Skeletal Muscle Anatomy and Physiology

Skeletal muscle is a complex organ. In order to optimize your workout routines, an understanding of skeletal muscle anatomy is essential.

Skeletal Muscle Anatomy

Skeletal muscle tissue is surrounded by connective tissue. It is separated from the skin by the superficial fascia, also known as the subcutaneous layer, which is composed of connective tissue and adipose (fat) tissue [1].

The adipose tissue in this superficial fascia is the body's main storage of triglycerides and serves as a protective layer for muscle. Blood vessels, lymphatic vessels, and nerves enter and exit muscles through this layer [1].

Under the superficial fascia lies the deep fascia. The deep fascia is irregular connective tissue that holds muscles which function with one another together [1].

Below The Deep Fascia There Are Three Layers Of Connective Tissue Which Strengthen Muscle:

* Epimysium - Outermost layer. Surrounds the entire muscle.
* Perimysium - Surrounds groups of muscle fibers called fascicles.
* Endomysium - Deepest layer. Separates individual muscle fibers.

Skeletal muscles are composed of many individual cells known as muscle fibers. Muscle fibers have multiple nuclei and are located below a plasma membrane called the sarcolemma. T (tranverse) tubules are tiny invaginations that run from the outside surface of the sarcolemma into the center of the muscle fiber. These T tubules propagate actions potentials throughout the muscle fiber, which causes muscle contraction.

Inside the sarcolemma is the fiber's cytoplasm, called the sarcoplasm. The sarcoplasm holds stored substances such as glycogen and oxygen, needed for muscle contraction.

Each individual muscle fiber is in turn composed of numerous smaller myofibrils. Myofibrils are divided into repeating functional units called sarcomeres. This repeating pattern gives skeletal muscle its striated appearance. A system of membranous sacs, called the sarcoplasmic reticulum (SR), surrounds the myofibrils. Calcium ions (Ca2+), the trigger for muscle contraction, are stored in the sarcoplasmic reticulum.

Three types of proteins form myofibrils (1) contractile, (2) regulatory, and (3) structural [1]. Contractile proteins are the force generators of muscle contraction. The two contractile proteins in myofibrils are actin and myosin, which are part of the thin filament and thick filament respectively.

The myosin filament is further classified as a motor protein because it creates force or movements by using the chemical energy stored in ATP molecules [1].

The myosin filament is often described to be shaped like "two golf clubs twisted together" with the "golf club handles" representing the myosin tail and the "golf club heads' representing the myosins heads or crossbridges [1].

The myosin cross-bridge (there are many crossbridges on the myosin filament) has an attachment site for actin and ATP (which will be discussed later). The myosin heads project out of the myosin tail towards the surrounding actin filaments. The helix actin filament, composed of individual actin molecules, each of which contains a myosin-binding site where the myosin heads can attach, is anchored to the Z discs**.

The two regulatory proteins troponin and tropomysium, which are also part of the thin filament, are involved in turning muscle contraction "on or off [1]."

When a muscle is relaxed, tropomysium blocks the myosin-binding sites on the actin proteins so the myosin heads cannot attach, and therefore the muscle cannot contract. Troponin holds the tropomysium proteins in place.

When calcium enters the cytoplasm of the muscle fiber, it can bind to a troponin molecule, which changes the troponin molecule's shape and "pulls" the tropomysium away from the myosin-binding site on each actin molecule [2]. When calcium is no longer present, the troponin molecule reconfirms to its original shape and the tropomysium again blocks the binding site.

The structural proteins are involved in the stability and elasticity of the myofibrils. The most notable of the structural proteins is titin. A titin protein is 50 times larger than an average protein [1] and spans from the Z disc to M line (half of the sarcomere).

Titin "anchors" the thick filament to the Z disc and M line, stabilizing its position [1]. The titin protein is also very elastic and served to help a stretched or contracted muscle return to its relaxed length [1]. There are about a dozen other structural proteins in each myofibril.

** The sarcomere is divided into various parts based on the filaments present.

* Z Line (discs) - Anchors the actin filaments. Separates sarcomeres.

* I Band - Contains the portion of the actin filaments that are not overlapping a myosin filament.

* A Band - Spans the entire myosin filament.

* H Zone - Contains the portion of the myosin filament that is not overlapped by actin filaments.

* M Band - Middle of the sarcomere.

Muscle Contraction

When a muscle contracts, the myosin cross-bridges are activated. This does not mean that the muscle fibers are shortening, but rather the mechanism that generates force and tension is active [2].

