Neuroscience Concepts in Synaptic Transmission

Synaptic Transmission

• Calcium ions (Ca2+) trigger synapse vesicles to fuse with the cell membrane, releasing neurotransmitters into the synaptic cleft.

Brain Organization

• Topographic Organization: Neurons are clustered for efficient local processing.
• Axon Length: Shorter axons allow more neurons, conserve resources, and optimize brain volume.
• Dense Connectivity: Enables efficient local processing.
• Lateral Inhibition: Enhances contrast and sharpens sensory signals.

Auditory System

• Humans can hear frequencies from 20 to 20,000 Hz.
• Sound waves travel from the outer ear to the eardrum, then to the cochlea.
• The cochlea transforms pressure waves into electrical signals.
• Mechanotransducers: Amplify cellular responses to membrane movement.
• Tonotopic Representation: Different frequencies activate specific cochlear zones.

Visual System

• 90% of optic nerve fibers terminate in the lateral geniculate nucleus (LGN).
• Retinotopic Mapping: Visual field is mapped onto LGN and V1 neurons.
• Simple Cells: Orientation selective, respond to bars and edges, have separate on and off regions, and can be monocular or binocular.
• Complex Cells: Orientation selective, no ON/OFF subregions, nearly all binocular.
• Cortical Magnification: Processing of the fovea (center of visual field) is more extensive than that of peripheral vision. This results in a larger representation of the fovea on the cortical map than its actual size in the retinal image.

Top-Down vs. Bottom-Up Processing

• Top-Down Processing: Uses prior knowledge to interpret sensory input.
• Bottom-Up Processing: Processes sensory information from receptors.
• 6% of visual input helps understand distorted images.
• Cortical Connectivity: Predominantly top-down, integrating multisensory input.
• McGurk Effect: Visual cues influence auditory speech perception.
• Retinotopy: Remapping of retinal image onto cortical surface.

Top-Down Auditory Processing

• Allows content and procedural demands to bias perception.
• Enables integration of multi-sensory input.

Ventral Stream

• Temporal lobe pathway for object identification.
• MTL Responses: Firing rates vary with familiarity of faces.
• Grandmother Cells: Hypothetical cells firing for specific familiar faces (debunked).
• Gnostic Cells: Cells processing simple features to form images.
• Local Coding: Specific features combined for object recognition.
• Object Discrimination: Ability to distinguish between different objects.
• Cell Ensemble: Group of cells firing together for image representation.

Nervous System

• Somatic Nervous System: Transmits information between the central nervous system and the sensory organs and muscles (external environment).
• Autonomic Nervous System: Controls the body's internal environment.
• Afferent Nerves: Input.
• Efferent Nerves: Output.
• Sympathetic Nervous System: Arouses the body (fight or flight).
• Parasympathetic Nervous System: Returns the body to a normal resting state (relaxed).

Spinal Nerves

• Cervical, thoracic, lumbar, sacral, coccygeal.
• Sensory information enters the spinal cord via the dorsal root, traveling along unipolar sensory neurons.
• Motor commands exit the spinal cord through the ventral horn, traveling along multipolar motor neurons.

Adult Human Forebrain

• Telencephalon
• Diencephalon

Midbrain

• Mesencephalon

Hindbrain

• Metencephalon: Forms the cerebellum.
• Myelencephalon: Forms the medulla.
• Pons

Neuron Structure

• External Features: Cell body, dendrites, axon, ions (sodium and chloride).
• Internal Features: Membrane potential (-70mV), ions potassium and proteins.

Neuron Function

• Ion Channels: Allow ions to flow across the membrane, leading to changes in membrane potential.
• Graded Potential: Varies in size.
• Action Potential: All-or-nothing response.
• Threshold: The level of depolarization needed to trigger an action potential.
• Receptor Activation: Triggers the opening of ion channels by signaling molecules (neurotransmitters) or environmental events (e.g., light, touch, sound).
• EPSP (Excitatory Postsynaptic Potential): Caused by inward flow of sodium (Na⁺) ions.
• Depolarization: Increase in membrane potential.
• Hodgkin-Huxley Cycle:
  • Synaptic or receptor potential
  • Depolarization of the membrane
  • Opening of voltage-gated Na⁺ channels
  • Na⁺ flows into the neuron.

Myelin

• Acts as electrical insulation for faster signal transmission and reduces interference from nearby axons.
• Composed of fat and protein.

