NROSCI/BIOSCI 1070 MSNBIO 2070 Human Physiology September 5, 2018 Muscle 2

Types of Muscle Skeletal muscle cells >> under voluntary control by the central nervous system; typically cause body movement during contraction. Cardiac muscle cells >> located in the heart; cause blood movement during contraction. Smooth muscle cells >> located in visceral organs; cause change in the shape of the organ, usually resulting in pressure changes inside the organ (e.g., bladder) or movement of materials through the organ (e.g., gastrointestinal tract).

Process of Contraction Two types of cytoskeletal filaments (actin and myosin) slide past each other The process is triggered by Ca2+ in the cytoplasm (sarcoplasm) The process requires the hydrolysis of ATP (ATP breaks into adenosine diphosphate (ADP) and phosphate (Pi), liberating energy

Act I: Smooth Muscle

Process of Contraction Actin filaments of smooth muscle: Oriented mainly parallel or oblique to the long axis of the cell. Are attached to the cell membrane, and joined at dense bodies. Myosin Actin

Process of Contraction A myosin filament is between a pair of actin filaments.  During contraction, the actin filament is ratcheted inward by the actions of myosin, causing a change in cell shape. Actin Myosin

Process of Contraction

How Does Ca2+ Trigger Contraction? Four Ca2+ ions bind to calmodulin (CaM), a  calcium-binding messenger protein 

How Does Ca2+ Trigger Contraction? The Ca2+-CaM complex activates the enzyme myosin light chain kinase (MLCK), which in turn phosphorylates the myosin head. This alters the conformation of the myosin head, which greatly increases its ATPase activity and allows it to interact with actin and to act as a molecular motor.

How Does Ca2+ Trigger Contraction? Once started, smooth muscle contraction continues even if sarcoplasmic Ca2+ levels drop. The enzyme myosin-light-chain phosphatase deactivates the myosin head, so it can no longer bind to actin.

What Regulates Sarcoplasmic Ca2+ ? Ca2+ is stored in many smooth muscle cells in the sarcoplasmic reticulum. Ca2+ is pumped against its concentration gradient into the sarcoplasmic reticulum by sarco(endo)plasmic reticulum ATPases (SERCA). SERCA uses the energy liberated by breaking the high-energy bond of ATP to pump Ca2+

What Regulates Sarcoplasmic Ca2+ ? Mechanisms at the cell surface determine whether the sarcoplasmic reticulum releases Ca2+ . Smooth muscle cells without an extensive saroplasmic reticulum depend on Ca2+ entering at the cell surface to trigger contraction. Hence, actions at the cell surface control smooth muscle contraction.

What Regulates Sarcoplasmic Ca2+ ? Ca2+ entering through channels on the cell surface can bind to the Ryanodine receptor in the sarcoplasmic reticulum, triggering the “calcium-induced calcium release.”

What Regulates Sarcoplasmic Ca2+ ? In addition, the sarcoplasmic reticulum of smooth muscle contains inositol triphosphate receptors (IP3Rs), which bind regulatory molecules called inositol triphosphates (IP3s).

Ca2+ Entry at the Cell Surface: Voltage-Gated Ca2+ Channels Voltage-gated Ca2+ channels are located in infoldings in the cell membrane called caveolae (Latin for “little caves”).

Ca2+ Entry at the Cell Surface: Voltage-Gated Ca2+ Channels Many of the voltage-gated Ca2+ channels at the cell surface are L-type: when opened they allow a long influx of Ca2+.

Voltage-Gated Ca2+ Channels Most cells have a negative intracellular charge due to the actions of Na+ - K+ ATPase. Voltage gated Ca2+ channels are closed at normal resting membrane potentials. However, depolarization of the membrane (making it less negatively charge) causes the voltage-gated Ca2+ channels to open.

What causes Membrane Depolarization? Electrical coupling to a pacemaker cell (membrane potential continually changes in a periodic fashion). Transmission of a depolarizing current from an adjacent smooth muscle cell. Binding of a ligand to an ionotropic (ligand-gated) receptor. If binding of a ligand to the ionotropic receptor allows cations (e.g., Na+) to enter the cell (or anions to leave), the membrane will become depolarized.

Ligand-Gated Ca2+ Channels There are also ligand-gated Ca2+ channels at the surface of many smooth muscle cells. The ligands that open these channels are usually specific hormones or paracrines.

Production of Inositol Triphosphates (IP3) Binding of hormones to IP3 receptors causes the formation of IP3, which opens Ca2+ channels in the sarcoplasmic reticulum.

So, MANY Factors Regulate Sarcoplasmic Ca2+ in Smooth Muscle Depolarization of the cell (via pacemakers or opening of a cation channel), which leads to opening of a voltage-gated Ca2+ channel on the cell membrane. Opening of voltage-gated Ca2+ channels can elicit a Ca2+-induced Ca2+ release if the smooth muscle cell has a well-developed sarcoplasmic reticulum.

