Muscle Properties, Functions, and Types

Muscle Properties

• Contractility allows muscles to shorten when stimulated
• Excitability is the ability to react to stimuli
• Extensibility means muscles can be stretched without harm
• Elasticity enables muscles to recoil from stretch
• Conductivity allows muscles to spread electrical impulses

Muscle Functions

• Muscles are responsible for movement
• Muscles help in opening and closing passageways
• Muscles provide posture and support
• Muscles generate heat
• Muscles pump blood out of the heart

Types of Muscle Tissue

• Skeletal muscle is attached to bones
• Skeletal muscle enables body movement, maintains posture, and generates heat
• Skeletal muscle is voluntary
• Skeletal muscle is not striated
• Cardiac muscle is located in the wall of the heart
• Cardiac muscle pumps blood throughout the body
• Cardiac muscle is involuntary
• Cardiac muscle is striated
• Smooth muscle is located in the walls of hollow organs
• Smooth muscle controls involuntary actions like digestion and blood vessel constriction
• Smooth muscle is involuntary
• Smooth muscle is not striated

Connective Tissue Coverings

• Epimysium wraps the entire muscle
• Perimysium wraps fascicles (bundles of muscle fibers)
• Endomysium wraps individual muscle fibers (cells)

Muscle Attachments

• Tendons connect muscle to bone
• Aponeurosis is a flat, sheet-like fibrous tissue that attaches muscle to bone or other muscles
• Fascia is dense fibrous tissue surrounding all structures in the body
• Superficial fascia is under the skin, connecting skin to muscle/tissue
• Deep fascia surrounds muscles/organs and provides support/separation
• Visceral fascia encloses hollow organs
• Parietal fascia lines the wall of body cavities, like the abdomen and chest

Muscle as an Organ Composition

• Muscle tissue
• Connective tissue
• Nerves

Muscle Fiber Structure

• Skeletal muscle is composed of muscle fascicles
• Muscle fascicles are composed of muscle fibers (cells)
• Muscle fibers are composed of myofibrils
• Myofibrils are composed of sarcomeres

Myofilaments

• Myofilaments are thick and thin filaments within myofibrils
• Thin filaments are made of ACTIN protein and regulatory troponin and tropomyosin proteins
• Thick filaments are made of MYOSIN protein, which has binding sites for actin and ATP hydrolysis

Other important components of the Muscle Fiber

• Striations are alternating thick and thin filaments
• T-tubules conduct nerve impulses to the sarcoplasmic reticulum (SR)
• Sarcoplasmic reticulum stores and releases calcium

Muscle Contraction Mechanism

• The sliding filament mechanism describes how filaments slide past thick filaments toward the M-line

Muscle Contraction Phases

• Motor neurons send action potentials (APs) to excite muscle fibers
• Thin (actin) filaments slide over thick (myosin) filaments, causing muscle contraction

Excitation

• A motor neuron sends an action potential (AP) to the neuromuscular junction
• Acetylcholine (ACh) is released from the motor neuron into the synapse
• ACh binds to ACh receptors on the muscle fiber
• Sodium (Na+) channels open, Na+ enters, and an AP is generated in the muscle fiber

Excitation-Contraction (EC) Coupling

• The AP in the muscle fiber triggers contraction
• The AP travels down the sarcolemma into T-tubules
• Calcium (Ca+2) is released from the sarcoplasmic reticulum (SR) and enters the sarcoplasm
• Ca+2 binds to troponin, removing the tropomyosin block
• Myosin heads of thick filaments bind to actin of thin filaments to form cross-bridges, initiating contraction

Contraction

• Cross-bridge formation occurs
• The power stroke ("sliding") happens
• Detachment occurs using ATP
• Myosin head resets

Disorders Affecting Muscle Contraction

• Clostridium botulinum prevents ACh release at the neuromuscular junction (NMJ)
• Rigor mortis: ATP production stops after death which prevents myosin from detaching from actin
• Tetanus (lockjaw): Toxin from Clostridium tetani blocks inhibitory signals in the nervous system

How Calcium Blockers Affect Muscle Contraction

• Calcium blockers prevent calcium release from the sarcoplasmic reticulum (SR)
• This stops troponin activation and blocks muscle contraction

Muscle Relaxation

• Relaxation happens in 5 stages
• Action potential (AP) from the motor neuron stops
• The brain stops sending signals, and the motor neuron no longer releases ACh
• Calcium (Ca+2) channels close
• Without AP, the SR closes calcium channels
• Return of Ca+2 to SR with Ca+2 ATPase pumps
• Active transport pumps use ATP to move calcium ions back to the SR
• Tropomyosin blocks myosin binding sites
• As calcium leaves, troponin releases tropomyosin, which shifts back to cover the myosin-binding sites on actin
• Thin filaments slide back, and the muscle relaxes
• Without cross-bridges, the thin filaments slide back to their resting position

Muscle Twitch

• A twitch is a muscle response to one action potential

Neuromuscular Junction

• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber

Motor Unit Activity

• A motor unit consists of a motor neuron and all the muscle fibers it controls
• The number of fibers varies from 2 to 2,000
• Each unit responds independently
• Follows an "all or none" law

Factors Influencing Whole Muscle Tension

• Frequency of stimulation (muscle twitch summation)
• Repeated stimulation before muscle relaxation leads to summation, increasing overall tension
• High frequency can lead to tetanus
• Length of the fiber at the onset of contraction
• Muscle fibers generate the most force at an optimal length
• Motor unit recruitment
• Activating more motor units increases total tension
• The body recruits small units first and then larger ones if needed
• Size of the muscle
• Larger muscles contain more muscle fibers and can generate greater force
• Extent of fatigue
• Fatigue from prolonged activity reduces ATP, calcium availability, and responsiveness

Tetanus from Increased Stimulation Frequency

• Rapid and repeated stimulation prevents full relaxation between twitches
• This causes twitch summation
• High enough frequency results in sustained contraction, which is tetanus

Muscle Fatigue

• Muscle fatigue occurs when muscles can't contract after too much exercise
• Extracellular potassium (K+) build-up
• Too much potassium builds up outside muscle cells
• This blocks calcium (Ca+2) release from SR
• Other reasons for fatigue
• Lactic acid build-up
• Inorganic phosphate build-up
• Central fatigue
• When the CNS no longer adequately activates motor neurons, muscles don't get the message to move

Length-Tension Relationship

• Overly contracted muscles (too short) generate very low tension
• Optimum resting length generates maximum tension
• Overly stretched muscles (too long) generate very low tension

Types of Contraction

• Concentric: muscle shortens while lifting a constant load
• Eccentric: muscle lengthens while controlling a constant load
• Isokinetic: muscle shortens at a constant speed
• Isometric: muscle generates tension but does not change length

Muscle Fiber Types

• Type I: Slow-oxidative
• Type IIa: Fast-oxidative
• Type IIx: Fast-glycolytic
• Whole muscles contain all 3 types of fibers
• All fibers within the same motor unit are the same type
• Recruitment order: slow-oxidative → intermediate (fast-oxidative) → fast-glycolytic