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II. Brain Structure If the brain were simple enough for us to understand, we would be too simple-minded to understand it. Anonymous Brain, Mind, and Belief:

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Presentation on theme: "II. Brain Structure If the brain were simple enough for us to understand, we would be too simple-minded to understand it. Anonymous Brain, Mind, and Belief:"— Presentation transcript:

1 II. Brain Structure If the brain were simple enough for us to understand, we would be too simple-minded to understand it. Anonymous Brain, Mind, and Belief: The Quest for Truth

2 Brain Structure: Topics  Components of the Brain  The cerebral cortex  Neurons, axons, dendrites  Synapses  Transmission of neural activity  Left brain and right brain  Front brain and back brain  Layers of cortex  Cortical columns  The essence of cortical geography

3 The nervous system  Central nervous system Spinal cord Brain  Peripheral nervous system Motor and sensory neurons connected to the spinal cord

4 The brain  Medulla oblongata – Myelencephalon  Pons and Cerebellum – Metencephalon  Midbrain – Mesencephalon  Thalamus and hypothalamus – Diencephalon  Cerebral hemispheres – Telencephalon Cerebral cortex Basal ganglia Basal forebrain nuclei Amygdaloid nucleus  More.. More..

5 The brain  Medulla oblongata – Myelencephalon  Pons and Cerebellum – Metencephalon  Midbrain – Mesencephalon  Thalamus and hypothalamus – Diencephalon  Cerebral hemispheres – Telencephalon *Brain Stem Alternative partition: Brain stem* Cerebellum Thalamus & hypothalamus Cerebral hemispheres

6 The brain  Medulla oblongata – Myelencephalon  Pons and Cerebellum – Metencephalon  Midbrain – Mesencephalon  Thalamus and hypothalamus – Diencephalon  Cerebral hemispheres – Telencephalon Cerebral cortex Basal ganglia Basal forebrain nuclei Amygdaloid nucleus

7 Thalamus and Cortex  The cortex is the area for High-level information processing Language  But the thalamus is also very important Timing and coordination of cortical activity Details not yet well understood  Metaphor: The cortex is the orchestra  A very large orchestra The thalamus is the conductor

8 Two hemispheres Left Right Interhemispheric fissure (a.k.a. longitudinal fissure)

9 Corpus Callosum Connects Hemispheres Corpus Callosum

10 Major Left Hemisphere landmarks Central Sulcus Sylvian fissure

11 The Sylvian Fissure opened up (it’s huge)

12 Major landmarks and the four lobes Central Sulcus Sylvian fissure Frontal Lobe Parietal Lobe Temporal Lobe Occipital Lobe

13 Primary motor and somatosensory areas Central Sulcus Sylvian fissure Primary Motor Area Primary Somato- sensory Area

14 Some terms..  Fissures and sulci (the “grooves”) Singular: sulcus – Plural: sulci The major sulci are usually called fissures  Interhemispheric fissure  Sylvian fissure  Sometimes the term Rolandic fissure is used for the central sulcus  Gyri Singular: gyrus – Plural: gyri

15 Alternatives terms for some fissures  Interhemispheric fissure Also known as Longitudinal fissure  Sylvian fissure Also known as Lateral sulcus  Central sulcus Also known as Rolandic fissure

16 Primary Areas Primary Somato- sensory Area Primary Motor Area Primary Auditory Area Primary Visual Area

17 Divisions of Primary Motor and Somatic Areas Primary Somato- sensory Area Primary Motor Area Primary Auditory Area Primary Visual Area Mouth Hand Fingers Arm Trunk Leg

18 Higher level motor areas Primary Somato- sensory Area Actions performed by hand Primary Auditory Area Primary Visual Area Mouth Hand Fingers Arm Trunk Leg Actions per- Formed by leg Actions performed by mouth

