The Muscular System

The Muscular System

• The muscular system consists of skeletal muscles (voluntary), cardiac muscles (involuntary), and smooth muscles (involuntary).
• All three muscle cell types are excitable and contract in response to chemical and/or electrical signals from other organ systems.
• Muscle cells contract to shorten and relax to lengthen.

Skeletal Muscles

• Skeletal muscles attach to the skeleton via tendons, create or resist all movements, and provide strength and mobility.
• Skeletal muscles represent about 40% of males body weight and 32% of females body weight.
• There are over 600 skeletal muscles in the human body.
• Skeletal Muscles are organized into pairs or groups that work together to perform the same movement.
• The smallest skeletal muscles assist in focusing our eyes.
• The largest skeletal muscles are responsible for shivering when we are cold.

Functions of Skeletal Muscles

• Skeletal muscles produce movement via voluntary control
• Example is picking up an object
• Skeletal muscles resist movement by generating force opposite to the force applied to the body.
• Example is standing
• Skeletal muscles generate heat.
• Contraction of muscles accounts for ¼ of the heat humans generate
• Heat generation is important for maintaining homeostasis of body temperature.
• Example is shivering to produce more heat in the body.

Synergistic Muscle Groups

• Synergistic muscle groups work together to create the same movement.
• The biceps brachii and the brachioradialis muscles work together to cause flexion in the arm.

Antagonistic Muscle Groups

• Antagonistic muscle groups are muscles that oppose one another.
• An example is the biceps brachii and triceps brachii where one flexes and the other extends the arm.

Major Muscles and Their Functions

• Masseter: Closes the jaw.
• Orbicularis oris: Closes lips, and is used for kissing and whistling.
• Pectoralis major: Draws arm forward and toward the body.
• Serratus anterior: Helps raise the arm, contributes to pushing, and draws the shoulder blade forward.
• Biceps brachii: Bends forearm at elbow.
• Rectus abdominus: Compresses abdomen, bends the backbone, and compresses the chest cavity.
• External oblique: Involved in lateral rotation of the trunk and compresses the abdomen.
• Adductor longus: Flexes thigh, rotates thigh laterally, and draws thigh toward the body.
• Sartorius: Bends thigh at hip, bends lower leg at knee, and rotates thigh outward.
• Quadriceps group: Flexes thigh at hips and extends leg at knee.
• Tibialis anterior: Flexes foot toward knee.
• Deltoid: Raises arm.
• Trapezius: Lifts the shoulder blade, braces the shoulder, and draws head back.
• Triceps brachii: Straightens forearm at elbow.
• Latissimus dorsi: Rotates and draws arm backward and toward the body.
• Gluteus maximus: Extends thigh and rotates thigh laterally.
• Hamstring group: Draws thigh backward and bends knee.
• Gastrocnemius: Bends lower leg at knee and bends foot away from knee.
• Achilles tendon: Connects gastrocnemius muscle to heel.

Quadriceps Group

• The quadriceps is comprised of the Rectus femoris, Vastus lateralis, Vastus intermedius, and Vastus medialis
• The Rectus femoris extends the knee and flexes the hip and lies overtop of vastus intermedius
• The Vastus lateralis extends the knee
• The Vastus intermedius extends the knee and is located in between the other two vastus muscles
• The Vastus medialis extends the knee and adducts the thigh

Hamstring Muscles

• The hamstring muscles consists of the Semitendinosus, Semimebranosus and the Biceps femoris
• The three muscles all work together to flex the knee and extend the thigh
• The Iliotibial band also contributes to knee stabilization

Origins and Insertions

• The origin of a muscle is where one end of a skeletal muscle joins to a bone that remains relatively stationary.
• The insertion of a muscle is where the other end of the muscle attaches to another bone across a joint.
• During contraction, the insertion is pulled towards the origin.

Muscle Terminology

• Whole Muscle: A group of individual muscle cells with the same origin, insertion, and function.
• Fascicle: A bundled arrangement of muscle cells/fibers enclosed by a sheath of fibrous connective tissue called "fascia".
• Fascicle: Each fascia contains anywhere from a few dozen to thousands of muscle cells.
• Muscle cells: Many inside each whole muscle, tube-shaped, larger than most cells (range from 1mm - 30cm in length), are multinucleated and packed with myofibrils.

The Myofibril

• Myofibrils are found inside individual muscle cells.
• Myofibrils are long, cylindrical structures arranged in parallel units.
• Myofibrils are packed with contractile proteins, allowing the muscle to contract and shorten.
• Each myofibril has a banded appearance that repeats at regular intervals.
• These regular intervals are known as a Z-line (the dark line)

Sarcomere

• A sarcomere is the segment of myofibril from one Z-line to the next.
• There can be over 100,000 sarcomeres in one single myofibril.
• A sarcomere contains two types of contractile protein filaments being actin and myosin.
• When the sarcomeres shorten, the muscle cell and the whole muscle will shorten (contract).

