Bio B Midterm 3 - 3/30 Flashcards

38.4 Muscle Contraction and Locomotion

muscle cells

-specialized for contraction

-allow for motions + facilitate bodily processes such as respiration and digestion

-three types of tissue: skeletal, cardiac, and smooth

skeletal (voluntary) muscle tissue

-forms skeletal muscles, which attach to bones or skin and control locomotion and any movement that can be consciously controlled

-long and cylindrical in appearance; when viewed under a microscope, skeletal muscle tissue has a striped or striated appearance

-striations are caused by the regular arrangement of contractile proteins (actin and myosin) along the length of myofibrils

-has multiple nuclei present in a single cell

actin

a globular contractile protein that interacts with myosin for muscle contraction

smooth (involuntary) muscle tissue

-occurs in the walls of hollow organs such as the intestines, stomach, and urinary bladder, and around passages such as the respiratory tract and blood vessels

-has no striations, is not under voluntary control, has only one nucleus per cell, is tapered at both ends

cardiac (involuntary) muscle tissue

-only found in the heart

-cardiac contractions pump blood throughout the body and maintain blood pressure

-striated, cannot be consciously controlled, one nucleus per cell, is branched, and is distinguished by the presence of intercalated disks

skeletal muscle fiber

-a skeletal muscle cell

-incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm

sarcolemma

-the plasma membrane of a skeletal muscle fiber

-the site of action potential conduction, which triggers muscle contraction

myofibrils

-long cylindrical structures that lie parallel to and are within each muscle fiber

-run the entire length of the muscle fiber, and because they are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one muscle fiber

-composed of smaller structures called myofilaments

I band

-light bands that, along with Dark A bands, repeat along myofibrils

-each ones has a dense line running vertically through the middle (Z disc/line)

sarcomeres

-the functional units of skeletal muscle

-one of these is the space between two consecutive Z discs and contains one entire A band and two halves of an I band, one on either side of the A band

what happens as sarcomeres individually contract?

the myofibrils and muscle cells shorten

myofilaments

two types:

1) thick filaments (myosin) - occur only in the A band of a myofibril

2) think filaments (actin, tropomyosin, troponin) - attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band

*central region of A band with only thick filaments = H zone

~vertical line in the middle of the H zone = M line → where accessory proteins hold thick filaments together

sliding filament model of contraction

-thick and thin filaments slide by one another, causing the sarcomere to shorten while the filaments remain the same length

-mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement

what happens when a sarcomere shortens?

-distance between the two Z discs/lines is reduced

-the H zone shortens

-the I band shortens

-the A band does not shorten but moves closer to those of other sarcomeres until they disappear

muscle contraction

1. myosin heads bind to actin and pull the actin inwards → requires ATP

2. ATP binding causes myosin to release actin

3. they detach + ATP converts to ADP; E released during ATP hydrolysis changes the angle of the myosin head into a “cocked,” high energy position

4. if acting binding sites are uncovered, a cross-bridge forms and the myosin head moves toward the M line, pulling the actin simultaneously (power stroke)

5. the sarcomere shortens and the muscle contracts

what is the configuration of myofibrils when the muscle is ina resting state?

actin and myosin are separated

tropomyosin

-blocks myosin binding sites on actin molecules, preventing cross-bridge formation and preventing contraction in a muscle without nervous input

-must change conformation to enable a muscle contraction, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation

troponin

binds to Ca2+ ions and tropomyosin + helps to position it on the actin molecule

excitation-contraction coupling

the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction

motor end plate

-the area of the sarcolemma on the muscle fiber that interacts with the neuron

-possesses junctional folds, folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors (Na+ channels)

refractory period

-during this time, the membrane cannot generate another action potential

-allows the voltage-sensitive ion channels to return to their resting configurations

tension

the pull exerted by a muscle

what is the primarily variable determining force production?

the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber

what happens if the sarcomere shortens too much?

thin filaments begin to overlap with each other—reducing cross-bridge formation even further, and producing even less tension

what happens if the sarcomere is stretched too much?

thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced