PA 544 Clinical Anatomy Tony Serino, Ph.D. Muscular System PA 544 Clinical Anatomy Tony Serino, Ph.D.

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

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 skeletal muscle cell must be innervated by a motor neuron to begin contracting.

Neuronal AP triggers release of ACh at neuromuscular junction (motor end plate).

Neuromuscular Junction (Motor End Plate)

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.

Biomechanics of Force Production Tension = force exerted on an object by a muscle Load = force exerted on muscle by the weight of an object Twitch = the mechanical response of a muscle to an AP Types of Contractions: Isometric = muscle increases tension without shortening Isotonic = muscle shortens with no further increase in tension Load Tension Bicep Fulcrum (pivot point) Weight of arm + object

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)

Smooth Muscle Cell

Visceral Smooth Muscle