Physiology I
Section 3C
Muscle Contraction
Muscle Mechanics, Smooth Muscle

Suggested Reading:  Guyton Chapter 6

Key Words

Length-Tension Relationship:  The increase in tension during contraction, called active tension, decreases as the muscle is stretched much beyond its normal length--that is to a sarcomere length greater than about 2.2 micrometers.   Guyton page 79

Preload:  I think this goes beyond (but similar to) the heart.

Afterload:  I think this goes beyond (but similar to) the heart.

Summation:  The adding together of individual twitch contractions to increase the intensity of overall muscle contraction.  This occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.  Guyton page 82

Isometric Contraction:  When the muscle does not shorten during contraction.  Guyton page 81

 Isotonic Contraction:  When the muscle does shorten and with the tension on the muscle remaining constant.  Guyton page 81

Force-Velocity Relationship:  

Fast Glycolytic Fibers:  
Much larger fibers for great strength of contraction.  
Extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate contraction.  
Large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process.  
Less extensive blood supply because oxidative metabolism is of secondary importance.  
Fewer mitochondria, also because oxidative metabolism is secondary.  Guyton page 82

Slow Oxidative Fibers:  
Smaller fibers.
Also innervated by smaller nerve fibers
More extensive blood vessel system and capillaries to supply extra amounts of oxygen
Greatly increased numbers of mitochondria, also to support high levels of oxidative metabolism
Fibers contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells.
Myoglobin combines with oxygen and stores it until needed, and it greatly speeds oxygen transport to the mitochondria.  The myoglobin gives the slow muscle a reddish appearance and the name red muscle, whereas the deficit of the red myoglobin in fast muscle gives the name white muscle.  Guyton page 82

Muscle Fatigue:  Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue.  Muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen.  Therefore, most fatigue probably results simply from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output.  However, studies also show that transmission of the nerve signal through the neuromuscular junction diminishes after prolonged muscle activity, thus further diminishing muscle contraction.
Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue in 1 or more minutes because of the loss of nutrient supply, especially loss of oxygen.  Guyton page 83

Series Elastic Component:  

Multi-Unit:  Smooth muscle composed of discrete smooth muscle fibers.  Each fiber operates independently of the others and often is innervated by a single verve ending, as occurs for skeletal muscle fibers.    The most important characteristic of multi-unit smooth muscle fibers is that each fiber can contract independently of the others, and there control is exerted mainly by nerve signals.  They seldom exhibit spontaneous contractions.  Guyton page 95

Single-Unit:  This does not mean single muscle fibers.  Instead, it means a whole mass of hundreds to millions of muscle fibers that contract together as a single unit.  The fibers usually are aggregated into sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next.  In addition, the cell membranes are joined by many gap junctions through which ions can flow freely from one cell to the next so that action potentials or simple ion flow can ravel from one fiber tot he next and cause the muscle fibers to contract together.

Unitary:  This does not mean single muscle fibers.  Instead, it means a whole mass of hundreds to millions of muscle fibers that contract together as a single unit.  The fibers usually are aggregated into sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next.  In addition, the cell membranes are joined by many gap junctions through which ions can flow freely from one cell to the next so that action potentials or simple ion flow can ravel from one fiber tot he next and cause the muscle fibers to contract together.  Guyton page 95

Visceral:  Unitary Smooth Muscles found in the walls of most viscera of the body, including the gut, bile ducts, ureters, uterus, and many blood vessels.  Guyton page 96

Syncytium:  Good Question????

Gap Junction:  Junction through which ions can flow freely from one cell to the next so that action potentials or simple ion flow can ravel from one fiber tot he next and cause the muscle fibers to contract together.  Guyton page 95

Actin Filament:  The "thin" filament.  Made up of 3 protein components: actin, tropomyosin, and troponin.  Actin filaments are based in the Z-line.   Guyton page 77

Myosin Filament:  It makes up the "thick" filament coming off of the M-line.  Composed of multiple myosin molecules wound together making a double-helix.  One end of the chains is folded into a globular polypeptide structure called the myosin head.  Guyton page 76

Dense Bodies:  Please read page 96.

