2.3 Muscle Contraction

Isometric Contractions

-A force generated by the muscle but no shortening of the muscle fiber -Sarcomeres do NOT shorten, but generate force

Troponin

-A globular, heterotrimeric protein making part of the thin filaments and accounting for the binding of calcium -Forms the calcium sensor of the sarcomere in striated muscle -Attached to tropomyosin in a 1:1 ratio -Each binds 4 Ca2+ ions allosterically -Under low Ca2+ concentrations, it positions tropomyosin so that the myosin binding sites on actin are occluded -When Ca2+ rises, troponin binds Ca2+ and causes a move of tropomyosin, which opens the binding sites for myosin on actin and allows for interaction

If Troponin C was constantly bound to Ca2+ what would occur?

-Cross bridge cycling and contraction -The muscle would maintain a full contraction, which would cause rigid paralysis -Cause muscles to maintain a tetanus and not respond to normal signaling

Shortening velocity depends on

-The number of sarcomeres in series

Each ATP hydrolysis, actin-myosin interaction =

1 crossbridge cycle

List the sequence of events in the sliding filament hypothesis

1. In response to increased Ca2+ in the cytopalsm, myosin binding site on actin is exposed. Myosin globular head binding to actin 2. Rotation of lever arm resulting in translational movement of thin filament past thick filament; ADP and P are released from myosin (filaments slide past each other) 3. ATP binds to myosin, which decreases the affinity of myosin for actin, and promotes detachment of crossbridge from thin filament 4. Arm swings back to original position and then the hydrolysis of the ATP (NOT dissociation of the ADP and P-stay bound to the crossbridge/myosin head), and myosin heads return to a resting conformation in the cocked state

A single crossbridge cycle involves an ___________nm shortening

11

Isotonic Contraction

A contraction where the force being generated stays constant but the muscle shortens. One of the most common types of muscle contractions.

Tropomyosin

A filamentous protein of the thin filaments that attaches to troponin and forms part of the calcium response element. Under low Ca2+ cytoplasmic concentrations, tropomyosin is found occluding the myosin binding sites of actin. Under high Ca2+ cytoplasmic concentrations, the tropomyosin is rotated by troponin so as to uncer the myosin binding sites of actin and allowing the myosin/actin interactions to occur.

Troponin T

A protein subunit of troponin which binds and can move the tropomyosin filaments

Troponin I

A protein subunit of troponin which binds strongly or weakly to actin, depending on whether troponin C is bound to calcium

Troponin C

A protein subunit of troponin which is responsible for the binding of Ca2+ from the cytoplasm

H Zone

A subdivision of the A Band in the center of the sarcomere where only thick filaments are present. This area shrinks during contraction.

Titin

A very large springy protein that helps to tether the thick filaments around the middle of the sarcomere Attach at the Z disk and contact the thick filament at the other end

What is the rate limiting step?

ADP release from the myosin head

The myosin heads have an intrinsic ________ activity

ATPase

A motor unit

All muscle cells innervated by the same motor neuron

What would happen if all the myosin heads were synchronized?

All tension would be developed over a very short period of time and so would all the shortening. As the next cycle began, all the myosin heads would let go of actin and so tension would drop and the sarcomeres would again lengthen before the cycle started again. It would lead to twitch like motions and it would be almost impossible for muscles to shorten substantially.

Contraction involves a population of crossbridges cycling __________________. This allows muscles to remain tone/force during contraction.

Asynchronously

What halts the cycle?

Decrease in Ca2+ Removal of ATP

ATP promotes detachment of crossbridge from thin filament by

Decreasing affinity of myosin for actin

Myosin alone has a ________ affinity for actin

High

Myosin-ADP+P has a ___________ affinity for actin

High

If you try to lift an object whose weight exceeded the maximal force that could be generated by your muscles, what type of contraction is this?

Isometric

Myosin-ATP has a _________ affinity for actin

Low

A disks move toward

M-Line

In response to increasing levels of calcium in the muscle cell cytoplasm, the myosin binding sites on actin become exposed and

Myosin ADP/P complex binds to Actin

When muscle is depleted of ATP, what happens?

Myosin crossbridge stays bound to actin; this is muscle rigor mortis

The speed of contraction of a muscle depends on the

Number of sarcomeres in series (muscle length), the myosin cycling rate, and the load on the muscle

Shortening Velocities

Refers to the conditions that can affect how quickly a muscle can shorten (and therefore only consdiders isotonic contraction)

The sarcoplasmic reticulum calcium ATPase activity determines the

Relaxation Rate

During an isometric contraction, the peak force is primarily dependent upon

Sarcomere Length

I Band

Straddle the Z line and are composed of thin filaments. During contractions, the this band shrinks.

The number of sarcomeres in a myocyte aligned in parallel may play a role in

Strength of contraction

Z Line

The partitions within a muscle fiber that section it into sarcomeres. Thin filaments (Actin) attach to these.

Z disks move towards

Thick Filaments

A Band

This area is composed of thick and thin filaments.It is the entire length of the thick filament. Does NOT shrink during muscle contraction.

Which two proteins help keep the filaments properly oriented?

Titin Nebulin

The Ca2+ bidning protein in striated muscle is

Troponin