Neural Circuits, EEG, and Seizures (Video Notes)

Neural Reflexes: Patellar Tendon (Knee-Jerk)

• Reflex is extremely fast due to the following anatomical and physiological features:

  • Sensory and motor axons are myelinated and large, increasing conduction velocity.

  • Sensory cells synapse directly onto motor neurons (monosynaptic reflex arc).

  • Both the central synapse (sensory to motor) and the neuromuscular junction are fast synapses.

• Reflex pathway (patellar tendon):

  • Stimulus: Tap on the patellar tendon stretches the quadriceps via the knee-jerk stimulus trigger zone.

  • Receptor: Muscle stretch receptor (muscle spindle) detects the stretch.

  • Afferent: Sensory neuron carries signal to the spinal cord.

  • Integration: Sensory neuron synapses directly on a motor neuron in the ventral horn of the spinal cord.

  • Efferent: Motor neuron fires and activates the quadriceps muscle.

  • Response: Quadriceps contracts, producing knee extension.

• Key components and signals:

  • Axon hillock and initial segment of the sensory neuron; unipolar sensory cell body; axon terminal.

  • Motor neuron cell body located in the ventral horn.

  • Action potentials drive the muscle fiber response.

  • Receptor potential and postsynaptic potential are part of the signaling chain leading to action potentials.

• Quantitative note from the slide:

  • Response magnitude shown as approximately 98\% of maximum in this reflex example.

Electroencephalography (EEG) and Event-Related Potentials (ERPs)

• EEG is the recording of spontaneous brain potentials (brain waves).

• ERPs are EEG responses to a single stimulus (e.g., a flash of light or a loud sound).

• ERPs have distinctive shapes and time delays (latency) relative to the stimulus.

• EEGs can distinguish sleep states and are used in the diagnosis of seizure disorders.

• Multichannel EEG example (conceptual):

  • Recordings from left and right hemispheres across regions.

  • Scale bar shown as about 200\,\mu\text{V} and a time scale of 1\,\text{s} per division.

  • The montage demonstrates simultaneous recordings from multiple cortical areas.

Electrical Storms in the Brain Can Cause Seizures

• In the normal brain, activity tends to be desynchronized across regions.

• Epilepsy is a brain disorder characterized by seizures.

• A seizure is a wave of abnormally synchronous electrical activity in the brain.

Types of Seizures

• 1) Tonic-clonic seizures: abnormal EEG activity throughout the brain.

  • Seizure activity is widespread across cortical regions; the sequence is shown as before, during, and after phases.

  • The slide includes a schematic across hemispheres illustrating broad involvement.

  • A slide annotation shows 97\% in the context of the example data.

• 2) Simple partial (focal) seizures: seizure activity shows patterns localized to a brain region for a short duration.

  • Duration: about 5\,\text{s} \le t \le 15\,\text{s} (5 to 15 seconds).

  • These seizures may occur several times a day.

  • Localization examples include left frontal (LT), left temporal (LT), left occipital (LO), right frontal (RF), right temporal (RT), and right occipital (RO).

• 3) Complex partial seizures: do not involve the entire brain; often preceded by an unusual sensation or aura.

  • The slide includes a complex partial seizure image and caption.

  • The same slide notes an associated statistic: 97\% (context-specific).

Treatment and Cortical Mapping

• Management options:

  • Many seizure disorders can be controlled with antiepileptic medications.

  • If medications fail and seizures are severe and frequent, neurosurgical removal of part of an awake patient’s brain may be considered.

  • Electrical stimulation of the cortex is used to help identify the seizure origin so that only the implicated region is removed.

• Wilder Penfield and cortical mapping:

  • Penfield was a pioneer in electrical stimulation mapping of the cortex.

  • He observed consistent body-part responses when stimulating identical cortical regions, revealing systematic functional organization.

  • This led to the concept of the homunculus—a visual representation of body parts mapped onto the cortex.

• The Homunculus (somatotopic map):

  • Demonstrates how body functions are organized on the brain.

  • The map is illustrated along the motor cortex (precentral gyrus) and the somatosensory cortex (postcentral gyrus).

  • Typical representations include:

  • From toe to head along the medial-to-lateral axis: toes → foot → leg → genitalia → trunk → neck → head → shoulder → arm → elbow → forearm → hand → fingers (including thumb) → eyes → nose → mouth parts like the upper lip, lower lip, chin, etc.

  • Emphasizes disproportionate representation for certain regions (e.g., hands and face) due to high functional demand.

• Practical significance:

  • In awake patients, electrical stimulation helps map function to avoid critical areas during surgical resections.

  • The homunculus provides a framework for understanding motor and sensory cortical organization and guiding neurosurgical planning.