Skeletal Muscle Functions and Characteristics

Functions of Skeletal Muscle

• Skeletal muscles perform five main functions: movement, posture, protection and support, regulation of materials, and heat production.
• Movement includes moving bones, facial expressions, speaking, breathing, and swallowing.
• Posture is maintained by stabilizing joints to keep you upright.
• Protection and support involves holding internal organs in place.
• Regulation of materials controls what enters and exits the body.
• Heat production helps maintain body temperature.

Characteristics of Skeletal Muscle

• Skeletal muscle has five special properties: excitability, conductivity, contractility, extensibility, and elasticity.
• Excitability is the ability to respond to signals from the nervous system.
• Conductivity is the ability to send electrical signals along the muscle.
• Contractility is the ability of muscle fibers to slide past each other to contract (shorten).
• Extensibility is the ability to stretch without damage.
• Elasticity is the ability 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 (bundles).
• 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 have three layers of connective tissue: epimysium, perimysium, and endomysium.
• 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

• 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 and aponeuroses.
• Tendons are strong, cord-like structures that connect muscle to bone.
• Aponeuroses are thin, flat sheets of connective tissue.
• Deep fascia surrounds and separates muscles.
• Superficial fascia separates muscles from the skin.

Blood Vessels and Nerves in Muscle

• Muscles need oxygen and nutrients, so 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 (Cell)

• Muscle fibers (cells) have special structures: sarcoplasm, sarcolemma, multiple nuclei, and T-tubules.
• Sarcoplasm is the cytoplasm (fluid inside the cell).
• Sarcolemma is the plasma membrane (outside layer) of the muscle cell.
• Muscle cells are long and have many nuclei because they form from fused cells.
• T-tubules are tubes that go deep into the muscle fiber and help electrical signals spread.

Sarcolemma and T-Tubules

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

Myofibrils and Myofilaments

• Inside each muscle cell are myofibrils, which are long, thread-like structures filled with proteins that allow contraction.
• Myofibrils are made of myofilaments: thick and thin filaments.
• Thick filaments are made of myosin (has heads that pull actin).
• Thin filaments are made of actin (binding sites for myosin), tropomyosin (blocks binding sites), and troponin (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.
• Important structures in the sarcomere: Z-disc, I-band, A-band, H-zone, and M-line.
• 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 are made of myosin proteins with heads that grab onto actin.
• Thin filaments consist of actin, tropomyosin (blocks binding sites), and troponin (moves tropomyosin when calcium binds).
• These filaments slide past each other during contraction.

Sarcomere Bands and Zones

• The I-band is a light area and contains only thin filaments (actin); it gets smaller during contraction.
• The A-band is a dark area and contains thick filaments (myosin) and overlapping thin filaments; it stays the same size.
• The H-zone is the middle of the A-band and contains only thick filaments; it disappears when the muscle fully contracts.
• Z-discs are the borders of a sarcomere; they 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 (membrane).
• Defective dystrophin causes muscular dystrophy (a disease that weakens muscles).

Clinical View - Muscular Dystrophy (DMD)

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

Energy Production in Muscle Cells

• Mitochondria produce 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 is one motor neuron plus all the muscle fibers it controls.
• Small motor units control fine movements (e.g., fingers, eyes).
• Large motor units control powerful movements (e.g., legs, back).

Neuromuscular Junction (NMJ) - Where Nerves and Muscles Meet

• The NMJ is where a neuron sends a signal to a muscle.
• Parts of the NMJ: synaptic knob, synaptic cleft, and motor end plate.
• The synaptic knob is the end of the neuron that releases ACh (a neurotransmitter).
• The synaptic cleft is the gap between the neuron and muscle.
• The motor end plate is 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 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 is waiting 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, creating an electrical signal that spreads along the sarcolemma and down T-tubules.
• Then potassium (K+) leaves the cell, restoring -90mV (repolarization).

Calcium Release - The Key to Contraction

• The signal causes the sarcoplasmic reticulum (SR) to release calcium (Ca2+).
• Calcium binds to troponin, which moves tropomyosin, exposing actin's binding sites.

Crossbridge Cycling - The Muscle Contracts!