Stress-Relaxation:  The ability to return nearly to its original force of contraction seconds or minutes after it has been elongated or shortened.  Guyton page 98

Slow-Wave Potential:  Good Question????

Hormonal Activation:  Some hormone receptors in the smooth muscle membrane open sodium or calcium ion channels and depolarize the membrane the same as after nerve stimulation.  Sometimes action potentials result, or rhythmical action potentials that are already occurring may be enhanced.  In many instances, depolarization occurs without action potentials; this depolarization is associated with calcium ion entry into the cell that promotes contraction.  Guyton page 101

Sarcoplasmic Reticulum:  An extensive endoplasmic reticulum, which in the muscle fiber is called the sarcoplasmic reticulum (SR).  This reticulum has a special organization that is extremely important in the control of muscle contraction.  Guyton page 74  Inside the SR, there is a very high concentration of calcium.

Learning Objectives

Define a "motor unit" and discuss its characteristics and significance:  Each motoneron that leaves the spinal cord innervates many different muscle fibers, the number depending on the type of muscle.  All the muscle fibers innervated by a singe motor nerve fiber are called a motor unit.  In general, small muscles that react rapidly and whose control must be exact have few muscle fibers (as few as two to three in some of the the laryngeal muscles) in each motor unity.  On the other hand, the large muscles that do not require very fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit.  An average figure for all the muscles of the body is questionable, but a good guess would be about 100 muscle fibers to the motor unit.  Guyton page 82

Identify the effects and significance of variations of motor unit action potential frequency and relative numbers of active motor units upon muscle contraction:  

Compare and contrast the general characteristics of fast and slow twitch muscle fibers:  
Fast Glycolytic Fibers:  
Much larger fibers for great strength of contraction.  
Extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate contraction.  
Large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process.  
Less extensive blood supply because oxidative metabolism is of secondary importance.  
Fewer mitochondria, also because oxidative metabolism is secondary.  Guyton page 82
Slow Oxidative Fibers:  
Smaller fibers.
Also innervated by smaller nerve fibers
More extensive blood vessel system and capillaries to supply extra amounts of oxygen
Greatly increased numbers of mitochondria, also to support high levels of oxidative metabolism
Fibers contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells.
Myoglobin combines with oxygen and stores it until needed, and it greatly speeds oxygen transport to the mitochondria.  The myoglobin gives the slow muscle a reddish appearance and the name red muscle, whereas the deficit of the red myoglobin in fast muscle gives the name white muscle.  Guyton page 82

Describe the relationship between the length of a muscle fiber (or sarcomere) at the onset of a contraction and the force of the contraction:  The increase in tension during contraction, called active tension, decreases as the muscle is stretched much beyond its normal length--that is to a sarcomere length greater than about 2.2 micrometers.   Guyton page 79

Identify the general relationship between load and velocity of shortening for skeletal muscle:   A muscle contracts extremely rapidly when it contracts against no load, to a state of full contraction in about 0.1 second for the average muscle.  When loads are applied, the velocity of contraction becomes progressively less as the load increases.  When the load increases to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction results, despite activation of the muscle fiber.

This decreasing velocity with load is caused by the fact that a load on a contracting muscle is a reverse force that opposes the contractile force caused by muscle contraction.  Therefore, the net force that is available to cause velocity of shortening is correspondingly reduced.  Guyton, page 79-80

Describe the basic functional anatomy and physiology of smooth muscle, and distinguish it from skeletal muscle:  Smooth muscle is composed of far smaller fibers than skeletal muscles, which can be 20 times larger.  Smooth muscle of each organ is distinctive from that of most other organs in several ways:  physical dimensions, organization into bundles or sheets, response to different types of stimuli, characteristics of innervation, and function.  Yet, for the sake of simplicity, smooth muscle can generally be divided into two major types:  Multiunit Smooth Muscle & Unitary (or single-unit) smooth muscle.  See above for these definitions.  Guyton, page 95

Below is a table comparing Smooth and Skeletal muscles Guyton, pages 97-98:  

Differentiate among the many types of activation in smooth muscle, and discuss each mechanism:  See below.  And:
Even without an action potential in the multi-unit smooth muscle fibers, the local depolarization, called the junctional potential, caused by the nerve transmitter substance itself spreads "electrotonically" over the entire fiber and is all that is needed to cause the muscle contraction.

