Muscle Contraction & NeuroMuscular Junction

• The study guide covers the muscular system, including types of muscle tissue, muscle tissue characteristics, skeletal muscle structure, neuromuscular junctions, sarcomere structure, myofilament structure, cross-bridge cycling, muscle fiber types, and respiration types.

Types of Muscle Tissue

• There are three types of muscle tissue: skeletal, cardiac, and smooth.
• Skeletal muscle is voluntary, cardiac and smooth are mainly involuntary.

Skeletal Muscle

• Cells are multinucleated with peripheral nuclei.
• It has voluntary control.
• Striated.
• Example: Biceps brachii
• Found attached to bones and sometimes skin.

Cardiac Muscle

• Cells branch and interconnect
• Cells are striated
• Completely involuntary control.
• Cells have central nuclei.
• Found in the heart.
• It is responsible for pumping blood through blood vessels.

Smooth Muscle

• Cells have central nuclei.
• Completely involuntary control.
• Found in intestines and stomach, urinary bladder, uterus, and vas deferens.
• Causes blushing and digestion.
• Slow contractions are called peristalsis.

Characteristics of Muscle Tissue

• Excitability is a muscle's ability to respond to stimuli, like an action potential from a motor neuron.
• Contractility is a muscle's ability to shorten.
• Extensibility is a muscle's ability to be stretched without tearing.
• Elasticity is a muscle's ability to return to its original shape after contracting or relaxing.
• Hamstrings responding to a brain message is an example of excitability.
• Triceps shortening when lifting a weight is an example of contractility.
• Stretching before a race and muscles not tearing is extensibility.
• A gymnast's muscles returning to their original shape after doing a split is elasticity.
• Intestines undergoing peristalsis is an example of contractility.
• The heart contracting and returning to its resting state every 0.7 seconds is elasticity.

Structure of Skeletal Muscle

• A muscle cell is also known as a muscle fiber.
• Tendons connect muscle tissue to a bone.
• A muscle is made of fascicles.
• A fascicle is made of muscle fibers.
• A muscle fiber is made of myofibrils.
• A myofibril is made of sarcomeres.
• A sarcomere is made of myofilaments.
• The largest to smallest: fascicle, muscle fiber, myofibril, sarcomere, myofilament.
• Actin and myosin are the two myofilaments.
• Myosin is thick; actin is thin.
• Actin and myosin together make up a sarcomere.
• A muscle is wrapped in a CT sheath called the epimysium.
• A fascicle is wrapped in a CT sheath called the perimysium.
• A muscle fiber is wrapped in a CT sheath called the endomysium.
• The sarcomere is the smallest part that contracts.

Neuromuscular Junction

• Each muscle fiber contains myofibrils.
• Muscle fibers contain organelles.
• The sarcolemma is the cell membrane of a muscle fiber.
• Receptors receive neurotransmitters.
• T-tubules are parts of the sarcolemma that penetrate deep into the interior of a muscle fiber.
• T-tubules allow action potential to reach the sarcoplasmic reticulum, for stronger contractions.
• The sarcoplasmic reticulum regulates calcium ion levels.
• When an electrical impulse reaches the sarcoplasmic reticulum, it releases calcium ions into the muscle fiber, which causes the muscle fiber to contract.
• Without an electrical impulse, the sarcoplasmic reticulum does not release calcium ions, so the muscle fiber relaxes.
• Action potential is another word for the electrical message that passes through neurons and to muscle fibers.
• Muscles respond due to excitability.
• The neuromuscular junction is where a motor neuron and a skeletal muscle fiber come together.
• Motor neurons carry action potentials from the brain to skeletal muscle fibers.
• The axon terminal is the part of a neuron that meets with a skeletal muscle fiber.
• A motor neuron and muscle fiber do not directly touch, which is false.
• A synaptic cleft is the space between the motor neuron and muscle fiber.
• Axon terminals release chemical messages to bridge the synaptic cleft, known as neurotransmitters.
• Acetylcholine is the specific neurotransmitter released by the axon terminal.
• Acetylcholine triggers a skeletal muscle fiber to contract.

Neuromuscular Junction Process

• The brain decides to contract a skeletal muscle fiber.
• Sends an action potential through motor neurons.
• Action potential travels down the spinal cord to the skeletal muscle.
• Reaches the skeletal muscle fiber at the neuromuscular junction.
• The axon terminal releases acetylcholine , into the synaptic cleft.
• Acetylcholine binds to receptors on the sarcolemma.
• This causes the action potential to travel along and down the T-tubules.
• Stimulates the sarcoplasmic reticulum to release calcium ions, triggering muscle fiber contraction.