So, MANY Factors Regulate Sarcoplasmic Ca2+ in Smooth Muscle Opening of ligand-gated Ca2+ channels on the cell membrane. Binding of ligands to an IP3 receptor on the cell membrane, which causes formation of IP3 and release of Ca2+ from the sarcoplasmic reticulum.

HENCE, many pharmacological agents can affect smooth muscle contraction

Types of Smooth Muscle Multi-unit Smooth Muscle: Individual muscle fibers contract independently, and the contraction is mainly controlled by neural inputs. Examples: smooth muscle that affects pupillary size and muscles that produce piloerection.

Types of Smooth Muscle Unitary Smooth Muscle (Syncytial or Visceral Smooth Muscle): Individual fibers are mechanically linked together in a sheet. Furthermore, gap junctions provide electrotonic coupling between adjacent fibers to assure that they contract as a “unit.”

Summary: Smooth vs Skeletal Muscle

Act II: Cardiac Muscle

How Cardiac Muscle Compares with Other Types Cardiac muscle, like skeletal muscle, is striated. Furthermore, both types of muscle are similar in that contraction is initiated when Ca++ binds to troponin. The sliding of cardiac and skeletal muscle actin and myosin filaments is also similar, and the rate limiting step is the breakdown of ATP by myosin ATPase.

How Cardiac Muscle Compares with Other Types In other respects, the contraction process is similar to that of smooth muscle. Both cardiac and smooth muscle cells are joined electrically by gap junctions, and physically (in the case of cardiac muscle cells, by the interdigitation of membranes at sites called intercalated disks)

How Cardiac Muscle Compares with Other Types In some ways, however, cardiac muscle differs completely from both smooth and striated muscle. Special cells in cardiac muscle are autorhythmic, in that they spontaneously generate action potentials without input from other sources. These so- called pacemaker cells set the rate of the heartbeat. Because of the existence of these specialized cells, the heart can contract in isolation, when removed from all neural and hormonal influences.

Structure of Cardiac Myocytes The intercalated disks hold the cardiac muscle cells into a latticework that maximizes pressure inside the heart chambers when the muscle cells contract.

Structure of Cardiac Myocytes Cardiac muscle cells also contain an abundance of a protein called titin, which has springlike properties and plays a key role in generating tension within the cells during contraction.

Mechanism of Cardiac Muscle Contraction Indirect active transport Ryanodine Receptor Ca++-induced Ca++ release

Cardiac Action Potential 1 During Phase 1, the fast sodium channels inactivate, and the cardiac transient outward potassium current (ITO) is initiated. + 50 mV 2 During Phase 2, ITO discontinues and L-type Ca++ channels open. The resulting Ca++ entry produces the Ca++-induced calcium release. Slow delayed rectifier K+ channels are also open so that there is no change in membrane polarity (inward and outward + charges balance). During Phase 3, the Ca++ channels inactivate, and two additional types of K+ channels open to repolarize the membrane. 3 4 Phase 4 is the resting membrane potential, which is ~-90 mV in a cardiac muscle cell. -90 mV 200-300 msec Phase 0 is a period of rapid depolarization, which results from the opening of fast Na+ channels.

Sarcoplasmic reticulum T-tubule Extracellular space Na+ K+ channel Cardiac Action Potential Na+ K+ Cytoplasm Na+ channel Ca2+ Na+ Na+ Cytosolic [Ca2+] Ca2+ Ca2+ Ca2+ Na/Ca exchanger Ca2+ Ca2+ Ca2+ Ca2+ Sarcoplasmic reticulum Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ T-tubule Ca2+ Ca2+ Ca2+ Ca2+ channel Ryanodine receptor Ca2+ PLB ATPase Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Cytoplasm loaded with Ca2+ fluorescent indicator Ca2+ Ca2+ Ca2+ ATPase Ca2+ K+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Na+ TnC TnC Na+ pump Contractile filaments

What is the importance of the long duration of the cardiac action potential?

Why Does Venous Return to the Heart Affect Cardiac Output? Unlike skeletal muscle, resting cardiac muscle is shortened well below its optimal length. As a result, cardiac muscle cells have a tendency to contract very weakly. When blood fills a heart chamber, however, the cardiac muscle is stretched and brought closer to its optimal length. As a result, the tension of contraction increases. This principle is the fundamental basis for the Frank-Starling Law of the Heart. This law states that as the volume of blood returning to a heart chamber increases (thereby stretching myocardial cells in that chamber to a more efficient resting length), then the force produced by contraction (and thus stroke volume) also increases. As a result, the heart will always pump out as much blood as is returned.

Why Does Venous Return to the Heart Affect Cardiac Output?

