GenEd: Lesson 14 Muscular System | Free LET Reviewer and Drill

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General Education

 Lesson 14: Muscular System

(Human Systems)   

What is the Muscular System?

The muscular system is a collection of muscles throughout your body that work to create movement and move substances. Muscles are tissues made up of specialized cells called muscle fibers that can contract (shorten) and relax (lengthen). When muscles contract, they pull on bones or other structures, creating movement. The muscular system is responsible for all the movements your body makes, from large movements like walking to tiny movements like blinking. Muscles also move substances through your body, like blood and food.

The muscular system is made of muscles that contract and relax to create all the movements your body makes.

Main Functions of the Muscular System

1. Moves limbs and other parts of the body
2. Moves substances through the body
3. Maintains posture
4. Generates heat
5. Protects organs

How Muscles Work?

Basic Muscle Function

Image from iStock

Muscles work by contracting (becoming shorter and tighter) and then relaxing (returning to normal length).

The process:

• Nerve signal arrives - a motor neuron sends an electrical signal to the muscle.
• Neurotransmitter is released - acetylcholine is released at the neuromuscular junction (where nerve meets muscle).
• Muscle fibers are stimulated - the neurotransmitter binds to receptors on the muscle fiber, causing it to depolarize (become electrically charged).
• Calcium is released - from the sarcoplasmic reticulum (storage area inside the muscle fiber).
• Contraction occurs - calcium allows myosin (thick filaments) to pull actin (thin filaments). This causes the muscle fiber to shorten.
• Muscle contracts - the entire muscle shortens and exerts force.
• Nerve signal stops - when the nerve signal stops, calcium is reabsorbed.
• Muscle relaxes - t
  he muscle fiber lengthens and the muscle returns to its original length.

Muscles contract when nerves tell them to, pulling on bones to create movement. When the signal stops, muscles relax.

Sliding Filament Theory

Image from GeeksforGeeks

Muscles contract through a process called the sliding filament theory:

Thick filaments (made of myosin)

• Have heads that can bind to thin filaments
• Pull thin filaments toward the center
• This sliding action shortens the muscle fiber

Thin filaments (made of actin)

• Slide toward the center when pulled by myosin
• The muscle fiber shortens
• Many muscle fibers shortening together creates the muscle contraction

During muscle contraction, thick and thin filaments slide past each other, making the muscle shorter and tighter.

Three Types of Muscle Tissue

Image from MidlinePlus

The muscular system has three different types of muscle tissue, each with different characteristics and functions.

Type 1: Smooth Muscle

Image from Open Textbook Publishing

Smooth muscle is involuntary muscle tissue found in internal organs and blood vessels. It is called "smooth" because it lacks the striped appearance of skeletal muscle.

Characteristics:

• Involuntary (you cannot control it consciously)
• Non-striated (no visible striations or stripes)
• Spindle-shaped muscle fibers
• Single nucleus per muscle fiber
• Smooth appearance under a microscope
• Contracts slowly but can remain contracted for long periods

Locations where smooth muscle is found:

• Stomach walls
• Small intestine walls
• Large intestine walls
• Blood vessels (arteries and veins)
• Esophagus
• Bladder
• Uterus (in females)
• Eyes (controls pupil size)
• Lungs (controls air passages)

Functions:

• Moves food through the digestive tract through peristalsis (wave-like contractions)
• Constricts blood vessels to control blood pressure
• Helps empty the bladder and bowels
• Controls pupil size in the eyes
• Adjusts air passages in the lungs

Smooth muscle is controlled by the autonomic nervous system (the part of your nervous system that works without your conscious control). You cannot make your stomach contract by thinking about it. It happens automatically.

Speed of contraction:

• Slow to contract
• Can remain contracted for extended periods
• Fatigue-resistant (can work for long times)

Smooth muscle is involuntary muscle found in your organs and blood vessels. It moves food through your digestive system and controls blood vessel size.

Type 2: Cardiac Muscle

Image from Britannica

Cardiac muscle is involuntary muscle tissue found ONLY in the heart. It is specialized to pump blood throughout your body. 

Characteristics:

• Involuntary (you cannot control it consciously)
• Striated (has visible stripes, like skeletal muscle)
• Branched muscle fibers
• Multiple nuclei per muscle fiber
• Intercalated discs connect muscle fibers to each other
• Striped appearance under a microscope

Location:

• ONLY found in the heart
• Comprises the heart wall (myocardium)
• Cannot be found anywhere else in the body

Functions:

• Contracts rhythmically to pump blood
• Right side pumps blood to the lungs
• Left side pumps blood to the body
• Creates the heartbeat
• Works continuously throughout your life without tiring

Cardiac muscle has a built-in electrical system that makes it contract automatically. The sinoatrial node (SA node) in the heart acts as a pacemaker, sending electrical signals that cause the heart to beat. The heart can beat on its own even if it is removed from the body (as long as it is kept alive in a laboratory).

