Quiz 4 Study Guide

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Lecture 19 : Muscular Organization

The Big Picture of Skeletal Muscle Organization

Remember this acronym:

Part of muscle: Layer it is surrounded by:

M uscle E pimysium

F asicle P erimysium

M uscle Fiber E ndomysium (ECM) + Sarcolemma

Muscle Fiber:

1. Endomysium (ECM)

2. Sarcolemma (Plasma Membrane)

extends deep into the cell aka muscle fiber to form the transverse tubules (T-tubules) that are flanked by two terminal cisternae from the sarcoplasmic reticulum (SR)

the t-tubule flanked on either side by 2 terminal cisternae makes up the triad

3. Sarcoplasm (Cytoplasm)

contains the organelles of the skeletal muscle fiber such as the myofibril, sarcoplasmic reticulum (SR), mitochondrion, and nucleus (multinucleated)

4.Myofibril

composed of many myofilaments

Thick filaments

myosin - contractile (produces tension) protein

has 2 globular heads and 2 intertwining tails

the myosin heads extend from tail on a neck

the neck is flexible where it meets the tails at the hinge

Thin filaments

actin - contractile (produces tension) protein

has an active site that can bind to the myosin head

tropomyosin - regulatory (controls when a muscle fiber can contract) protein

at rest, covers the active sites on actin

troponin - regulatory controls when a muscle fiber can contract) protein

holds tropomyosin in place

Elastic filaments

titin - structural (provides structural stability for the myofibril and holds the myofilaments in place) protein

runs through the core of the thick filaments to structurally stabilize the myofibril

Myofilament Arrangement and the Sarcomere

The thick and thin myofilaments forms an alternation light and dark bands in the myofibril producing the sarcomere (the functional unit of skeletal muscle contraction)

The I Band : contains only thin filaments

Contains the dark lines known as the Z disc : the attachment point for the elastic filaments; attach myofibrils to one another; its proteins anchor the thin filaments in place and to one another

The A Band : contains the entire thick filaments

Contains the 2 zones of overlap : contain both thick and think filaments

Contains the H zone : contains only thick filaments

Contains the dark line known as the M line : has proteins that hold the thick filaments in place and anchors the elastic filaments

A sarcomere extends from one Z-disc to the next Z-disc.

The Sliding Filament Mechanism of Contraction

the sliding filament mechanism is known as the interaction of the thin and thick filaments during muscle contraction

During contraction, the I Bands and the H Zone narrow

The A Band remains unchanged

the myosin heads grab the thin filaments and pull toward the M line ---> the Z discs move closer together and causes the sarcomere to shorten

as individual sarcomeres simultaneously contract, the whole muscle fiber contracts

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Lecture 20: Muscular Physiology

Membrane Potential in our cells

In muscle cells, near the sarcolemma, is a thin layer of both:

negative ions in the cytosol (NC)

positive ions in the extracellular fluid (PE)

| PRODUCES AN

electrical gradient

​​​​​​​ | IS A SOURCE OF POTENTIAL ENERGY KNOWN AS AN

electrical potential

​​​​​​​ | A DIFFERENCE IN ELECTRICAL POTENTIAL BTW. 2 POINTS IS CALLED A

voltage (the size of the voltage indicates the size of the potential )

membrane potential : the electrical potential of cells and in measured in millivolts (mV)

resting membrane potential : the eletrical potential across the sarcolemma in a muscle fiber at rest

measures around -85 mV

means the cytosol is 85 mV more negative than the extracellular fluid

here the membrane is considered to be polarized ​​​​​​​

The Na+/K+ ATPase Pump and the [Na+] and [K+] Gradients

the Na+/K+ ATPase Pump actively transports these ions against their concentration gradient:

Na+ exits the cell (3 are released into the ECF as ...)

K+ enters the cell (2 are binded from the ECF and pumped into the cytosol )

this maintains the high extracellular [Na+] and high intracellular [K+] which maintains the resting membrane potential

Action Potential and Steps in an Action Potential

The Neuromuscular Junction

Skeletal Muscle Contraction

Excitation Phase

end plate potential is similar to the action potential ; the entry of Na+ depolarizes the sarcolemma locally producing an end plate potential

Excitation-Contraction Coupling : the connection between the excitation and contraction phases where the end plate potential leads to an action potential

Contraction Phases