smooth muscle

Examples of smooth muscle in the following topics:

• Types of Muscle Tissue

  • The function of muscles is movement, but the types of movement elicited differ between skeletal, cardiac, and smooth muscle.
  • There are three kinds of muscle tissue: skeletal, smooth, and cardiac.
  • Smooth muscle is named because it does not have any striations.
  • Smooth muscle is found in the walls of the hollow organs.
  • Although cardiac muscle is involuntary in nature, it is structurally different from smooth muscle.

• Adrenergic Neurons and Receptors

  • Smooth muscle behavior is variable depending on anatomical location.
  • One important note is the differential effects of increased cAMP in smooth muscle compared to cardiac muscle.
  • Increased cAMP will promote relaxation in smooth muscle, while promoting increased contractility and pulse rate in cardiac muscle.
  • Specific actions of the α1 receptor mainly involve smooth muscle contraction.
  • Other areas of smooth muscle contraction are as follows:

• Characteristics of Muscle Tissue

  • The three types of muscle tissue are skeletal, smooth, and cardiac.
  • Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
  • Cardiac and skeletal muscle are both striated in appearance, while smooth muscle is not.
  • Both cardiac and smooth muscle are involuntary while skeletal muscle is voluntary.
  • Differentiate among the structure and location of skeletal, smooth, and cardiac muscles

• Local Regulation of Blood Flow

  • Blood flow is regulated locally in the arterioles and capillaries using smooth muscle contraction, hormones, oxygen, and changes in pH.
  • Blood flow is regulated by vasoconstriction or vasodilation of smooth muscle fibers in the walls of blood vessels, typically arterioles.
  • Arterioles contain smooth muscle fibers in their tunica media which allows for fine control of their diameter.
  • Local responses to stretch, carbon dioxide, pH, and oxygen also influence smooth muscle tone and thus vasoconstriction and vasodilation.
  • However, the arterioles of skeletal muscle, cardiac muscle, and the pulmonary circulation vasodilate in response to these hormones acting on beta-adrenergic receptors.

• Esophagus

  • Swallowing is a voluntary act that utilizes the muscles of the mouth and tongue to push food into the esophagus.
  • Food then moves down the esophageal tube through waves of muscle movement, or peristalsis, until it reaches the stomach.
  • The esophageal tube in humans is comprised of two main layers of smooth muscle, though striated muscle comprises the tube near the pharynx.
  • The innermost layer of smooth muscle is arranged in a series of concentric rings, while the outermost layer is arranged longitudinally.
  • In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus) from the pharynx to the stomach.

• Arrangement of Fascicles

  • Skeletal muscles are grouped into fascicles, which are bunches of muscle fibers surrounded by a perimysium.
  • Skeletal muscle tissue is composed of numerous muscle fibers which are separated from adjacent muscles and other tissues by a layer of dense, elastic connective tissue termed the fascia.
  • This fascia can project beyond the end of the muscle and attach to bones, other muscles, and other tissues.
  • The fascia surrounding a muscle or muscle group does not contain many blood vessels, but is rich with sensory receptors.
  • Whilst both cardiac and smooth muscles are also wrapped in connective tissue, they are not differentiated in the same way as skeletal muscles.

• Overview of the Musculoskeletal System

  • These are skeletal, smooth, and cardiac muscle.
  • Only skeletal and smooth muscles are considered part of the musculoskeletal system.
  • Examples of smooth muscles include those found in intestinal and vessel walls.
  • Cardiac muscles are found in the heart.
  • Cardiac and smooth muscle are characterized by involuntary movement (not under conscious control).

• Abnormal Contractions of Skeletal Muscle

  • Involuntary muscle contractions are referred to as spasms, and can be due to abnormal activity of the nerve or the muscle.
  • Examples of spasms include muscle contractions due to abnormal nerve stimulation, or abnormal activity of the muscle itself.
  • In this case, the hypertonic muscle tone is excessive and the muscles are unable to relax.
  • This is an episodic pain due to spasms of smooth muscle in a particular organ (e.g., the bile duct).
  • Hypertonic muscle spasms is the state of chronic, excessive muscle tone, or tension in a resting muscle – the amount of contraction that remains when a muscle is not actively working.

• Muscle Tone

  • Muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons.
  • The main regulator of muscle tone is the muscle spindle, a small sensory unit that is closely associated with and lies parallel to a muscle.
  • If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction.
  • Smooth and cardiac muscles do not have specialized muscle spindles.
  • Muscle tone ensures that even when at rest the muscle is at least partially contracted.

• How Skeletal Muscles Are Named

  • Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber.
  • In Pennate muscles, the tendon runs through the length of the muscle.
  • However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
  • If the central tendon branches within a pennate muscle, the muscle is called multipennate.
  • Skeletal circular muscles are different from smooth muscle equivalents due to their structure and because they are under voluntary control