Intercalated discs:

• Special connections between cardiac muscle fibers
• Allow muscle fibers to contract together as a unit
• Transmit electrical signals between muscle fibers
• Contain desmosomes and gap junctions
• Make the heart function as one coordinated pump

Speed of contraction:

• Contracts at a regular rhythm (about 60-100 beats per minute at rest)
• Can increase with exercise or stress
• Automatically adjusts to body's needs

Unique feature - Built-in rhythm: unlike skeletal muscle (which needs a nerve signal to contract), cardiac muscle has its own built-in rhythm. Even without nerve signals, it will continue beating. The autonomic nervous system modulates the rate and strength of contractions but is not required to initiate them.

Cardiac muscle is involuntary muscle found ONLY in the heart. It pumps blood throughout your body continuously without you thinking about it.

Type 3: Skeletal Muscle

Image from Posture Geek

Skeletal muscle is voluntary muscle tissue that connects to bones and allows you to move. It is called "skeletal" because it is attached to the skeleton.

Characteristics:

• Voluntary (you can control it consciously)
• Striated (has visible stripes)
• Long, cylindrical muscle fibers
• Multinucleated (multiple nuclei per muscle fiber)
• Striped appearance under a microscope
• Contracts quickly
• Fatigues more easily than smooth muscle

Locations where skeletal muscle is found:

• Limbs (arms and legs)
• Chest
• Back
• Neck
• Face
• Abdominal wall
• Diaphragm (for breathing)
• Tongue
• All voluntary movement muscles

Functions:

• Moves limbs and other body parts
• Maintains posture
• Generates heat through shivering
• Produces movement in response to conscious decision
• Moves bones at joints

How skeletal muscle is organized:

• Muscle fiber: Individual contractile cell. Very long (can be several inches). Contains myofibrils and mitochondria.
• Myofibril: Structure inside the muscle fiber. Contains sarcomeres arranged in a line.
• Sarcomere: The basic contractile unit of muscle. Extends from Z-disc to Z-disc. Contains thick filaments (myosin) and thin filaments (actin). Shortens when the muscle contracts.

Skeletal muscle is voluntary muscle attached to bones that you control consciously. It allows you to move your limbs and body.

Speed of contraction:

• Contracts quickly (milliseconds to seconds)
• Can produce rapid movements
• Fatigues with repeated use
• Recovers with rest and nutrition

Skeletal Muscle Structure

Image from SEER Training Modules

Skeletal muscles have a complex structure that allows for efficient contraction:

1. Muscle: Made up of many muscle fibers. Surrounded by fascia (connective tissue). Connected to bones by tendons.
2. Muscle Fiber (Myofiber): Individual contractile cell. Contains sarcomeres. Contracts when stimulated by nerve signal.
3. Myofibril: Structure within muscle fiber. Contains sarcomeres in series.
4. Sarcomere: Basic contractile unit of muscle. Contains thick filaments (myosin) and thin filaments (actin). Z-discs mark the boundaries.
5. Thick Filament (Myosin): Made of myosin protein. Has heads that bind to thin filaments. Pulls thin filaments causing contraction.
6. Thin Filament (Actin): Made of actin protein. Slides toward center when pulled by myosin. Movement creates muscle contraction.

Skeletal muscles are attached to bones all over your body. Contracting these muscles moves your bones in different directions, creating all your voluntary movements.

Muscle Contraction Types

Image from Clientel3

Muscles can contract in different ways:

1. Isotonic Contraction: Muscle changes length. Movement occurs. Examples: lifting a weight, running, throwing.
2. Isometric Contraction: Muscle changes tension but not length. No movement occurs. Examples: pushing against a wall, holding a plank position. Useful for building strength and endurance.
3. Eccentric Contraction: Muscle lengthens while contracting. Resists movement against resistance. Examples: lowering a heavy weight, walking downstairs. Creates the most tension and can cause muscle soreness.

Muscles can contract in different ways, some move bones, some resist movement, and some change tension without moving.

Muscle Fiber Types in Skeletal Muscle

Image from Chicago Athletic Club

Skeletal muscles contain two main types of muscle fibers:

Type 1: Slow-Twitch (Red) Fibers

• Contract slowly. Produce less force. Very fatigue-resistant. Rich in myoglobin (oxygen-carrying protein) making them red. High in mitochondria for energy production. Aerobic metabolism (uses oxygen).
• Maintain posture. Perform endurance activities. Long-distance running. Sustained activities.

Type 2: Fast-Twitch (White) Fibers

• Contract quickly. Produce more force. Fatigue more easily. Lower in myoglobin making them white/pale. Lower in mitochondria. Anaerobic metabolism (works without oxygen).
• Produce rapid, powerful movements. Sprinting. Weight lifting. Quick movements.

