Muscular System Main functions: Movement Production of heat Protection of other vital organs Helps in posture
TRIVIA! How many muscles are there in the human body? Answer: 640 Muscles The muscles make up about 40 % of the body mass. What is the longest muscle in the body? Answer: The Sartorius The Sartorius runs from the outside of the hip, down and across to the inside of the knee. It twists and pulls the thigh outwards. What is the smallest muscle in the body? Answer: The Stapedius The Stapedius is located deep in the ear. It is only 5mm long and thinner than cotton thread. It is involved in hearing. What is the biggest muscle in the body? Answer: The Gluteus Maximus The Gluteus Maximus is located in the buttock. It pulls the leg backwards powerfully for walking and running.
Muscle Classification Functionally Voluntarily – can be moved at will Involuntarily – can’t be moved intentionally Structurally Striated – have stripes across the fiber Smooth – no striations
The 3 Types of Muscles 3 Types of Muscles Skeletal Muscle Smooth Muscle Cardiac Muscle
Smooth Muscle Fibers are thin and spindle shaped. No striations Single nuclei Involuntary Contracts slowly
Smooth Muscle They fatigue… but very slowly Found in the circulatory system Lining of the blood vessels Helps in the circulation of the blood Found in the digestive system Esophagus, stomach, intestine Controls digestion Found in the respiratory system Controls breathing Found in the urinary system Urinary bladder Controls urination
Cardiac Muscle Cells are branched and appear fused with one another Has striations Each cell has a central nuclei Involuntary
Cardiac Muscle Found ONLY in the heart Contractions of the heart muscles pump blood throughout the body and account for the heartbeat. Healthy cardiac muscle NEVER fatigues → or else…
Skeletal Muscle ⚫ Fibers are long and cylindrical ⚫ Has many nuclei ⚫ Has striations ⚫ Have alternating dark and light bands ⚫ Voluntary
Skeletal Muscle Attached to skeleton by tendons Causes movement of bones at the joints. And yes… they do fatigue Muscle fatigue activity → what substance forms causing muscle fatigue???
Functions of Skeletal Muscle Movement – muscle move bones by pulling not pushing. Synergists – any movement is generally accomplished by more than one muscle. All of the muscles responsible for the movement are synergists. The one that is most responsible for the movement is the Prime Mover (agonist).
Functions of Skeletal Muscle Movement Antagonists – muscles and muscle groups usually work in pairs – example the biceps flex your arm and its partner the triceps extend your arm. The two muscles are antagonists, i.e. cause opposite actions. – when one contracts the other relaxes. Levators – muscle that raise a body part.
Functions of Skeletal Muscle Maintenance of posture or muscle tone We are able to maintain our body position because of tonic contractions in our skeletal muscles. These contractions don’t produce movement yet hold our muscles in position. Heat production – contraction of muscles produces most of the heat required to maintain body temperature.
Structure of Skeletal Muscle Composed of striated muscle cells (=muscle fibers) and connective tissue. Most muscles attach to 2 bones that have a moveable joint between them. The attachment to the bone that does not move is the origin. The attachment to the bone that moves is the insertion. Tendons anchor muscle firmly to bones. Tendons are made of dense fibrous connective tissue. Ligaments connect bone to bone at a joint.
Structure of Skeletal Muscle Bursae – small fluid filled sacs that lie between some tendons and the bones beneath them. They are made of connective tissue and are lined with synovial membrane that secretes synovial f luid.
Structure of Skeletal Muscle Contribution of the nervous system Electrochemical impulses travel from the frontal lobes of the cerebrum via motor nerves to the muscle fibers and cause them to contract. Sensation is a function of the brain – impulses are integrated in the parietal lobes of the cerebrum (conscious muscle sense) and in the cerebellum (unconscious). These activities promote coordination.
MUSCLES ARE ATTACHED TO THE BONES BY TENDON. THEY ARE MADE UP OF CONNECTIVE TISSUE COMPRISEDOF COLLAGEN. THE WHOLE MUSCLE IS COVERED BY A CONNECTIVE TISSUE CALLED EPIMYSIUM. EACH MUSCLE CELL IS COVERED BY A CONNECTIVE TISSUE SHEATH CALLED ENDOMYSIUM.
BENEATH THE EPIMYSIUM EACH MUSCLE CONSISTS OF MANY MUSCLE FIBRES IN BUNDLES CALLED FASCICULE. THE BUNDLE OF FASCICULI IS SURROUNDED BY A TOUGH EXTERNAL LAYER CALLED FASCIA. THE MIDDLE PART OF A MUSCLE BUNDLE IS OFTEN THIKER AND CALLED BELLY.
