Skeletal Muscle Functions and Characteristics

Functions of Skeletal Muscle

• Movement occurs as skeletal muscles move bones and contribute to facial expressions, speaking, breathing, and swallowing
• Posture is maintained by skeletal muscles which stabilize joints to keep you standing or sitting upright
• Protection & Support is provided as skeletal muscles hold internal organs in place
• Regulation of Materials happens as skeletal muscles control what enters and exits the body, such as bladder control of urine
• Heat Production occurs as skeletal muscles help maintain body temperature

Characteristics of Skeletal Muscle

• Excitability allows skeletal muscles to respond to signals from the nervous system
• Conductivity allows skeletal muscles to send electrical signals along the muscle
• Contractility allows muscle fibers to slide past each other to contract (shorten)
• Extensibility allows skeletal muscles to stretch without damage
• Elasticity allows skeletal muscles to return to its normal shape after stretching

Gross Anatomy of Skeletal Muscle

• A skeletal muscle is an organ made up of different types of tissue
• Muscle fibers are grouped into fascicles
• A whole muscle consists of many fascicles
• A fascicle contains many muscle fibers
• A muscle fiber is another name for a muscle cell

Connective Tissue Layers of Muscle

• Muscles possess three layers of connective tissue
• The Epimysium surrounds the whole muscle
• The Perimysium surrounds each fascicle and contains blood vessels and nerves
• The Endomysium surrounds each muscle fiber (cell) and helps with electrical signals and support

Skeletal Muscle Structure

• Muscle fibers are grouped into fascicles and surrounded by connective tissue layers
• Muscles are organized from large to small parts: Muscle → Fascicle → Muscle Fiber → Myofibril → Myofilaments (actin & myosin)

Muscle Attachments

• Muscles attach to bones, skin, or other muscles in two ways
• Tendons are strong, cord-like structures that connect muscle to bone
• Aponeuroses are thin, flat sheets of connective tissue, such as in the abdomen
• Deep fascia surrounds and separates muscles
• Superficial fascia separates muscles from the skin

Blood Vessels and Nerves in Muscle

• Muscles require oxygen and nutrients, therefore they have many blood vessels
• Nerves control muscle contractions
• Somatic motor neurons send signals from the brain/spinal cord to muscles
• The axon (nerve fiber) branches and connects to muscle fibers at a neuromuscular junction
• Skeletal muscle is voluntary, meaning you control it

Microscopic Anatomy of a Muscle Fiber

• Muscle fibers have special structures
• Sarcoplasm is the cytoplasm inside the cell
• Sarcolemma is the plasma membrane (outside layer) of the muscle cell
• Muscle cells are long with multiple nuclei formed from fused cells
• T-tubules are tubes that go deep into the muscle fiber, helping electrical signals spread

Sarcolemma and T-Tubules

• The sarcolemma has voltage-gated ion channels, which means electrical signals travel along it
• T-tubules allow the signal to go deep into the muscle, telling it to contract
• The sarcoplasmic reticulum (SR) stores calcium needed for muscle contraction

Myofibrils and Myofilaments

• Myofibrils are long, thread-like structures filled with proteins that allow contraction and are located inside each muscle cell
• Myofibrils are made of myofilaments
• Thick filaments are made of myosin which has heads that pull actin
• Thin filaments containing actin, binding sites for myosin, tropomyosin, which blocks binding sites, and troponin, which moves tropomyosin when calcium is present

Sarcomere – The Functional Unit of Muscle

• Sarcomeres are the basic units of muscle contraction and are made of repeating thick and thin filaments
• The Z-disc marks the boundary of a sarcomere
• The I-band contains only thin filaments (actin)
• The A-band contains thick filaments (myosin) and some overlapping thin filaments
• The H-zone is the center of the A-band and contains only thick filaments
• The M-line is the middle of the sarcomere and holds myosin in place

Myofilaments – Thick and Thin Filaments

• Thick filaments consist of myosin proteins with heads that grab onto actin
• Thin filaments consist of actin, plus tropomyosin, which blocks binding sites, and troponin, which moves tropomyosin when calcium binds
• These filaments slide past each other during contraction

Sarcomere Bands and Zones

• I-band is a light area with only thin filaments (actin) and gets smaller during contraction
• A-band is a dark area with thick filaments (myosin) and overlapping thin filaments which stays the same size
• H-zone is the middle of the A-band that contains only thick filaments which disappears when muscle fully contracts
• Z-discs are borders of the sarcomere, and move closer during contraction