Most muscles contain a mix of both fiber types, giving them varied abilities.

Slow-twitch fibers are good for endurance and posture. Fast-twitch fibers are good for power and speed.

How Muscles Move Substances Through the Body

Image from Innerbody Research

Muscles do not just move limbs. They also move substances through your body.

1. Digestive Movement

• Peristalsis: Wave-like contractions of smooth muscle in the digestive tract. Pushes food from mouth to stomach to small intestine to large intestine. Propels food even against gravity. You do not control this is involuntary.
• Mechanical digestion: Muscles of the jaw chew food. Muscles in the stomach churn food. These are voluntary and involuntary.

2. Circulation

• Heart pumping: Cardiac muscle contracts to pump blood. Left ventricle pumps blood to the body. Right ventricle pumps blood to the lungs.
• Skeletal muscle pump: When you exercise, skeletal muscles contract. This squeezes veins and helps blood return to the heart. Prevents blood from pooling in legs.

3. Respiratory Movement

• Diaphragm: Large skeletal muscle below the lungs. Contracts to pull air into the lungs during inspiration. Relaxes during expiration.
• Intercostal muscles: Between the ribs. Help diaphragm with breathing.

4. Urinary and Fecal Elimination

• Bladder contraction: Smooth muscle in bladder wall contracts. Pushes urine through the urethra. Controlled by involuntary smooth muscle and voluntary skeletal muscle.
• Bowel movement: Smooth muscle in intestines contracts through peristalsis. Moves feces to the rectum. Voluntary skeletal muscle (external anal sphincter) allows you to control defecation.

5. Lymphatic Movement

• Muscle contraction: Skeletal muscles compress the lymphatic vessels. Pushes lymph (clear fluid) through the lymphatic system. Helps immune function.

Muscles move more than just limbs, they push food through your digestive system, pump blood, help you breathe, and eliminate waste.

Muscle Pairs and Antagonistic Movements

Image from Parallel Coaching

Muscles usually work in pairs to create opposite movements:

• Agonist (Prime Mover): The muscle that contracts to produce movement. Creates the desired action.
• Antagonist: The muscle that relaxes. Does the opposite action. Must relax for the agonist to move the joint.
• Synergist: Muscles that assist the agonist. Help produce the desired movement.
• Stabilizers: Muscles that keep joints stable. Prevent unwanted movement.

Example: Bending Your Elbow

• Biceps (agonist) contracts and bends the elbow
• Triceps (antagonist) relaxes
• The biceps is the prime mover
• The triceps must relax for this movement to occur

Example: Straightening Your Elbow

• Triceps (agonist) contracts and straightens the elbow
• Biceps (antagonist) relaxes
• The roles reverse

Muscles work in pairs, one contracts to create movement while its partner relaxes. They must work together for smooth, coordinated movement.

Muscle Fatigue and Recovery

Image From Nonstop Neuron

Muscle fatigue is the decline in the ability of a muscle to produce force after repeated contractions.

Causes of muscle fatigue:

• Depletion of ATP (energy molecule)
• Accumulation of lactic acid
• Depletion of glycogen (stored glucose)
• Accumulation of ions (potassium, sodium)
• Accumulation of metabolic waste products
• Nerve fatigue (the motor neuron stops firing)
• Central fatigue (the brain stops sending signals)

Recovery from muscle fatigue:

• Rest allows muscles to replenish ATP
• Glycogen is resynthesized from glucose
• Lactic acid is cleared from the muscle
• Metabolic waste products are removed
• Sleep helps muscle recovery

Avoiding muscle fatigue:

• Proper nutrition (carbohydrates for glycogen, protein for muscle repair)
• Adequate rest between exercise sessions
• Gradual increase in exercise intensity
• Proper hydration
• Stretching and cool-down after exercise

Muscle fatigue happens when muscles run out of energy and accumulate waste products. Rest and nutrition help your muscles recover.

Muscle Disorders and Injuries

Image from Ergohealth

• Muscle strains: Overstretching or tearing of muscle fibers. Caused by overexertion. Results in pain, swelling, and reduced function.
• Muscle cramps: Involuntary, painful contraction of a muscle. Caused by dehydration, electrolyte imbalance, or overuse. Can be relieved by stretching and hydration.
• Muscular dystrophy: Genetic disorder causing progressive muscle weakness. Muscle fibers gradually degenerate. Various types affecting different muscles.
• Myositis: Inflammation of muscle tissue. Caused by infection, injury, or autoimmune disease. Results in pain and weakness.
• Atrophy: Decrease in muscle size due to disuse. Happens when muscles are not used. Can be reversed with exercise.
• Hypertrophy: Increase in muscle size due to exercise. Muscle fibers increase in size. Common result of strength training.

 

Muscular System Quiz: click here

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