The human body contains three distinct types of muscle tissue: skeletal, cardiac, and smooth muscle. Smooth muscle is responsible for the involuntary movements of our internal organs and is found within the walls of hollow structures. A specific orientation of this tissue, known as longitudinal smooth muscle, features fibers that run parallel to the long axis of an organ. This arrangement enables numerous bodily systems to function without conscious control. The contractions of this muscle type are slow and steady, contributing to the transport of substances through various tracts.
Where Longitudinal Smooth Muscle is Found
Longitudinal smooth muscle is a component of the walls of many hollow organs throughout the body. In the digestive system, it forms a distinct layer in the esophagus, stomach, small intestine, and large intestine. The urinary system also relies on this tissue, with longitudinal smooth muscle present in the ureters, which carry urine from the kidneys, and in the urinary bladder, where it contributes to the organ’s ability to expand and contract.
The reproductive systems of both males and females utilize longitudinal smooth muscle. It is found in the uterus and in the fallopian tubes, guiding the ovum. In males, the vas deferens contains this muscle to help propel sperm. Longitudinal smooth muscle is also present in some larger arteries and veins, contributing to their structure, and in the respiratory tract within the walls of the bronchi.
Key Functions Across Body Systems
The primary role of longitudinal smooth muscle is to facilitate the movement of substances by altering the dimensions of the organ it surrounds. In the digestive tract, its contraction shortens and widens a segment of the intestine. This action works with the circular muscle layer to produce peristalsis, which propels food, and segmentation, which mixes contents to aid digestion and nutrient absorption.
Within the urinary system, the contraction of longitudinal smooth muscle in the ureters helps move urine towards the bladder. In the bladder wall, its contraction contributes to the overall squeezing force required to expel urine from the body. This function is part of the micturition reflex, an involuntary process that empties the bladder when it becomes full. The muscle’s ability to maintain tone also prevents the organ from becoming flaccid when empty.
Coordinated contractions of uterine longitudinal smooth muscle are part of childbirth. In the fallopian tubes, subtle contractions are thought to help move the egg from the ovary toward the uterus. In the male reproductive tract, contractions within the vas deferens propel sperm forward during ejaculation. In large blood vessels, the longitudinal fibers can influence vessel length and tension, complementing circular muscles to regulate blood flow and pressure.
Microscopic Structure and Contraction Mechanism
Each longitudinal smooth muscle cell is spindle-shaped, meaning it is wide in the middle and tapers at the ends. Unlike skeletal or cardiac muscle, these cells contain a single, central nucleus and lack a striped, or striated, appearance. This non-striated look is due to the arrangement of its contractile proteins, actin and myosin.
The contractile filaments are not organized into sarcomeres but are instead stretched between structures called dense bodies in the cytoplasm and attachment plaques on the cell membrane. An internal network of intermediate filaments provides structural support. Cells are electrically connected to their neighbors through gap junctions, which allow for synchronized contractions, enabling a sheet of muscle to contract as a single unit.
Contraction is initiated by an increase in intracellular calcium ions (Ca2+), which bind to a protein called calmodulin. This complex activates an enzyme, myosin light chain kinase (MLCK), which phosphorylates the myosin heads. This action enables them to bind to actin and perform the cross-bridge cycling that generates force and shortens the cell. This calcium-calmodulin system differs from the troponin-tropomyosin complex used by skeletal muscle.
Interaction with Circular Muscle and Neural Control
In many tubular organs, longitudinal smooth muscle works in a coordinated manner with an adjacent layer of circular smooth muscle. This relationship is evident in the peristaltic movements of the digestive tract. When the longitudinal muscle layer contracts, it shortens the tube’s length and increases its diameter. This is followed by the contraction of the circular muscle layer, which constricts the lumen and elongates the tube, pushing the contents forward in an alternating sequence that creates waves of movement.
The activity of longitudinal smooth muscle is regulated involuntarily by the autonomic nervous system, which has both sympathetic and parasympathetic divisions. These divisions can have either excitatory or inhibitory effects, depending on the specific organ and the receptors present. Neurotransmitters like acetylcholine or norepinephrine are released at nerve endings to stimulate or relax the muscle cells.
Longitudinal smooth muscle activity is also influenced by hormones circulating in the blood, such as oxytocin stimulating uterine contractions. Local chemical factors released by nearby cells can also trigger contraction or relaxation. Some smooth muscle exhibits myogenic activity, meaning it can initiate contractions spontaneously without any external stimulus, a property that contributes to the baseline tone of many organs.