Introduction to Neurons

1. Ling 411 – 02 Introduction to Neurons Types, Structure, Operation Cortical Columns Connectivity

2. Video of basic cortical anatomy http://www.youtube.com/watch?v=HVGlfcP3ATI&NR=1&feature=fvwp From Medical Legal Art (2009)

3. Coronal Section Gray matter White matter

4. Schematic of coronal section

5. Coronal section Gray matter White matter Sylvian fissure Insula

6. 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

7. 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

8. Therefore, the linguistic systemoperates by means of connections • A person’s linguistic system is largely represented in his/her cerebral cortex • The cerebral cortex is a neural network • A linguistic system is therefore represented as a neural network • Therefore, any component of the system does what it does by virtue of its connections to other components • The first big secret to understanding how the linguistic system operates

9. 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

10. 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

11. 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

12. View of the cortex by Ramon y Cahal

13. 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

14. 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!

15. 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

16. 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 • Enough to go to the moon 7 times Big lesson:Connectivity rules!

17. Types of cortical neurons • Cells with excitatory output connections • Pyramidal cells (about 70% of all cortical neurons) • Spiny stellate cells • Cells with inhibitory output connections • Large basket cells (two subtypes) • Columnar basket cells • Double bouquet cells • Chandelier cells • Other

18. Types of cortical neurons Neuron types

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

20. Structure of pyramidal neuron Apical dendrite Cell body Axon Myelin

21. 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!)

22. Diagram of synaptic structure

23. Release of neurotransmitter Presynaptic terminal releases neurotransmitter

24. Video of Synaptic Transmission http://www.youtube.com/watch?v=HXx9qlJetSU&feature=related By Jokerwe

25. 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

26. Inhibitory connections Axosomatic Axoaxonal

27. More on the pyramidal neuron Dendrites Myelin Cell body Axon hillock

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

29. 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

30. Transmission of activation (sensory neuron) Kandel 28

31. Spread of activation • Activation moves across links • Physical reality: from neuron to neuron • Abstract model: from node to node • At larger scale, across multiple links • In speech production, • from meanings to their expression • For a listener, • From expression to meaning

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

33. 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

34. 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

35. Three views of the gray matter Different stains show different features

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

37. 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

38. Some long-distance fiber bundles(schematic)

39. Arcuate Fasciculus (From: www.rice.edu/langbrain)

40. Coronal Section Grey matter White matter

41. 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

42. 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

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

44. Sequence of development in the cortex

45. Large-scale hierarchy in the cortex • 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

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47. Information Card.. LastName, FirstName email address Major(s) Class Previous linguistics courses Previous neuroscience courses Previous cognitive science courses Languages Why this course?