Key Features of the Muscular System
Major Tissue Type: Skeletal muscle tissue
Characteristic Cell Type: Skeletal muscle cell also called "muscle fiber"
Functional Unit: Sarcomere
Confusing Terms: Muscle fibers, myofibrils, myofilaments
You already know quite a bit about skeletal muscle from a lifetime of experience. For example, you know that skeletal muscles are attached to the skeleton--the skeletal part of their name--and that you can control them. The fact that you can will a muscle to move means that these muscles are "under voluntary control". However, skeletal muscles also move reflexly. If the doctor taps a spot on your knee, can you stop your leg from jerking forward? The best possible definition that I can give you of voluntary is that this type of muscle needs its nerve supply to contract. In fact, skeletal muscle is totally dependent on its nerve supply for good health. If the nerve is damaged, muscle fibers atrophy.
Muscle cells, also called muscle fibers, are arranged parallel to one another in a muscle belly. The two ends of the muscle belly attach to bone by means of a muscle tendon. The bone that remains stable during a movement is called the origin. The bone that is moved when the muscle belly contracts is called the insertion. Muscle action occurs when the insertion moves toward the origin as the muscle belly shortens. Deep fascia (aka the epimysium) is the dense, irregular connective tissue that surrounds the entire muscle belly and extends onto the muscle tendons. Another connective tissue sheath called the perimysium surrounds groups, or fascicles, of muscle fibers within a muscle belly. The endomysium lies outside the cell membrane of the muscle cell, and surrounds each individual muscle fiber.
The elongated, multinucleated, skeletal muscle cell contains many of the same organelles as any other cell. Two prefixes, sarco- and myo- are applied to some of the organelles with special significance in these cells. The sarcolemma or cell membrane surrounds the sarcoplasm. The sarcoplasmic reticulum or smooth endoplasmic reticulum contains calcium ions. The specialized arrangement of this organelle into terminal cisterns on either side of extensions of the sarcolemma called transverse or T tubulescauses the formation of a triad. The sarcoplasm contains many myofibrils consisting of myofilaments. Myofilaments are threadlike structures in the sarcoplasm. They may be thick, thin and elastic filaments. Think and thin myofilaments are contractile elements; elastic myofilaments anchor or stabilize thick myofilaments.
The only thing a muscle can do is contract. This means that the muscle shortens, and its resting length decreases. The smallest element of a muscle fiber that is capable of shortening or contracting is the sarcomere--the functional unit of skeletal muscle.
In lab, you recognized striations in longtidunal sections of skeletal muscle. You noticed a clear space, called the sarcomere between adjacent striations. However, analysis of individual myofibrils in special preparations for both the light and the electron microscope have given us a clearer picture of this "banding pattern"; and provided us with my favorite alphabet soup nomenclature!
. ;-)A bands are regions of a sarcomere that appear dark with the polarized light microscope. Myosin-containing, thick filaments lie in the center of an A band. Thin filaments containing the proteins actin, tropomyosin and troponin overlap thin filaments at the edges of the A band and extend into the I band to insert in the Z lines (some books call this region Z disks). The I band appears light in polarized light.
If a muscle fiber is at rest, the fiber is elongated. A region containing only thick filaments is visible in the center of the A band--this region is called the H zone. Proteins connect adjacent thick filaments. These proteins form a line in the center of the H zone called the M line.
(Study hint: Review this paragraph while you look at the drawing in your Syllabus or a similar figure in your textbook.)
Let's look at some alternative definitions of a sarcomere.
- Functional unit of skeletal muscle (the smallest unit of skeletal muscle that can contract)
- Region between adjacent striations (You saw this in lab.)
- Region between adjacent Z-lines (or Z-disks)
Each sarcomere contains:
- Thick filaments
- Thin filaments
- One A band
- Two 1/2 I bands
- One H zone
- One M line
The two contractile proteins, myosin in thick filaments and actin in thin filaments, work together to shorten the sarcomere during muscle contraction. The model of this rhythmic process is called the Sliding Filament Mechanism. Study the shape of the myosin molecule. Notice its heads which act as cross bridges as they bind to their myosin-binding site on the actin molecule, but only during contraction. At rest, the myosin-binding site on the actin molecule is covered by the tropomysin-troponin complex. During muscle contraction, the influx of calcium ions and the use of energy (ATP) causes the complex to shift so that the myosin-binding site can attach to the myosin cross bridge (head). The myosin cross bridges move thin filaments toward each other as the sarcomere shortens. Z lines move closer together, and the H zone disappears. (Look for a more detailed explanation of this process in Physiology next semester.)
The neuromuscular junction is our first example of a specialized type of contact called a synapse. Remember that voluntary muscle is innervated, that is, supplied, by axons from the brain of spinal cord. These axons are called motor axons; they end as a specialized region called an axon terminal. Axon terminals are easy to identify (find a picture in your textbook), because they contain structures called synaptic vesicles. These specialized vesicles contain the neurotransmitter, acetylcholine or ACh. When the motor nerve is stimulated, the synaptic vesicles release (exocytose) ACh. ACh combines with receptors on the muscle cells in a region called the motor end-plate. This process, a typical ligand-receptor interaction, produces a series (cascade) of events that leads to sarcomere shortening and muscle contraction.
Motoneurons in both the brain and spinal cord innervate individual muscle fibers (muscle cells). Every muscle fiber in a muscle belly must participate in a synapse with an axon terminal at its motor end-plate region--the neuromuscular junction described above. Each motoneuron contacts more a group of muscle fibers. Its axon branches into several axon collaterals, and each axon collateral synapses with a muscle fiber.
A motor unit is defined as all of the muscle fibers supplied by a single motoneuron, and therefore, by a single axon and its branches.
| Motor Units Vary in Size! | |
|---|---|
| Muscles responsible for fine coordination have small motor units. | For example, 1 axon supplies 12 different muscle fibers in the same muscle |
| Muscles responsible for gross movements have large motor units. | For example, 1 axon supplies 250 different muscle fibers in the same muscle |