Neuromuscular Junction and Muscle Contraction

Muscle cells possess a unique romance between actin and myosin, driving all body movements.
Muscle tissues convert chemical potential energy into mechanical energy through contraction and relaxation.

Smooth Muscle:
- Involuntary control.
- Found in the walls of hollow visceral organs (e.g., stomach, airways, blood vessels).
- Functions to push fluids and materials.
Cardiac Muscle:
- Involuntary control.
- Striated appearance.
- Specific to the heart for blood pumping.
Skeletal Muscle:
- Mostly voluntary control via the somatic nervous system.
- Striated appearance.
- Attached to the skeleton to facilitate movement.
- Examples: biceps brachii, vastus lateralis, gluteus maximus.

Skeletal muscles are organs composed of muscle tissue, connective tissue, blood vessels, and nerve fibers.
Each muscle has a dedicated nerve for stimulation and an artery/vein for nutrient supply.
Structure:
- Myofibrils: Tiny, parallel threads.
- Muscle Fibers: Muscle cells formed by myofibrils, containing mitochondria, multiple nuclei, and a sarcolemma (cellular membrane).
- Fascicles: Bundles of muscle fibers.
- Muscle Organ: Formed by fascicles (e.g., biceps brachii).
Connective tissue sheaths support and protect muscles during movement.

Myofibrils are divided into segments called sarcomeres.
Sarcomeres contain actin (thin filaments) and myosin (thick filaments) myofilaments.
Z lines: Borders of the sarcomere, formed by alternating thin filaments.
Muscle contraction occurs as sarcomeres contract, bringing Z lines closer.

At rest, actin and myosin filaments do not touch but have an affinity for each other.
Myosin heads seek to bind with actin, leading to muscle contraction.
Tropomyosin and troponin proteins block actin's binding sites, acting as obstacles.
These obstacles are overcome by ATP and calcium.

ATP: Molecular currency that provides energy for muscle contraction.
- Muscle cells have many nuclei and mitochondria to produce ATP.
Sarcoplasmic Reticulum: Specialized endoplasmic reticulum in muscle cells that stores calcium ions.
- Contains calcium pumps that use ATP to store calcium.
- Contains calcium channels linked to voltage-sensitive proteins.

The site where the nervous system connects to the muscular system.
Every skeletal muscle fiber is innervated by a motor neuron at the neuromuscular junction.
The only way to activate the fiber to contract.
Action potential travels down the axon to the axon terminal.
Axon terminal meets the muscle fiber.

Upper Motor Neurons: Originate in the brain, carry signals to lower motor neurons.
Lower Motor Neurons: Located in the brainstem and spinal cord, mediate between the nervous system and muscles.

Action potential travels down the motor neuron axon.
The axon terminal branches and ends at a neuromuscular junction along the muscle fiber.
The axon terminal contains mitochondria and vesicles filled with acetylcholine.
Acetylcholine is released from the axon terminal, triggering changes in the sarcolemma and muscle fiber contraction.
Synaptic Cleft: The space between the axon terminal and sarcolemma.
Motor End Plate: Part of the sarcolemma that contains acetylcholine receptors.
Acetylcholinesterase: Enzyme in the synaptic cleft and motor end plate that breaks down acetylcholine to end muscle contraction.

Axon Terminal: The end of the axon.
Synaptic Vesicles/Neurotransmitter Vesicles: Storage areas for neurotransmitters.
Neurotransmitter: Chemical messenger (acetylcholine in this context).

Acetylcholine: Specific neurotransmitter involved in neuromuscular junction and muscular contraction.
Synaptic Cleft: Small space separating the neuron and target cell, enabling nerve impulse transmission.
Presynaptic Cell: The sending end of the synapse.
Postsynaptic Cell: The receiving end of the synapse.
Acetylcholine release: Vesicles merge with the cell membrane, releasing acetylcholine into the synaptic cleft, where it binds to receptors.
Motor End Plate: Surface with receptors for acetylcholine.
Sarcolemma: Plasma membrane covering striated muscle fibers.
T-Tubule (Transverse Tubule): Carries action potential to the interior of the cell.
Sarcoplasmic Reticulum (SR): Stores calcium in muscle cells.
Sarcoplasm: Cytoplasm of muscle cells.

Arrival of Action Potential:
- Action potential reaches the axon terminal.
- Triggers influx of calcium ions.
- Synaptic vesicles move toward the synaptic cleft.
Acetylcholine Release:
- Synaptic vesicles release acetylcholine into the synaptic cleft.
Acetylcholine Binding:
- Acetylcholine binds to receptors on the motor end plate.
- Triggers change in membrane permeability, allowing sodium influx.
Action Potential Generation:
- Sodium influx causes a shift in membrane charge, generating an action potential in the sarcolemma.
Acetylcholinesterase Action:
- Acetylcholinesterase breaks down acetylcholine in the synaptic cleft.
- Ends the contraction cycle.
Action Potential Propagation:
- Action potential travels along the T tubules.
Calcium Release:

Actin Filaments: Have active sites for myosin binding.
Troponin and Tropomyosin: Troponin holds tropomyosin, blocking active sites on actin.
Calcium Binding: Calcium binds to troponin, causing it to change shape and roll tropomyosin away from the active sites.
Myosin Binding: Myosin binds to exposed active sites on actin, initiating contraction.