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
•
That is, about 400 sq inches
•
•
Range: 1,130 – 1,610 grams
Average: 1,370 grams
•
Female brain grows faster than male during 1st 4 yrs
•
•
Range: 1.4 – 4.0 mm
Average: 2.87 mm
 Surface of the cortex – about 2600 sq cm
 Weight of cortex –
 Brain mass nears adult size by age six yrs
 Thickness of cortex – (inf. from Mountcastle 1998)
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 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
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 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
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
interneural
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:
• 1–100 mm per ms
• Faster for myelinated axons
• Faster for thicker axons
 Synaptic delay: ½ – 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 30–50 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
Wernicke’s 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 Wernicke’s area to Broca’s area
Arcuate Fasciculus
(From: www.rice.edu/langbrain)
Some long-distance fiber bundles
(schematic)
Coronal Section
Grey
matter
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
end
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The Anatomy of Language Sydney Lamb Rice University