Cell Membrane Structure and Transport

Structure & Composition of the Cell Membrane

The cell (plasma) membrane is a dynamic barrier that separates the cell's interior (intracellular fluid, ICF) from its external environment (extracellular fluid, ECF). Its structure is essential for maintaining cellular homeostasis and regulating transport.

• Phospholipid Bilayer: Composed of two layers of amphipathic phospholipids.

  • Phosphate Head: Polar, hydrophilic; faces the aqueous ICF and ECF.

  • Fatty Acid Tails: Nonpolar, hydrophobic; oriented inward, away from water. Saturated and unsaturated tails influence membrane fluidity.

• Cholesterol: Interspersed within the bilayer, stabilizes membrane structure and modulates fluidity.

• Fluid Mosaic Model: Describes the lateral movement of lipids and proteins within the membrane, contributing to its flexibility and functionality.

Membrane Proteins & Their Functions

Membrane proteins are integral to the cell membrane's diverse functions, including transport, signaling, and cell recognition.

• Integral Proteins: Embedded within the membrane; may span the bilayer (transmembrane).

  • Channel Proteins: Form selective pores for ions or water.

  • Carrier Proteins: Bind and transport specific molecules via conformational changes.

  • Receptors: Bind ligands (e.g., hormones, neurotransmitters) to initiate intracellular responses.

• Glycoproteins & Glycocalyx:

  • Glycoproteins: Proteins with attached carbohydrate chains; extend into the ECF.

  • Glycocalyx: Carbohydrate-rich zone on the cell surface; functions in cell recognition and immune response.

• Peripheral Proteins: Loosely attached to the membrane surface; often serve as enzymes or structural supports.

Selective Permeability Overview

The cell membrane is selectively permeable, allowing certain substances to cross more easily than others based on size, charge, and polarity.

• Passes Easily: Small, nonpolar molecules (e.g., O2, CO2, lipids, alcohol).

• Requires Assistance: Large polar molecules (e.g., glucose) and ions (e.g., Na+, K+, Ca2+, Cl-).

Passive Transport (No Energy Required)

Passive transport moves substances down their concentration gradient (from high to low concentration) without using cellular energy (ATP).

• Simple Diffusion: Direct movement of small, nonpolar molecules through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or charged molecules via channel or carrier proteins.

• Osmosis: Diffusion of water across a semipermeable membrane from lower to higher solute concentration.

  • Tonicity:

    • Isotonic: Equal solute concentration; no net water movement.

    • Hypertonic: Higher solute outside; cell shrivels (crenation).

    • Hypotonic: Lower solute outside; cell swells and may burst (lysis).

• Filtration: Movement of fluid and small solutes across a membrane driven by hydrostatic pressure (e.g., in kidneys).

Active Transport (Requires ATP)

Active transport moves substances against their concentration gradient (from low to high concentration) or transports bulk materials, requiring energy input (ATP).

• Primary Active Transport: Direct use of ATP to power membrane pumps.

  • Sodium-Potassium Pump (Na+/K+ ATPase): Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed; maintains resting membrane potential.

• Secondary Active Transport: Uses the energy stored in ion gradients (established by primary active transport) to move other substances.

  • Symporters: Move two substances in the same direction (e.g., Na+-glucose symporter).

  • Antiporters: Move two substances in opposite directions.

Transport Type Comparison Table

Key Equations

• Osmolarity:

Basic Structure and Function of the Nervous System

Anatomical Divisions of the Nervous System

The nervous system is divided into central and peripheral components, each with specialized roles in processing and relaying information.

• Central Nervous System (CNS): Brain and spinal cord; main processing center.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; connects CNS to limbs, organs, and skin.

CNS vs. PNS Terminology Table

Gray Matter vs. White Matter

• Gray Matter: Contains neuron cell bodies and dendrites; site of information processing.

• White Matter: Composed of myelinated axons; facilitates rapid signal transmission.