Proteins That Support the Sarcomere

• Connectin (Titin) is a stretchy protein that helps the muscle return to its normal length
• Dystrophin connects myofibrils to the sarcolemma
• Defective dystrophin causes muscular dystrophy, which weakens muscles

Clinical View – Muscular Dystrophy

• Duchenne Muscular Dystrophy (DMD) is the most common type
• Duchenne Muscular Dystrophy is caused by a genetic mutation in the dystrophin protein
• Symptoms of Duchenne Muscular Dystrophy include muscle weakness, trouble walking, and posture issues
• Most patients with Duchenne Muscular Dystrophy don't live past 30 years old

Energy Production in Muscle Cells

• Mitochondria produces ATP for energy
• Myoglobin stores oxygen in muscle cells for ATP production
• Glycogen stores sugar for energy
• Creatine Phosphate (CP) quickly regenerates ATP when needed

Motor Units – Controlling Muscle Contraction

• A motor unit consists of one motor neuron and all the muscle fibers it controls
• Small motor units control fine movements, like fingers and eyes
• Large motor units control powerful movements, like legs and back

Neuromuscular Junction

• The NMJ is where a neuron sends a signal to a muscle
• Parts of the NMJ include the synaptic knob that releases ACh, the synaptic cleft, and the motor end plate that receives the signal
• The synaptic knob is the end of the neuron and releases ACh (a neurotransmitter)
• The synaptic cleft is the gap between the neuron and muscle
• The motor end plate is the part of the muscle fiber that receives the signal

How the Nerve Signal Triggers Muscle Contraction

• A nerve signal arrives at the synaptic knob
• Calcium (Ca2+) enters, making vesicles release acetylcholine (ACh)
• ACh binds to receptors on the muscle, opening ion channels
• Sodium (Na⁺) enters the muscle, starting an electrical signal
• The electrical signal travels down T-tubules and causes the sarcoplasmic reticulum to release calcium

Resting Muscle Fibers – The Starting Point

• Muscle fibers have a resting membrane potential (RMP) of about -90mV
• Sodium (Na+) is outside the cell, and potassium (K+) is inside
• The muscle waits for a signal to contract

Excitation – Getting the Signal to Contract

• A nerve signal arrives at the synaptic knob
• Calcium (Ca2+) enters, triggering ACh release into the synaptic cleft
• ACh binds to muscle receptors, opening sodium (Na+) channels
• Sodium rushes in, making the inside of the cell less negative (depolarization)

Action Potential – The Signal Spreads

• The change in charge causes voltage-gated Na⁺ channels to open
• An electrical signal spreads along the sarcolemma & down T-tubules
• Then potassium (K+) leaves the cell, restoring -90mV (repolarization)

Calcium Release – The Key to Contraction

• The signal makes the sarcoplasmic reticulum release calcium (Ca2+)
• Calcium binds to troponin, which moves tropomyosin and exposes actin's binding sites

Crossbridge Cycling – The Muscle Contracts

• Crossbridge Formation involves myosin heads attaching to actin
• The Power Stroke involves myosin pulling actin toward the center, which shortens the sarcomere
• Myosin Head Release involves ATP binding to myosin, which makes it let go of actin
• Reset occurs as ATP is split into ADP + Pi, which re-cocks the myosin head

Muscle Relaxation – Stopping the Contraction

• The nerve signal stops, so ACh is broken down by acetylcholinesterase (AChE)
• Calcium is pumped back into the SR
• Troponin moves tropomyosin back, blocking actin's binding sites
• The muscle relaxes and returns to its resting length

Energy Sources for Muscle Contraction

• ATP runs out in 5 seconds
• Muscles use three ways to produce more ATP
• Creatine Phosphate (CP) System is the fastest, lasting 10-15 seconds
• Glycolysis (Anaerobic) requires no oxygen and makes 2 ATP from glucose
• Aerobic Respiration requires oxygen and makes 30 ATP per glucose

Oxygen Debt – Why You Breathe Hard After Exercise

• Extra oxygen replenishes ATP, creatine phosphate, and glycogen
• Oxygen is needed to convert lactic acid back to glucose