A muscle is contracting when a dumbbell is held in one position while the muscle neither shortening nor lengthening. In most cases, such as curling a dumbbell, the muscle (in this example the biceps brachii) does shorten while contracting. This is described in the "Sliding-filement mechanism."

Sliding-Filament Mechanism

When a muscle shortens during contraction, the myosin cross-bridge attaches to the actin filament. The movement of the cross-bridge is often described to move in an arc like the rowing of a boat oar [2], pulling the two successive Z lines towards the center of the sarcomere.

Each individual "stroke" of the cross-bridge only produces a small amount of movement (pulling the Z lines towards the center), but if the muscle stays activated, the cross-bridge continues its stroking motion, resulting in a larger movement [2]. Many cross-bridges are formed when a muscle contracts. The process of cross-bridge attachment and movement is known as the cross-bridge cycle.

Cross-Bridge Cycle

The cross-bridge cycle, initiated when calcium from the SR enters the cytoplasm, consists of four steps [2]:

1. Myosin cross-bridge attaches to actin filament A + M·ADP·Pi » A·M·ADP·Pi

2. Cross-bridges moves, creating tension A·M·ADP·Pi » A·M + ADP + Pi

3. Cross-bridge detaches from actin filament A·M + ATP » A + M·ATP (The binding of ATP detaches the myosin from actin)

4. Cross-bridge is "energized" and reattaches to actin filament if calcium is still present A + M·ATP » A + M·ADP·Pi

* A = Actin; M = Myosin cross-bridge; · = bound to

An important note is that each cross-bridge goes through this cycle independently of the other cross-bridges [2]. Therefore, during muscle contraction some cross-bridges are attached to an actin filament while others are not.

ATP provides the energy for the movement of the cross-bridge when it is hydrolyzed. It also breaks the bond between myosin and actin when it binds to myosin.

ATP is required for muscle contraction. Calcium is also required for muscle contraction. It initiates the actually physical contraction. Calcium release from the SR is trigger by an action potential.


Neuromuscular Signaling

Muscle contractions are signaled by action potentials (electrical signals) in the plasma membranes of muscle fibers. This signal is transmitted by nerve cells, known as neurons, from the central nervous system (CNS) to the muscle fiber. Nerve cells that innervate muscle fibers are called motor neurons.

A motor neuron originates in the CNS and spans to a muscle, where it divides into multiple branches. Each branch forms a junction with a single muscle fiber. A motor neuron plus all the muscle fibers innervated by its branches is known as a motor unit [2].

When a motor neuron propagates (transmits) an action potential from the CNS to the muscle fibers it innervates, all the fibers in its motor unit contract [2].

When the axon reaches a muscle fiber, it splits into "short processes" that embed into the surface of the muscle fiber [2]. The axon terminals contain vesicles that hold the neurotransmitter acetylcholine (ACh). The muscle fiber plasma membrane under the axon terminal is known as the motor end plate [2].

The axon terminal and the motor end plate form a junction known as a neuromuscular junction. When an action potential is propagated to a motor neuron axon terminal, it depolarizes it, which opens voltage-sensitive calcium channels allowing Ca2+ in the extracellular fluid to enter the axon terminal. This entry of Ca2+ signals the release of ACh from the vesicles in the axon terminal.

The ACh then diffuses from the axon terminal across the neuromuscular junction to the motor end plate and binds to ACh receptors. This binding of ACh opens ions channels through which sodium (Na+) can enter.

There is an electrochemical gradient across the muscle fiber's plasma membrane controlled by the concentration of ions on both sides of the membrane. The resting potential (no electrical signal present) a muscle fiber is negative relative to the extracellular fluid.

Opening of the ion channels and movement of ions causes the membrane to depolarize (membrane potential becomes less negative) and produces an end plate potential (EPP) [2]. The EPP depolarizes the plasma membrane adjacent to the motor end plate causing an action potential to propagate over the entire muscle fiber and along the T-Tubules.

The action potential in the T-tubules triggers the release of Ca2+ from the SR. This Ca2+ binds to the regulatory protein troponin and initiates muscle contraction and described above. Cross-bridge cycling can continue as long as Ca2+ is bound to troponin.

With an understanding of how muscle contracts, we can now examine other aspects of exercise physiology, biomechanics, and motor control.

References

1. Gerard J. Tortora. Principles of Human Anatomy (9th Ed.)
2. Widmaider, E. Raff, H., Strang, K. Human Physiology The Mechanism of Body Function. (9th Ed.)