Basic Functions of the Nervous System

1. Sensation (Sensory Input): Detects changes in the environment via receptors.

2. Integration: Processes and interprets sensory input, formulates responses.

3. Response (Motor Output): Executes actions via muscle contraction or gland secretion.

Functional & Control Divisions

• Somatic Nervous System (SNS): Voluntary control of skeletal muscles; includes conscious and reflex actions.

• Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands; maintains homeostasis.

• Enteric Nervous System (ENS): Specialized network in the digestive tract; regulates gastrointestinal function.

Nervous Tissue: Structure and Function

Overview of Nervous Tissue Cells

• Neurons: Excitable cells that transmit electrical impulses and communicate via chemical signals.

• Glial Cells (Neuroglia): Support, protect, and nourish neurons; maintain the neural environment.

Structural Parts of a Neuron

• Soma (Cell Body): Contains nucleus and organelles; integrates signals.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts action potentials away from the soma.

  • Axon Hillock: Site where action potentials originate.

  • Axon Terminal & Synaptic End Bulbs: Release neurotransmitters to communicate with target cells.

• Myelin Sheath: Insulating layer that increases conduction speed.

• Nodes of Ranvier: Gaps in myelin; facilitate rapid signal conduction.

Structural Classification of Neurons

• Unipolar (Pseudo-unipolar): Single process; sensory neurons in PNS.

• Bipolar: Two processes; found in special senses (e.g., retina, olfactory epithelium).

• Multipolar: One axon, multiple dendrites; most common type (motor neurons, interneurons).

• Specialized Multipolar Subtypes: Pyramidal cells (cerebral cortex), Purkinje cells (cerebellum).

Glial Cells of the CNS and PNS

The Function of Nervous Tissue

Sequence of Events: Sensory-Motor Pathway

Nervous tissue links stimulus detection to response through a defined pathway:

1. Stimulus detection by sensory receptors (e.g., thermoreceptors in skin).

2. Graded potential reaches threshold, triggering an action potential in the sensory neuron.

3. Signal enters the spinal cord, synapses with interneurons.

4. Ascending signal relayed to the thalamus (sensory relay station).

5. Signal reaches the cerebral cortex for conscious perception.

6. Integration with memories and emotions; response plan formulated.

7. Motor command sent from upper motor neuron in cortex down the spinal cord.

8. Lower motor neuron activates target muscle for response.

Key Terminology

• Thermoreceptor: Detects temperature changes.

• Graded Potential: Localized change in membrane potential proportional to stimulus strength.

• Threshold: Membrane voltage required to trigger an action potential.

• Thalamus: Brain region that relays sensory signals to the cortex.

• Precentral Gyrus: Initiates voluntary motor movements.

• Upper/Lower Motor Neuron: Relay motor commands from CNS to muscles.

The Action Potential

Ion Distribution & Resting Membrane Potential

• Resting Membrane Potential (RMP): Typically -70 mV inside the cell relative to outside.

• Ion Distribution: High Na+ outside, high K+ inside, with negatively charged proteins and phosphates inside.

• Sodium-Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in per ATP.

• Leakage Channels: Allow passive movement of Na+ and K+; gradients maintained by active transport.

Gated Ion Channels

• Ligand-Gated: Open in response to neurotransmitter binding.

• Mechanically Gated: Open in response to physical deformation (e.g., touch).

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

• Channel Selectivity: Determined by charge and size exclusion.

Phases of the Action Potential

1. Resting State: -70 mV; Na+ and K+ channels closed.

2. Threshold: -55 mV; initial Na+ channels open.

3. Depolarization: Na+ influx raises voltage to +30 mV.

4. Repolarization: Na+ channels inactivate; K+ channels open, voltage returns toward -70 mV.

5. Hyperpolarization: K+ channels slow to close, voltage dips below -70 mV before stabilizing.

Key Properties of Action Potentials

• All-or-None Principle: Action potentials fire fully if threshold is reached; otherwise, not at all.

• Intensity Coding: Stronger stimuli increase firing frequency, not amplitude.

• Refractory Periods:

  • Absolute: No new AP possible; Na+ channels inactivated.

