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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 1

2 Brain Structure: Topics  Components of the Brain  The cerebral cortex  Neurons, axons, dendrites  Synapses  Transmission of neural activity  Left brain and right brain 2

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

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.. 4

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 5

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 6

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 7

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

9 Corpus Callosum Connects Hemispheres Corpus Callosum 9

10 Major Left Hemisphere landmarks Central Sulcus Sylvian fissure 10

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

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

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

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 14

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 15

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

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 17

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 18

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

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 20

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) 21

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

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 23

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 24

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 25

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,

27 View of the cortex by Ramon y Cahal 27

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 28

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! 29

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 30

31 Connecting fibers of pyramidal neurons Apical dendrite Basal dendrites Axon 31

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 32

33 Types of cortical neurons 33

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

35 Structure of pyramidal neuron Apical dendrite Cell body Axon Myelin 35

36 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 36

37 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! 37

38 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 38

39 Interconnections of pyramidal neurons Input from distant cells Input from neighboring columns Output to distant cells 39

40 Synapses  The connections between neurons Neurotransmitters cross from pre-synaptic terminal to post-synaptic terminal Synaptic cleft – about 20 nanometers 40

41 Quantity of synapses in the cortex  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 41

42 Diagram of synaptic structure 42

43 Release of neurotransmitter Presynaptic terminal releases neurotransmitter 43

44 Video of Synaptic Transmission By Jokerwe 44

45 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 45

46 Inhibitory connections Axosomatic Axoaxonal 46

47 Myelin (and other features) Dendrite Nucleus Soma Myelin sheath Schwann cell Node of Ranvier Axon terminal 47

48 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 48

49 Transmission of activation (sensory neuron) Kandel 28 49

50 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 50

51 Another kind of neurotransmitter Released into interneural space, has global effect – e.g. serotonin, dopamine 51

52 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 52

53 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 53

54 Gray matter and white matter Gray matter White matter

55 Coronal section magnified From top to bottom, About 3 mm Has 6 layers

56 Further magnification: Layers of the Cortex From top to bottom, about 3 mm 56

57 Microscopic views 57 Different stains show different features

58 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 58

59 Gray matter and white matter (coronal section) Grey matter White matter 59

60 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 60

61 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 61

62 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 62

63 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 63

64 Sequence of development in the cortex 64

65 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 65

66 Left hemisphere vs. right hemisphere  Left hemisphere Analytical thinking Exact Digital Heightened contrast Proof  Right Hemisphere Holistic thinking Metaphorical Analog Fuzzy boundaries Hunches, intuition 66

67 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 67

68 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 68

69 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 two weeks: Brain function 69

70 T h a n k s f o r y o u r a t t e n t i o n ! 70


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