• Crossbridge Formation: Myosin heads attach to actin.
• Power Stroke: Myosin pulls actin toward the center (sarcomere shortens).
• Myosin Head Releases: ATP binds to myosin, making it let go of actin.
• Reset: ATP is split into ADP + Pi, re-cocking 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

• With ATP running out in 5 seconds, muscles use three ways to make more: Creatine Phosphate (CP) System, Glycolysis (Anaerobic), and Aerobic Respiration.
• Creatine Phosphate (CP) System is the fastest and lasts 10-15 seconds.
• Glycolysis (Anaerobic) needs no oxygen and makes 2 ATP from glucose.
• Aerobic Respiration needs oxygen and makes 30 ATP per glucose.

Oxygen Debt - Why You Breathe Hard After Exercise

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

Muscle Fiber Types - Different Kinds of Muscles

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

Muscle Twitch - A Single Muscle Contraction

• Latent Period: Time before contraction starts.
• Contraction Period: Myosin pulls actin, tension increases.
• Relaxation Period: Calcium is pumped back, muscle relaxes.

Motor Unit Recruitment - More Strength = More Fibers

• Light activity (writing) activates small motor units.
• Heavy activity (lifting weights) recruits large motor units.
• Maximum contraction activates all motor units.

Wave Summation - More Signals = Stronger Contraction

• If signals come too fast, the muscle doesn't fully relax between contractions.
• Incomplete tetany means muscle twitches build up.
• Complete tetany means no relaxation at all (sustained contraction).

Muscle Tone - Muscles Never Fully Relax

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

Isometric vs. Isotonic Contractions

• Isometric contractions involve the muscle contracting but not moving (holding a heavy bag).
• Isotonic contractions involve the muscle contracting and moving (lifting a weight).
• Concentric contractions involve the muscle shortening (lifting a weight).
• Eccentric contractions involve the muscle lengthening (lowering a weight).

Muscle Fatigue - Why Muscles Get Tired

• Muscle fatigue is caused by a lack of glycogen (energy storage), not enough calcium at the NMJ, and ion imbalances affecting electrical signals.

Effects of Exercise on Muscles

• Endurance Training leads to more mitochondria & capillaries, which is better ATP production.
• Strength Training leads to muscles growing (hypertrophy) by adding proteins.
• No Exercise leads to muscles shrinking (atrophy).

Aging and Muscle Loss

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

Cardiac Muscle - The Heart's Special Muscle

• Cardiac muscle is striated and involuntary.
• Cardiac muscles have one or two nuclei per cell.
• They are connected by intercalated discs (for coordinated contractions).
• A self-exciting pacemaker controls contraction.

Smooth Muscle - Found in Organs

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

How Smooth Muscle Works

• Uses calmodulin instead of troponin.
• The latchbridge mechanism allows it to stay contracted without using much ATP.

Single-Unit vs. Multi-Unit Smooth Muscle

• Single-unit smooth muscle contracts as a group (digestive organs, blood vessels).
• Multi-unit smooth muscle contracts independently (eyes, large arteries, hair muscles).

Muscle Relaxation - Stopping Contraction

• Nerve signal stops, leading to no more ACh 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 its resting state.

ATP Production - How Muscles Get Energy

• Creatine Phosphate (CP) provides quick energy for 10-15 seconds.
• Glycolysis (Anaerobic) needs no oxygen and makes 2 ATP per glucose.
• Aerobic Respiration uses oxygen and makes 30 ATP per glucose.

Isometric vs. Isotonic Contractions

• Isometric contractions involve no movement, just tension (holding a plank).
• Isotonic contractions involve movement.
• Concentric contractions involve the muscle shortening (lifting a weight).
• Eccentric contractions involve the muscle lengthening (lowering a weight).

Muscle Fatigue - Why Muscles Get Tired

• Low levels of Glycogen
• Low calcium at neuromuscular Junctions
• Ion imbalances

Effects of Exercise

• Endurance exercise means more mitochondria, resulting in better ATP production.
• Strength training means muscles grow (hypertrophy)
• No exersice means muscles shrink (atrophy)

Aging and Muscle Loss

• After age 30 muscles shrink due to inactivity
• Slower healing

Cardiac Muscle

• Striated, involuntary, contracts by itself

Smooth mucle

• No striations, involutary, contracts slowly bur for long periods

Smoth Muscle Function

• Uses clmodulin rather than toponin
• Latchbridge mechanism means they can contract contantly wihout ATP

Single/Multi Unit smoothmuscle

• Single unit contracts as a group
• Mutil unit contracts independently