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 Intro to Cognitive Science
Jamuna Subramaniam
Department of Biological Sciences and Bioengineering
Indian Institute of Technology
Kanpur
Higher order brain functions
Language
Learning and memory
Thought
Emotion
Parts of the brain
Diseases led to the discovery
of the regions involved in specific functions.
Brain imaging
EEG ­ collecting the peripheral brain electrical activity
PET­ positron emission tomography – radioactive tracers in the water
deutrium­ more active areas more blood flow­ increased radiotracer
signal
Individual neurotransmitters –radiotracers –receptor binding,occupancy and turnover molecular level studies possible at the systems level in humans.
MRI&fMRI­Based on bloodflow – paramagnetic property of hemoglobi
Active brain regions oxygen bound blood flow will be higher. MRI
Human brain
The nervous system of a vertebrate
Structure and functional areas of the cerebrum
Brain regions involved in language
Left lobe
Wernicke’s area
Auditory input understanding speech
Broca’ s area
Motor output Indelligible speech
Areas active during different language tasks
Language learning disability ­ Dyslexia
Molecular and cellular network
level understanding !!!!!!
NERVOUS SYSTEMS
Three overlapping functions
sensory input
integration
motor output
Interconnected network of neurons
Overview of a vertebrate nervous system
•
Networks of neurons with intricate connections form nervous systems
Neuron Structure and Synapses.
• The neuron is the structural and functional unit of the nervous system.
• Nerve impulses are conducted along a neuron.
• Dendrite → cell body → axon hillock → axon
• Some axons are insulated by a myelin sheath.
• Axon endings are called synaptic terminals.
• They contain neurotransmitters which conduct a signal across a synapse. A synapse is the junction between a presynaptic and postsynaptic cell.
Aplysia neuron
• Types of Nerve Circuits.
• Single presynaptic neuron → several postsynaptic neurons.
• Several presynaptic neurons → single postsynaptic neuron.
• Circular paths.
Presynaptic neurons­ muscles – neuromuscular junctions
Types of neurotransmission
1. Electrical 2. Chemical Every cell has a voltage, or membrane potential, across its plasma membrane
• A membrane potential is a localized electrical
gradient across the membrane.
• Anions are more concentrated within a cell.
• Cations are more concentrated in the extracellular fluid.
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• Measuring Membrane Potentials.
Fig. 48.6a
• An unstimulated cell usually have a resting potential of ­70mV.
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How a Cell Maintains a Membrane Potential?
• Cations
• K+ is the principal intracellular cation.
• Na+ is the principal extracellular cation.
• Anions
• Proteins, amino acids, sulfate, and phosphate are the principal intracellular anions.
• Cl– is the principal extracellular anion.
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The basis of the membrane potential
Fig. 48.7
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Types of gated ion channels
• Chemically­gated ion channels open or close in response to a chemical stimulus.
• Voltage­gated ion channels open or close in response to a change in membrane potential.
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Graded Potentials: Hyperpolarization and Depolarization
• Graded potentials are changes in membrane potential
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Hyperpolarization
• Gated K+ channels open → K+ diffuses out of the cell → the membrane potential becomes more negative.
Fig. 48.8a
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Depolarization
• Gated Na+ channels open → Na+ diffuses into the cell → the membrane potential becomes less negative.
Fig. 48.8b
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The Action Potential: All or Nothing Depolarization
• If graded potentials sum to ≈ ­55mV a threshold potential is achieved.
• This triggers an action potential.
• Axons only.
Fig. 48.8c
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• Step 1: Resting State.
Fig. 48.9
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• Step 2: Threshold.
Fig. 48.9
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• Step 3: Depolarization phase of the action potential.
Fig. 48.9
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• Step 4: Repolarizing phase of the action potential.
Fig. 48.9
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• Step 5: Undershoot.
Fig. 48.9
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Propagation of the action potential
Fig. 48.10
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Chemical or electrical communication between cells occurs at synapses
• Electrical Synapses
• Action potentials travel directly from the presynaptic to the postsynaptic cells via gap junctions.
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Chemical Synapses
• More common than electrical synapses.
• Postsynaptic chemically­gated channels exist for ions such as Na+, K+, and Cl­.
• Depending on which gates open the postsynaptic neuron can depolarize or hyperpolarize.