Multiple Sclerosis

• Demyelination/damage to the myelin sheath disrupts nerve impulse flow.
• Symptoms include:
  • Numbness or weakness in limbs
  • Vision loss
  • Double vision
  • Tingling or pain
  • Fatigue
  • Dizziness

Visual Perception

• Photopic Vision: High acuity color vision in bright light.
• Scotopic Vision: Low acuity achromatic vision in dim light.
• Cone-fed Circuits: Low convergence; one ganglion cell per cone.
• Rod-fed Circuits: High convergence; one ganglion cell feeds multiple rods.
• ipRGC (Intrinsically Photosensitive Retinal Ganglion Cells): Contain melanopsin.
• Melanopsin: Photopigment sensitive to blue light in ipRGC.
• Circadian Rhythms: Biological processes regulated by light exposure.
• Blind Spot: Located 15 degrees from the center of the fovea.

Neurotransmitters

• Vesicle Steps: Loading, storage, mobilization, docking, priming, fusion.
• Glutamate: Neurotransmitter activating Na+ channels for EPSP.
• GABA: Neurotransmitter activating K+ channels for IPSP.

Retinal Cells

• Amacrine Cells: Release GABA to inhibit retinal signals.
• Horizontal Cells: Support cells releasing GABA in the retina.
• Lateral Inhibition: Surrounding cells inhibit a cell's response.

Color Vision

• Monochromatic View: Vision limited to shades of gray.
• Dichromatic View: Vision limited to blues and greens.
• Trichromatic View: Vision including blues, greens, and reds.
• Tetrachromatic Vision: A visual system with four different types of cone cells.
• Color Blindness: Affects 5-10% of males and 0.1% of females.
  • Deuteranomaly: Weakness in green perception.
  • Protanopia: Inability to perceive red color.
• Opponent-Process Theory: Color perception is controlled by opposing color systems.

Homeostasis

• Self-regulation to maintain internal stability.
  • Hot: Sweating
  • Cold: Shivering

Presynaptic Calcium Influx

• Ca2+ arrives at the pre-synaptic terminal and depolarization occurs, causing the action potential to drive the voltage-gated ion Ca2+ channel to open.

Pattern Recognition

• Pattern Completion: Recognizing incomplete patterns based on context.
• Pattern Separation: Distinguishing between similar patterns or objects.
• Generalization: Recognizing objects in varied contexts and positions.
• Trade-off in Processing: Balancing pattern completion, separation, and generalization.
• Dense Encoding: Requires overlap for pattern completion and generalization.
• Sparse Encoding: Requires separation for distinct object representation.

Facial Recognition

• Face Patches: Specific areas in the brain (middle lateral, middle fundus, and anterior medial) contain cells that respond when certain facial features are altered.
• Chang and Tsao Experiment: Demonstrated distributed facial representation in cells.
• Face Space: Dimensional space representing facial features.
• Selective Cells: Cells responding to specific features in faces.
• Pareidolia: Perceiving familiar patterns, like faces or animals, in objects.

Brain Statistics

• 86 Billion Neurons: Total number of neurons in the human brain.
• 40 Million Neurons: Number of neurons in the hippocampus.
• Chang and Tsao Experiment: Found that single cells are linked to one axis of face space and are blind to changes orthogonal to this axis. Multiple cells are active for a particular face, each coding a particular feature dimension.

Dorsal Stream

• Parietal lobe pathway for object location and movement.

Learning

• Hebbian Learning: Cells that fire together strengthen their connections.

Motion Perception

• Akinetopsia: Inability to perceive motion.

Visual Processing Areas

• V4 Area: Brain region for color detection and processing.
• V5 Area: Responsible for motion perception in the visual system.

Binding Problem

• Linking features to form a unified object.
• Temporal Binding: Synchronization of neural firing links features together.
• Cell Assemblies: Groups of neurons firing together to represent objects.

Visual Perception

• Ambiguous Figure: Visual illusion where perception shifts between interpretations.
• Foreground-Background Segmentation: Distinguishing main objects from their surroundings.
• 3D Shape Computation: Determining the shape of objects in three dimensions.
• Looming: Perception of approaching objects based on visual angle.

Fear Response

• Activation of the amygdala due to perceived threats.

Memory

• Short Term Memory: Retention of information for 10-20 seconds.