A STRIATED MUSCLE FIBRE IS AN ELONGATED CELL WHICH IS MULTINUCLEATED AS THE SARCOPLASM CONTAINS MANY NUCLEI. ITS MEMBRANE IS CALLED SARCOLEMMA. CYTOPLASM PRESENT IN MUSCLE CELL IS TERMED AS SARCOPLASM THAT CONTAIN NUMEROUS MITOCHONDRIA CALLED SARCOSOMES. THE SARCOLEMMA CONSISTS OF A TRUE CELL MEMBRANE, CALLED THE PLASMA MEMBRANE AND OUTER COAT MADE UP OF THIN LAYER OF COLLAGEN FIBRES
CYTOPLASM IN A MUSCLE IS TERMED AS SARCOPLASM. THE SARCOPLASM HAVE AN EXTENSIVE ENDOPLASMIC RETICULUM TERMED AS SARCOPLASMIC RETICULUM. PRIMARY FUNCTION OF THIS IS TO STORE CALCIUM IONS. THEY ARE ABUNDANT IN SKELETAL MUSCLE CELLS AND IS CLOSELY ASSCIATED WITH MYOFIBRILS. HERE, THE T-TUBULES CONDUCTS IMPULSES FROM THE SARCOLEMMA DOWN INTO THE CELL CALLED SARCOPLASMIC RETICULUM
THE MUSCLE FIBRE HAS A LARGE NUMBER OF PARALLELLY ARRANGED FILAMENTS IN THE SARCOPLASM CALEED MYOFIBRILS. THEY ARE THIN, ROD SHAPED & BEAR CROSS STRIATIONS. THEY ARE COMPOSED OF 2-TYPES OS MYOFILAMENTS : THICK & THIN . THICK MYOFILAMENTS ARE ALWAYS SURROUNDED BY 6 THIN MYOFILAMENTS. EACH MYOFIBRIL IS ABOUT 1-2 MICROMETRES IN DIAMETRE AND EXTENDS THE ENTIRE LENGTH OF MUSCLE FIBRE. THE MYOFIBRIL IS ATTACHED TO THE PLASMA MEMBRANE BY THE INTERVENTION OF SPECIALIZED PROTEINS
ACTIN IS A GLOBULINE PROTEINAND HAS LOW MOLECULAR WEIGHT. ACTIN FILAMENTS, WHICH ARE STRUCTURAL PROTEINS MADE UP OF MULTIPLE SUB UNITS, HELP MUSCLES CONTRACT & CELLS MAINTAIN THEIR SHAPE. THEY ARE SECONDARY NARROW SMOOTH FILAMENTS ARISED FROM Z- LINE. THEY CONSISTS OF PROTEINS LIKE : F- ACTIN, TROPONIN COMPLEX & TROPOMYOSIN. GENERALLY, ACTIN OCCURS IN TWO FORMS; MONOMERIC G-ACTIN & POLYMERIC F-ACTIN.
THE THICK FILAMENT CONSISTS MAINLY OF MYOSIN PROTEIN. MYOSIN CONTRIBUTES 50% OF MUSCLE PROTEIN BY WEIGHT. THE MYOSIN MOLECULE IS COMPOSED OF 6 POLYPEPTIDE CHAINS, 2 IDENTICAL HEAVY CHAINS, & 4 LIGHT CHAINS. THE 2 HEAVY CHAINS WRAP SPIRALLY AROUND EACH OTHER TO FORM A DOUBLE HELIX. HOWEVER, ONE END OF EACH CHAINS IS FOLDED INTO A GLOBULAR PROTEIN MASS CALLED MYOSIN HEAD. THE ELONGATED PART OF THE COILED HELIX IS CALLED THE TAIL.
MYOFIBRILS ARE ARRANGED IN SEVERAL CONTRACTILE UNITS ARRANGED END TO END ALONG THE LENGTH OF MUSCLE FIBRE. THEY ARE CALLED SARCOMERES. ADJACENT SARCOMERES ARE SEPARATED BY ZIGZAG Z-LINE / Z MEMBRANE. SARCOMERS HAVE TWO TYPES OF LONGITUDINALLY RUNNING PARALLEL FILAMENTS, THICK PRIMARY MYOSIN FILAMENTS & THICK ACTIN FILAMENTS. A TYPICAL SARCOMERE LENGTH - ~ 1.8 um. THE SARCOMERE CONSISTS OF Z-LINE, I- BAND, A-BAND, M-LINE & H-ZONE.
EACH MYOFIBRIL OF STRAITED MUSCLE FIBRE APPEARS TO BE FORMED OF LIGHT & DARK CROSS BANDS. HERE, THE LIGHT BAND IN WHICH ACTIN IS PRESENT ARE CALLED I-BAND / ISOTROPIC BAND. EACH I-BAND HAS AT ITS CENTRE A DARK MEMBRANE CALLED Z-LINE. THE I-BANDS BECOME SHORTER AND FINALLY DISAPPEAR WHEN THE FIBRE IS FULLY CONTRACTED. THE ACTIN FILAMENTS OF I-BAND ON EITHER SIDE OVERLAP BETWEEN MYOSIN FILAMENTS OF A-BAND.