Probably at least half of all smooth muscle contraction is initiated not by action potentials but by stimulatory factors acting directly on the smooth muscle contractile machinery.  The two types of non-nervous and non-action potential stimulating factors most often involved are (1) local tissue factors and (2) various hormones.
Local Tissue Factors:  Small vessels such as arterioles, meta-arterioles, and precapillary sphincters, have little or no nervous supply.  The smooth muscle is highly contractile, responding rapidly to changes in local conditions in the surrounding interstitial fluid.  In this way, a powerful local feedback control system control the blood flow to the local tissue area.
Hormones:  Most of the circulating hormones in the body affect smooth muscle contraction to some degree, and some have profound effects.  Some of the more important blood-borne hormones that affect contraction are norepinephrine, epinephrine, acetylcholine, angiotensin, vasopressin, oxytocin, serotonin, and histamine.

Describe the electrical potentials of smooth muscle, and the role of each:  The quantitative value of the membrane potential of smooth muscle is variable from one type of smooth muscle to another and depends on the momentary condition of the muscle.  In the resting state, the membrane potential is usually about -50 to -60 mV, or about 30 mV less negative than in skeletal muscle.

Action potentials occur in unitary smooth muscle, such as visceral muscle, in the same way that they occur in skeletal muscle.  They do not normally occur in many, if not most, multi-unit types of smooth muscle.  The action potentials of visceral smooth muscle occur in two forms: (1) spike potentials and (2) action potential with plateaus.
Spike Potentials:  Typical spike action potentials, such as those seen in skeletal muscle, occur in most types of unitary smooth muscle.  Such action potentials can be elicited in many ways, for example, by electrical stimulation, by the action of hormones on the smooth muscle, by the action of transmitter substances from nerve fibers, by stretch, or as a result of spontaneous generation in the muscle fiber itself.
Action Potentials with Plateaus:  The onset of this action potential is similar to that of the typical spike potential.  However, instead of rapid repolarization of the muscle fiber member, the repolarization is delayed for several hundred to as much as 100 milliseconds.  
Calcium Channels:  The smooth muscle cell membrane has far more voltage-gated calcium channels than does skeletal muscle but but few voltage-gated sodium channels.  Therefore, sodium participates little in the generation of the action potential in most smooth muscle.  Instead, the flow of calcium ions to the interior of the fiber is mainly responsible for the action potential.  This occurs in the same self-regenerative way as occurs for the sodium channels in nerve fibers and in skeletal muscle fibers.  However, calcium channels open many times more slowly than do sodium channels, but they also remain open much longer.  This accounts in large measure fro the slow action potentials of smooth muscle fibers.  Another important feature of calcium entry into the cells during the action potential is that the same calcium acts directly on the smooth muscle contractile mechanism to cause contraction.
Slow Wave Potentials:  Some smooth muscle is self-excitatory.  That is, action potentials arise within the smooth muscle itself without an extrinsic stimulus.  This is often associated with a basic slow wave rhythm of the membrane potential, especially in intestinal wall smooth muscle.  The slow wave itself is not an action potential.  That is, it is not a self-regenerative process that spreads progressively over the membranes of the muscle fibers.  Instead, it is a local property of the smooth muscle fibers that make up the muscle mass.
Excitation by Stretch:  When visceral (unitary) smooth muscle is stretched sufficiently, spontaneous action potentials are usually generated.  The result from a combination of the normal slow wave potentials plus a decrease in the negativity of the membrane potential caused by the stretch itself.  

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Last Updated 04/10/00 12:26:59 PM