Neurology II – Neurophysiology Vocabulary

Overview of Neural Transmission

• Five sequential stages outline information flow within neurons:
  1. Resting potential
  2. Local (graded) potential
  3. Action potential
  4. Synaptic activity
  5. Information processing
• Direction of signal is unidirectional: dendrites → cell body → axon → synapse → next cell.
• Each stage depends on specific ion channel behavior and membrane potentials.

Membrane Potential Fundamentals

• Membrane potential = electrical charge difference across the plasma membrane.
  • Inside: high K^+, high negatively charged proteins.
  • Outside: high Na^+.
• Represented as voltage (millivolts, mV).
  • Convention: interior relative to exterior; negative interior at rest.
• Key principle: Potential changes only when ions cross the membrane through channels.

Ion Channel Types & Stimuli

• Chemically-gated (ligand-gated) channels
  • Located mainly on dendrites & cell body.
  • Open when a neurotransmitter/chemical binds.
• Voltage-gated channels
  • Dense at the axon hillock/trigger zone, along axon, and axon terminals.
  • Open when adjacent membrane reaches a specific voltage threshold.
• Mechanically-gated channels
  • Often present in sensory dendrites.
  • Respond to deformation/pressure.
• Channel opening leads to depolarization (membrane becomes less negative) or hyperpolarization (more negative) depending on ion flow.

Electrical States of a Neuron

1. Resting Potential

• Undisturbed neuron maintains V_{rest} \approx -70\,\text{mV}.
• Established by Na⁺/K⁺ ATP-ase pumps and leak channels.

2. Local (Graded) Potential

• Stimulus (usually chemical) opens chemically-gated channels on soma.
• Local change in membrane potential is:
  • Proportional to stimulus strength.
  • Greatest at the site of stimulation; decays with distance (electrotonic spread).
• Can be depolarizing or hyperpolarizing; summation at axon hillock determines outcome.

3. Action Potential (AP)

• Requires threshold depolarization at trigger zone: V_{threshold} \approx -55\,\text{mV}.
• Sequence in voltage-gated channels:
  1. Rapid Na^+ influx → rising phase.
  2. Na^+ channels inactivate; K^+ channels open → falling phase.
  3. After-hyperpolarization as K^+ exit overshoots V_{rest}.
• All-or-none: amplitude independent of stimulus strength once threshold reached.

Propagation of the Action Potential

• AP propagates along the entire axon; cannot reverse because of refractory period.

Continuous Propagation (Unmyelinated Fibers)

• AP spreads incrementally along membrane.
• Conduction velocity: \approx 1\,\text{m!/s}.

Saltatory Propagation (Myelinated Fibers)

• Myelin (created by oligodendrocytes in CNS) insulates internodes, forcing AP to "jump" node to node.
• Results in rapid conduction up to 180\,\text{m!/s}.
• Energy-efficient: fewer ions moved overall → reduced pump workload.

Clinical Correlation — Multiple Sclerosis (MS)

• Autoimmune destruction of oligodendrocytes & myelin sheaths.
• Proposed mechanism: molecular mimicry/cross-reactivity with pathogen antigens.
• Loss of insulation → impaired or blocked saltatory conduction.
• Common symptoms:
  • Sensory deficits (numbness, visual problems)
  • Spasticity, motor coordination loss
  • Bladder & intestinal dysfunction
  • Cognitive or psychological issues

Diagnostic Workflow for MS

1. Evaluate neurological symptoms profile.
2. Blood tests to exclude inflammatory, infectious, chemical etiologies.
3. Lumbar puncture (spinal tap): look for microglia activation & inhibitory proteins (e.g., oligoclonal bands).
4. MRI: detect demyelinating lesions & assess extent of CNS damage.
5. Evoked potential tests: measure slowed conduction velocities in visual or somatosensory pathways.

Linking Forward

• Concludes Neurology II content; next lecture (Neurology III) will build on these foundations, likely expanding into synaptic mechanisms, neural integration, or clinical applications.

Concept Map / Integration Tips

• Always relate ion channel type ➔ location ➔ stimulus ➔ resulting potential change.
• Remember key voltages: -70\,\text{mV} (rest), -55\,\text{mV} (threshold).
• Myelination influences speed and energy cost of signaling; pathological loss explains MS symptomatology.
• Use real-world parallels: electrical insulation on wires ≈ myelin on axons; breaks in insulation cause current leak & signal degradation.