AS YOU KNOW A MYOFIBRIL HAS DARK & LIGHT BANDS. HERE, THE DARK BAND CONTAINING MYOSIN ARE TERMED AS A-BAND / ANISOTROPIC BAND. THEY ARE NAMED FOR THEIR PROPERTIES UNDER A POLARIZATION MICROSCOPE. MYOSIN ARE FREE AT BOTH SIDES IN AN A- BAND. AT THE CENTRE OF A-BAND A COMPARATIVELY LESS DARK ZONE CALLED H-ZONE. THE DARK COLOR OF A- BAND IS DUE TO THE OVERLAPPING OF MYOSIN FILAMENTS BY ACTIN FILAMENTS. THE LENGTH OF AN A-BAND REMAINS CONSTANT
EACH I-BAND AT ITS CENTRE A DARK MEMBRANE CALLED Z-LINE. THE Z-LINE IS ALSO CALLED Z DISK OR KRAUSE’s MEMBRANE OR DOBIES LINE. ADJACENT SARCOMERES ARE SEPARATED BY ZIGZAG Z-LINE / Z MEMBRANE. SARCOLEMMA IS INVAGINATED AT THE LEVEL OF Z-LINE TO FORM T-TUBULES. Z-LINE ACTUALLY BISECTS THE I-BAND. THE GIANT PROTEIN TITIN EXTENDS FROM THE Z-LINE OF THE SARCOMERE.
AT THE MIDDLE OF A-BAND & H-ZONE, THEIR EXISTS A FIBROUS CONNECTION BETWEEN MYOSIN FILAMENTS CALLED M-LINE. ALSO TERMED AS MITTLE SCHIEBE. HALF WAY BETWEEN TWO Z - DISCS IS AN M-LINE. MYOSIN THICK FILAMENTS ARE ATTACHED TO M-LINE.
MIDDLE OF EACH DARK / A-BAND HAS A VARIABLE LESS DARK ZONE CALLED H- ZONE /HENSEN’s LINE. H-ZONE CONTAIN ONLY MYOSIN.
Cardiac muscle Intercalated discs- special gap junction Provide direct electrical coupling Longer contraction than skeletal (up to 20x) Skeletal muscle- action potential triggers contraction, do not control duration Cardiac muscle- duration of action potential controls duration of contraction
Smooth muscle ◦ No striations ◦ Filaments have a spiral aarangement ◦ Less myosin than skeletal and not associated to actin ◦ NO T tubule system and well-developed sarcoplasmic reticulum ◦ Calcium ions enters via the plasma membrane through an action potential ◦ Slow, long contraction ◦ Found in tubular organs
The muscle
Some terms (actually a lot of terms) Skeletal muscle- consists of bundle of muscle fiber Fascicle- a bundle of muscle fiber surrounded by a connective tissue Muscle fiber- a single muscle cell (syncytium) Myofibril – makes up a muscle fiber ◦ Divided into two myofilaments (creates the striation) Thin filaments Thick filaments
Still on terms...... Thin filaments- consists of two strands of actin and one strand of regulatory protein Thick filament- consists of staggered array of myosin Sarcomere- the basic contractile unit of a muscle
The sarcomere Z line- border of the sarcomere (striation visible in a light microscope) Thin filaments- attached to the Z line Thick filaments- center of the sarcomere I band- area where there are only thin filaments A band- shows the entire length of the thick filament H zone- area that contains thick filament only M line- the center of the thick filament
The sliding-filament model Contraction of a muscle is a result of the shortening of a sarcomere The distance of two Z lines decreases A bands do not change in length I bands shortens and H zone disappears
The sliding-filament model Neither the thin nor the thick filaments shorten The filaments slide past each other increasing the degree of overlap Only the distance occupied specifically by both filaments disappears/shortens (I band, H zone)
The sliding-filament model Based on the interaction of the myosin head in the thick filament and actin molecules in the thin filament Binding and hydrolysis of ATP is responsible for the change in shape of the myosin molecule The myosin molecule goes into an energized stage Formation of cross-bridge between the myosin and actin Bond is broken when new ATP binds to myosin
Energy for contraction Stored ATP in muscle cell Glycogen that can be degraded and form ATP Phosphagens- supplies a phosphate to ADP to turn it into ATP Vertebrate phosphagen- Creatine phosphate
Control of muscle contraction Contraction is due to stimulation from a motor neuron At rest- tropomyosin blocks binding site in actin molecules Troponin complex- controls position of tropomyosin Calcium ions- binds to troponin ◦ Releases the active site of actin ◦ High calcium in cytosol- contraction can occur ◦ Low calcium in cytosol- contraction cannot occur
The Sarcoplasmic Reticulum Specialized endoplasmic reticulum Controls calcium ion concentration Membrane- actively transports calcium ions ◦ Cytosol into the interior of the reticulum
Action potential Stimulus of contraction- action potential from a motor neuron Motor neuron- a type of neuron that is connected to a motor cell (as oppose to sensory) Action potential is due to a change in the net charge of the surrounding muscle cell
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