19 Video of basic cortical anatomy From Medical Legal Art (2009)

20 The brain operates by means of connections  Neurons do not store information  Rather they operate by emitting activation To other neurons to which they connect  Via synapses Proportionate to activation being received  From other neurons via synapses  Therefore, a neuron does what it does by virtue of its connections to other neurons The first big secret to understanding how the brain operates

21 The cerebral cortex is a very large network Made up of interconnected neurons Very large Dynamic Changes take place in connection strengths  Every neuron is connected (directly or indirectly) to every other neuron Therefore, all of the information in it has the form of a network  The information is in the connectivity  (stay tuned for further details)

22 Gray matter and white matter (coronal section) Gray matter White matter

23 Some brain quantities  The cortex accounts for 60-65% of the volume of the brain But has only a minority of the total neurons of the brain  Surface of the cortex – about 2600 sq cm That is, about 400 sq inches  Weight of cortex – Range: 1,130 – 1,610 grams Average: 1,370 grams  Brain mass nears adult size by age six yrs Female brain grows faster than male during 1st 4 yrs  Thickness of cortex – (inf. from Mountcastle 1998) Range: 1.4 – 4.0 mm Average: 2.87 mm

24 Cortical Neurons  Cells, but quite different from other cells Multiple fibers, branching in tree-like structures  Input fibers: Dendrites  Output fibers: Axons Great variation in length of fibers  Short ones — less than one millimeter  Long ones — several centimeters Only the pyramidal cells have such long ones

25 Cellular Communication: How to communicate with other cells  Method One (Nervous System): Fibers projecting from cell body  Branching into multiple fibers  Input fibers – dendrites Allow cell to receive from multiple sources  Output fiber – axon Allows cell to send to multiple destinations  Method Two: Circulation  Circulatory system Endocrine system  Lymphatic system

26 Santiago Ramon y Cajal   Spanish neuroscientist “The father of modern neuroscience”  Used microscope to examine brain tissue Was skilled at drawing Many of his drawings are still used today in teaching neuroscience  Nobel Prize in Medicine, 1906

27 View of the cortex by Ramon y Cahal

28 Some quantities relating to neurons  Number of neurons In cortex: ca. 27 billion (Mountcastle) Beneath 1 sq mm of cortical surface: 113,000  Synapses 440 million synaptic terminals/mm 3 in visual area Each neuron receives avg 3,400 synaptic terminals

29 Formation of neurons in the fetus  500,000 neurons are formed per minute in the developing fetus (from a program on PBS, 2002)  By 24 weeks, the brain has most of its neurons  Checking: 500,000 per minute 30 million per hour 720 million per day 5 billion per week 96 billion in 24 weeks Checks!

30 Brains of the young and very young  At about 7 months, a child can recognize most sound distinctions of the world’s languages  By 11 months the child recognizes only those of the language of its environment  At 20 months the left hemisphere is favored for most newly acquired linguistic information  Brain mass nears adult size by age six yrs Female brain grows faster than male during 1st 4 yrs

31 Neuronal fibers  Estimated average 10 cm of fibers per neuron A conservative estimate Times 27 billion neurons in cortex Amounts to 2.7 billion meters of neural fibers in cortex (27 billion times 10 cm) Or 2.7 million kilometers – about 1.68 million miles  Enough to encircle the world 68 times  Seven times the distance from the Earth to the moon Big lesson: Connectivity rules!

32 Types of cortical neurons  Cells with excitatory output connections (spiny) Pyramidal cells (about 70% of all cortical neurons) Spiny stellate cells  Cells with inhibitory output connections (non-spiny) Large basket cells (two subtypes) Columnar basket cells Double bouquet cells Chandelier cells Others

33 Types of cortical neurons

34 Pyramidal neurons About 70% of cortical neurons are of this type Microelectronic probe

35 Structure of pyramidal neuron Apical dendrite Cell body Axon Myelin

36 Synapses  The connections between neurons Neurotransmitters cross from pre-synaptic terminal to post-synaptic terminal Synaptic cleft – about 20 nanometers  40,000 synapses per neuron (4x10 4) And 27 billion neurons  i.e., 27,000,000,000 = 27x x10 15 (over 1 quadrillion) synapses per cortex (4x10 4 x 2.7x10 10 = 11x10 14 ) (Big lesson: Connectivity rules!)

