Selective Permeability of Plasma Membrane

Selective Permeability of Plasma Membrane

• The plasma membrane is highly permeable to potassium ions (K+), allowing them to easily move across it.
• The membrane is less permeable to sodium ions (Na+).
• The negative charge within the cell is due to large, non-diffusible anions (PO43-, SO42-, organic acids, proteins) that cannot escape the cell.
• Na+/K+ pumps actively move 3 Na+ ions out of the cell for every 2 K+ ions brought in, constantly working against the concentration gradient and requiring a significant amount of ATP.

Ion Channels

• Plasma membranes of all cells contain proteins that act as ion channels, facilitating the movement of ions across the membrane.
• Excitable cells (nerve and muscle cells) have ion channels that respond to specific stimuli, triggering impulses or action potentials.
• When these channels are open, they form pathways for the flow of ions, similar to how electrical wires connect to a battery to allow electron flow.

Two Basic Types of Ion Channels

• Leakage channels are always open, continuously allowing the passive movement of ions.
• Gated channels open and close in response to specific stimuli, controlling the flow of ions.
  • Chemically-gated channels are activated by chemical messengers like neurotransmitters and hormones, found in neuron cell bodies and dendrites, as well as receptors for taste and smell.
  • Mechanically-gated channels are triggered by mechanical vibrations and pressure, found in touch and pressure receptors in the skin.
  • Light-gated channels are activated by light, found in rods and cones in the retina of the eye.
  • Voltage-gated channels open and close in response to changes in the membrane potential, highly concentrated at the axon trigger zone.

Local Potentials

• Localized changes in membrane potential, triggered by stimuli like chemicals, light, heat, or mechanical disturbance.
• Depolarization occurs when gated Na+ channels open, allowing Na+ ions to rush into the cell, decreasing the potential across the membrane.
• This influx of Na+ creates a local potential, a short-lived change in voltage along the membrane.
• The duration and magnitude of local potentials vary depending on the strength of the stimulus.

Action Potentials

• Dramatic changes in membrane potential triggered when a graded potential reaches a critical threshold and activates voltage-gated ion channels.
• A segment of membrane undergoes depolarization, reversing its polarity, and then repolarizes.
• This rapid change in membrane potential is known as an action potential.

Distinguishing Local Potentials from Action Potentials

• Local potentials are graded: Their magnitude depends on the strength of the stimulus.
• Local potentials are decremental: They become weaker as they spread from the point of stimulation.
• Local potentials are reversible: They disappear if the stimulation ceases.
• Local potentials can be excitatory or inhibitory: Inhibitory local potentials cause hyperpolarization, increasing the negative potential.

Action Potential Steps

• When Na+ ions reach the axon hillock, they depolarize the membrane, creating a rising local potential.
• If this local potential reaches the threshold (~-55 mV), voltage-gated Na+ channels open, initiating the action potential.
• As Na+ enters the cell, further depolarization occurs, opening more voltage-gated Na+ channels, creating a positive feedback loop that rapidly increases the membrane voltage.
• As the membrane potential passes 0 mV, Na+ channels inactivate, and slow-opening K+ gates fully open.
• K+ ions rush out of the cell, repolarizing the membrane and shifting the voltage back to negative values.
• K+ channels remain open longer than Na+ channels, causing a brief hyperpolarization (more negative than resting potential) before returning to resting potential.

Synapse

• Junction between two neurons, where the first neuron (presynaptic neuron) releases a neurotransmitter onto the second neuron (postsynaptic neuron), which then responds to the neurotransmitter.
• Synapses can be axodendritic, axosomatic, or axoaxonic depending on the connection between the axon and the target neuron.

Supporting Cells

• Supporting cells in the nervous system, known as neuroglia or glia, are 10-50 times more numerous than neurons.
• They are much smaller than neurons and play a crucial role in supporting and maintaining neurons.

Types of Supporting Cells:

• Central Nervous System (CNS):
  • Astrocytes: Provide structural support, regulate neurotransmitter levels, and contribute to the blood-brain barrier.
  • Ependymal cells: Line cavities in the brain and spinal cord, producing cerebrospinal fluid (CSF).
  • Microglia: Immune cells of the CNS, engulfing cellular debris and pathogens.
  • Oligodendrocytes: Form myelin sheaths around CNS axons, improving signal transmission speed.
• Peripheral Nervous System (PNS):
  • Schwann cells: Form myelin sheaths around PNS axons, promoting rapid signal transmission.
  • Satellite cells: Surround neuron cell bodies in the PNS, providing structural support and regulating the environment around the neuron.

Myelinated & Unmyelinated Axons

• Myelinated axons are wrapped in multiple layers of plasma membrane, forming a myelin sheath.
• Each layer of the myelin sheath is formed by a separate neurolemmocyte (Schwann cell in the PNS, oligodendrocyte in the CNS).
• The myelin sheath is rich in fat, providing excellent insulation and allowing for faster nerve impulse transmission.
• Unmyelinated axons rest in invaginations of oligodendrocytes or Schwann cells without forming a complete myelin sheath.

White Matter vs. Gray Matter

• White matter is dominated by myelinated axons, appearing white due to the myelin sheaths.
• Gray matter is dominated by unmyelinated axons and neuron cell bodies, appearing gray.
• The cortex is a layer of gray matter on the outer surface of the brain, while nuclei are collections of gray matter within the brain.

Electrical Signals

• Neural communication relies on the production of electrical potentials and currents.
• Electrical potential is a difference in the concentration of charged particles across a membrane.
• Electrical current is the flow of charged particles from one point to another within the cell.
• Living cells are polarized, meaning there is an unequal distribution of charge across the membrane, resulting in a resting membrane potential.

Ionic Basis of Resting Membrane Potential

• Na+ ions are highly concentrated outside the cell (ECF), while K+ ions are highly concentrated inside the cell (ICF).

Resting Membrane Potential

• The resting membrane potential is the electrical potential difference across the membrane when a neuron is not being stimulated, typically around -70 mV.
• This potential is maintained by the uneven distribution of ions between the ICF and ECF, driven by:
  • Diffusion of ions down their concentration gradients.
  • Active transport of ions across the membrane (e.g., Na+/K+ pump).