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Notes to look at: Kalman Filters and Poisson Processes

There will be several circuit analogies in understanding neurons—resistors, capacitors, RC, etc…

Introduction to Neuroscience

• The human brain is a highly interconnected network of ~100 billion neurons
• A brief history
  • Galvani (1700s) - frog legs
  • Golgi and Ramon y Cajal - saw network of discrete cells
  • Ramon y Cajal’s neuron doctrine - individual neurons
  • Harrison - two processes of cell: dendrites (input) and axon (output)
  • Wernicke et. al: 1850: cellular connectionism: individual neurons are signaling units
    • Opposed to aggregate field (entire brain makes movements, vision, etc.)
    • Phineas Gage (pole stuck through head), could keep motion, but not executive functioning
• Brain has distinct functional regions
  • CNS is bilateral and symmetrical (left and right hemispheres)
  • Modern imaging techniques confirm different regions specialized for different functions
  • Cerebral cortex (top of brain) 20e9 neurons
  • Cerebellum (bottom back of brain) 80e9 neurons
    • Used for motor feedback, high neuron density
    • Control non-voluntary motor functions
  • Spinal cord maps info from cerebral/cerebellum to other parts of body
  • Brain stem is segmented based on function for parts of body
    • Lumbar for legs, cerebral for face, etc.
• Focusing on the Cerebral Cortex
  • Thin sheet 1.3 sq ft and 2-4 mm thick
    • Wraps around top of brain, underneath the skull
    • Volume underneath supporting cells for neurons
  • Cognitive abilities
  • Furrowed gray matter covering two cerebral hemispheres
  • Folds increase surface area gyri (crests), sulci (grooves)
  • Central sulcus: largest groove
  • Anterior: closer to front, posterior, closer to back
  • Motor cortex just anterior to central sulcus
  • Somatosensory cortex just posterior
  • 4 major lobes of the brain
    • Frontal lobe: planning future action and control of movement
      • Contains motor cortex
    • Parietal label: relating position of body to extra-personal space
      • Has somatosensory
      • Can tell where limbs are in space (proprioception)
    • Temporal lobe
      • Hearing, learning, memory and emotion
      • Bottom
    • Occipital: vision
      • Back of brain
  • Control is contralateral
    • Left cortex controls right hand and vise versa
    • About 10% is ipsilateral (same side)
  • Hemispheres are similar, but not completely symmetrical in structure
    • Language in left, etc.
  • Brodmann’s Areas
    • Functional areas of cortex based on variations in structure of cells
    • 52 anatomically and functionally distinct in cerebral cortex
    • Still used today
      • Motor cortex is area 4
    • Used as functional localization
      • Example: PET scan as reading
      • Thinking of words, reading words, different areas are lighted up
• Neurons
  • Basic unit of brain
  • $10^{11}$ neurons in the human brain
  • 1000 different types but basic arch
  • Complexity from anatomical circuits not specialization
  • Rather like any single transistor in VLSI digital circuit
  • Nervous System two cell classes
    • Neurons and Glia
    • Glia are support, outnumber 50 to 1
    • Do not information process
    • Microglia (20%), Macroglia (80%)
      • Microglia support immune system (essentially macrophages), scavenging for neurotransmitters (like dopamine)
      • Macroglia astrocytes (blood neuron barrier), oligodendrocytes wrap around axon provides electrical insulation for neurons
        • Degradation of latter has to do with ALS
  • Neurons have 4 main parts
    • Cell body (soma) - metabolic center with nucleus
      • Center
    • Dendrites (inputs of neuron), synapses from other neurons connect (output)
    • Axon hillock (below cell body), sums up voltages and sends out action potential if above certain value
    • Axon (below hillock)
      • Convey signals long distances
      • Conveys action potential (spike) ~100mV, ~1ms pulses
      • Propagate without distortion or failure 1-100m/s
      • Actively regenerated
    • Presynaptic terminals (after axon) releases dopamine, and binds to dendrites of other neurons
      • Not connected
  • Action potentials
    • Signals which brain receives, analyzes and conveys info
    • Not by shape of voltage, just pattern
    • 0 or 1, like digital circuits!
    • 1 lasts about 1 ms (highest frequency 1000 Hz), highest 200 spikes/second
  • Neuron shapes vary considerably
    • Axon hillock may or may not be part of body, could be right next to dendrites
    • Motor cells may take lots of input to one output (Purkinje cell with lots of dendrites) (fan-in)
    • Sensory (fan-out)
• Membrane potential
  • Knee-Jerk Spinal Reflex Circuit
    • On tap, stretches quadriceps muscle
    • Muscle spindle
    • Sends out sensory neuron active
    • To another motor neuron
    • Stretch quadriceps more
    • Another inhibitory path toward hamstring to relax it
    • Only in spinal cord
    • Hence spinal cord is used for quick movements
    • Cats walk without cerebral cortex????
  • Divergence (fan-out, take sensory, send to different parts)
  • Convergence (fan in, integrate diverse info, send to one source)
  • Sensory signals are afferent (going in) or motor are efferent (going out)
  • Resting membrane potential
    • Maintain difference in electrical potential across cell membrane
    • More positive charges outside cell than inside (separated by cell wall)
    • Extracellular side vs cytoplasmic side
    • Form capacitor on cell wall!
    • Electric field from outside cell to inside cell
    • Cell wall across entire dendrites and cell body
    • $V_n$ membrane potential approx -65mV
  • How do ions flow?
    • Only allows Na+ to flow (ion channel)
    • Voltage increases
    • Diffusion current, will distribute uniformly if can, so Na+ flows in since more outside than inside
    • Also drift current due to electrical driving force since repelled from positive and attracted to negative