Muscle cells possess the ability to convert chemical energy into mechanical energy. This process relies on adenosine triphosphate (ATP) to generate force and perform work. Muscle tissue can be categorized into three main types: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal muscle is responsible for movements involving the skeleton. It allows for joint movement and is under voluntary control. It is involved in various activities such as maintaining posture, locomotion, speech, and respiration. Under a microscope, skeletal muscle displays transverse striations, which are caused by the organized arrangement of actin and myosin molecules within the muscle cells. This makes skeletal muscle a striated muscle.

Cardiac muscle makes up the heart and, like skeletal muscle, is also striated. However, it is an involuntary muscle that is controlled by an intrinsic pacemaker and influenced by the autonomic nervous system.

Smooth muscle, which lacks the striations seen in skeletal and cardiac muscle, is also involuntary. It is commonly found lining hollow organs like blood vessels and the intestine. In all three muscle types, the generation of force involves the interaction between actin and myosin molecules, a process that requires an increase in intracellular calcium levels.

This chapter focuses on the molecular mechanisms involved in skeletal muscle contraction and the regulation of contraction force. To understand these mechanisms, it is crucial to first comprehend the organization of skeletal muscle.

The organization of skeletal muscle can be illustrated using the example of muscles spanning the elbow joint, as shown in Fig. 12.1. These muscles are attached to bones on either side of the joint. The point of attachment closest to the spine is called the origin, while the point of attachment on the far side of the joint is known as the insertion. Tendons, which are connective tissues, facilitate these attachments at the end of the muscle. Notably, the insertion point is located near the elbow joint, allowing for a wide range of motion. The joint is spanned by a flexor muscle on one side and an extensor muscle on the opposite side. Consequently, the contraction of the flexor muscle, exemplified by the biceps muscle in Fig. 12.1, leads to a decrease in the angle of the elbow joint, bringing the forearm closer to the shoulder. Conversely, the contraction of the extensor muscle, represented by the triceps muscle in Fig. 12.1, has the opposite effect.
Describe the organization of skeletal muscle, including the structural features/proteins within the skeletal muscle fiber that link the contractile elements to the extracellular matrix and bone to effect movement. While describing the various linkages, identify congenital conditions that commonly affect particular structures and how they might contribute to a myopathy.
2.
Describe the molecular mechanisms by which an action potential in the α motor neuron in the ventral horn of the spinal column can lead to contraction of a skeletal muscle.
3.
Describe the mechanisms by which the force of skeletal muscle contraction increases.
4.
Compare skeletal muscle fiber types in terms of recruitment pattern, metabolic characteristics, contractile characteristics, and thus their suitability for various types of activity.
5.
Discuss the signaling pathways that contribute to the expression of the slow-twitch muscle phenotype versus the fast-twitch muscle phenotype.
6.
Describe general signaling pathways that contribute to hypertrophy or atrophy.
7.
Discuss mechanisms underlying the development of muscular fatigue.
8.
Describe mechanisms underlying the monosynaptic reflex.
9.
Discuss the length-tension curves and force-velocity curves for skeletal muscle, including the molecular bases of both curves.
Muscle cells are highly specialized for the conversion of chemical energy to mechanical energy. Specifically, muscle cells use the energy in adenosine triphosphate (ATP) to generate force or do work. Because work can take many forms (such as locomotion, pumping blood, or peristalsis), several types of muscle have evolved. The three basic types of muscle are skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle acts on the skeleton. In limbs, for example, skeletal muscle spans a joint, thereby allowing a lever action. Skeletal muscle is under voluntary control (i.e., controlled by the central nervous system) and plays a key role in numerous activities such as maintenance of posture, locomotion, speech, and respiration. When viewed under the microscope, skeletal muscle exhibits transverse striations (at intervals of 2 to 3 µm) that result from the highly organized arrangement of actin and myosin molecules within the skeletal muscle cells. Thus skeletal muscle is classified as a striated muscle. The heart is composed of cardiac muscle, and although it is also a striated muscle, it is an involuntary muscle (i.e., controlled by an intrinsic pacemaker and modulated by the autonomic nervous system). Smooth muscle (which lacks the striations evident in skeletal and cardiac muscle) is an involuntary muscle typically found lining hollow organs such as the intestine and blood vessels. In all three muscle types, force is generated by the interaction of actin and myosin molecules, a process that requires transient elevation of intracellular [Ca++].
In this chapter, attention is directed at the molecular mechanisms underlying contraction of skeletal muscle. Mechanisms for regulating the force of contraction are also addressed. To put this information into perspective, it is important to first examine the basic organization of skeletal muscle.
Organization of Skeletal Muscle
Fig. 12.1 illustrates skeletal muscles spanning the elbow joint. The muscles are attached to bone on either side of the joint. The point of attachment closest to the spine (proximal) is called the origin, whereas the point of attachment on the far side of the joint (distal) is called the insertion. These points of attachment occur through tendons (connective tissue) at the end of the muscle. Note that the point of insertion is close to the elbow joint, which enables a broad range of motion. Also note that the joint is spanned by a flexor muscle on one side and an extensor muscle on the opposite side of the joint. Thus contraction of the flexor muscle (see the biceps muscle in
Fig. 12.1 ) results in a decrease in the angle of the elbow joint (bringing the forearm closer to the shoulder), whereas contraction of the extensor muscle (see the triceps muscle in