Why Does Venous Return to the Heart Affect Cardiac Output? In addition to mechanically affecting contraction strength by altering the relationship between the contractile proteins actin and myosin in cardiac muscle cells, stretch of a ventricle affects the sensitivity of the calcium-binding protein troponin for calcium. This heightened sensitivity increases the rate of cross- bridge attachment and detachment, and the amount of tension developed by the muscle fiber .

Note: The Frank-Starling Effect Has a Cost: ATP! The Frank-Starling effect facilitates cross-bridge cycling in cardiac muscle cells Each cross bridge cycle requires ATP Thus, the Frank-Starling effect results in more ATP use by cardiac muscle cells, along with stronger contraction Although the Frank-Starling effect is very practical to stabilize cardiac output, it is hazardous in patients with reduced coronary blood flow, who cannot deliver enough oxygen to cardiac muscle cells to generate large quantities of ATP

Action Potential Generation by Autorhythmic Cells  Cardiac autorhythmic cells do not have a stable resting potential.  This is mainly due to the fact they have If (f=funny) channels that open spontaneously when the membrane is polarized to -60 mV.  The If channels allow both Na+ and K+ ions to flux, but at negative membrane voltages inward Na+ influx exceeds K+ efflux. As a result, the membrane slowly depolarizes.

Action Potential Generation by Autorhythmic Cells  The depolarization causes a few voltage-gated Ca++ channels to open, and this depolarization eventually promotes more Ca++ channels to open, producing a full-blown action potential.  The Ca++ channels then close, repolarization begins due to opening of K+ channels, the membrane returns to a negative potential, and the process begins again.

How does the Parasympathetic Nervous System Affect Cardiac Contraction? Release of ACH from vagal efferents acts to slow heart rate. The transmitter binds to muscarinic receptors on autorhythmic cells that influence both K+ and Ca++ channels. Potassium permeability increases, so the membrane potential becomes more negative after each action potential. At the same time, Ca++ permeability decreases. Thus, it takes more time for the cell to reach threshold for generating the Ca++-mediated action potential.

How does the Sympathetic Nervous System Affect Cardiac Contraction? The binding of epinephrine or norepinephrine to β-receptors on autorhythmic cells results in the activation of a second messen-ger system (c-AMP), whose effect is to cause phosphorylation of If and Ca++ channels. The net result is that cation entry speeds up, which decreases the time required for the voltage-gated Ca++ channels to become activated during each action potential cycle. As a result, the time between action potentials decreases.

How does the Sympathetic Nervous System Affect Cardiac Contraction? Phospholamban Acts Here Makes This More Efficient Typical myocardial cells are also affected by binding to β-receptors, which induces the production of cAMP. The phosphorylation of voltage-gated Ca++ channels increases the likelihood that they will open. In addition, phosphorylation of a regulatory protein, phospholamban, enhances Ca++—ATPase activity in the sarcoplasmic reticulum, so that more Ca++ is sequestered in the sarcoplasmic reticulum, and thus more Ca++ can be released when the myocardial cell reaches threshold. As a result, the actin filaments slide further along myosin, and contraction tension is increased.

Other Effects of the Sympathetic Nervous System on Myocardial Cells The Ca2+ channels on the myocardial cell membrane open faster. The Ca2+ channels in the sarcoplasmic reticulum (ryanodine receptors) open faster. Cross bridge cycling occurs faster. Net Effect: Myocardial contraction is faster and more intense; the myocardial action potential is shortened in duration.

The Staircase Effect (Bowditch effect; Treppe; Frequency-dependent inotropy) Increases in heart rate cause an automatic increase in the tension generated by contracting myocardial cells, even when all neural and hormonal influences are eliminated. This phenomenon is called the Bowditch effect after the physiologist (Henry Bowditch) that discovered it.

The Staircase Effect (Bowditch effect; Treppe; Frequency-dependent inotropy) 3 Na+ per Ca2+ Most think the Bowditch effect is due to the impaired ability to remove Ca++ from the sarcoplasm. Although most of the Ca++ is transported into the sarcoplasmic reticulum via the Ca++-ATPase, some is removed at the cell surface by indirect active transport. When contraction rates are high, the Na+/K+ ATPase is overwhelmed, and levels of Na+ climb in the cell near the plasma membrane. Thus, there is less driving force for indirect active transport, so intracellular Ca++ levels increase.

Heart Rate & Cardiac Action Potential Duration When heart rate increases, there is an automatic shortening of the cardiac action potential (mainly the plateau phase). This is due to the intracellular Ca2+ accumulation at higher heart rates, which affects the L-type Ca2+ channels. These channels inactivate quicker, which shortens the plateau phase.

Heart Rate & Cardiac Action Potential Duration High heart rates induced by the sympathetic nervous system diminish the protective value of the long duration cardiac action potential (which becomes shorter than the period of contraction). Other safety mechanisms are also needed to minimize the possibility of sustained myocardial contraction (as we will see in a subsequent lecture). 1 sec 0.5 sec