Musclebvlecture(FMTC)1002
MUSCLE & BLOOD VESSELS
(Foundation module; Dr T Cowen; © University College London)3 types of muscle distinguished on the basis of morphological and functional characteristics: skeletal or striated, cardiac and smooth.
SKELETAL MUSCLE
Highly elongated cells (up to 30cm); multinucleate (formed by the fusion of embryonic myoblasts); dia. of 10-100um; peripheral nuclei; under voluntary nervous control.
Subdivisions of structure:
*DIAG. ARCHITECTURE OF SKELETAL MUSCLE (Junquiero, p.235).
1) Groups of fibres (cells) enclosed in a fasciculus:
- A skeletal muscle is made up of muscle fasciculi bound by a connective tissue epimysium.
- Each fascicle is made up of muscle fibres; these are the muscle cells.
2) Organisation within the fibre (cell):
- Muscle fibrils or myofibrils (range 0.2-2um**) NB not membrane-bound; separated by sarcoplasmic reticulum.
- Myofilaments: myofibrils are made up of filaments of contractile proteins. NB these cannot be seen in the LM, although the striations show where they are.
*LM MUSCLE FASCICULI, TS & LS.
*RETURN TO SLIDE 1
- striations: alternating myosin (thick) and actin (thin) filaments arranged in myofibrils where all myofilaments are arranged in parallel so that actin (I) and myosin (A) bands form striations right across the fibre
*HP LM LS OF SKELETAL MUSCLE
- peripheral nucleus (also NB connective tissue nuclei).
*HP EM A and I bands, mitochondria
- The actin filaments join to the adjacent actin filaments at the Z line. The region between adjacent Z lines is called a sarcomere (length in relaxed mammalian muscle 2-3um).
- The area where there are no actin filaments within the A band is called the H zone within which can be seen the darker M line.
- During contraction, the actin filaments slide into the myosin filaments, consequently the I bands shorten and disappear, while sarcomere width increases.
*HP EM MYOSIN HEADS
Conformational changes in the actin molecule mediated by changes in Ca2+, regulated by the sarcoplasmic reticulum -a network of cisternae running around each myofibril (ie intracellular) - associated with a system of transverse (T) tubules:
*DIAG SARCOPLASMIC RETICULUM.
- T-tubules are invaginations of the sarcolemma, running transversely around the fibre.
- T-tubule are associated with 2 vesicles of sarcoplasmic reticulum forming the triad, ensuring rapid, synchronous contractions of ALL myofibrils in each fibre.
*ATPase OF DIFFERENT SKELETAL MUSCLE
- 3 types of skeletal muscle: red (slow, longterm); white (fast, shortterm); intermediate. Demonstrated by histochemical staining for ATPase (high = slow; low = fast).
Nerves in skeletal muscle:
*EM NEUROMUSCULAR JUNCTION
- note close apposition, folded pre- and post-junctional membranes to increase SA, vesicles containing ACh
*TS MUSCLE SPINDLE
- sensory apparatus to respond to altered status of contraction/stretching for (simple reflex) eg postural control
II. SMOOTH MUSCLE
2nd major group of muscle tissues: the involuntary muscle of gut, blood vessels and urinogenital systems.
*LM SMOOTH MUSCLE CELLS
*Elongated, fusiform cells, with single, centrally placed nucleus; no fusion of adjacent cells.
*LM SMCs IN GUT (LS/TS), BVS & UTERUS
- Note absence of striations; position of nucleus; organelles grouped at pole of nucleus.
*EM SMC
- actin and myosin, as in skeletal muscle, but fibrils are not arranged in parallel; actin filaments are joined at dense bodies (as Z line) in cytoplasm and at membrane. Plus [non-contractile] intermediate filaments (10nm) composed of the protein desmin, also vimentin in vascular muscle.
-
- T-tubule system represented by simpler system of s.e.r. at plasma membrane.
*EM BASAL LAMINA OF SMC'S & C/T INTERCONNECTIONS
- Basal lamina (type IV) and reticular fibres (type III) collagen provides anchor for type I collagen and cell-to-cell mechanical connections.
- Note pinocytotic vesicles (exchange).
*HP EMs SHOWING GAP JUNCTIONS
- Cell-to-cell communication provided by gap junctions (electrical and electro-chemical); important in coordination of bands of muscle involved in, for instance, peristalsis, vasoconstriction, uterine and bladder contraction.
*EM AUTONOMIC NEUROEFFECTOR JUNCTION
- not all SMCs are innervated; junctions innervate some and impulse is spread to neighbours via gap junctions - 'electrotonic coupling'
III. CARDIAC MUSCLE
Intermediate in type between skeletal and smooth muscle:
*LM CARDIAC MUSCLE (STRIATIONS)
- striations, but not so prominent as skeletal muscle.
