chapter 10

Muscular System

• Skeletal muscles, however, are not the only places where muscle tissue is found.

• Muscle tissue is distributed almost everywhere in the body and is responsible for the movement of materials within and throughout the body.

• This vital tissue propels the food we eat through the gastrointestinal tract, expels the waste products we produce, adjusts the diameter of blood vessels to regulate blood pressure, and pumps blood to body tissues.

• The three types of muscle tissue-skeletal muscle, cardiac muscle, and smooth muscle-were

Skeletal muscle constitutes

• a significant component of body weight, typically about 30-40%, but the percentage is dependent on a variety of factors that include an individual's age, health, activity levels, and sex hormone levels.

• It is primarily attached to the skeleton but is also found in other locations, such as the openings of the gastrointestinal and urinary tracts.

• We begin our discussion on skeletal muscle by describing both its general functions and the characteristics of the skeletal muscle cells that primarily compose it.

Functions of Skeletal Muscle

• The hundreds of skeletal muscles within your body perform a wide range of functions.

• The five general functions are:

  • Move the body.

  • Maintain posture.

  • Protect and support.

  • Regulate elimination of materials

  • Produce heat

Move the body

• Contraction of your skeletal muscles generates large body movements, such as those of walking, and the smaller, more precise body movements such as picking up an object.

• Skeletal muscle is also responsible for the highly developed movements involved in communicating that occur when speaking, writing, and changing facial expressions; the movements associated with breathing

Maintain posture

• Contraction of specific skeletal muscles stabilizes your trunk, pelvis, legs, neck, and head to keep you erect.

• These postural muscles contract continuously when you are awake to keep you from collapsing.

Protect and support.

• Skeletal muscle is arranged in layers within the walls of the abdominal cavity and the floor of the pelvic cavity

• These layers of muscle protect the internal organs and support their position within the abdominopelvic cavity.

Regulate elimination of materials

• Circular muscle bands, called sphincters (sfingk'ter; sphincter = a band) contract and relax to regulate passage of material.

• These skeletal muscle sphincters at the orifices (or'i-fis; orificium = opening) of the gastrointestinal and urinary tracts allow you to
  voluntarily control the expulsion of feces and urine.

Produce heat

• Energy is required for muscle tissue contraction, and heat is always produced by this energy use (the second law of thermodynamics)

• Thus, muscles are like small furnaces that continuously generate heat and function to help maintain your body temperature.

• You shiver when you are cold because involuntary skeletal muscle contraction gives off heat.

• Likewise, you sweat during exercise to release the additional heat produced by your working muscles

characteristics of skeletal muscle

• Skeletal muscle is composed primarily of skeletal muscle cells that exhibit these characteristics-excitability, conductivity, contractility, extensibility, and elasticity:

• Excitability

• Conductivity

• Contractility

• Extensibility

• Elasticity

Excitability

• is the ability of a cell to respond to a stimulus (e.g., chemical, stretch).

• The stimulus causes a local change in the resting membrane potential by triggering the movement of ions across the plasma membrane of the excitable cell

• A skeletal muscle cell responds when its receptors bind the neurotransmitter acetylcholine, which is released from a motor neuron

Conductivity

• involves an electrical signal that is propagated along the plasma membrane as voltage-gated channels open sequentially.

• These electrical signals functionally connect the plasma membrane of the muscle cell (where stimulation occurs) to the interior of the muscle cell

Contractility

• is exhibited when contractile proteins within skeletal muscle cells slide past one another.

• Contractility is what enables muscle cells to cause body movement and to perform the other functions of muscles.

Extensibility

• is the lengthening of a muscle cell.

• This lengthening is possible because the contractile proteins slide past one another to decrease their degree of overlap.

• Muscle's extensibility is exhibited when we stretch our muscles, such as before exercising.

Elasticity

• is the ability of a muscle cell to return to its original length following either shortening or lengthening of the muscle.

• Elasticity of muscle cells is dependent upon the release of tension in the springlike connectin protein associated with contractile proteins