Kharkov National Medical University Department of histology, cytology and embryology Muscle tissue Lecture N 7

Types of movement in nature and in the human body - by movement of cilia and flagella - by contraction of cells of the body Muscle cells satisfy requirement of the body in movement.

Classification – 3 types of muscle tissue: 1. skeletal muscle 2. cardiac muscle 3. smooth muscle groups: Striated Smooth

Why do muscles contract? Muscle cells have contractile proteins - actin and myosin, troponin, tropomyosin and some another. The interaction of actin and myosin mediates the contraction of muscle cells.

Why do muscles contract? Actin and myosin form myofilaments: Myosin - thick, dark and Anisotropic (A) Actin – thin, light and Isotropic (I) Actin and myosin myofilaments are arranged parallel to the direction of cellular contraction, and form special organelles – myofibrils, responsible for muscle contraction.

SMOOTH MUSCLE

Locations: walls of visceral hollow organs (stomach, uterus, ureter, blood vessels, eye) Functions: involuntary movement -- changes of lumen of hollow organs, movement of its contens, i.e.: - churning of food - peristalsis, - movement of urine from the kidney to the bladder, -parturition, - blood pressure, -accomodation and adaptation of the eye (The innervation -- by autonomic nervous system)

SMOOTH MUSCLE Unit – myocyte (cell) Cells are spindle-shaped. Individual cells are organized in sheaths In hollow organs they form layers: - longitudinal, - circular, - oblique. Contraction is usually slow and may take minutes to develop.

Smooth Muscle The largest smooth muscle cells occur in the uterus during pregnancy (12x600 µm). The smallest are found around small arterioles (1x10 µm).

Origin of smooth muscle 1. Smooth muscle cells arise from mesenchymal cells. 2. These cells differentiate first into mitotically active cells, myoblasts, which contain a few myofilaments. 3. Myoblasts differentiate into mature smooth muscle cells. Very good regenerates!

Striated muscles

See: regular organization of the myofibrils gives rise to the cross-striations, which characterize skeletal and cardiac muscle.

CARDIAC MUSCLE Location: the heart Function: involuntary, rhythmic contraction Unit – cardiomyocyte (cell)

Cardiac muscle cells: 3 types: Contractile, Conducting, Secretory. Regeneration - intracellular

CARDIAC MUSCLE cardiac muscle cells are cylindrical, connected end-by-end, and form “functional fiber”, which often branches at acute angles.

CARDIAC MUSCLE They are connected by special junction - intercalated discs – that consist of gap junctions and desmosomes.

Excitation in cardiac muscle A special system of modified cardiac muscle cells – conducting myocytes – they form conducting system - Purkinje fibers.

Excitation in cardiac muscle Modified nodal muscle cells, which are called P-cells (pacemaker or pale-staining), generate rhythm of the heart contraction The rhythm can be modified by the autonomic nervous system, which accelerates (sympathetic) or decelerates (parasympathetic) heart rate.

SKELETAL MUSCLE

Location Muscles which are associated with the skeleton (are connected to bones by tendons). Platysma and mimic muscles Voluntary sphincters of inner organs

SKELETAL MUSCLE --- is innervated by the somatic nervous system – voluntary!! ---- consists of very long tubular cells (also called muscle fibres).

SKELETAL MUSCLE Skeletal muscle fibers run the full length of a muscle. The average length of skeletal muscle cells in humans is about 3 cm (sartorius muscle up to 30 cm, stapedial muscle only about 1 mm). Their diameters vary from 10 to 100 µm.

Nuclei Skeletal muscle fibres contain many nuclei (up to several hundred ) placed beneath the plasma membrane (sarcolemma)

Myofibrils Mechanism of contraction: Sliding filaments theory

Myofibrils has some bands and lines depending on the distribution and interconnection of myofilaments -- : I-band - actin filaments, A-band - myosin filaments which may overlap with actin filaments Z-line -- band of connections between actin filaments; zone of apposition of actin filaments belonging to two neighbouring sarcomeres; M-line - band of connections between myosin filaments. H-zone - zone of myosin filaments only (no overlap with actin filaments) within the A-band

Sarcomeres are smallest contractile units of myofibrils. Sarcomere formula: S = ½ I + A + ½ I

Sarcomere after contraction S = A (- ½ I, - ½ I, - H)

Mechanism of contraction

Skeletal muscles (C) are stimulated by nerve impulses carried by axons (A) of motor neurons. Axon forms synapse – motor end plate (B). The excitatory transmitter is acetylcholine. Invaginations of the sarcolemma form the T-tubule system which "leads" the excitation into the muscle fiber. Close to the border between A- and I-bands of the myofibrils T-tubules are in close apposition with cisternae of sarcoplasmatic reticulum. This association is called a triad.

The impuls of excitation from T-tubule opens the calcium channels of the sarcoplasmatic reticulum. Calcium can now move from stores in the sarcoplasmatic reticulum into the cytoplasm surrounding the myofilaments.

Ultrastructure of skeletal muscle

Sites of interaction between actin and myosin are in resting muscle cells "hidden" by tropomyosin. Tropomyosin is kept in place by a complex of proteins collectively called troponin. The binding of calcium to troponin-C induces a conformational change in the troponin- tropomyosin complex which permits the interaction between myosin and actin and, as a consequence of this interaction, contraction.

ATP-dependent calcium pumps in the membrane of the sarcoplasmatic reticulum typically restore the concentration of Ca to resting levels within 30 milliseconds after the activation of the muscle fiber.

muscle cells convert chemical energy (ATP) into mechanical energy by the contractile apparatus of muscle cells.

Origin of skeletal muscle The myoblasts of all skeletal muscle fibers originate from the paraxial mesoderm - myotome.

1. Myoblasts undergo frequent divisions and coalesce with the formation of a multinucleated, syncytial muscle fiber or myotube. The nuclei of the myotube are still located centrally in the muscle fiber. 2. In the course of the synthesis of the myofilaments and myofibrils, the nuclei are gradually displaced to the periphery of the cell.

Regeneration. Satellite cells Satellite cells are small cells which are closely apposed to muscle fibers within the basal lamina which surrounds the muscle fiber. Satellite cells are believed to represent persistent myoblasts. They may regenerate muscle fibers in case of damage.

Muscle as an organ Muscle fibers in skeletal muscle occur in bundles, fascicles, which make up the muscle.

Muscle The muscle is surrounded by a layer of connective tissue, the epimysium, which is continuous with the muscle fascia.

Connective tissue from epimysium extends into the muscle to surround individual fascicles (perimysium) from which a delicate network of reticular fibers surrounds each individual muscle fiber (endomysium).

The dense regular connective tissue of tendon transduces the force generated by the muscle fibers to the bone. Tendons join muscle with bone

Red and white Red muscle fibers are comparatively thin and contain large amounts of myoglobin and mitochondria. Red fibers contain an isoform of myosin with low ATPase activity, i.e. the speed with which myosin is able to use up ATP. Contraction is therefore slow. Red muscles are used when sustained production of force is necessary, e.g. in the control of posture.

Red and white White muscle cells, which are predominantly found in the white muscles, are thicker and contain less myoglobin. ATPase activity of the myosin isoform in white fibres is high, and contraction is fast.

Motor units Motor units - groups of muscle fibers innervated by one motor neuron and its branches. All muscle fibers of a motor unit are of the same type. Skeletal muscle fibers do not contract spontaneously.