The Anatomy of Language Sydney Lamb Rice University, Houston, USA
Ling 411 02 Introduction to Neurons Types, Structure, Operation Cortical Columns Coronal Section Gray matter White matter
Schematic of coronal section Coronal section Gray matter White matter Sylvian fissure Insula 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 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 Therefore, the linguistic system operates by means of connections A persons 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 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 Communicating with other cells Method one: 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
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 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! Brains of the young and very young At about 7 months, a child can recognize most sound distinctions of the worlds 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 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! 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 Types of cortical neurons
Neuron types Pyramidal neurons Microelectronic probe About 70% of cortical neurons are of this type Structure of pyramidal neuron
Apical dendrite Cell body Myelin Axon 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!) Diagram of synaptic structure
Release of neurotransmitter Presynaptic terminal releases neurotransmitter Seven steps of neurotransmitter action 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 Inhibitory connections Axosomatic Axoaxonal More on the pyramidal neuron
Dendrites Cell body Axon hillock Myelin 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 Transmission of activation (sensory
neuron) Kandel 28 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 Another kind of neurotransmitter
Released into interneura l space, has global effect e.g. serotonin, dopamine 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: 1100 mm per ms Faster for myelinated axons Faster for thicker axons Synaptic delay: 1 ms
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
Three views of the gray matter Different stains show different features Layers of the Cortex From top to bottom,
about 3 mm The (Mini)Column Extends thru the six cortical layers Three to four mm in length The entire thickness of the cortex is accounted for by the columns Roughly cylindrical in shape About 3050 m in diameter
If expanded by a factor of 100 Like a tube with diameter of 1/8 inch and length of one foot If expanded by a factor of 8,000 Like a telephone pole with diameter of 10 inches and length of 80 feet
Cortical Columns A graphic model, not an anatomical diagram From M. vanLandingham, unpublished Features of the cortical (mini)column 75 to 110 neurons 70% of the neurons are pyramidal
The rest include Other excitatory neurons Several different kinds of inhibitory neurons For further information: Vernon Mountcastle, Perceptual Neuroscience (1998) Cortical minicolumns: Quantities
Diameter of minicolumn: 30 microns Neurons per minicolumn: 75-110 Minicolumns/mm2 of cortical surface: 1413 Minicolumns/cm2 of cortical surface: 141,300 Approximate number of minicolumns in Wernickes area: 2,825,000
Estimates based on Mountcastle More quantities Number of neurons in cortex: 27.4 billion Number of minicolumns: 368 million Neurons per minicolumn: average 75-80 Neurons beneath 1 mm2 of surface: 113,000 Mountcastle 96 Cortical column connectivity
The neurons of a column are mutually interconnected a whole column is active together the column acts as a single functional unit The neurons of a column are connected to: adjacent columns inhibitory and excitatory connections (gray matter connections) distant columns, by means of long distance excitatory connections (the white matter)
Columns and neurons At the small scale.. each column is a little network At a larger scale.. each column is a node of the cortical network The cerebral cortex: Gray matter columns of neurons and connections to adjacent columns White matter: Long-distance inter-column connections
N.B.: The cortex operates by means of connections! Quotation from Mountcastle My general hypothesis is that the minicolumn is the smallest processing unit of the neocortex. (165) Vernon Mountcastle, Perceptual Neuroscience Harvard University Press, 1998
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 Wernickes area to Brocas area
White matter Topology of the Gray Matter Each hemisphere is like a thick napkin, with Thickness varying from 2 to 4 mm (avg. 3 mm ca. 1/8 in.) Area of about 1300 square centimeters (200 sq. in.) Subdivided into six layers The thickness is accounted for
entirely by cortical columns 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 Major features of cortical anatomy Each hemisphere appears to be a threedimensional structure, but.. Each hemisphere is very thin and very broad The grooves sulci are there because the cortex is crumpled so it will fit inside the skull
Topological essence of cortical structure Two dimensions for the array of the columns Viewed this way the cortex is an array a two-dimensional structure of interconnected columns Dimensionality of the cortex Two dimensions: The array of nodes The third dimension:
The length (depth) of each column (through the six cortical layers) The cortico-cortical connections (white matter) Functional layout of the two dimensions Primary areas: Visual (occipital)
Auditory (temporal) Somatosensory (parietal) Motor (frontal) Secondary areas Association areas Executive area, in prefrontal lobe Primary and other areas Primary Motor Area
Primary Somatosensory Area All other areas are secondary, association , or executive areas Primary Auditory Area
Primary Visual Area Sequence of development in the cortex 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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