Term
| List and describe the anatomical components that make up a muscle fiber. |
|
Definition
Plasmalemma: A plasma membrane that surrounds the muscle fiber. As part of a larger unit called the sacrolemma, the plasmalemma fuses with the tendon and inserts into the bone at the end of each fiber. Its functions include folding and stretching to allow contraction and extension without damage, and assists in transfering action potential from the motor neuron to the muscle fiber. Lastly, it helps maintain acid-base balance and transports metabolites from the capilary blood vessels to the muscle fiber.
Sarcoplasm: A gelatin-like substance that fills spaces between and inside the myofibrils. As the cytoplasm of the muscle fiber, (the fluid part), it primarily contains dissolved proteins, minerals, fats, necessary organelles and a large quantity of stored glycogen and myoglobin, which serves as a oxygen binding compound that's similar to hemoglobin in red blood cells.
Transverse Tubules- Extensions of the Plasmalemmal housed in the sarcoplasm. These pass laterally through the muscle fiber and are interconnected as they pass through the myofibrils, allowing nerve impulses to rapidly transmit to individual myofibrils after being received by the plasmalemmal. They also provide paths from the outside to the inside of the fiber, allowing substances to enter and wastes to leave.
Sarcoplasmic Reticulum: A longitudinal network of tubules that parallel the myofibrils and loop around them within the muscle fiber. It stores calcium, which is essential for muscle contraction.
Myofibrils: Small fibers made up of contractile elements of sacromeres, (basic contractile elements of skeletal muscle). They appear as long strands of sacromeres under electron microscopes. Several hundreds to several thousand can be found within each muscle fiber.
Sarcomeres: The basic functional unit of a myofibril and the basic contractile unit of muscle. They are joined end to end at the Z-disk, and includes several elements between each pair of Z-disk as follows:
An I-Band (light zone, only thin filaments composed primarily of actin.)
An A-band (dark zone, Contains both thick and thin filaments)
An H-zone (in the middle of the A-band, only thick filaments composed primarily of myosin.)
An M-line in the middle of the H-zone. (composed of proteins that serve as the attachment site for thick filaments. Assists in stabilizing sarcomere structure.)
The rest of the A-band.
A second I-band.
Z-disks appear on each end of the sarcomere; are composed of proteins and provide points of attachment and stability for thin filaments alongside titin and nebulin. |
|
|
Term
| List the components of a motor unit. |
|
Definition
Alpha-motor neuron (a-unit)
Dendrites.
Axon hillock.
Axon terminal.
Motor end plates.
Myofibrils.
Muscle fibers. |
|
|
Term
| What are the steps in excitation-contraction coupling? |
|
Definition
| The process is initiated by an action potential, or a nerve impulse from the spinal cord or brain to the alpha-motor neuron. It reaches the neuron belonging to the a-motor neuron, then travels down the axon to the axon terminals. Once there, these nerve endings release a neurotransmitter, (a signaling molecule,) called acetylcholine, (ACh). With enough ACh bound to the receptors, the action potential extends to the full length of the muscle fiber while ion gates open in the muscle cell membrane to allow sodium to enter. |
|
|
Term
| What is the role of calcium in muscle contraction? |
|
Definition
| Once calcium is released from the SR after the action potential creates an electrical charge that causes this reaction to take place, the calcium ions bind to the troponin on the actin molecules. This causes the troponin to initiate the contraction process by moving the tropomyosin molecules off of the myosin binding sites on the actin molecules. This allows myosin heads to attach to the binding sites of actin molecules once the tropomyosin have been lifted from the binding sites. |
|
|
Term
| Describe the sliding filament theory. How do muscle fibers shorten? |
|
Definition
| The sliding filament theory states that myosin cross-bridges bind with actin once activated, which results in a conformational change in the cross bridge. This causes the myosin head to tilt and drag the thin filament towards the sarcomere's center. |
|
|
Term
| What are the basic characteristics that differ between type I and type II muscle fibers? |
|
Definition
Type I fibers take about 110 ms to reach peak tension, consist of only one form, are recruited more frequently than any other muscle fiber type, and appear black in a micrograph. Type II
fibers take about 50 ms to reach peak tension, (due to different forms of ATPase,) is made up of two types of fibers, (fast twitch type a and fast twitch type x, with a third subtype called type IIc,) and are unstained with a white appearance in a micrograph.
In addition, type II fibers have a more highly developed SR, and are more adept at delivering calcium into the muscle cell. Type I a motor neurons have a smaller cell body, which makes it so type I fibers innervate less than 300 muscle fibers while type II innervate more than 300. |
|
|
Term
| What is the role of genetics in determining the proponents of muscle fiber types and the potential for success in selected activities? |
|
Definition
| For the most part, muscle fiber is genetically determined with little change up until middle age. Muscle fibers become specialized after innervation according to the type of a-motor neuron that stimulates them, which we likely inherit from our parents. |
|
|
Term
| Describe the relation between muscle force development and the recruitment of type I and type II motor units. |
|
Definition
| Based on the size principle, motor units are recruited based on size, so smaller ones are used first. Type I units are smaller, so they're recruited for very low to very high force production rates, then type II are used as the force needed for the movement increases. |
|
|
Term
| Explain and give examples of how concentric, static and eccentric contractions differ. |
|
Definition
| Concentric contraction is when a muscle shortens, producing joint movement. A primary example would be when flexing an arm. Static contraction is when a muscle generates force, but its length doesn't change. An example is when you try to lift something that's too heavy. Eccentric contractions are when muscles exert force when lengthening. An example is in the biceps brachii extends slowly to lower something heavy. |
|
|
Term
| What two mechanisms are used by the body to increase force production in a single muscle? |
|
Definition
| Summation is the process of increasing force by creating a series of three rapid stimuli before complete relaxation, resulting in even more force or tension. Rate coding is the process where a motor unit can vary in the amount of tension up to tetanus by increasing the frequency of stimulation. |
|
|
Term
| What is the optimal length of a muscle for maximal force development? |
|
Definition
| The optimal length of a muscle for maximal force development is the length at which there is optimal overlap of the thick and thin filaments. |
|
|
Term
| What is the relation between maximal force development and the speed of shortening (concentric) and lengthening (eccentric) contractions? |
|
Definition
| During concentric contractions, maximal force decreases with higher speeds. For example, people tend to lift heavy weights slowly. The opposite is true with eccentric contractions, allowing maximum force with higher speeds. |
|
|
Term
| In muscle contraction, what roles are played by the protein titin? |
|
Definition
| Titin is considered the third filament. Updated theories suggest that titin is actively involved in the regulation of skeletal muscle force generation by winding around and rotating thin filaments. It stabilizes sarcomeres and centers myosin filaments, proved increased force when muscles are stretched, and prevents overstretching by creating a resistance. |
|
|
