Muscle Tissue and Neuromuscular Junction: Study Notes for Anatomy & Physiology

Muscle Tissue

Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and heat production in the body.

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

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original length after contraction or extension.

Organization of Muscle Structure

Muscle tissue is organized hierarchically from large to small components.

• Muscle (whole organ)

• Muscle fiber (individual cell)

• Myofibril (contractile threads within muscle fiber)

• Myofilaments (protein filaments: actin and myosin)

Order of decreasing size: Muscle > Muscle fiber > Myofibril > Myofilament

Muscle Contraction Mechanism

Sarcomere Structure and Function

The sarcomere is the basic contractile unit of striated muscle tissue. It is defined as the segment between two Z lines and contains organized arrays of actin (thin) and myosin (thick) filaments.

• Role in contraction: Sarcomeres shorten during muscle contraction as actin and myosin filaments slide past each other.

• Key proteins:

  • Troponin: Binds calcium and regulates actin-myosin interaction.

  • Tropomyosin: Blocks myosin binding sites on actin until moved by troponin.

  • Actin: Thin filament, interacts with myosin for contraction.

  • Myosin: Thick filament, forms cross-bridges with actin.

  • Calcium: Initiates contraction by binding to troponin.

Equation for force generation:

$\text{Force} \propto \text{Number of cross-bridges formed}$

Neural Control of Muscle Contraction

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

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

Neuromuscular Junction (NMJ)

Structure of the NMJ

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber. It is essential for transmitting the neural signal that initiates muscle contraction.

• Presynaptic cell: Motor neuron

• Postsynaptic cell: Muscle fiber

• Synaptic cleft: Space between neuron and muscle

• Neurotransmitter: Acetylcholine (ACh)

• Receptor: Nicotinic acetylcholine receptor (nAChR) on muscle cell membrane

Cell types at the NMJ:

• Motor neuron

• Muscle fiber

Action Potential Initiation at the NMJ

When an action potential reaches the axon terminal of the motor neuron, it triggers the release of acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the muscle cell membrane, leading to depolarization and initiation of an action potential in the muscle cell.

• Steps:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated calcium channels open; calcium enters neuron.

  3. Acetylcholine is released into synaptic cleft.

  4. Acetylcholine binds to receptors on muscle cell membrane.

  5. Muscle cell depolarizes, generating an action potential.

Steps of Muscle Contraction

Sequence from Neural Signal to Muscle Contraction

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

1. Action potential travels down motor neuron to NMJ.

2. Acetylcholine is released and binds to muscle cell receptors.

3. Muscle cell membrane depolarizes; action potential spreads along sarcolemma.

4. Action potential travels down T-tubules.

5. Calcium is released from sarcoplasmic reticulum.

6. Calcium binds to troponin, moving tropomyosin and exposing myosin binding sites on actin.

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

8. Myosin heads pivot, pulling actin filaments toward center of sarcomere (power stroke).

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

Example: Recording yourself explaining these steps can help ensure you understand the sequence and terminology.

Cross-Bridge Cycling and Muscle Relaxation

Cross-bridge cycling is the process by which myosin heads bind to and pull on actin filaments, resulting in muscle contraction.

• ATP: Required for myosin to detach from actin and "break" cross-bridges.

• Relaxation: Occurs when neural stimulation ceases, calcium is pumped back into the sarcoplasmic reticulum, and tropomyosin covers myosin binding sites on actin.

Equation for ATP hydrolysis:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: Key Components of Muscle Contraction

Component	Function
Motor Neuron	Initiates action potential for muscle contraction
Acetylcholine	Neurotransmitter that activates muscle cell
Calcium	Triggers exposure of myosin binding sites on actin
ATP	Provides energy for cross-bridge cycling and relaxation
Troponin/Tropomyosin	Regulate actin-myosin interaction

Additional info: Some steps and terminology have been expanded for clarity and completeness.