General histo theory

Lecture 6 (4th -10th November 2016)

Muscular Tissues
Muscle tissue is composed of differentiated cells containing contractile proteins. The structural biology of these proteins generate the forces necessary for cellar contraction, which drives movement within certain organs and the body as a whole. Most muscle cells are of mesodermal origin, and they are differentiated mainly gradual process of lengthening, with simultaneous synthesis of myofibriller proteins. It also is defined as a soft tissue that composes of muscles in human and animal bodies, and provides the muscles ability to contract. This is opposed to other components or tissues in muscle such as tendons or perimysium. It is formed during embryonic development through a process known as myogenesis.

(Figure 1): 3D skeletal muscle fibers morphology and their link to relevant bone.

Muscle tissue varies with function and location in the body. In mammals the three types are: skeletal (striated muscle); smooth (non-striated muscle) and cardiac muscle, which is sometimes known as (semi-striated). Smooth and cardiac muscle contracts involuntarily, without conscious intervention. These muscle types may be activated both through interaction of the central nervous system (CNS) as well as by receiving innervation from peripheral plexus or endocrine (hormonal) activation. Striated or skeletal muscle only contracts voluntarily, upon influence of the CNS. Reflexes are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the CNS, albeit not engaging cortical structures until after the contraction has occurred.
The different muscle types vary in their response to neurotransmitters and endocrine substances i.e. acetyl-choline, noradrenalin, adrenalin, nitric oxide and among others depending on muscle type and the exact location of the muscle. Sub-categorization of muscle tissue is also possible, depending on among other things the content of myoglobin, mitochondria, myosin ATPase etc.
Structures of Muscles:
Muscles are made up of small units called (myocytes) which are elongated cells ranging from several millimeters to about 10 centimeters in length and from 10-100 micrometers in width (diameter). These cells are joined together in tissues that may be either striated or smooth, depending on the presence or absence, respectively, of organized, regularly repeated arrangements of myofibrillar contractile proteins called myofilaments. Striated muscle is further classified as either skeletal or cardiac muscle. Striated muscle is typically subject to conscious control, while smooth muscle is not. Thus, muscle tissue can be described as being one of three different types:



(Fig.2): Schematic diagram of the 3 types of Muscular tissues, (a). Skeletal, (b). Cardiac and (c). Smooth muscles. Note the structural variations amongst them.
(1). Skeletal muscle: are striated in structure and under voluntary control, is anchored by tendons (by aponeuroses at a few places) to bone and is used to effect skeletal movement such as locomotion and to maintain posture. (Though postural control is generally maintained as an unconscious reflex-see proprioception-the muscles responsible also react to conscious control like non-postural muscles). An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass). It also has unlike smooth muscle, striations.

LS of skeletal muscle fibers

(Fig. 3): Skeletal muscle fibers in (a). Longitudianl (LS); and (b) in cross section (CS). Note the peripheral location of nuclei inside the muscle cytoplasm (Sarcoplasm) and the endomysium between the muscle fibers (dark arrows); perimysium (white arrows).
Striated skeletal muscle cells in microscopic view. The myofibers are oriented vertically; the horizontal striations (lighter and darker bands) that are visible result from differences in composition and density of fibrils within the cells. The short dark patches to the side of the myofibers are cell nuclei.
Skeletal muscle is further divided into several subtypes:
(i): Type-I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity. Type-I muscle fiber are sometimes broken down into Type-I and Type-Ic categories, as a result of recent research.
(ii): Type-II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed: Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red when deoxygenated. Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.
(iii): Type-IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.

(Fig.4): Fine details of the skeletal muscle fibers.

(2): Smooth muscles are neither striated in structure nor under voluntary control, is found within the walls of organs and structures such as the esophagus, stomach, intestines,bronchi, uterus, urethra, bladder, blood vessels, and the erector in the skin (in which it controls erection of body hair). The smooth muscle fibres taper at both ends and do not show striation. Cell junctions hold them together and they are bundled together in a connective tissue sheath. The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue. Smooth muscles are involuntary.




(Fig.5): Schemtaic diagram of smooth muscle fibers in various directions. Note the centrally located nuclei.
(3): Cardiac muscle (myocardium): found only in the heart, is a striated muscle similar in structure to skeletal muscle but not subject to voluntary control. Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
Cardiac muscle cells are joined end to end. The resulting fibers are branched and interconnected in complex networks. Each cell has a single nucleus. At its end, where it touches another cell, there is a specialized intercellular junction called an intercalated disc, which occurs only in cardiac tissue. Cardiac muscle is controlled involuntarily for pumping blood through the heart chambers into the blood vessels.



(Fig.6): Cardiac muscle fibers in LS. Note the fine structures of the fibers and compare with other types of muscles.

(Fig. 7): Comparison between the 3 types of muscle fibers in both low and high powers.

Functional aspects of cardiac muscles:
Cardiac muscles also have gap junctions between adjacent fibers. The intercalated discs bind all cardiac muscle fibers, so that the myocardium contracts as a whole.
Sympathetic nerves increase the heart rate and raise the blood pressure while parasympathetic nerves slow the heart rate lower the blood pressure.

Applied Aspects:

Fibrillation: is the abnormal contraction of cardiac muscles. The cardiac chambers do not contract as a whole resulting in the disruption of pumping action. In arterial fibrillation, there is rapid and uncoordinated contraction of atria, ineffective pumping and abnormal contraction of the AV node. Ventriclar fibrillation is characterized by very rapid and disorganized contraction of ventricle. This leads to disruption of ventricular function.
Angina pectoris: Episodes of chest pain due to temporary ischemia of cardiac muscles. It is usually relieved by rest.

Myocardial ischemia: Persistent ischemia due to blockage of more than one arteries results in necrosis (death) of the cardiac muscle, pain, not relieved by rest, gets referred to the left arm, chest and neighboring areas.


The variation in diameter of skeletal muscle fibers depends on factors such as the specific muscle and the age and sex, state of nutrition, and physical training of the individual. It is a common observation that exercise enlarges the musculature and decrease fat depots. The increase in muscle thus obtained is caused by formation of new myofibrils and a pronounced growth in the diameter of individual muscle fibers. This process, characterized by augmentation of cell volume, is called hypertrophy (Hyper=above + trophe=nourishment); tissue growth by an increase in the number of cells is termed hyperplasia (Hyper+plasia=molding). Hyperplasia does not occur in either skeletal or cardiac muscle but does take place in smooth muscle, whose cells have not lost the capacity to divide by mitosis. Hyperplasia is rather frequent in organs such as the uterus, where both hyperplasia and hypertrophy occur during pregnancy.

(Fig. 7): General characterization of 3 types of muscles.

(Table-1): Comparison between the 3 types of muscle fibers.

Skeletal Muscles

Smooth Muscles
Cardiac Muscles.
Shape
Cylindrical
Fusiform
Short cylindrical
Striation
Transverse and longitudinal
Only longitudinal
Faint longitudinal and transverse
Nucleus
Multiple peripheral fattened
Single central fusiform
Single, large, oval, central, perinuclear space may be seen
Branching
None
None
Branched
Cell to cell connections
None
Through nexus
Through intercalated discs.
Practical activities (In laboratory):
Draw the transverse section of a skeletal muscle showing the CT coverings.
Examine the skeletal muscle of the tongue and see the muscle fibers in various planes.
Draw a few cardiac muscle fibers under high power magnification.
Draw a few smooth muscle fibers under high power magnification.
Draw a few skeletal muscle fibers in longitudinal plane under high power magnification.
Show your works done to the laboratory stuffs for checking.