37 Diagram of synaptic structure

38 Release of neurotransmitter Presynaptic terminal releases neurotransmitter

39 Video of Synaptic Transmission By Jokerwe

40 Connections to other neurons  Excitatory Pyramidal cells and spiny stellate cells Output terminals are on dendrites or cell bodies of other neurons Neurotransmitter: Glutamate  Inhibitory All other cortical neurons Output terminals are on cell bodies or axons of other neurons Neurotransmitter: GABA GABA: gamma-aminobutyric acid

41 Inhibitory connections Axosomatic Axoaxonal

42 Myelin (and other features) Dendrite Nucleus Soma Myelin sheath Schwann cell Node of Ranvier Axon terminal

43 Integration of neural inputs  Takes place at the axon hillock  Excitatory inputs are summed  Inhibitory inputs are subtracted  Result of this summation is the amount of incoming activation  Determines how much activation will be transmitted along the axon (and its branches), hence to other neurons  Degree of activation is implemented as frequency of spikes

44 Transmission of activation (sensory neuron) Kandel 28

45 Spread of activation  Activation moves across links  At the small scale from neuron to neuron  At larger scale, across multiple links In vision  From retina to conceptual area of cortex In speech production,  from meanings to their expression For a listener,  From expression to meaning

46 Another kind of neurotransmitter Released into interneural space, has global effect – e.g. serotonin, dopamine

47 Events in short time periods  Duration of one action potential: about 1 ms  Frequency of action potentials: 1–100 per sec  Rate of transmission of action potential: 1–100 mm per ms Faster for myelinated axons Faster for thicker axons  Synaptic delay: ½ – 1 ms

48 Traveling the pathways of the brain  Neuron-to-neuron time in a chain (rough estimate) Neuron Hz  Time for activation to reach ends of axon mm/ms = 1 ms  Time to activate post-synaptic receptor – 1 ms Neuron 2  Activation reaches firing threshold – 4 ms (??) Hence, overall neuron-to-neuron time – ca. 6 ms  Time required for spoken identification of picture Subject is alert and attentive Instructions: say what animal you see as soon as you see the picture Picture of horse is shown to subject Subject says “horse” This process takes about 600 ms

49 Three views of the gray matter Different stains show different features

50 Layers of the Cortex From top to bottom, about 3 mm

51 Long-distance cortico-cortical connections  White matter – Long-distance inter-column connections  Example: the arcuate fasciculus A bundle of fibers very important for language  Connects Wernicke’s area to Broca’s area

52 Gray matter and white matter (coronal section) Grey matter White matter

53 The White Matter  Provides long-distance connections between cortical columns  Consists of axons of pyramidal neurons  The cell bodies of those neurons are in the gray matter  Each such axon is surrounded by a myelin sheath, which.. Provides insulation Enhances conduction of nerve impulses  The white matter is white because that is the color of myelin

54 Functional layout of the gray matter  Primary areas: Visual (occipital) Auditory (temporal) Somatosensory (parietal) Motor (frontal)  Secondary areas  Association areas  Executive area, in prefrontal lobe

55 Primary and other areas Primary Somato- sensory Area Primary Motor Area Primary Auditory Area Primary Visual Area All other areas are secondary, association, or executive areas

56 The cortical network has a hierarchical structure  At ‘bottom’, the primary systems Somatosensory, visual, auditory, motor  In ‘middle layers’ the association areas and ‘higher-level’ motor areas  At ‘top’ (prefrontal cortex) the supra-modal association area Frontal lobe comprises 1/3 of the area of the cortex Prefrontal cortex is nearly 1/4 of the whole cortex Prefrontal functions  Planning, anticipation, mental rehearsal, prediction, judgment, problem solving

