Muscle and Nervous System: Structure, Function, and Physiology

Muscle Microanatomy

Functions of Muscle

Muscles are specialized tissues responsible for producing movement, maintaining posture, stabilizing joints, and generating heat. These functions are essential for daily activities such as walking, breathing, and manipulating objects.

• Movement: Muscles contract to move bones and body parts.

• Posture: Continuous muscle contractions maintain body posture.

• Joint Stabilization: Muscles reinforce and stabilize joints.

• Heat Production: Muscle activity generates heat, helping maintain body temperature.

Antagonist Muscles

An antagonist muscle opposes the action of a prime mover (agonist). This opposition is crucial for controlled movement and joint stability.

• Example: The triceps brachii acts as an antagonist to the biceps brachii during elbow flexion.

Structural Organization of Muscle

Muscle tissue is organized hierarchically from the whole muscle to microscopic filaments:

• Whole Muscle → Fascicle → Muscle Fiber (cell) → Myofibril → Myofilaments (actin and myosin)

Specialized Organelles in Muscle

• Sarcoplasm: Cytoplasm of a muscle fiber, containing glycosomes and myoglobin.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions, essential for muscle contraction.

• T-tubules (Transverse tubules): Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.

Connective Tissue Sheaths

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds each fascicle (bundle of muscle fibers).

• Endomysium: Surrounds individual muscle fibers.

Skeletal vs. Smooth Muscle (Cellular Level)

• Skeletal Muscle: Long, cylindrical, multinucleated, striated.

• Smooth Muscle: Spindle-shaped, single nucleus, non-striated.

Mechanism of Contraction

Sarcomere Structure

The sarcomere is the functional unit of skeletal muscle, defined as the region between two Z-discs. It contains organized arrays of thick (myosin) and thin (actin) filaments.

• Z-disc: Boundary of the sarcomere.

• A band: Region containing thick filaments.

• I band: Region containing only thin filaments.

• H zone: Central region with only thick filaments.

Thick and Thin Filament Specializations

• Thick Filaments (Myosin): Have heads that bind to actin and hydrolyze ATP.

• Thin Filaments (Actin): Contain binding sites for myosin; regulated by tropomyosin and troponin.

Crossbridge Cycling

Muscle contraction occurs via the cyclical interaction of myosin heads with actin filaments, known as crossbridge cycling.

1. Attachment: Myosin head binds to actin (crossbridge forms).

2. Power Stroke: Myosin head pivots, pulling actin filament toward the center of the sarcomere.

3. Detachment: ATP binds to myosin, causing it to detach from actin.

4. Reactivation: ATP hydrolysis re-cocks the myosin head.

Chemical Events: ATP hydrolysis provides energy; calcium ions bind to troponin, shifting tropomyosin and exposing binding sites on actin.

Muscle Activation

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber. It converts an electrical signal (action potential) into a chemical signal (neurotransmitter release), initiating muscle contraction.

• Steps:

  1. Action potential arrives at axon terminal.

  2. Acetylcholine (ACh) is released into the synaptic cleft.

  3. ACh binds to receptors on the muscle fiber, triggering an action potential in the sarcolemma.

Motor Units

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Small Motor Units: Few fibers per neuron; allow fine control (e.g., eye muscles).

• Large Motor Units: Many fibers per neuron; generate powerful contractions (e.g., thigh muscles).

Summation and Recruitment

• Temporal Summation: Increased frequency of stimulation increases muscle tension.

• Recruitment (Multiple Motor Unit Summation): Increasing the number of active motor units increases force.

Key Principle: Muscle force increases with the number of crossbridges formed.

Muscular Biochemistry

ATP Regeneration Mechanisms

Muscle fibers regenerate ATP through three main pathways:

• Direct Phosphorylation: Creatine phosphate donates a phosphate to ADP to form ATP.

• Anaerobic Pathway (Glycolysis): Glucose is broken down to pyruvate, yielding ATP and lactic acid (when oxygen is limited).

• Aerobic Pathway: Glucose, fatty acids, or amino acids are oxidized in mitochondria, producing ATP, CO2, and H2O.

Pros and Cons of ATP Pathways

Pathway	Pros	Cons
Direct Phosphorylation	Very rapid	Limited supply (10-15 sec)
Anaerobic	Quick ATP production	Lactic acid buildup; less efficient
Aerobic	High ATP yield; sustainable	Slower; requires oxygen

Muscle Adaptations to Exercise

• Aerobic Exercise: Increases capillary density, mitochondrial number, and myoglobin content, enhancing endurance.

• Resistance Exercise: Increases muscle fiber size (hypertrophy) and strength, favoring anaerobic pathways.

Cells of the Nervous System

Axon Structure

• Axon Hillock: Site of action potential initiation.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Afferent vs. Efferent

• Afferent (Sensory): Carry information toward the CNS.

• Efferent (Motor): Carry commands away from the CNS to effectors (muscles/glands).

Nervous System Organization

• Somatic Sensory: Sensory input from skin, muscles, joints.

• Visceral Sensory: Sensory input from organs.

• Somatic Motor: Voluntary control of skeletal muscles.

• Visceral Motor (Autonomic): Involuntary control of smooth/cardiac muscle and glands.

Nuclei, Ganglia, Nerves, and Tracts

• Nuclei: Clusters of neuron cell bodies in the CNS.

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Nerves: Bundles of axons in the PNS.

• Tracts: Bundles of axons in the CNS.

Dendrite and Axon Specializations

• Dendrites: Receive incoming signals; increase surface area for synaptic input.

• Axons: Conduct action potentials away from the cell body.

Myelination

• Myelin Sheath: Insulating layer around axons; increases speed of action potential conduction.

• Cells Responsible: Schwann cells (PNS), oligodendrocytes (CNS).

Neuroglia (Glial Cells)

• Astrocytes: Support neurons, regulate extracellular environment, form blood-brain barrier.

• Microglia: Immune defense in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• Oligodendrocytes: Myelinate CNS axons.

• Schwann Cells: Myelinate PNS axons.

• Satellite Cells: Support PNS neuron cell bodies.

Action Potentials

Resting Membrane Potential

The resting membrane potential is maintained by leakage channels and the sodium-potassium pump:

• Leakage Channels: Allow passive movement of ions (mainly K+).

• Na-K Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining negative interior.

$V_{rest} \approx -70\ \mathrm{mV}$

Ion Channels and Activation

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically-Gated Channels: Open in response to neurotransmitter binding.

Depolarization vs. Hyperpolarization

• Depolarization: Membrane potential becomes less negative (e.g., influx of Na+).

• Hyperpolarization: Membrane potential becomes more negative (e.g., efflux of K+ or influx of Cl-).

Action Potential Graph Interpretation

• Phases: Resting state, depolarization, repolarization, hyperpolarization, return to rest.

• Key Events: Threshold reached, rapid Na+ influx, K+ efflux, restoration of resting potential.

Chemistry of Action Potentials

• Driven by changes in permeability to Na+ and K+ through voltage-gated channels.

Propagation of Action Potentials

• Depolarization at one segment of the axon triggers opening of voltage-gated channels in the next segment, propagating the action potential.

• Myelination enables saltatory conduction, where action potentials jump between nodes of Ranvier, increasing speed.

$\text{Na}^+ / \text{K}^+ \text{ pump:} \ 3\ \text{Na}^+ \ \text{out},\ 2\ \text{K}^+ \ \text{in}$