Smooth Muscle PDF

Smooth muscle Dr. Rasha Abuelgasim Babiker MBBS, MSc, PhD Physiology Office 216 Department of Physiology [email protected]  Learning Objectives 1. Describe the structure and function of smooth muscle types. 2. Explain how smooth muscle myofilaments are regul...

Smooth muscle Dr. Rasha Abuelgasim Babiker MBBS, MSc, PhD Physiology Office 216 Department of Physiology [email protected]  Learning Objectives 1. Describe the structure and function of smooth muscle types. 2. Explain how smooth muscle myofilaments are regulated. 3. Name three ways in which smooth muscle contraction is initiated. 4. Explain spontaneous electrical activity (pacemaker) in smooth muscle. 5. Contrast single unit and multiunit smooth muscles.  Thin filaments contain actin. Thick filaments contain myosin which has two subunits 1. The heavy chain contains the myosin ATPase activity. 2. The light chain regulates this myosin ATPase. - When the light chain is phosphorylated by an enzyme (myosin light chain kinase), cross bridges form between myosin and actin; contraction begins. - The myosin light chain kinase is regulated by Ca++. Consequently, the thick filament regulates contraction in smooth muscle. Characteristic of smooth muscle Smooth muscle produces slow, sustained contractions using only 10% of the ATP that skeletal muscle would require for the same work. A special characteristic of smooth muscle is the variability of the tension  Membrane activation Contraction of smooth muscle, like skeletal muscle, is dependent on a rise of cytosolic Ca++ due to changes in the plasma membrane. However, smooth muscle does not have T tubules. Instead Ca++ enters from the ECF by diffusion through calcium channels in the plasma membrane. These Ca++ channels include: voltage-gated channels, ligand-gated channels and mechano-gated channels. The inputs that regulate contraction include: 1. Autonomic nervous system (parasympathetic, sympathetic and enteric) via voltage gated Ca++ channels. 2. Hormones via ligand-gated Ca++ channels. 3. Stretch via mechano-gated Ca++ channels.  At any one time, multiple inputs, some excitatory and others inhibitory, can be activated in a single cell. The net effect is dependent on the relative intensity of these inputs. Note that the intracellular Ca++ of smooth muscle can increase (or decrease) due to changes in the membrane potential from graded depolarization, hyperpolarization, or an action potential. Spontaneous pacemaker potentials Some smooth muscle exhibits spontaneous contractile activity in the absence of either nerve or hormonal stimuli. The plasma membranes of these fibers do not maintain a stable resting membrane potential. Instead the resting membrane potential gradually drifts towards threshold where it triggers an action potential. Following repolarization the membrane again begins to depolarize. This is property is called pacemaker activity. Pacemakers are found within the GI tract. Single versus multi- unit fibers - Smooth muscle fibers do not have a specific neuro-muscular junction. Instead as the autonomic nerve enters the region of the smooth muscle it divides into many branches each containing a series of swellings (called varicosities) filled with vesicles of neurotransmitters.  In the multi-unit smooth muscle, each fiber is innervated independently. The fibers are not connected by gap junctions. Depolarization of one fiber is followed by contraction of that fiber only. These fibers are richly innervated by the autonomic nervous system. Nervous stimuli and hormones cause contraction (or relaxation) of these fibers, not stretch. 1. The smooth muscle of the lung airways 2. walls of large arteries 3. attached to the hair of the skin are multi-unit fibers.  In the single unit smooth muscle, the fibers are connected by gap junctions. Depolarization of one fiber triggers synchronous depolarization throughout the bundle followed by contraction of the fiber bundle. That is, many fibers act as one sheet. Single unit fibers are found in the: 1. walls of small blood vessels, the 2. GI tract 3. uterus where stretching of one fiber creates a coordinated contraction.  KEY CONCEPTS Smooth muscle is an involuntary, non-striated type associated with blood vessels and visceral organs. Cardiac muscle is the involuntary, striated type that forms the heart wall. Both smooth and cardiac muscle have two sets of overlapping protein myofilaments, actin and myosin, the relative sliding of which produces shortening and generates force. This process involves cross bridge formation between actin and myosin which is driven by ATP. In both smooth and cardiac muscle, coupling between the membrane action potentials and contraction is mediated by calcium ions. In cardiac muscle Ca++ regulates the thin filament (actin) to enable cross bridge formation. In smooth muscle, Ca++ regulates the thick filament (myosin) to enable cross bridge formation and contraction.  Smooth muscle is regulated by the autonomic nervous system. Some smooth muscle is regulated by stretch and hormones. Relaxation of cardiac and smooth muscle, like skeletal muscle, is by removal of Ca++. Thank You