57 Sequence of development in the cortex

58 Major anatomical-functional dichotomies  Left hemisphere vs. Right hemisphere Left  Analytical, linguistic, digital  Maintains existing beliefs Right  Metaphorical, artistic, analog  Open to new data and ideas  Front (anterior) vs. Back (posterior) Front  Action and planning of action  Process oriented Back  Perception  Perceptual integration  Object oriented

59 Left hemisphere vs. right hemisphere  Left hemisphere Analytical thinking Exact Digital Heightened contrast Proof  Right Hemisphere Holistic thinking Metaphorical Analog Fuzzy boundaries Hunches, intuition

60 Corpus Callosum (revealed by excision of top of RH) Corpus Callosum

61 Cerebral dominance for language  Linguistic abilities are subserved by the left hemisphere in about 97% of people 99% of right-handed people A majority of left-handers  But this is just a first approximation

62 The Role of RH in semantics  Conceptual information, even for a single item, is complex Therefore, widely distributed A network Occupies both hemispheres  RH information is more connotative LH information more exact

63 Faulty thinking in cognitive science (among some but not all practioners)  The brain (likewise the mind) is like the computer  An example of the misapplied metaphor

64 The Cortex is a Network Entirely different structure than that of computers  Connectivity as key property of brain structure  Symbol-manipulation is the key property of computers  The cortex operates by means of connections Transmission of activation along neural pathways Changes in connection strengths

65 Computers and Brains: Different Structures, Different Skills  Computers Exact, literal Rapid calculation Rapid sorting Rapid searching Faultless memory Do what they are told Predictable  Brains Flexible, fault tolerant Slow processing Association Intuition Adaptability, plasticity Self-driven activity Unpredictable Self-driven learning

66 Things that brains but not computers can do  Acquire information to varying degrees “Entrenchment” How does it work?  Variable connection strength  Connections get stronger with repeated use  Perform at varying skill levels Degrees of alertness, attentiveness Variation in reaction time Mechanisms:  Global neurotransmitters (next slide)  Variation in blood flow  Variation in available nutrients  Presence or absence of fatigue  Presence or absence of intoxication

67 Global neurotransmitters Released into interneural space, has global effect – e.g. serotonin, dopamine

68 Neuronal Structure and Function: Connectivity  White matter: it’s all connections Far more voluminous than gray matter Cortico-cortical connections  The fibers are axons of pyramidal neurons  They are all excitatory White since the fibers are coated with myelin  Myelin: glial cells  There are also grey matter connections Unmyelinated Local Horizontal, through gray matter Excitatory and inhibitory

69 Pyramidal neurons and their connections  Connecting fibers Dendrites (input): length 2mm or less Axons (output): length up to 10 cm  Synapses Afferent synapses: up to 50,000  From distant and nearby sources Distant – to apical dendrite Local – to basal dendrites or cell body Efferent synapses: up to 50,000  On distant and nearby destinations Distant – main axon, through white matter Local – collateral axons, through gray matter

70 Connecting fibers of pyramidal neurons Apical dendrite Basal dendrites Axon

71 Interconnections of pyramidal neurons Input from distant cells Input from neighboring columns Output to distant cells

72 Neuronal Structure and Function: Connectivity  Synapses of a typical pyramidal neuron: Incoming (afferent) – 50,000 (5 x 10 4 ) Outgoing (efferent) – 50,000  Number of synapses in cortex: 28 billion neurons (Mountcastle’s estimate)  i.e., 28 x 10 9  Synapses in the cortex (do the math) 5 x 10 4 x 28 x 10 9 = 140 x = 1.4 x Approximately 1,400,000,000,000,000 i.e., over 1 quadrillion

73 How does all this complex structure work?  A structure sui generis – quite unlike computers and in fact unlike anything else we have ever known  Extraordinarily complex Billions of neurons Trillions of interconnections  How can we make sense of it?  Stay tuned: Next week: Brain function

74 T h a n k s f o r y o u r a t t e n t i o n !


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