  • Relative: AP possible with stronger stimulus; K+ channels closing.

Propagation & Conduction Speed

• Continuous Conduction: Stepwise depolarization along unmyelinated axons (slow).

• Saltatory Conduction: AP "jumps" between Nodes of Ranvier on myelinated axons (fast).

• Factors Increasing Speed: Myelination and larger axon diameter.

Communication Between Neurons

Graded Potentials

• Depolarizing: Move membrane potential toward threshold (e.g., Na+ influx).

• Hyperpolarizing: Move membrane potential away from threshold (e.g., K+ efflux, Cl- influx).

• Types:

  • Generator Potential: In sensory dendrites; can trigger AP.

  • Receptor Potential: In sensory receptor cells; triggers neurotransmitter release.

  • Postsynaptic Potential (PSP): In postsynaptic neurons; can be excitatory (EPSP) or inhibitory (IPSP).

Summation

• Spatial Summation: Multiple inputs from different locations add together.

• Temporal Summation: Rapid, repeated inputs from one source add together.

Synaptic Transmission Mechanics

1. AP arrives at presynaptic terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters.

3. Ca2+ triggers exocytosis of neurotransmitter vesicles.

4. Neurotransmitter diffuses across synaptic cleft, binds to postsynaptic receptors.

5. Signal terminated by enzymatic breakdown, reuptake, or glial uptake.

Major Neurotransmitter Systems

• Cholinergic (Acetylcholine): Nicotinic (ionotropic, always excitatory) and muscarinic (metabotropic, excitatory or inhibitory) receptors.

• Amino Acid: Glutamate (excitatory), GABA & Glycine (inhibitory).

• Biogenic Amines: Serotonin (mood, sleep), Dopamine (motor control, reward), Norepinephrine/Epinephrine (fight-or-flight).

• Neuropeptides: Modulate pain and hormone pathways (e.g., endorphins).

Receptor Mechanisms

• Ionotropic Receptors: Ligand-gated ion channels; direct, rapid effects (e.g., nicotinic ACh receptor).

• Metabotropic Receptors: G protein-coupled; indirect, slower, and longer-lasting effects via second messengers (e.g., muscarinic ACh receptor).

Additional Key Concepts and Tables

Graded Potentials vs. Action Potentials

IPSP vs. EPSP

• EPSP (Excitatory): Depolarizes membrane, moves toward threshold (usually via Na+ influx).

• IPSP (Inhibitory): Hyperpolarizes membrane, moves away from threshold (usually via K+ efflux or Cl- influx).

Somatic vs. Autonomic Nervous System

• Somatic: Voluntary control of skeletal muscle.

• Autonomic: Involuntary control of cardiac/smooth muscle and glands; subdivided into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

Afferent vs. Efferent Neurons

• Afferent (Sensory): Carry information toward CNS.

• Efferent (Motor): Carry commands away from CNS to effectors.

Key Voltages in Neuronal Membrane Potential

• Resting:

• Threshold:

• Peak Depolarization:

• Hyperpolarization:

Example: Sodium-Potassium Pump Cycle

1. 3 Na+ bind to pump (cytosolic side).

2. ATP phosphorylates pump; conformational change releases Na+ outside.

3. 2 K+ bind (extracellular side); dephosphorylation returns pump to original shape, releasing K+ inside.

Ion Exchange Ratio: 3 Na+ out, 2 K+ in per ATP.

Example: Synaptic Transmission Steps

1. AP arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ influx.

3. Neurotransmitter vesicles fuse with membrane; release contents into synaptic cleft.

4. Neurotransmitter binds to postsynaptic receptors; initiates graded potential.

5. Signal terminated by reuptake, enzymatic degradation, or diffusion.

Additional info: This guide integrates foundational concepts from cell membrane transport and neurophysiology, including PhysioEx simulation highlights and clinical/biotechnological applications. For further study, review the mechanisms of neurotoxins, the role of myelination in disease, and the pharmacology of neurotransmitter systems.