Histology                                                                                                         Stanley Shostak
BioSc 1450                                                                                                      Spring 05

Lecture 5: Muscle

*: identify epithelial-like characteristics

also note linguistic anachronisms:

nerve = sinew;
aponeuroses = derived from sinew  = ct sheath around muscle and arteries

special terminology: sarcodina = amoeba

plasma lemma: sarcolemma

cytoplasm: sarcoplasm

endoplasmic reticulum: sarcoplasmic reticulum

mitochondria: sarcosomes

bear in mind:
epi-/peri-/endomysium: from outside to inside

3 types: skeletal (voluntary; striated); smooth (visceral and circulatory; involuntary; ); cardiac (striated)

Skeletal muscle: Anatomy (typically described for relax muscle)

muscles have names; (mass): bundles of fasciculi; covered by epimysium;

fascicule (fasciculi): bundles of muscle fibers covered by perimysium;

muscle fibers (10 to 100 µm in diameter/up to 35 cm in length): parallel arrangement; multinucleate cells = syncytium (no cell division); elongate; unbranched; flattened nuclei located at fairly regular intervals beneath sarcolemma; (products of cell fusion during development) covered by endomysium; shrinkage  artifact (= endomysial space)

*basement membrane: = external lamina

fibers have fibrils; fibrils have filaments (thin and thick consist of major proteins)

myofibrils: (the  fine dots seen x section; longitudinal lines in longitudinal section) elongated cylindrical structures; lie parallel to one another in sarcoplasm; in register giving regular striations seen with light microscope in longitudinal section.

myofilaments: contractile proteins

sarcomeres (myomeres) : = contractile unit: interval between adjacent Z bands

mitochondria (sarcosomes): rows in sarcoplasm; between sarcomeres; lie over I band predominantly; in immediate association with thick and thin filaments

sarcoplasmic reticulum (pattern more regular in slow-twitch [type I; red; small in x sect] muscle [relying on aerobic metabolism - hence mitochondria, myoglobin {oxygen storage molecule} and succinic dehydrogenase and ATP-ase; richer blood supply] than in 'fast twitch' [type II; white; large in x sect] muscle [relying on anaerobic pathways; rich in glycogen & glycolytic enzymes; intense but sporadic contraction]): between T tubules within a sarcomere; network embraces myofibril

Terminal cisternae: on either side of T tubule; one on A band side; one on I band side

connecting meshwork: wider one covers A band; narrower one covers I band (less regular than meshwork covering A band)

T (tubule) system:

T tubules: transverse, tubular extension of sarcolemma; surrounds each myofibril at junction of A and I bands (i.e., near each end of sarcomere [Z bands in amphibia and cardiac muscle]) triad: terminal cisterna on either side of a T tubule

arrangement of contractile proteins > striations seen with light microscope

light microscopic level

A: anisotropic; broad, dark; remains constant in width despite degree of contraction

I: isotropic; broad, light; only thin filaments (no thick); narrows during contraction

Z: (Zwischenscheiben) bisects I band; drawn together during contraction

vague at light microscopic level; seen at electron microscopic level

H: (heller) light band bisects A band; only thick filaments (no thin); narrows during contraction

M: (Mittelscheibe) denser band bisects H band

sliding filament theory: during contraction, thick and thin filaments slide over one another, causing shortening of sarcomere.

thick (mainly myosin; attached [in middle] to disc-like zone in M line) and thin (mainly of actin; attached [by ends] to disc-like zone in Z line): retain length during contraction
 

Development   Develop from myoblasts following mitosis and end to end fusion multinucleate myotubules (up to 100 nuclei)
proteins laid down in central axis displace nuclei peripherally; growth due to increase in bulk of existing muscle
satellite cells: persistent myoblasts
  Nerves of skeletal muscle   Motor unit: group of muscle fibers innervated by collection of individual nerve branches within muscle; contracts simultaneously.
Neuromuscular junction: required for growth and maintenance; determines type of metabolism (red vs white) by frequency of impulses; any one motor nerve supplies fibers of one type only; all fibers of particular motor unit are of same metabolic type; may be experimentally manipulated

Neuromuscular spindles: (sensory) stretch receptors in muscle
Neuromuscular junction = motor end plate motor end plate: contains concentration of mitochondria and rER
  sole plate: recess in effector (muscle) cell surface; covered by extension of last neurolemmocyte; external lamina of neurolemmocyte merges with that of muscle fiber; endoneureum merges with endomycium

secondary synaptic clefts: deep folds in postsynaptic membrane; contain acetylcolinesterase

postsynaptic membrane: concentration of receptors for acetylcholine

depolarization of sarcolemma > disseminated through T system > release Ca2+ from sarcoplasmic reticulum > activates sliding filament mechanism > contraction

