Muscle Tissue and Neuromuscular Junction: Study Guide

Muscle Tissue and Neuromuscular Junction

Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement in the body. It possesses several unique characteristics:

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

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

Example: Skeletal muscles contract to move bones at joints.

Organization of Muscle Structure

Muscle structure is organized hierarchically from largest to smallest components:

• Muscle (organ)

• Muscle fascicle (bundle of muscle fibers)

• Muscle fiber (single muscle cell)

• Myofibril (rod-like unit within muscle fiber)

• Myofilaments (actin and myosin filaments)

Example: Myofibrils are composed of repeating units called sarcomeres.

Sarcomere: Structure and Function

The sarcomere is the basic contractile unit of striated muscle fibers. It is defined as the segment between two Z lines.

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

• Key Proteins:

  • Actin: Thin filament

  • Myosin: Thick filament

  • Troponin and Tropomyosin: Regulatory proteins on actin that control binding of myosin

  • Calcium: Binds to troponin, causing conformational change that moves tropomyosin and exposes binding sites for myosin

Equation:

$\text{Muscle Contraction} \propto \text{Number of Sarcomeres Shortening}$

Example: During contraction, myosin heads bind to actin, forming cross-bridges and pulling actin filaments inward.

Neural Stimulation of Skeletal Muscle

Skeletal muscle contraction is initiated by stimulation from motor neurons (specifically, somatic motor neurons).

• Motor Neuron: Transmits action potentials from the central nervous system to muscle fibers.

• Neuromuscular Junction (NMJ): The synapse where the motor neuron communicates with the muscle fiber.

Cell Types at the Neuromuscular Junction (NMJ)

Two main cell types interact at the NMJ:

• Motor Neuron Terminal: The axon terminal of the motor neuron, which releases neurotransmitter.

• Muscle Fiber (Postsynaptic Cell): The muscle cell membrane (sarcolemma) that receives the signal.

Structure of the Neuromuscular Junction (NMJ)

The NMJ is a specialized synapse between a motor neuron and a skeletal muscle fiber. Key components include:

• Synaptic Knob: Swollen end of the motor neuron containing synaptic vesicles filled with acetylcholine (ACh).

• Synaptic Cleft: Small gap between the neuron and muscle fiber.

• Motor End Plate: Specialized region of the muscle fiber membrane with ACh receptors.

• Neurotransmitter: Acetylcholine (ACh) is released from the neuron and binds to receptors on the muscle fiber.

• Receptor Type: Nicotinic acetylcholine receptors (ligand-gated ion channels).

Example: When ACh binds to its receptor, it triggers an action potential in the muscle fiber.

Action Potential Transmission at the NMJ

The action potential in the motor neuron leads to muscle contraction through the following steps:

• Action potential arrives at the axon terminal of the motor neuron.

• Voltage-gated calcium channels open; calcium enters the neuron.

• Synaptic vesicles release ACh into the synaptic cleft.

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

• Sodium ions enter the muscle fiber, generating an action potential.

• The action potential propagates along the sarcolemma and into the T-tubules.

• This triggers calcium release from the sarcoplasmic reticulum, initiating contraction.

Steps of Muscle Contraction (Excitation-Contraction Coupling)

Muscle contraction involves a sequence of events from neural stimulation to sarcomere shortening:

1. Action potential travels down the motor neuron to the NMJ.

2. ACh is released and binds to receptors on the muscle fiber.

3. Muscle fiber depolarizes, generating an action potential.

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

5. Calcium is released from the sarcoplasmic reticulum.

6. Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

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

8. ATP is hydrolyzed, causing myosin heads to pivot and pull actin filaments (power stroke).

9. New ATP binds to myosin, causing it to release actin and reset for another cycle.

10. Contraction continues as long as calcium and ATP are present.

Example: Recording yourself explaining these steps can help reinforce your understanding.

Role of ATP in Cross-Bridge Cycling

ATP is essential for both the formation and release of cross-bridges during muscle contraction:

• Binding of ATP to Myosin: Required for myosin to detach from actin after the power stroke.

• Hydrolysis of ATP: Provides energy for the myosin head to return to its high-energy conformation.

Equation:

$\text{Myosin-Actin Detachment} + \text{ATP} \rightarrow \text{Myosin-ATP Complex}$

Muscle Relaxation

Relaxation of muscle fibers occurs when stimulation ceases and calcium is removed from the cytoplasm:

• ACh is broken down by acetylcholinesterase in the synaptic cleft.

• Calcium ions are actively transported back into the sarcoplasmic reticulum.

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

• Muscle fiber returns to its resting state.

Example: Without ATP, muscles cannot relax, leading to rigor mortis after death.