LO4 Notecards - Chapters 10, 11, 12

In order to contract, a skeletal muscle must:

- be simulated by a nerve ending - propagate an electrical current along its sarcolemma - have an increase in intracellular calcium levels

Fast fibers

- contain large glycogen reserves - majority of skeletal muscle fibers in the body are these - use vast quantities of ATP - do not require oxygen, anaerobic reactions - relatively few mitochondria - pale in color (lack of myoglobin) - fatigue rapidly (short duration, sprinting, lifting weights)

Slow fibers

- needs lots of oxygen (requires oxygen to make ATP) - contracts slowly - specialized to continue contracting for extended period of time - fatigue resistant (endurance, marathon runner) - red color fibers (large amounts of myoglobin)

Summary of muscle contraction

1. A nerve impulse from the axon terminal triggers release of ACh from the synaptic knob into the synaptic cleft. ACh binds to ACh receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber. 2. As the muscle impulse spreads quickly from the sarcolemma along T-tubules, calcium ions are released from terminal cisternae into the sarcoplasm. 3. Calcium ions bind to troponin causing troponin to change shape. As troponin changes shape, it simultaneously moves the tropomysin molecule to which it is attached, thus exposing the active sites on the G-actin molecules. Myosin heads of thick filaments attach to exposed active sites to form crossbridges. 4. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds myosin heads and is broken down into ADP and P. Myosin heads detach from thin filaments and return to their pre-pivot position. The repeating cycle of attach-pivot-detach-return slides thick and thin filaments past one another. The sarcomere shortens and the muscle contracts. The cycle continues as long as calcium ions remain bound to troponin to keep active sites exposed. 5. When the impulse stops, calcium ions are pumped back into the sarcoplasmic reticulum, and the troponin-tropomyosin complex moves back to its original conformation where the tropomyosin once again blocks active sites on actin.

Fascicle

A bundle of muscle fibers

Elasticity

A contracted muscle cell recoils to its resting length when the applied tension is removed. Thus, elasticity is not the muscle's ability to stretch, but its ability to return to its original length when tension is released.

Extensibility

A muscle cell must be capable of extending in length in response to the contraction of opposing muscle cells.

M-line

A thin transverse protein meshwork that appears as a dark protein disc in center of H zone where thick filaments attach; does not shorten when muscle contracts

A-band

Dark band in the middle of the sarcomere; thick filaments stretch across the whole band, while thin filaments are only present on its lateral ends; does not shorten when muscle contracts

Z-disc

Dark proteins in the center of the I band where thin filaments attach; move closer together when the I band shortens

Myoblasts

Embryonic skeletal muscle cells; each contains a single nucleus; they fuse together to form a skeletal muscle fiber (this now has many nuclei)

Role of ATP in muscle contraction

Energy to drive the myosin movement is provided in the form of ATP. Myosin head attachment and pivoting do not require energy, but ATP is needed for the myosin head crossbridge to detach from actin. Energy is released to power the detachment and recocking of the myosin head when ATP binds to the myosin head and is broken down into ADP and P.

Three connective tissue sheaths

Epimysium Perimysium Endomysium

List the 4 functional characteristics shared by all muscle tissues

Excitability Contractility Elasticity Extensibility

Parallel Muscles

Fascicles are parallel to the long axis of the muscle; high endurance, not very strong (e.g., rectus abdominis, biceps brachii)

Endomysium

Innermost layer that surrounds each muscle fiber

I-band

Light band containing thin filaments and titin proteins; shortens when muscle contracts

H-zone

Lighter region in the middle of the A band; contains thick filaments only; shortens (disappears) when muscle contracts

Myofibril

Long, cylindrical structures within the sarcoplasm of a skeletal muscle fiber; hundreds to thousands are present and within each there are bundles of short myofilaments

List the methods used in naming muscles

Muscle action Specific body regions Muscle attachments Orientation of muscle fibers Muscle shape and size Muscle heads/tendons of origin

Pennate Muscles

Muscle body has one or more tendons; fascicles at oblique angle (neither parallel nor perpendicular) to tendon; pulls harder than a parallel muscle of equal size

Epimysium

Outer most layer; layer of dense irregular connective tissue that surrounds the whole skeletal muscle

Sarcolemma

Plasma membrane of a skeletal muscle fiber

Troponin

Regulatory protein that holds tropomyosin in place and anchors to actin; when calcium ions bind to one of its subunits, this protein changes shape, causing the tropomyosin to move off the actin active site, and this permits myosin binding to actin

Intermediate fibers

These exhibit properties that are somewhere between those of slow fibers and fast fibers. The intermediate fibers contract faster than the slow fibers and slower than the fast fibers. Histologically, intermediate fibers resemble fast fibers; however, they have a greater resistance to fatigue.