Actin and Myosin

• Actin filaments are thin and attached to the Z-line.
• Actin filaments come in two strands wound around each other
• Myosin filaments are thick with club-like heads
• Myosin filaments are interspersed at regular intervals between actin filaments.
• The myosin filaments are located entirely within sarcomeres and stretch between two actin filaments.

The Act of Contraction

• During contraction, each sarcomere shortens just a little
• All sarcomeres work in tandem during muscle contraction.
• The major points to remember for muscle contraction are:
• Muscle must be activated by a nerve, and will not contract on its own.
• Nerve activation increases calcium concentration near contractile proteins.
• Calcium is required for contraction to occur
• If no longer stimulated by nerve, the muscle stops contracting.

Nerves and Muscle Contraction

• Skeletal muscle cells are stimulated to contract by motor neurons.
• Motor neurons release acetylcholine (ACH), a neurotransmitter.
• Acetylcholine (ACH) is released at the neuromuscular junction
• ACH binds to receptors causing an electrical impulse to travel along the cell membrane and T tubules.
• The electrical impulse gets delivered deep into the interior of the cell via the T tubules.

Calcium and Muscle Contraction

• Ionic calcium (Ca2+) is stored inside the sarcoplasmic reticulum.
• The sarcoplasmic reticulum is a membrane bound chambers that are in close contact with the T tubules.
• The electrical impulse from the T tubule triggers the release of calcium from the sarcoplasmic reticulum.
• Calcium diffuses into the cell cytoplasm and then comes in contact with the myofibrils
• The myofibrils begin the contracting process by initiating movement in the contractile proteins
• The movement of contractile proteins is a process called the Sliding Filament Mechanism

Sliding Filament Mechanism

• Myosin heads "stand up" and come in contact with actin
• Contact forms "cross bridges" which bend
• Myosin heads bend towards the shaft pulling actin filaments towards the centre of the sarcomere

Preventing Constant Contraction

• Calcium is an important factor for muscle contraction
• Calcium must be present for contraction to occur.
• Two protein molecules (troponin & tropomyosin ) are closely associated with actin filaments.
• Together troponin and tropomyosin form the troponin-tropomyosin protein complex.
• The troponin-tropomyosin complex interferes with cross-bridge formation when calcium is absent.

The Troponin-Tropomyosin Complex

• During contraction, calcium attaches to troponin.
• Calcium pulls the tropomyosin out of the myosin binding sites.
• This allows myosin heads to bind to actin forming the cross-bridge.

Muscle Relaxation

• Nerve activity ends and no electrical impulse is sent, so calcium is no longer released.
• Calcium is transported back into the sarcoplamic reticulum via active transport (requires ATP).
• Troponin-tropomyosin protein complex shifts back to block actin as calcium concentrations decrease.
• Myosin returns to its original position and cross-bridges disappear.
• Sarcomere stretches passively to its resting length, and the muscle cell relaxes.

Muscle Contraction Requires Energy

• Muscles require a great deal of energy to contract.
• Many sources provide this required energy.
• ATP is the first source that is used to start a contraction

ATP Usage in Muscle Contraction

• ATP is bound to myosin in a relaxed state
• Nerve stimulation causes ATP to release phosphate and cross-bridge forms.
• ADP leaves myosin and causes myosin to tilt back to normal position.
• Power stroke & sarcomere shortens as a result
• New ATP binds to myosin when stimulation ends.
• Myosin then detaches from actin.

Nerve Impulse End

• When the never impulse ends Calcium release is stopped, and the muscle relaxes
• ATP molecules attach to myosin and cause it to return to relaxed position, breaking all cross-bridges.
• Troponin-tropomyosin shifts back to original position and prevents cross-bridge formation.
• ATP is then used to pump calcium back into sarcoplasmic reticulum so that relaxation of myosin heads can occur.
• Muscles will be in constant contraction if there is no ATP to detach or relax the muscles.

Rigor Mortis

• Rigor Mortis translates to "rigid death" in Latin
• The body becomes stiff in the four hours to several days after death
• Calcium leaks out of sarcoplasmic reticulum
• The muscles contract
• There is no Present ATP so Myosin doesn't detach from muscle
• Muscle becomes "locked" in contracted state
• Eventually the rigidity declines due to cellular degeneration

Energy Sources for Muscles

• Muscles need multiple energy sources made available to be able to act
• Store ATP offers 2–3 seconds of maximal activity
• Stored creatine phosphate
• Stored glycogen
• Aerobic metabolism.

