II. Brain Structure

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

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..

5. The brain *Brain Stem • Medulla oblongata – Myelencephalon • Pons and Cerebellum – Metencephalon • Midbrain – Mesencephalon • Thalamus and hypothalamus – Diencephalon • Cerebral hemispheres – Telencephalon 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 Right Left 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 Parietal Lobe Frontal Lobe Occipital Lobe Temporal Lobe Sylvian fissure

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

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 Leg Primary Motor Area Trunk Arm Hand Fingers Mouth Primary Auditory Area Primary Visual Area

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

19. Video of basic cortical anatomy http://www.youtube.com/watch?v=HVGlfcP3ATI&NR=1&feature=fvwp 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 • 1852-1934 • 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/mm3 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 Microelectronic probe About 70% of cortical neurons are of this type

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 (4x104) • And 27 billion neurons • i.e., 27,000,000,000 = 27x109 • 1.1x1015 (over 1 quadrillion) synapses per cortex (4x104 x 2.7x1010 = 11x1014) (Big lesson: Connectivity rules!)

37. Diagram of synaptic structure

38. Release of neurotransmitter Presynaptic terminal releases neurotransmitter

39. Video of Synaptic Transmission http://www.youtube.com/watch?v=HXx9qlJetSU&feature=related 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 Axon terminal Node of Ranvier Soma Schwann cell Myelin sheath Nucleus

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 1 fires @ 100 Hz • Time for activation to reach ends of axon • 10 mm @ 10 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