• Brought about by binding of neurotransmitters released from the presynaptic terminal to the postsynaptically localized chemically­gated channels
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fundamental currencies of the living cell
DNA/RNA­ string of nucleotides­ nitrogenous base (four different bases ­ pentose sugar and phosphate)
Protein : string of amino acids (20 different amino acids)­ secondary,
tertiary and quarternary structures
DNA
RNA
Protein
R
NH2­(CH)n­COOH
Excitatory neurotransmission
Positive event­ brings about membrane depolarization­ action potential generation – an action happening
Major excitatory neurotransmitter in the CNS: glutamate
Glutamate receptors : Ionotrophic (Na+ channel coupled) functional receptor complex is multimeric Three different types: Kainate, AMPA and NMDA receptors
Diseases : conitnous firing­ excitotoxicity­ neuronal death in stroke, epilepsy Inhibitory neurotransmission
Hyperpolarization – modulates – no action potential generation
Inhibitory anion : Cl ­
Inhibitory neurotransmitter : GABA­ gamma­aminobutyric acid
GABAA receptor complex ­ pentameric –
several types of subunits: alpha, beta, gamma, delta and rho – subunit subtypes.
Have modulatory sites – anesthetics, antianxiety drugs
In vitro patch clamp recording
Nobel Price: Neher and Sakmann, 1991
The Major Known Neurotransmitters
Integration of multiple synaptic inputs
• Summation: graded potentials (EPSPs and IPSPs) are summed to either depolarize or hyperpolarize a postsynaptic neuron.
Fig. 48.14
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Cellular communication at synapse
Summation of input
Action potential generation
Output­ action
Biological clock
Emotions
Learning and memory
Human brain
The Hypothalamus and Circadian Rhythms
The biological clock is the internal timekeeper.
• The clock’s rhythm usually does not exactly match environmental events.
• Experiments in which humans have been deprived of external cues have shown that biological clock has a period of about 25 hours.
• In mammals, the hypothalamic suprachiasmatic nuclei (SCN) function as a biological clock.
• Produce proteins in response to light/dark cycles.
• This, and other biological clocks, may be responsive to hormonal release, hunger, and various external stimuli.
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The limbic system (emotions)
Learning and memory
Short­term memory stored in the frontal lobes.
The establishment of long­term memory involves the hippocampus.
­The transfer of information from short­term to long­term memory is enhanced by repetition (remember that when you are preparing for an exam).
­Influenced by emotional states mediated by the amygdala.
­Influenced by association with previously stored information.
Synaptic signaling
1. Fast neurotransmission 2. Slow neurotransmission
Signal transduction: Mode of regulation
Phosphorylation ­ dephosporylation
Second messenger : cAMP, calcium, IP3, cGMP
Figure 11.5 Overview of cell signaling (Layer 3)
Overview of cell signaling
(Layer 3)
Figure 11.18 The specificity of cell signaling
Cross talk between different signal cascades
Signal amplification
Learning and memory
A Nobel laureate’s perspective
Eric Kandel
Selection of the problem: Learning and behavior
easily tractable
Selection of the organism: less complicated
easy to observe behavior and molecular events
Selection of behavior: Memory storage behavior
Memory storage behavior
Neural circuit
Critical neurons
Cellular and molecular changes in specific neurons
Aplasia ­ marine snail
Model­behavior Gill and siphon withdrawal upon tactile stimuli
Aplasia neuron visible to naked eye
Types of memory
Short term: lasts only minutes
Long term: lasts for days
By practice (timed repetition)
Short term memory
Long term memory
Behavior taught learned fear
Sensitization shock in the tail, withdrawal of siphon and gill Neuronal circuit in gill withdrawal 24 mechanosensory neurons integrate siphon skin
6 monosynaptic connection to gill motor neuron
Neurons in in vitro culture
Injection of signal cascade components
Molecular events
Serotonin, dopamine
Involvement of cAMP cascade
Serotonin, dopamine
cAMP levels
Modulatory pathway
involves cAMP cascade
Sensory neurons release glutamate ­ memory facilitation
Synaptic plasticity
Increase in strength.
Increase in synapse number.
Behavior : Spatial memory
Hippocampal activation
Diseases of Cognition
Dementia – Loss of memory
Alzheimer’s disease
Cerebral cortex and hippocampus;
Amyloid plague accumulation.
Most prevalent –
Complex neurodegenerative disease
defects in the neurotransmitter
acetylcholine and glutamate !
Huntington’s disease
Diseases of the thought process
Psychiatirc diseases
• Schizophrenia • delusions, hallucinations
• Too much of dopamine neurotransmitter signaling. Spinal cord injury
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