Muscle Fiber Types

• Slow Oxidative (Type I) are small endurance muscles, such as needed for posture and long-distance running
• Fast Oxidative (Type IIa) are medium-power muscles, such as needed for walking and sprinting
• Fast Glycolytic (Type IIx) are large, powerful muscles such as needed for weightlifting and jumping

Muscle Twitch – A Single Muscle Contraction

• The Latent Period is the time before contraction starts
• The Contraction Period occurs as myosin pulls actin, and tension increases
• The Relaxation Period happens as calcium is pumped back, and muscle relaxes

Motor Unit Recruitment – More Strength = More Fibers

• Light activity involves small motor units activated
• Heavy activity involves large motor units recruited
• Maximum contraction equals all motor units activated

Wave Summation – More Signals = Stronger Contraction

• If signals come too fast, the muscle doesn't fully relax between contractions
• Incomplete tetany occurs as muscle twitches build up
• Complete tetany involves no relaxation at all, also known as sustained contraction

Muscle Tone – Muscles Never Fully Relax

• Even at rest, muscles stay slightly contracted for posture and readiness
• Muscle tone stops during deep sleep

Isometric vs. Isotonic Contractions

• Isometric contraction occurs as the muscle contracts but doesn't move.
• Isotonic contraction occurs as the muscle contracts and moves
  • Concentric contraction occurs when the muscle shortens, such as lifting a weight
  • Eccentric contraction occurs when the muscle lengthens, such as lowering a weight

Muscle Fatigue

• Caused by a lack of glycogen (energy storage)
• Caused by there not being enough calcium at the NMJ
• Caused by Ion imbalances that affect electrical signals

Effects of Exercise on Muscles

• Endurance Training results in more mitochondria & capillaries and better ATP production
• Strength Training results in muscles that grow, known as hypertrophy, by adding proteins
• No Exercise results in muscles that shrink, known as atrophy

Aging and Muscle Loss

• After age 30, muscles weaken and shrink due to less activity
• There is slower recovery from injuries
• Muscle is replaced by fat and connective tissue (fibrosis)

Cardiac Muscle

• Cardiac muscle is striated & involuntary
• Cardiac muscle possesses one or two nuclei per cell
• Cardiac muscle is connected by intercalated discs for coordinated contractions
• Self- exciting pacemaker controls contraction

Smooth Muscle

• Smooth muscle has no striations and is involuntary
• Smooth muscle is found in blood vessels, intestines, bladder, uterus, etc
• It contracts slowly but for long periods

How Smooth Muscle Works

• Smooth Muscles use calmodulin instead of troponin
• Latchbridge mechanism, which is can stay contracted without using much ATP

Single-Unit vs. Multi-Unit Smooth Muscle

• Single-unit contracts as a group, such as in digestive organs and blood vessels
• Multi-unit contracts independently, such as in eyes, large arteries, and hair muscles

Muscle Relaxation – Stopping Contraction

• The nerve signal stops, which means no more ACh is released
• ACh is broken down by acetylcholinesterase (AChE)
• Calcium is pumped back into the sarcoplasmic reticulum
• Tropomyosin moves back, blocking actin’s binding sites
• The muscle returns to resting state

ATP Production – How Muscles Get Energy

• Muscles get energy through the following ways:
  • Creatine Phosphate (CP) provides quick energy for 10-15 seconds
  • Glycolysis (Anaerobic) which needs no oxygen, makes 2 ATP per glucose
  • Aerobic Respiration uses oxygen, and makes 30 ATP per glucose

Lactate Formation

• If no oxygen is available, pyruvate turns into lactate (lactic acid)
• Lactate can be converted back to glucose in the liver
• This cycle is called the Cori cycle

Muscle Fiber Types

• Slow Oxidative (Type I) muscles are small endurance muscles (long distanced running)
• Fast Oxidative (Type IIa) are medium and quick but oxygen-using muscles
• Fast Glycolytic (Type IIx) are large, powerful, fast-fatiguing muscles (weightlifting)

Muscle fatigue causes

• Low glycogen, or fuel for the muscle
• Low calcium at NMJ
• Ion imbalances

Final review topics

• Muscle Structure which includes the Epimysium, Perimysium, and Endomysium
• Motor Units and Neuromuscular Junctions
• Muscle Contraction regarding Excitation, Calcium, and Crossbridge Cycle