Circular Muscles

These muscle fibers are concentrically arranged around an opening or recess. Functions as a sphincter to close a passageway or opening (e.g., orbicularis oris)

Components of a thick filament

Thick filaments are assembled from bundles of the protein myosin. Each strand has a free, globular head and an attached, elongated tail.

How are troponin, tropomyosin and G-actin molecules put together?

Thin filaments are primarily composed of two strands of the protein actin twisted around each other to form a helical shape. Two regulatory proteins, tropomyosin and troponin, are part of the thin filaments. The tropomyosin molecule is a short, thin, twisted filament that covers small sections of the actin strands (actin sites). The troponin has three functions: Structurally, it (1) attaches to actin to anchor itself in place, and (2) attaches to tropomyosin to hold it in place over the surface of the actin; (3) functionally, troponin provides a binding site for calcium ions.

sarcomere

This is defined as the distance from one Z disc to the next adjacent Z disc. It is the contractile unit of a muscle fiber; shortens as the muscle fiber contracts

Excitability

This is equated with responsiveness, muscle cells are very responsive to input from stimuli.

Sliding filament theory

This says that when a muscle contracts, thick and thin filaments slide past each other, and the sarcomere shortens. Thick and thin filaments maintain their same length, whether the muscle is relaxed or contracted. However, during muscle fiber contraction, the relative position between the thick and thin filaments within the sarcomeres changes markedly.

Convergent Muscles

Triangular muscle with common attachment site, does not pull as hard as equal-sized parallel muscle (e.g., pectoralis major of the chest)

Components of a thin filament

Tropomyosin Troponin G-actin

Synergist muscles

a muscle that assists the agonist in performing its action

Antagonist

a muscle whose actions oppose those of the agonist. If the agonist produces extension, the antagonist produces flexion.

unipennate muscle

all of the muscle fibers are on the same side of the tendon (e.g., extensor digitorum longus)

Agonist

also called a prime mover, it is a muscle that contracts to produce a particular movement. Triceps brachii would be an example of this when extending the forearm.

What are the different fascicle arrangements of skeletal muscle?

circular convergent parallel pennate

motor unit

composed of a single motor neuron and all of the muscle fibers it controls.

T-tubules (transverse tubules)

distribute muscle impulse throughout the inside of the muscle fiber; sandwiched by terminal cisternae

Actin

double stranded contractile protein; binding site for myosin to shorten a sarcomere

Tropomyosin

double stranded regulatory protein; covers the active sites on actin, preventing myosin from binding to actin when muscle fiber is at rest

Titin

filaments of an elastic protein; help return myofilaments to resting position after contraction; also maintains positions of myofilaments in sarcomere

multipennate muscle

has branches of the tendon within the muscle (e.g., deltoid)

autorhythmic cells

individual cells can generate a muscle impulse without nervous stimulation

Prime mover

muscle primarily responsible for generating a specific movement

What are the components of the sliding filament mechanism?

myosin heads actin site crossbridges ATP

What is the difference between a tendon and a ligament?

one connects bone to muscle, and the other connects bone to bone

List the components of a typical motor unit

one motor neuron all the muscle fibers it controls neuromuscular junction

Myoglobin

oxygen-binding, reddish-appearing protein found in heart and skeletal muscles

Thin filament

protein myofilament composed of actin, troponin, and tropomysoin

Thick filament

protein myofilament composed of bundles of myosin; they bind to thin filament and cause contraction

Terminal cisternae

reservoirs storing calcium ions required for muscle contraction

What is the contractile unit of a muscle fiber called?

sarcomere

What are the three different muscle tissue types

skeletal muscle smooth muscle cardiac muscle

What are the 3 major ions involved in muscle contraction?

sodium potassium calcium

Triad

term for one centrally placed T-tubule sandwiched between two terminal cisternae

Origin

the less mobile attachment of a muscle

Insertion

the more mobile attachment of a muscle

bipennate muscle

the most common type, has muscle fibers on both sides of the tendon (e.g., rectus femoris)

What is the neuromuscular junction

the point where a motor neuron meets a skeletal muscle fiber for the purpose of transmitting a nerve impulse to the muscle fiber

Excitation-contraction coupling

the stimulation of a muscle fiber by a nerve impulse results in a series of events that culminates in muscle fiber contraction

Name the three types of pennate muscles

unipennate muscle bipennate muscle multipennate muscle

Sarcoplasm

Cytoplasm of a skeletal muscle fiber

Components of a neuromuscular junction

Synaptic knob ACh Motor end plate Synaptic cleft ACh receptor AChE

Types of skeletal muscle fibers

Slow (type I, slow oxidative) Intermediate (type IIa, fast aerobic) Fast (type IIb, fast anaerobic)

Contractility

Stimulation of muscle cells generates tension within the cell (contraction), which may cause the cell to shorten. This shortening results in either a pull on bones of the skeleton or the movement of specific body parts.

Sarcoplasmic reticulum

Stores calcium ions needed to initiate muscle contraction

Perimysium

Surrounds the fascicles