Synaptic Transmission: How Neurons Communicate (Step-by-Step)

Integral membrane proteins serve critical functions: channel proteins form pores allowing water-soluble molecules to pass; carrier proteins bind specific molecules and change shape for transport; receptor proteins receive signals from outside and transmit them inside; enzymatic proteins catalyze reactions at the membrane surface. Antigens are surface molecules serving as identification markers for the immune system, crucial for distinguishing self from non-self cells and determining blood group compatibility. Cell membranes are selectively permeable, allowing some substances while blocking others, which maintains cellular homeostasis by controlling ion concentrations and solute levels. This selective permeability ensures essential molecules enter while harmful substances are kept out, maintaining the stable internal environment necessary for cellular function.

Cells need channels or gates to move substances from outside to inside. Cell membrane has protein channels and pumps that allow specific substances to move into the cell. Channels are specific to particular substances - for example, glucose has its own channel while other substances may not have channels. This selective permeability is essential for cellular function and survival. Living cells require cell membrane, cytoplasm, DNA, ribosomes, and protein channels to function properly. Without any of these essential components, the cell cannot function and will die.

Membrane proteins are essential for cellular function and are classified as peripheral (attached to one side, providing mechanical support or forming glycoproteins/glycolipids for cell recognition) or integral (spanning the entire membrane width). Integral proteins facilitate molecular transport through two mechanisms: channel proteins create water-filled pores allowing ions to pass, while carrier proteins bind to molecules like glucose and undergo conformational changes to transport them across. The membrane exhibits selective permeability, permitting only lipid-soluble molecules (oxygen, CO2, water, certain hormones) to diffuse directly through the bilayer, while water-soluble and large polar molecules require protein-mediated transport.

Transport proteins enable selective permeability by moving specific substances across membranes. Ion channels allow passive ion passage, while active transport proteins move substances against concentration gradients using energy. The sodium-potassium pump (Na+/K+ ATPase) uses ATP to move three sodium ions out and two potassium ions into the cell, creating concentration gradients essential for membrane repolarization after electrical signals. Hydrophilic substances cannot pass directly through the lipid bilayer; only specific properties allow passage (like corticosteroids). This selective permeability maintains cellular integrity and enables precise control over internal environment.

Membrane proteins serve multiple critical functions: integral proteins penetrate the lipid bilayer and act as channels, carriers, enzymes, and receptors; peripheral proteins attach to the membrane surface. The membrane is selectively permeable, allowing some substances to pass while blocking others. This selectivity is maintained by the hydrophobic lipid bilayer, which prevents polar molecules and ions from passing freely. Channel proteins form hydrophilic passages, while carrier proteins bind specific molecules and change shape to transport them. This selective permeability is essential for maintaining cellular homeostasis and proper internal concentrations.

Cells maintain concentration gradients where potassium tends to exit, while sodium, chloride, and calcium tend to enter. The Nernst equation calculates equilibrium potentials: potassium at -92 mV, sodium at +67 mV, chloride at -86 mV, and calcium at +123 mV. Real cells are permeable to multiple ions simultaneously, with potassium, sodium, chloride, and calcium being the primary contributors to resting membrane potential. This multi-ion permeability distinguishes real cells from simplified models.

Cells maintain concentration gradients for key ions: sodium (145 mM outside, 15 mM inside), potassium (4 mM outside, 150 mM inside), and calcium (10^-4 mM outside, 10^-4 mM inside). These gradients, combined with selective membrane permeability, create the resting membrane potential of approximately -20 mV in non-excitable cells and -90 mV in muscle cells. Potassium's high intracellular concentration drives its diffusion out of cells, establishing the electrical potential.

Membrane potential is the electrical capacity of cell membranes to receive, generate, or transmit impulses. All cells exhibit electrical polarity with negative interior and positive exterior in resting states, typically ranging from -70 to -90 mV. Cells maintain concentration gradients: potassium (140 mEq/L inside, 4 mEq/L outside) and sodium (142 mEq/L outside, 10 mEq/L inside). Cations (positively charged ions like Na+ and K+) must be accompanied by anions (negatively charged ions like Cl- and proteins) to maintain electrical neutrality. The chemical gradient drives ions from high to low concentration, while the electrical gradient opposes this movement as charges separate.