Structure of a Sarcomere

• Sarcomere is the smallest unit of a muscle to contract.
• Actin is the thin myofilament.
• Myosin is the thick myofilament, containing "hands."
• Actin looks like two strands of pearls and is attached to the Z line.
• The H zone contains only thick filaments.
• The I band contains only thin filaments.
• The Z line is the boundary between sarcomeres.
• The A band contains both actin and myosin and runs the length of myosin.
• The M line is down the middle of myosin.
• When a sarcomere contracts, actin and myosin stay the same length.
• Calcium ions are needed for myosin to grab and pull actin.
• The I band shortens and disappears during contraction.
• The Z lines get closer together during contraction.
• The A band does not change during contraction.
• The H zone shortens and potentially disappears during contraction.
• During sarcomere contraction, myosin moves closer to the Z line.

Structure of Myofilaments

• Actin and myosin are the two myofilaments.
• Together, myofilaments make up a sarcomere.
• Myofilaments themselves do not shorten during contraction.
• Myosin has "hands" to grab actin.
• Actin is attached to the Z line and looks like two strands of pearls.
• Myosin is powered by ATP.
• Tropomyosin and troponin are attached to the actin myofilament.
• Tropomyosin is rod-like and runs the length of actin, blocking myosin binding sites.
• Troponin has a binding site for calcium ions and three binding sites.
• Troponin stiffens actin and can move the other one.
• When calcium binds to troponin, troponin changes shape and rolls tropomyosin off the myosin binding sites.
• Myosin uses ATP for power to cock up and bind once the myosin binding sites are uncovered.
• With ATP and calcium present, myosin heads bind, forming a cross-bridge for contraction.
• Without ATP and calcium, myosin heads cannot bind, causing relaxation.
• Sarcomere relaxes when calcium is absent, and the myosin-binding sites are covered by tropomyosin.
• Sarcomere relaxes when ATP is absent because myosin heads cannot cock up and attach.

Cross-Bridge Cycling

• First, an action potential occurs at the neuromuscular junction.
• A cross-bridge is where myosin heads and myosin binding sites meet.
• A cross-bridge forms when ATP and calcium ions are present.
• Myosin pulls actin, causing the sarcomere to shorten.
• The steps of cross-bridge cycling are:
• Calcium ions are released by the sarcoplasmic reticulum.
• Calcium ions bind to troponin molecules on actin.
• The binding of calcium causes troponin to change shape.
• Triggering tropomyosin to roll away from the myosin binding sites, exposing them.
• The myosin head cocks, forming a cross-bridge between the myosin head and myosin binding site.
• Myosin pulls actin toward the M-line (power stroke).
• ATP binds to myosin, allowing it to break the cross-bridge.
• The process repeats as long as both calcium ions and ATP are present.

Muscle Fiber Types

• Fast-twitch and slow-twitch are the two types of muscle fibers.
• Not all athletes have equal amounts of both fiber types, which is false.
• The more a fiber type is works, the more dominant it becomes.
• Sprinters use fast-twitch fibers.
• Marathon runners use slow-twitch fibers.

Fast twitch muscle fibers

• Great for powerful bursts
• Contract quickly for short burst of time
• Grow more easily
• Lack myoglobin
• White
• Lack oxygen
• Must use anaerobic respiration
• Fatigues more quickly

Slow-twitch muscle fibers

• Great for endurance
• Contracts slowly for longer periods
• Have myoglobin
• Red
• Has oxygen
• Use aerobic respiration
• Less likely to fatigue

Types of Respiration

• Aerobic respiration is better for endurance.
• Anaerobic respiration is better for powerful bursts.
• Aerobic respiration uses oxygen, while anaerobic respiration occurs without oxygen.
• Aerobic respiration makes more ATP (38 per cycle) than anaerobic respiration (2).
• Anaerobic respiration produces lactic acid causing the burn.
• Fatigue happens quickly when using anaerobic respiration due to a lack of oxygen and ATP; lactic acid also causes fatigue.
• Aerobic respiration is used for a 5-mile walk.
• Anaerobic respiration is used for lifting weights.

Explore muscle contraction, the role of calcium ions in cross-bridge cycling, and the function of myofilaments. Learn how ATP binding affects myosin detachment and the role of acetylcholine at the neuromuscular junction. Also, the process of acetylcholine release and the sarcolemma stimulation.