Muscle Microanatomy, Mechanisms of Contraction, Muscle Activation, Biochemistry, and Nervous System Organization

Muscle Microanatomy

Functions of Muscle and Daily Life Applications

Muscles play a vital role in movement, maintaining posture, joint stability, and heat generation (shivering).

• Movement: Muscles contract to produce movement of body parts.

• Posture: Continuous muscle contractions maintain posture.

• Joint Stability: Muscles help stabilize joints.

• Heat Generation: Muscle activity produces heat, important for thermoregulation.

Antagonist Muscles and Their Function

Antagonist muscles perform the opposite action of another muscle, allowing for controlled movement.

• Example: Triceps brachii (extension) is the antagonist of biceps brachii (flexion).

• Importance: Maintains position, controls rapid movement.

Structural Organization of Muscle

Muscle structure is hierarchical, from whole organ to myofilaments.

• Muscle → Fascicle → Fiber (cell) → Myofibril → Myofilament

Specialized Organelles in Muscle

Muscle cells contain unique organelles for contraction.

• Myofibrils: Contain sarcomeres, the contractile units.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Cytoplasm of muscle cell.

• Sarcoplasmic Reticulum: Stores and releases calcium for contraction.

• T-tubules: Invaginations of sarcolemma that transmit signals.

Connective Tissue Organization

Connective tissue sheaths support and organize muscle fibers.

• Epimysium: Surrounds entire muscle.

• Perimysium: Surrounds fascicles.

• Endomysium: Surrounds individual muscle fibers.

Mechanism of Contraction

Sarcomere Structure and Importance

The sarcomere is the fundamental contractile unit of muscle, composed of overlapping thick (myosin) and thin (actin) filaments.

• Sliding Filament Theory: Muscle contraction occurs as myosin heads bind to actin and pull, shortening the sarcomere.

Thick and Thin Filament Specializations

• Thin Filaments:

  • Attach to Z discs

  • Composed of actin

  • Myosin binding sites

  • Regulatory proteins: tropomyosin (blocks binding sites), troponin (binds calcium)

• Thick Filaments:

  • Composed of myosin

  • Myosin heads bind to actin

  • Attachment site for ATP

Crossbridge Cycling

Crossbridge cycling is the process by which myosin heads bind to actin, pull, and release, resulting in muscle contraction.

• ATP binds to myosin head, causing detachment from actin.

• ATP hydrolysis "cocks" the myosin head.

• Myosin binds to actin, forming a crossbridge.

• Power stroke occurs as ADP is released.

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

Chemical Equation:

• $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (energy released for power stroke)

Muscle Activation

Electrical to Chemical Signal Conversion

Muscle activation begins with an electrical signal from a neuron, which is converted to a chemical signal at the neuromuscular junction.

• Action potential travels down motor neuron.

• Release of acetylcholine (ACh) at synaptic cleft.

• ACh binds to receptors on muscle fiber, triggering depolarization.

• Depolarization leads to calcium release and muscle contraction.

Motor Units

A motor unit consists of a single motor neuron and all the muscle fibers it innervates.

• Small motor units: Fine control, few muscle fibers.

• Large motor units: Gross movement, many muscle fibers.

Temporal Summation and Recruitment

• Temporal Summation: Increased frequency of stimulation increases force.

• Recruitment: Increased number of active motor units increases force.

Muscular Biochemistry

ATP Regeneration Mechanisms

Muscle cells regenerate ATP through three main pathways:

• Direct Phosphorylation: Creatine phosphate donates phosphate to ADP.

• Anaerobic Respiration: Glycolysis produces ATP without oxygen.

• Aerobic Respiration: Uses oxygen, produces more ATP.

Equations:

• $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• $\text{Glucose} \rightarrow \text{Pyruvate} + \text{ATP}$ (anaerobic)

• $\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ATP}$ (aerobic)

Anaerobic vs. Aerobic Pathways

• Anaerobic: Fast, less efficient, produces lactate.

• Aerobic: Slower, more efficient, produces more ATP.

Cells of the Nervous System

Basic Parts of an Axon

• Function: Transmits signals.

• Axon Hillock: Initiates action potential.

• Axon Terminals: Transmit signals to other cells.

Efferent vs. Afferent

• Afferent: Towards brain/CNS (sensory).

• Efferent: Away from brain/CNS (motor).

Organization of the Nervous System

• CNS: Brain and spinal cord.

• PNS: All nerves outside CNS.

Somatic vs. Visceral

• Somatic Sensory: Sensory input from skin, muscles.

• Visceral Sensory: Sensory input from organs.

• Somatic Motor: Voluntary control of skeletal muscle.

• Visceral Motor: Involuntary control (e.g., heart).

Nuclei, Ganglia, Nerves, and Tracts

• Nuclei: Clusters of neuron cell bodies in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

• Nerves: Bundles of axons in PNS.

• Tracts: Bundles of axons in CNS.

Dendrites and Axon Specializations

• Dendrites: Receive inputs, transmit to soma.

• Axon: Transmits signal away from soma.

Myelination

• Function: Increases speed of signal transmission.

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

• Nodes of Ranvier: Gaps in myelin sheath, allow for saltatory conduction.

Neuroglia Functions

• Support cells: Astrocytes, microglia, oligodendrocytes, Schwann cells, ependymal cells.

• Roles: Support, protect, nourish neurons.

Action Potentials

Leakage Channels and Na-K Pumps

• Leakage Channels: Allow passive diffusion of ions.

• Na-K Pump: Actively transports Na+ out and K+ in, maintains resting potential.

Channel Activation Mechanisms

• Leakage: Always open.

• Mechanically Gated: Open in response to physical deformation.

• Ligand Gated: Open in response to chemical binding.

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

Hyperpolarization vs. Depolarization

• Hyperpolarization: Membrane potential becomes more negative.

• Depolarization: Membrane potential becomes less negative.

Action Potential Propagation

• Action potentials are all-or-nothing events.

• Propagation involves sequential depolarization along the axon.

• Each segment of axon generates its own action potential.

Graph Interpretation: Be able to interpret changes in membrane potential over time.

Key Equation:

• $\text{Resting~potential} \approx -70~\text{mV}$

Summary Table: Muscle and Nervous System Structures

Structure	Location	Function
Epimysium	Muscle	Surrounds entire muscle
Perimysium	Muscle	Surrounds fascicles
Endomysium	Muscle	Surrounds muscle fibers
Schwann Cell	PNS	Myelinates axons
Oligodendrocyte	CNS	Myelinates axons
Astrocyte	CNS	Support, blood-brain barrier
Microglia	CNS	Immune defense

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.