happening

Is the intracellular or extracellular more negative?

intracellular (Inside more negative)

How does K+ want to move?

[inside] > [outside]; wants to move outside (down concentration gradient) but held out by electrical gradient

What ion is the cell membrane more permeable?

K+

How does Na+ want to move?

[inside] < [outside]; travel down electrical and concentration gradient

What are ion channels?

always open; ions diffuse through channel down gradient