Membrane Transport and Bioelectricity

Cell Membrane Structure and Permeability

The cell membrane acts as a selective barrier, separating the intracellular environment from the extracellular fluid. This separation is crucial for maintaining distinct ion concentrations and cellular function.

• Intracellular fluid: High in potassium (K+) and negatively charged proteins.

• Extracellular fluid: High in sodium (Na+), chloride (Cl-), and calcium (Ca2+).

• Permeability: Refers to the ease with which substances cross the membrane. Membranes can be freely permeable, impermeable, or selectively permeable based on size, charge, and solubility.

Transport Mechanisms

Substances move across the cell membrane via passive or active transport mechanisms.

• Passive transport: Does not require energy. Includes diffusion and facilitated diffusion.

• Active transport: Requires ATP. Includes ion pumps and secondary active transport.

Diffusion and Osmosis

Diffusion is the movement of molecules from high to low concentration. Osmosis is the diffusion of water across a semi-permeable membrane in response to solute differences.

• Simple diffusion: Small, nonpolar molecules (e.g., O2, CO2) cross the lipid bilayer.

• Electrochemical gradient: For ions, movement is influenced by both chemical and electrical gradients.

• Osmotic pressure: The force driving water movement into a solution.

Tonicity

Tonicity describes the effect of extracellular solutions on cell volume.

• Isotonic: No net osmosis; cell volume unchanged.

• Hypotonic: Net gain of water; cell may swell and burst (cytolysis).

• Hypertonic: Net loss of water; cell shrinks (crenation).

Muscle Tissue and Structure

Types of Muscle Tissue

Muscle tissue is specialized for contraction and exists in three forms:

• Skeletal muscle: Striated, voluntary, attached to bones.

• Cardiac muscle: Striated, involuntary, found only in the heart.

• Smooth muscle: Non-striated, involuntary, lines hollow organs.

Gross Anatomy of Skeletal Muscle

Skeletal muscles are organized into connective tissue layers that support and protect muscle fibers.

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Encloses bundles of fibers (fascicles).

• Epimysium: Encloses the entire muscle.

Development and Structure of Skeletal Muscle Cells

Skeletal muscle fibers are formed by the fusion of embryonic myoblasts, resulting in long, multinucleate cells. Satellite cells aid in muscle repair.

Microscopic Structure of Muscle Fibers

Muscle fibers contain myofibrils composed of myofilaments (actin and myosin). The sarcolemma is the cell membrane, and the sarcoplasmic reticulum stores calcium ions.

Myofilament Arrangement

Thick (myosin) and thin (actin) filaments are organized into sarcomeres, the functional units of muscle contraction.

• Thick filaments: Bundles of myosin molecules with heads that form cross-bridges during contraction.

• Thin filaments: Twisted strands of actin, with tropomyosin covering active sites and troponin regulating access.

Striated Sarcomeres

Sarcomeres are defined by Z lines and contain alternating bands:

• I band: Light, only thin filaments.

• A band: Dark, thick filaments and overlapping thin filaments.

• H band: Only thick filaments.

Sliding Filament Model of Muscle Contraction

Sarcomere Structure and Function

Muscle contraction occurs as thin filaments slide over thick filaments, shortening the sarcomere and thus the muscle fiber.

Sliding Filament Theory

The sliding filament theory explains the steps of muscle contraction:

1. Contraction Cycle Begins: Calcium ions bind to troponin, exposing actin's active sites.

2. Active-Site Exposure: Troponin changes shape, moving tropomyosin and exposing actin's active sites.

3. Cross-Bridge Formation: Energized myosin heads bind to exposed actin sites.

4. Myosin Head Pivoting (Power Stroke): Myosin head pivots, pulling actin toward the M line and releasing ADP and Pi.

5. Cross-Bridge Detachment: ATP binds to myosin, causing it to detach from actin.

6. Myosin Reactivation: ATP is hydrolyzed, re-cocking the myosin head.

Role of ATP in Muscle Contraction

ATP provides energy for myosin head movement and is essential for both contraction and relaxation.

• ATP hydrolysis:

• Myosin heads bind ATP to detach from actin and re-cock for another cycle.

Muscle Contraction Mechanisms

Length-Tension Relationship

The amount of tension a muscle can produce depends on the overlap between actin and myosin filaments and the room for movement during contraction.

Frequency of Stimulation

Muscle tension increases with repeated stimulation:

• Twitch: Single, brief contraction.

• Summation: Increased tension due to repeated stimulation.

• Incomplete tetanus: Tension rises to a peak with brief relaxation periods.

• Complete tetanus: Tension plateaus at maximum level with no relaxation.

Motor Units and Tension Production

Motor units consist of a motor neuron and all the muscle fibers it innervates. Small motor units provide precise control, while large units provide gross movement.

Muscle Tone and Types of Contractions

• Isotonic contraction: Muscle length changes as tension rises.

• Isometric contraction: Tension rises but muscle length remains constant.

Energy Use and Muscle Contraction

ATP and Creatine Phosphate

Muscle cells store limited ATP and creatine phosphate (CP) for rapid energy release. Additional ATP is generated as needed.

• Creatine phosphate reaction:

Aerobic and Anaerobic Metabolism

• Aerobic respiration: Occurs in mitochondria, uses O2, produces CO2 and ATP.

• Anaerobic metabolism: Produces ATP rapidly but inefficiently, leading to lactic acid buildup and fatigue.

Types of Skeletal Muscle Fibers

Slow, Fast, and Intermediate Fibers

• Slow fibers (Type I): Small diameter, abundant mitochondria, fatigue-resistant.

• Fast fibers (Type II-B): Large diameter, rapid contraction, fatigue quickly.

• Intermediate fibers (Type II-A): Fast contraction, greater resistance to fatigue with training.

Muscle Fiber Distribution and Training Effects

• Pale (white) muscles: Mostly fast fibers, suited for rapid, powerful contractions.

• Dark (red) muscles: Dominated by slow fibers, suited for endurance activities.

• Training increases intermediate fibers and fatigue resistance.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle diameter due to more actin and myosin.

• Atrophy: Decrease in muscle diameter due to loss of myofilaments.

Muscle Disorders and Aging

Muscular System Disorders

• Botulism: Blocks ACh release, causing paralysis.

• Myasthenia gravis: Autoimmune disorder affecting ACh receptors.

• Polio: Loss of motor neurons.

• Multiple sclerosis: Damage to myelin, reducing action potential efficiency.

• Tetanus: Excessive stimulation of motor neurons.

Aging and the Muscular System

• Decrease in muscle fiber number and size

• Reduced ATP, CP, glycogen, and myoglobin content

• Increased fibrosis and decreased elasticity

• Lower exercise tolerance and increased fatigue

Smooth and Cardiac Muscle

Smooth Muscle Structure and Function

Smooth muscle is present in most organ systems and is non-striated. Thick filaments are scattered, and thin filaments attach to dense bodies.

Cardiac Muscle Structure and Function

• Striated, involuntary, found only in the heart.

• Cells linked by intercalated disks (desmosomes and gap junctions).

• Automaticity: Can contract without neural stimulation.

Control of Contraction

• Cardiac muscle: Controlled by pacemaker cells and autonomic nervous system.

• Smooth muscle: Controlled by pacesetter cells, hormones, and autonomic nervous system.

Summary Table: Muscle Tissue Types