مقدمة في علم وظائف الأعضاء الخلوي

Introduction to Cellular Physiology

• Cellular physiology is the study of the dynamic processes of life.
• It explores the physical and chemical factors that contribute to the origin and continuation of life.
• The study ranges from the simplest organisms to the most complex, including viruses, bacteria, plants, and animals, culminating in humans.

Claude Bernard

• Claude Bernard (1813-1878) is considered the father of physiology.
• His work on pancreatic juices in digestion earned him an award from the French Academy of Sciences.
• A notable finding was that the liver performs glycogenic action, in addition to bile production.
• He discovered the vaso-motor system, involving vaso-constrictor and vaso-dilator nerves, while experimenting on the effects of temperature changes in different parts of the body.
• Bernard emphasized the importance of homeostasis, the internal milieu or internal environment, for the maintenance of life.

Body Fluid Compartments

• Body water accounts for about 60% of body weight (approximately 42 liters in a 70 kg adult).
• Intracellular fluid (ICF) comprises about 40% of body weight (about 28 liters), representing two-thirds of total body water.
• Extracellular fluid (ECF) accounts for approximately 20% of body weight (about 14 liters), which is one-third of the total body water.
• ECF is further divided into plasma (25% of ECF) and interstitial fluid (ISF) (75% of ECF).

Composition of Body Fluids

• Extracellular fluid and intracellular fluid have different compositions of electrolytes and other molecules.
• Key electrolytes like sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) play crucial roles in maintaining osmotic balance and nerve impulses.
• Other components like glucose, amino acids, cholesterol, and phospholipids are also present in characteristic amounts in both compartments for normal bodily function.

Homeostasis

• Homeostasis is the maintenance of a stable internal environment in the body.
• This is crucial for the proper functioning of cells and organs.
• Homeostasis uses primarily negative feedback loops, which oppose the initial change, and less commonly, positive feedback loops, which amplify the initial change.
• Examples of negative feedback in the body include regulating blood pressure, blood glucose, and body temperature

Gain

• Gain describes the efficiency of a control system in the body, especially for physiological parameters like blood pressure.
• A system with high gain is more effective at maintaining stability.
• The gain of a system is calculated by dividing the amount of correctional response (Correction) by the initial error in the system (Error).

Examples of Regulatory Systems

• Calcium homeostasis, controlled by parathyroid hormone (PTH) and vitamin D.
• Blood glucose regulation by insulin and glucagon . These hormones and mechanisms work in concert with the aforementioned regulatory systems in the body

Membrane Structure and Function

• Cell membranes define cell boundaries enabling compartmentalization and selective permeability , regulating which substances can enter and leave.
• The fluid mosaic model describes the dynamic nature of the cell membrane consisting of a phospholipid bilayer with embedded proteins.
• The proteins within the membrane are integral or peripheral carrying out a variety of functions from signaling to transporting nutrients between internal and external cellular environments.

Membrane Proteins

• Integral membrane proteins are embedded within the lipid bilayer and play crucial roles in maintaining membrane structure and function.
• Peripheral membrane proteins adhere to the membrane surface and are primarily involved in signaling pathways.
• These proteins support various functions from transport to enzymatic activity facilitating critical cellular processes.

Factors affecting Membrane Fluidity

• Chemical factors such as cholesterol, influencing membrane fluidity's proper functioning.
• Physical factors like temperature and pressure, affecting the arrangement and movement of phospholipids.
• Physiological factors like the cell cycle, cellular differentiation, and aging, all influence properties of the cell membrane.

Membrane Transport Mechanisms

• Passive transport mechanisms include simple diffusion, facilitated diffusion, and osmosis, which don't require energy.
• Active transport mechanisms, such as primary active transport involving ATP, and secondary active transport using energy from another ion's concentration gradient, do need energy.
• Specialized transport mechanisms like endocytosis and exocytosis transport large molecules across membranes.

Types of Membrane Channels

• Channels are specialized proteins that allow specific ions to pass through the cell membrane.
• Ion channels can be non-gated (always open, like leak channels) or gated (open or close in response to stimuli).
• Gated channels can be voltage-gated, ligand-gated, or mechanically gated, each reacting to different stimuli.

Resting Membrane Potential

• The resting membrane potential is the electrical difference across the cell membrane when it's at rest.
• It is determined by the concentration gradient and permeability of the membrane to various ions especially Na+, K+, and Cl-.
• The cell membrane is selectively permeable to certain ions in a state of rest and the concentration difference between these ions are essential in maintaining the RMP.
• The Nernst equation determines the equilibrium potential for a single ion, while the Goldman-Hodgkin-Katz (GHK) equation accounts for multiple ions.

Action Potential

• An action potential is a rapid and transient change in the membrane potential, typically seen in excitable cells like neurons and muscle cells.
• It's characterized by depolarization, where the membrane potential becomes more positive, followed by repolarization, where it returns to its resting state.
• Action potentials are propagated along the membrane, allowing for rapid communication between cells.

Refractory Periods

• Refractory periods are phases of reduced or absent excitability in a cell after an action potential.
• The absolute refractory period prevents the generation of another action potential during a specific interval thus maintaining a unidirectional propagation .
• The relative refractory period allows for the generation of another action potential if the stimulus is sufficiently strong.

Muscle Contraction

• Muscle contraction in skeletal muscles is regulated by the release of neurotransmitters at neuromuscular junctions .
• It involves the interaction of actin and myosin filaments to enable muscle contraction.
• The process also requires ATP for energy.

Types of Muscle Tissues

• Skeletal muscle, smooth muscle, and cardiac muscle are the three main types of muscle tissue.

• Skeletal muscles are voluntary and striated, controlled by the somatic nervous system.

• Smooth muscles are involuntary and non-striated, regulated by the autonomic nervous system.

• Cardiac muscles are involuntary and striated, responsible for the coordinated contractions needed for efficient blood pumping and function.

Smooth Muscle Differences

• Smooth muscle exhibits differences in structure when compared to skeletal muscle.
• The presence of a unique structure, known as dense bodies, replaces the Z-discs structure.
• It is possible to exhibit a steady/prolonged state of contraction-without significant energy expenditure and the presence of different receptors to differing stimuli, such as hormones, stretching, and chemicals.

علم وظائف الأعضاء الخلوي هو دراسة العمليات الديناميكية للحياة. يستكشف العوامل الفيزيائية والكيميائية التي تساهم في أصل واستمرار الحياة. يغطي الدراسة الكائنات الحية من الفيروسات والبكتيريا إلى النباتات والحيوانات والبشر.