Creatine

• Creatine phosphate is a high energy molecule with phosphate attached
• The phosphate can be quickly be transferred to an ADP to form ATP
• The reaction is reversal and ATP can convert back to into creatine phosphate for short term storage
• Energy + ADP forms ATP
• Creatine Phosphate and ATP can provides enough energy for 10-12 seconds

Glycogen

• Glycogen is a Complex polysaccharide made up of many small molecules of glucose
• When there is demand Glucose molecules are removed from the glycogen and ATP is synthesized to provide extra energy
• Glycogen kicks into gear 10-12 seconds heavy activity (after all stored ATP and creatine phosphate are used)
• Glycogen get approximately 3-5 minutes of sustained activity
• Glycogen is mostly via anaerobic metabolism (without oxygen)
• Glycogen Is fast, but only produces 2 ATP molecules per glucose molecule
• Lactic acid produced which contributes to the "burning" sensation in the muscles

Aerobic Metabolism

• Aerobic metabolism is the most efficient, long-term energy source
• Aerobic metabolism also involves the glucose, fatty acids, and lactic acid
• As well to function it also Requires oxygen and Occurs in the mitochondria.
• With all the factors the results is a high ATP yield and Yields 36 ATP molecules per glucose
• This Metabolism starts when the other ATP sources have been exhausted for approximately 3-5 minutes into exercise

Energy Source Timeline

• ATP offers 2-3 seconds of use in the body
• Creatine phosphate offers 8-10 seconds of use in the body
• Anaerobic offers use in 3–5 minutes to intense exercise in the body
• Aerobic metabolism is always present

The Oxygen Debt

• Continue use of muscles leads to a "oxygen debt"
• As the body is reversing the used resources after exercise.
• Oxygen: used in during activities.
• ATP early: Are "repaying" the amount of oxidation of the lactic acid produced by early anaerobic metabolism
• Replenishing energy storage of ATP & creatine phosphate in muscles occurs Allows muscles to perform at near maximal rates faster

Muscle Activity

• Functions of muscles are to move body parts or maintain certain positions with the help of bones.
• How well we do this depends on these factors like Whether bones move (isotonic)or not (isometric)
• Nerve Stimulation, Type of muscle fiber, , Exercise effects (Fit Planning lecture)

Isotonic Movement

• Isotonic contractions offer "same" + "strength" or "tone" movements Muscle length shortens while bones and objects will move

Isometric Movement

• Isometric offer 'length" movements
• Force is generated, and the muscle tension increases but objects do not move
• Isometric contractions also stabilize skeleton and posture

The Motor Neuron

• Individual muscle cells all work together and are controlled: by a Motor neuron unit
• An individual motor neuron units Is the single nerve cell that controls the movement of a group of muscle cells
• The Motor unit Consists of the motor neuron and all of the muscle cells it controls

Nerve Activation and Muscle Tensions

• Nerve activation has an influence on the amount of mechanical force generated by muscle
• Factors like the Motor unit Size, The # of motor units active, The stimulation of motor units

Motor Unit and Size

• Is relative from Muscle and Motor
• For Thighs/Strength the motor unit varies at 1000's of cells
• For Fine control the motor unit varies at 10 cells
• Larger motor units have more muscle cells, leading to more force
• Smaller motor units have less control over the muscle

Motor unit activation

• Known as the " All Or None Principle." This has the same response and cant be disobeyed
• This will give the response to complete with contraction & relaxation on a twitch
• This muscle will provide muscle tone to the area, know what its doing at its core
• Motor Unit activation - Need the activation of recruiting More Motor Units.

Frequency of Stimulation

• Latent Period happens with the time delay between neural stimulation and contraction
• Contraction: Actin filaments are pulled towards the center of sarcomere and muscle cell will shortens Relaxation: Ca2+is transported back into sarcoplasmic reticulum and the Sarcomere will stretch passively to relaxed state. This will only get completed when the Additional stimulus are transferred back in before the sarcoplasmic reticulum occurs.

Muscle Stimulus (TETANUS OR SUMMATION)

• Summation - Total force generated to be greater than producing twitch action
• Tetanus - Causing maximum contraction in the muscle/ and creating Fatigue. This occur when when stimulation becomes too frequent and the muscle remains in a state of "NO Relaxation".

Muscle Fiber Composition

• Two primary types of skeletal muscle fibers exist being the Slow twitch and fast twitch Distinguished by which quickly ATP is put to use in order to produces Contraction
• Most muscles will have a mixture of those kinds
• Also Ratios Dependant on function in the muscle (inherited influence).

The muscle Twitch

• Slow Twitch - Breakdowns ATP slow and contract slowly with lots of mitochondria

• The ATP provide oxygen for aerobic with and it stored /supplied myoglobin

• Often called the Red muscle with lots endurance

• Fast Twitch - Breakdowns ATP fast for the quick contraction with the help of reliance on anaerobic metabolism

• The ATP has a to Little to nothing supplied/stored

• Often called White muscle with the use of little amount for creatine.