Muscle Tissue and Neuromuscular Junction: Study Guide

Muscle Tissue

Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. The main characteristics include:

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

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

Organization of Muscle Structure

Muscle tissue is organized hierarchically from largest to smallest components:

• Muscle (whole organ)

• Muscle fascicle (bundle of muscle fibers)

• Muscle fiber (single muscle cell)

• Myofibril (organelle within muscle fiber)

• Myofilaments (actin and myosin proteins)

Example: The biceps brachii muscle contains many fascicles, each made up of muscle fibers, which in turn contain myofibrils composed of myofilaments.

Sarcomere Structure and Function

The sarcomere is the basic contractile unit of muscle fiber. It is defined as the region 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, tropomyosin, actin, and myosin regulate contraction.

• Calcium: Binds to troponin, causing tropomyosin to move and expose binding sites on actin for myosin.

Equation:

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

Neural Control of Muscle Contraction

Types of Neurons Stimulating Skeletal Muscle

Motor neurons are responsible for stimulating skeletal muscle contraction. They transmit action potentials from the central nervous system to muscle fibers.

Cell Types at the Neuromuscular Junction (NMJ)

The NMJ is a specialized synapse where a motor neuron communicates with a muscle fiber. The two main cell types are:

• Motor neuron (presynaptic cell)

• Muscle fiber (postsynaptic cell)

Structure of the Neuromuscular Junction (NMJ)

The NMJ consists of several key components:

• Axon terminal of the motor neuron

• Synaptic cleft (space between neuron and muscle)

• Motor end plate (region of muscle fiber membrane with acetylcholine receptors)

• Neurotransmitter: Acetylcholine (ACh)

• Receptor type: Nicotinic acetylcholine receptor

Muscle Contraction Mechanism

Action Potential Initiation in Muscle Cell

An action potential in the motor neuron leads to the release of acetylcholine at the NMJ, which binds to receptors on the muscle fiber, initiating an action potential in the muscle cell.

Steps of Muscle Contraction

The process from neural stimulation to muscle contraction involves several steps:

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

2. Acetylcholine is released into the synaptic cleft.

3. Acetylcholine binds to receptors on the motor end plate, causing depolarization.

4. Action potential spreads along the sarcolemma and down T-tubules.

5. Calcium ions are released from the sarcoplasmic reticulum.

6. Calcium binds to troponin, shifting tropomyosin and exposing actin binding sites.

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

8. Myosin heads pivot, pulling actin filaments (power stroke).

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

Example: During a biceps curl, these steps occur rapidly to produce muscle shortening and movement.

Requirements for Cross-Bridge Cycling

For myosin to break cross-bridges and continue the contraction cycle, ATP must bind to the myosin head.

Muscle Relaxation

Relaxation occurs when:

• Acetylcholine is broken down by acetylcholinesterase.

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

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

Equation:

$\text{ATP} + \text{Myosin} \rightarrow \text{Myosin-ATP} \rightarrow \text{Release of Actin}$

Summary Table: Key Components of Muscle Contraction

Component	Function
Motor Neuron	Initiates action potential
Acetylcholine	Neurotransmitter at NMJ
Muscle Fiber	Receives signal, contracts
Sarcomere	Site of contraction
Calcium	Triggers contraction
ATP	Provides energy for contraction and relaxation

Additional info: Some steps and details have been expanded for clarity and completeness, based on standard Anatomy & Physiology curriculum.