Muscle Tissue and Neuromuscular Junction: Study Guide

Muscle Tissue and Neuromuscular Junction

Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction and plays a vital role in movement, posture, and heat production.

• Excitability: Ability to respond to stimuli by generating electrical signals.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original length after stretching or contracting.

• Conductivity: Ability to propagate electrical signals along the membrane.

Organization of Muscle Structure

Muscle tissue is organized in a hierarchical manner from largest to smallest components.

• Muscle (organ)

• Muscle fascicle (bundle of muscle fibers)

• Muscle fiber (muscle cell)

• Myofibril (organelle within muscle fiber)

• Myofilaments (actin and myosin filaments)

Order (largest to smallest): Muscle > Muscle fascicle > Muscle fiber > Myofibril > Myofilament (actin/myosin)

Sarcomere: Structure and Function

The sarcomere is the basic contractile unit of striated muscle fibers.

• Definition: The region between two Z lines in a myofibril.

• Role in Contraction: Sarcomeres shorten during muscle contraction, pulling Z lines closer together.

• Key Proteins:

  • Actin: Thin filament

  • Myosin: Thick filament

  • Troponin: Regulatory protein that binds calcium

  • Tropomyosin: Blocks myosin-binding sites on actin

  • Calcium: Binds to troponin, causing tropomyosin to move and expose binding sites

Example: During contraction, calcium ions bind to troponin, shifting tropomyosin and allowing myosin heads to bind to actin, forming cross-bridges.

Neural Stimulation of Skeletal Muscle

Skeletal muscle contraction is initiated by signals from the nervous system.

• Type of Neuron: Somatic motor neurons stimulate skeletal muscle contraction.

Cell Types at the Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber.

• Presynaptic cell: Motor neuron

• Postsynaptic cell: Muscle fiber (muscle cell)

Structure of the Neuromuscular Junction (NMJ)

The NMJ is a specialized synapse that allows communication between the nervous system and skeletal muscle.

• Motor neuron axon terminal: Releases neurotransmitter (acetylcholine, ACh)

• Synaptic cleft: Space between neuron and muscle fiber

• Motor end plate: Region of muscle fiber membrane with ACh receptors

• Neurotransmitter: Acetylcholine (ACh)

• Receptor: Nicotinic acetylcholine receptor (ligand-gated ion channel)

Action Potential Initiation in Muscle Cell

The action potential in the neuron leads to muscle cell excitation.

• Action potential arrives at axon terminal of motor neuron.

• Voltage-gated calcium channels open; calcium enters axon terminal.

• Vesicles release ACh into synaptic cleft.

• ACh binds to receptors on motor end plate, opening ion channels.

• Sodium enters muscle cell, generating an action potential in the muscle fiber.

Steps of Muscle Contraction

Muscle contraction involves a series of steps from neural stimulation to sarcomere shortening.

1. Action potential travels down motor neuron to NMJ.

2. ACh released into synaptic cleft.

3. ACh binds to receptors on muscle fiber, generating muscle action potential.

4. Action potential travels along sarcolemma and down T-tubules.

5. Calcium released from sarcoplasmic reticulum.

6. Calcium binds to troponin; tropomyosin shifts, exposing actin binding sites.

7. Myosin heads bind to actin, forming cross-bridges.

8. Power stroke: Myosin heads pivot, pulling actin filaments toward center of sarcomere.

9. ATP binds to myosin, causing it to detach from actin.

10. ATP hydrolyzed; myosin head returns to cocked position.

11. Cycle repeats as long as calcium and ATP are present.

Additional info: This process is known as the sliding filament theory.

Breaking Cross-Bridges

ATP is required for myosin heads to detach from actin after the power stroke.

• Key Point: ATP must bind to myosin to break the cross-bridge and allow the cycle to repeat.

Muscle Relaxation

Relaxation occurs when stimulation ends and calcium is removed from the cytoplasm.

• Motor neuron stops releasing ACh.

• ACh is broken down by acetylcholinesterase.

• Calcium is pumped back into the sarcoplasmic reticulum.

• Troponin and tropomyosin return to resting positions, blocking myosin-binding sites on actin.

• Muscle fiber returns to resting state.