Cardiac cells maintain specific ion concentration gradients: sodium (Na+) is higher outside, potassium (K+) is higher inside, and calcium (Ca2+) is higher outside. These gradients are established by the sodium-potassium pump and sodium-calcium exchanger. The IK1 potassium channel is always open, allowing potassium to flow outward according to its concentration gradient, establishing the resting membrane potential of approximately -90 mV. Ion flow direction is determined by concentration gradients: sodium and calcium flow inward while potassium flows outward.

Membrane potentials are electrical phenomena across cell membranes fundamental to cellular excitability. The main types include: equilibrium potential (single electrolyte effect, Nernst equation), resting membrane potential (combined electrolyte effect, Goldman equation), electrotonic potentials (stimulus-induced variations), and action potentials (all-or-nothing response). Potassium is the most important electrolyte for resting membrane potential due to its leak channels. Potassium is more abundant intracellularly (140 mEq/L) than extracellularly (3.5-5 mEq/L), creating a strong gradient that drives potassium out of the cell. Leak channels are always open, constantly allowing potassium efflux that makes the inside of the cell negative. The resting membrane potential of neurons is approximately -70 mV, less negative than potassium's equilibrium potential (-93 mV) because not all potassium channels are open. A concentration gradient is the difference in electrolyte concentration between inside and outside the cell. Electrolytes move down their gradient (from high to low concentration) through channels without requiring energy. When extracellular potassium increases, the gradient decreases, so less potassium flows out. When extracellular potassium decreases, the gradient increases, so more potassium flows out. Sodium is more abundant extracellularly (135-145 mEq/L) than intracellularly (14 mEq/L), creating a strong gradient that drives sodium into the cell. Calcium is more abundant extracellularly (4.25-5.25 mEq/L) than intracellularly (0.0001 mEq/L), with an extremely strong gradient. However, at rest, very few calcium channels are open, so calcium's effect on resting membrane potential is minimal.

The action potential propagates along the axon through a chain reaction mechanism. Voltage-gated sodium channels trigger opening first, allowing rapid sodium influx that depolarizes the membrane. After an initial spike, sodium channels close and potassium channels open, causing potassium efflux that repolarizes the membrane. Finally, potassium channels close and the membrane returns to resting state through leakage and ion pump activity. This local process repeats sequentially along the axon.

The action potential is the electrical signal that travels along the axon. Its mechanism involves: (1) Depolarization phase: Sodium channels open, sodium ions rush in, making the inside of the cell positive. (2) Repolarization phase: Sodium channels close, potassium channels open, potassium ions rush out, restoring the negative charge inside. (3) Hyperpolarization phase: Potassium channels remain open briefly, making the inside more negative than resting potential. (4) Return to resting potential: The sodium-potassium pump restores the original ion distribution. This process regenerates the signal at each point along the axon.

After depolarization, sodium channels close and potassium channels open, causing repolarization. However, potassium channels close slowly, leading to hyperpolarization (membrane potential dropping to approximately -90 millivolts). The sodium-potassium pump gradually restores the resting potential. During the absolute refractory period (when sodium channels are open or inactivated), no new action potential can be initiated regardless of stimulus strength. During the relative refractory period (hyperpolarization), a new action potential requires a stronger-than-normal stimulus. Action potentials propagate unidirectionally along axons due to refractory periods. When depolarization occurs at one point, that region becomes refractory and cannot be re-excited. Depolarization spreads to adjacent regions, which are not yet refractory, causing them to depolarize and become refractory in turn. In humans, myelinated axons enable saltatory conduction, where action potentials 'jump' between nodes of Ranvier (gaps in myelin), which is much faster than continuous conduction.