Muscular System PA 481 C Anatomy & Physiology Tony Serino, Ph.D.

Muscular System Functions:  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 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)

ACh is released and diffuses across gap

ACh bind to the nicotinic receptor and triggers a MEPP

The MEPP triggers an AP that races along the sarcolemma 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 AP reaches synaptic terminal triggering an influx of Ca ++ The Ca ++ stimulates the release of ACh ACh diffuses across cleft and binds to nicotinic receptors in motor end plate This causes Na + channels to open; causing the generation of a MEPP The MEPP triggers an AP along sarcolemma and into T-tubules This deplorarizes the SR cisternae which releases stored Ca ++ into the cytoplasm

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

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

H Band M Line Z Line Myofibril Anatomy

Myofibril Structure (cross section) Cross sections: H Band M Line

Overlapping Tick and Thin Filaments

Thick Filament Structure

Thin Filament Structure: Twisted bead chain of actin proteins

Thin Filament: Actin, Tropomyosin and Troponin

Calcium is trigger

Detachment Contraction Events

Detachment Reset: energize myosin head

Detachment Reset Attachment

Detachment Reset Attachment Power Stroke

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

Single Muscle Twitch

Factors Affecting Muscle Fiber Performance  Load –affects velocity of contraction Increasing load  decreases velocity  Frequency of stimulation  Initial Length of muscle fiber  Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

Load Effect on Degree and Duration of Contraction

Load vs. Velocity of Contraction

Factors Affecting Muscle Fiber Performance  Load –affects velocity of contraction Increasing load  decreases velocity  Frequency of stimulation  Initial Length of muscle fiber  Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

Mechanical (Wave) Summation Increase frequency of stimulation allows tension to add to previous contraction’s tension

Factors Affecting Muscle Fiber Performance  Load –affects velocity of contraction Increasing load  decreases velocity  Frequency of stimulation  Initial Length of muscle fiber  Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

Initial Length of Muscle Fiber: affects the maximum tension that can be developed due to degree of overlap between thick and thin filaments

Factors Affecting Muscle Fiber Performance  Load –affects velocity of contraction Increasing load  decreases velocity  Frequency of stimulation  Initial Length of muscle fiber  Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

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

Control of Whole Muscle Tension dependent on: Tension developed by each fiber –Dependent on fiber type, initial length and degree of wave summation Amount of fibers stimulated to contract –The number of motor units responding is directly related to amount of tension produced –If the body needs more power, it recruits more motor units to respond –Known as recruitment (motor unit summation)

Energy Use: stored ATP in muscle used quickly so re-supply is crucial to function 1.Creatine Phosphate –quick re-supply, allowing time for aerobic respiration to gear up 2.Aerobic Respiration –oxidative phosphorylation dependent on adequate blood supply of oxygen, uses different sources for energy: a)Stored glycogen b)Glucose and fatty acids from blood c)Fatty acids from blood 3.Anaerobic Respiration -becomes dominant as need for oxygen exceeds ability of blood to transport it into muscles After exercise, energy continues to be consumed at increased levels to re- build reserves, etc., this is the oxygen debt incurred during the exercise

Fatigue –inability to maintain contraction tension even while being stimulated. Two kinds: Primary Fatigue –due to accumulation of lactic acid in sarcoplasm, this changes the cytoplasm pH and begins to change protein configurations which ends contraction. Secondary Fatigue –related to the loss of energy reserves in the body, as seen in day after soreness. Why this triggers a low intensity pain signal (a dull ache) is unknown.

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