CT of skeletal muscle: contains both collagen & elastin fibers (most numerous in muscles attached to soft tissues, e.g., tongue and face); becomes continuous with tendons and muscle attachment (anchorage) sites endomysium: surrounds individual muscle fibers: mainly reticulin fibers and small amount collagen; surrounds muscle fibers; conveys small blood vessels, lymphatics and nerves; stains blue with Trichrome (Masson's)

perimysium: loose ct; surrounds fascicle

epimysium: surrounds muscle: white sheath = deep fascia

myotendinous junction: fibers end; fascicles merge with tendon

 Smooth: visceral and circulatory muscle (visceral: arranged in orthogonal layers;  longitudinal outermost (reverse urinary system); circulatory: tunica media, especially in arteries)
visceral: specialized for contraction of entire muscle mass (rather than of individual motor units); inherent rhythmic or wave-like; influence of autonomic nervous system, hormones, local metabolites superimposed
circulatory: adjusts blood pressure by determining bore of arterioles
Cells (= fibers): small, elongate, spindle-shaped (tapering ends); occasionally bifurcate; individual nuclei in center of fiber (depending on state at contraction may appear cork-screw shaped); paucity of mitochondria and organelles; contractile proteins not arranged in myofibrils
  caveolae: (analogous to T tubules) flask-shaped  infoldingof plasma membrane regular in shape and distribution; associated with tubular structures

*superficial dense bodies: maintain longitudinal orientation of contractile filaments (analogous to M band?)
cytoplasmic dense bodies: maintain longitudinal orientation of contractile filaments (analogous to M band?)

*Nexus (gap) junctions: mediate spread of excitation;

*Adhering junctions: resembling zona adherens anchorage points for actin and desmin, intermediate filaments of smooth muscle.

  CT: surrounds fibers; binds fibers in irregular branching fasciculi; functional contractile units; surrounded by CT Cardiac muscle striations (see above)

Cells: long, cylindrical, one or at most two nuclei, centrally located; ends of fibers split longitudinally into small number branches; ends abut on similar branches of adjacent cells; sarcoplasm rich in glycogen

*External Lamina

contractile proteins: analogous to striated muscle; system of

T tubules: originate as indentations at Z line (scallop) and ramify throughout the cardiac muscle cytoplasm at Z lines

sarcoplasmic reticulum: leaks Ca2+ after recovery from previous contraction

triads: poorly defined compared to skeletal muscle

mitochondria: typically abundant, closely packed cristae

intercalated discs: coincides with Z line; anchorage for myofibrils; permit extremely rapid spread of contractile stimuli from cell to cell

3 types of membrane specializations:

*fascia adherens: resembles zonula adherens of epithelial junction (but more extensive and less regular); actin filaments insert into and transmit contractile force to adjacent cell

*desmosomes: anchorage for intermediate filaments

*gap (nexus) junctions: mainly in longitudinal portions of interdigitations; sites of low electrical resistance; pass excitation between cells

Cardiac conductile system (Purkinge system): pacemaker region and cardiac muscle conduction myofiber = Purkinje fibers)

CT: between fibers, analogous to endomyscium; abundant blood supply
 

NOTE: Goodson, H. V., Molecular evolution of the myosin superfamily: application of phylogenetic techniques to cell biological questions. In D. M. Fambrough, ed., Molecular Evolution of Physiological Processes, 47th Annual Symposium Society of Physiologists, Vol. 49. New York: Rockerfeller University Press; 141-157; 1994.

pp. 151/152: '[V]ertebrate smooth muscle myosin is more similar to nonmuscle myosin than to striated muscle myosin, both in sequence and in biochemical characteristics. This fact suggests that smooth muscle is more primitive and led many to assume that smooth muscle is the older of original form of muscle and that striated muscle evolved from it. However, · smooth muscle and striated muscle myosins branch independently from nonmuscle myosin. Thus, smooth muscle myosin cannot be the progenitor of striated muscle myosin, and, in fact, the two types of muscle myosin do not appear to be related, except through nonmuscle myosin. The independent origin of the muscle myosins suggests that the two types of muscle tissue may also be independently derived from nonmuscle tissue, and that any similarities between these tissues may be the result of convergent evolution.

'[S]mooth muscle myosin seems to have arisen relatively recently, apparently after the divergence of lines leading to flies and / vertebrates ( · vertebrate smooth and nonmuscle myosins split after the divergence of fly and vertebrate nonmuscle myosins). This suggests that smooth muscle tissue (at least of the types found in vertebrates) may also have arisen recently, possibly after the protostome-deuterostome divergence. Although this may seem unlikely, morphological studies of Drosophila support this idea, showing that all muscles identified in this well-characterized organism are striated muscles, including the gut muscles (Crossley, 1978).'

last revised: 01-15-05