Phys Lab M7 Muscle Contraction

Neuromuscular Junction (NMJ)

• Interface between the nervous system and the muscular system

• Where motor neurons transmit signals to muscle fibers to initiate muscle contraction

NMJ 3 main components

1. 1. Axon terminal - where nerve impulse arrives

2. 2. Synaptic cleft - gap between neuron and muscle fiber

3. 3. Motor end plate - specialized region of muscle fiber membrane (sarcolemma) that contains receptors for neurotransmitters

Process of signal transmission at the NMJ

1. 1. Action potential in the motor neuron

2. 2. Calcium influx at the axon terminal

3. 3. Release of Acetylcholine (ACh)

4. 4. Binding of Acetylcholine to receptors

5. 5. Generation of an End Plate Potential (EPP)

6. 6. Action potential in the muscle fiber

7. 7. Acetylcholine breakdown

8. 8. Calcium release from the Sarcoplasmic Reticulum

Muscle Contraction

Allows muscles to generate force and produce movement

Driven by protein filaments: Actin & Myosin → sliding filament theory

Process of muscle contraction

Describe the sequence of events leading from motor neuron stimulation to muscle contraction.

• Acetylcholine (ACh) is released at the neuromuscular junction and binds to receptors on the muscle fiber membrane.

• This causes depolarization and generates an action potential.

• The action potential travels along the membrane and into the T-tubules.

• The action potential triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum.

• Ca²⁺ binds to troponin, causing tropomyosin to move and expose binding sites on actin.

• Energized myosin heads (powered by ATP hydrolysis) bind to actin, forming cross-bridges.

• The myosin heads perform a power stroke, pulling actin toward the center of the sarcomere, shortening it and causing muscle contraction.

How does the cross-bridge cycle continue and how does muscle relaxation occur?

• After the power stroke, a new ATP molecule binds to the myosin head, causing it to release actin.

• ATP is hydrolyzed, recharging the myosin head for another cross-bridge cycle.

• This cycle repeats as long as Ca²⁺ levels remain elevated and ATP is available.

• When the action potential stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum.

• As Ca²⁺ concentration decreases, tropomyosin re-blocks the actin-binding sites, and the muscle fiber relaxes.

Joints

Connections between two or more bones → allow for movement and provide structural support throughout the body

1. 1. Fibrous

2. 2. Cartilaginous

3. 3. Synovial

Fibrous Joints

Fibrous joints are connected by dense connective tissue composed primarily of collagen. They allow little to no movement, providing stability and protection. The three types are:

• Sutures: Immovable joints found in the skull that hold skull bones together.

• Syndesmoses: Slightly movable joints found between long bones such as the tibia and fibula.

• Gomphoses: Joints that anchor teeth to the jawbone, providing a stable connection.

Cartilaginous Joints

Cartilaginous joints are connected by cartilage and allow limited movement. The two types are:

• Synchondroses: Bones are united by hyaline cartilage, such as the growth plates in long bones, allowing slight movement during bone development.

• Symphyses: Bones are united by fibrocartilage, providing slight movement and shock absorption. Examples include the pubic symphysis and intervertebral discs between the vertebrae.

Synovial Joints + their characteristics

Synovial joints are the most common and most mobile type of joint in the body. They contain a joint cavity filled with synovial fluid and allow a wide range of movement.

What are the types of synovial joints and their functions?

• Ball-and-Socket Joints: Allow movement in multiple directions, including rotation (e.g., shoulder and hip).

• Hinge Joints: Allow movement in one direction (e.g., elbow and knee).

• Pivot Joints: Allow rotational movement around a single axis (e.g., between the first and second cervical vertebrae).

• Condyloid Joints: Allow flexion, extension, and side-to-side movement, but no rotation (e.g., wrist).

• Saddle Joints: Allow movement in different planes and a wide range of motion (e.g., thumb joint).

• Gliding (Plane) Joints: Allow bones to slide past each other (e.g., carpal bones of the wrist and tarsal bones of the foot).

These joint types provide varying degrees of mobility, stability, and protection according to the body's needs.

What is synovial fluid and what are its major functions?

Synovial fluid is a viscous, slippery substance found in the cavities of synovial joints. Its major functions are:

• Lubrication: Reduces friction and allows smooth movement between bones, protecting articular cartilage.

• Shock Absorption: Distributes compressive forces and cushions load-bearing joints like the knees and hips.

• Cartilage Nourishment: Supplies oxygen and nutrients to chondrocytes (cartilage cells) since cartilage lacks a blood supply.

• Protection: Contains hyaluronic acid and lubricin, which increase viscosity, reduce friction, and preserve joint function and integrity.

How do synovial fluid components contribute to joint health?

• Synovial membrane: Produces synovial fluid.

• Hyaluronic acid: Increases fluid thickness and resistance to mechanical stress.

• Lubricin: Further reduces friction between joint surfaces.

• Movement of the joint: Squeezes synovial fluid into and out of cartilage, promoting nutrient exchange and supporting cartilage maintenance and repair.

What are ligaments and what are they composed of?

Ligaments are tough, fibrous bands of connective tissue that connect bone to bone at a joint. They are composed primarily of:

• Collagen fibers, which provide strength and some flexibility.

• Elastin fibers, which provide elasticity.

Ligaments are slightly elastic, allowing limited stretching, but their primary role is to stabilize joints and prevent excessive movement.

What is the function of ligaments in joint stability?

Ligaments maintain proper bone alignment during movement and limit excessive motion to prevent injuries such as dislocations. For example, the anterior cruciate ligament (ACL) in the knee prevents the tibia from moving too far forward relative to the femur. When ligaments are overstretched or torn, joint stability is compromised, leading to sprains or more severe injuries that affect mobility.

What are tendons and what are they composed of?

Tendons are strong, flexible bands of fibrous connective tissue that attach muscle to bone. They are primarily composed of collagen fibers, which provide tensile strength. Tendons are less elastic than ligaments and are designed to withstand the pulling forces generated by muscle contractions.

What is the function of tendons and what happens when they are injured?

Tendons transmit the force generated by muscle contractions to bones, producing movement at joints. For example, the Achilles tendon connects the calf muscles to the heel bone, enabling movements such as walking, running, and jumping. Healthy tendons allow efficient movement, while injuries such as tendonitis or tears can cause pain, impair function, and limit mobility.