Introduction to Muscles

From MyMCAT

Introduction

Muscles are the contractile cells of the body which generate the force required for the mechanical movement of the body and its internal parts. In this section we will broadly discuss the different types of muscles and how muscles generate movement.

Types of Muscles

Skeletal muscle

Skeletal muscles (or voluntary muscles) are anchored by tendons to bone and are used to effect skeletal movement such as locomotion and in maintaining posture. Skeletal muscles can be controlled at a conscious level and thus are often called voluntary muscles implying that we can voluntarily move them. Skeletal muscles are said to be "striated" in that they contain sarcomeres and are packed into highly-regular arrangements of bundles which are often visible under the microscope. It is these bundles which drive contraction of the entire cell and thus produce the pulling force necessary to mechanically move the body.

Smooth muscle

Smooth muscles (or involuntary muscles) are found within the walls of organs and structures such as the esophagus, stomach, intestines, bladder, and blood vessels. Unlike skeletal muscle, smooth muscle is not under conscious control and instead is stimulated by the autonomic nervous system. Smooth muscles lack the striated arrangement of sarcomeres but functions nonetheless in a similar manner of contracting spindles of fibers inside each cell. Unlike striated muscle which contract and relax in short intense bursts, smooth muscle can sustain long contractions for near indefinite periods of time.

Cardiac Muscle

Cardiac muscle is also an involuntary muscle but is more akin in structure to skeletal muscle and is unique to the heart. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs).

Organization of Skeletal Muscle Fibers

An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. Each compartment contains a bundle of muscle fibers and each bundle of muscle fiber is called a fasciculus. In turn, these fasciculus contain an individual muscle cell, called a muscle fiber, and is surrounded by connective tissue called the endomysium. The connective tissue covering furnish supports and protects the delicate cells and provides pathways for the passage of blood vessels and nerves.

Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle, contraction. Before a skeletal muscle fiber can contract, it has to receive an impulse from a nerve cell. Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries.

The Basics of Contraction

The three types of muscle (skeletal, cardiac and smooth) have significant differences, however, all three use the movement of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.

Muscular activity accounts for much of the body's energy consumption. All muscle cells produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads. Muscles conserve energy in the form of creatine phosphate glycogen and rapidly metabolize these to produce ATP when energy is required for sustained, powerful contractions.