Muscular System Comparative Anatomy Tony Serino, Ph.D. Biology Dept. Misericordia Univ.

Muscular System Functions: Attributes: Movement –generation of force and/or shortening Maintenance of posture Joint stabilization Heat Generation Attributes: contractility, irritability, extensibility, and elasticity

Types of Muscle Cells Skeletal Muscle –voluntary, striated Cardiac Muscle –involuntary, striated Smooth Muscle –involuntary, no striations

Whole Muscle Structure

Muscles wrapped with CT, that is continuous with tendon and periosteum

The elasticity of the CT sheaths, tendon and the muscle cells = the Series Elastic Component

Antagonistic Muscle Arrangement This arrangement plus the series elastic component allows the muscle to return to its original length.

Skeletal Muscle Cells Long, cylindrical, non-branching, multinucleated 10-100 mcm wide and up to 35 cm long Voluntary, no spontaneous depolarization normally Contractile proteins (myosin & actin) arranged in bundles called myofibrils

Develop as a fusion of myoblasts, which accounts for multinucleated cells, extra myoblasts remain as satellite cells.

Unique Muscle Cell Structures Sarcomere

Each myofibril consists of overlapping thick and thin filaments arranged in units called sarcomeres.

Myofibril Structure

Each skeletal muscle cell must be innervated by a motor neuron to begin contracting. A motor neuron plus all the muscle cells it innervates is a motor unit.

Motor End Plate Neuronal AP triggers release of ACh at neuromuscular junction (motor end plate). ACh bind to the nicotinic receptor and triggers a depolarization of sarcolemma which spreads across entire cell and down the T-tubules.

The depolarization affects the SR cisternae which releases Ca++ into the cytoplasm. The rise of intracellular Ca++ triggers the mechanical events of contraction.

Muscle Cell Contraction (Excitation-Contraction Coupling) A motor neuron is stimulated to fire an AP It releases ACh at the motor end plate ACh diffuses across cleft and binds to nicotinic receptors in motor end plate This causes a depolarization of the sarcolemma and T-tubules This deplorarizes the SR cisternae which releases stored Ca++ into the cytoplasm

Muscle Contraction: Mechanical Events (Sliding Filaments) Calcium ions from SR flood the myofibrils This causes the thick and thin filaments to bind each other (generates tension) and may cause them to slide past each other This causes the sarcomere to shorten The H-band shrinks as the filaments slide past each other.

Overlapping Arrangement

Thick Filament Structure

Thin filament

Calcium binding rolls tropomyosin

Accessory Proteins Titin –large elastic protein, forming elastic lattice that attach to z-line and align thick filaments with respect to thin filaments α-actinin –anchoring protein of z-line Nebulin –helps to anchor thin filaments to z-line Tropomodulin –caps thin filament and regulates filament length Desmin- (not shown) form lattice that surrounds sarcomere at level of z-lines attaching them to other myofibrils and sarcolemma Myomesin and C-protein -attaches and aligns individual myosin molecules to one another Dystrophin –is a cell membrane associated protein which helps to connect the cytoskeleton to the extracellular matrix

Detachment

Detachment Reset: energize myosin head

Detachment Reset Attachment

Detachment Reset Power Stroke Attachment

Muscle Contraction: Review

Muscles are arranged as Motor Units Motor Unit = 1 motor neuron + all the muscle fibers it controls (innervates) The size of the motor unit depends on the degree of control needed in that particular whole muscle.

Types of Muscle Fiber: each motor unit consists of only one type of muscle fiber Slow twitch, red (oxidative) fibers (SO) –small diameter, weakest, slow ATPase, much myoglobin and mitochondria, abundant blood supply, fatigue resistant Fast twitch, red (oxidative) fibers (FO) –medium diameter, moderate strength, fast ATPase, abundant mitochondria and myoglobin, good blood supply, moderate fatigue resistance Fast twitch, white (glycolytic) fibers (FG) –largest diameter, great strength, fast ATPase, low amount of myoglobin or mitochondria, decreased blood supply, high in glycolytic enzymes, tire quickly

Cardiac Muscle Striated, single nucleus, branched cells, connected together by intercalated discs (with many gap junctions) Spontaneously contracts, needs no innervation, involuntary

Smooth Muscle No sarcomeres, therefore, no striations, single nucleated, small spindle shaped cells Spontaneously contracts, involuntary control, can remain contracted for long periods of time without fatiguing Two types: Visceral (single unit) –united by gap junctions Multi-unit –needs innervations, behaves like skeletal muscle (Ex. Iris)

Electric organs Electrocytes are modified skeletal muscle cells arraned in columns. Each column of cells are connected in series thus multiplying the voltage created, some eels can generate more than 600V.