- one or two nuclei per cell, centrally placed.
- note intercalated discs joining Y-shaped cells to eachother, end-to-end.
*LM resin section of CARDIAC MUSCLE CELLS AND I/C DISC
- The intercalated disc zigzags between cells, having lateral and transverse faces
*HP EM OF I/C DISC
Details of I/C disc:
a) Fascia adherens (3) or intercalated disc junctions are the anchoring point for actin filaments (= Z line) in the middle of the I band.
b) Desmosomes (1) anchor adjacent cell membranes (as opposed to myofilaments) together [may also be on lateral surface].
c) Gap junctions (2) allow ionic continuity between cells and thus cell-to-cell communication. Often found connecting excitable cells.
T-tubule system comprises a single pouch of sarcoplasmic reticulum forming a DIAD with the T-tubule.
CONDUCTING SYSTEM IN HEART
Impulses pass from the sino-atrial node (pacemaker) to the atrio-ventricular
node, and thence to the a-v bundle of His. These tracts are made up of
specialised nerve fibres (some argument about origin). The nerve fibres of
the bundles of His merge distally with the Purkinje fibres which are modified
cardiac myocytes.
Purkinje fibres ramify in the myocardium and subendocardial space. They are
different from cardiac myocytes in the following ways:
- larger
- contain more glycogen (can be stained)
- have gap junctions, not intercalated discs between cells
- contain less contractile proteins (some striations visible, but peripheral)
REGENERATIVE CAPACITY OF MUSCLE:
Smooth muscle has good powers of regeneration: de-differentiation, mitosis, re-differentiation.
Skeletal muscle: also regenerates, but from a specialised group of mononucleate cells, the satellite cells, which are undifferentiated myoblasts. Injury trigures mitosis followed by differentiation and fusion to form new muscle cells.
Cardiac muscle: no powers of regeneration, hence danger of cardiac myopathy.
BLOOD VESSELS
Four sets of tubes: arteries, veins, capillaries and lymphatics. All derived from embryonic mesoderm
CAPILLARIES: the business end of the system
- to supply nutritional needs of the tissues and remove excretory products
- exchange of these substances takes place in the capillaries
- capillary endothelium is crucial to this process
*Ag STAIN OF ENDOTHELIUM
3 TYPES OF CAPILLARY:
- wall structure combined with blood pressure combine to control exchange processes.
1. Continuous endothelium eg. in muscle and other tissues
*DIAG. CONT. & FENESTRATED CAPILLARIES
- cells possess pinocytotic vesicles for exchange:
*EM CONTINUOUS ENDOTHELIUM
and PINOCYTOTIC VESICLES
- note basal lamina, intercellular junctions (NB blood brain barrier), pericytes
2. Fenestrated eg. in renal glomeruli, glands, gut.
- associated with rapid exchange of ions, water and macromolecules.
- pores covered with membrane.
3. Sinusoidal capillaries:
*EM LIVER SINUSOID
- discontinuous endothelium and basal lamina, leaving open spaces between
blood and tissues eg. liver, spleen and bone marrow, allowing direct passage
of blood cells and antibodies to and from blood stream.
LARGE VESSELS
Vary greatly in size and structure, but ALL (including heart but not
capillaries) have a wall made up of three layers:
*SMALL ARTERIOLE AND VEIN
*LARGE ARTERY
1) Intima: endothelium and subendothelial connective tissue, including the
internal elastic lamina where present.
- function: mediates exchange between blood and vessel wall
- damage induces thrombosis.
- site of atherosclerosis (subendothelial).
2) Media: spirally arranged smooth muscle cells and interspersed connective
tissue.
- contractile and elastic elements; changing vessel diameter ---> vasomotor
control; and smooths pulse wave ('Windkessel' function).
- in disease or old age, collagen increases at the expense of smooth muscle and wall stiffens (need for exercise).
3) Adventitia: connective tissue sheath.
- function to prevent overdistension
- adventitia ruptures in aneurism
*ELASTIN AND COLLAGEN IN AORTA
- note lamellae of smooth muscle, elastin and Type I collagen
*HP LM VEINS AND VENOUS VALVES
- note thin walled large venules and valves in (some) veins eg. leg.
*HP LM LYMPHATICS
- lymphatic system consists of capillaries and vessels which run from the tissues to the thoracic and right lymphatic ducts which drain into the veins of the neck (read yourselves)
- note even thinner-walled vessels - endothelium only, no muscle or pericytes