Introduction%20to%20Neurons

1
Introduction to Neurons
Ling 411 02

• Types, Structure, Operation
• Cortical Columns

2
Coronal Section
Gray matter White matter
3
Schematic of coronal section
4
Coronal section
Gray matter White matter Sylvian fissure Insula
5
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

6
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

7
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

8
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

9
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

10
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

11
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!

12
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

13
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!
14
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

15
Types of cortical neurons
Neuron types
16
Pyramidal neurons
Microelectronic probe
About 70 of cortical neurons are of this type
17
Structure of pyramidal neuron
Apical dendrite Cell body Axon
Myelin
18
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!)
19
Diagram of synaptic structure
20
Release of neurotransmitter
Presynaptic terminal releases neurotransmitter
21
Seven steps of neuro- transmitter action
22
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

23
Inhibitory connections
Axosomatic
Axoaxonal
24
More on the pyramidal neuron
Dendrites Myelin
Cell body Axon hillock
25
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

26
Transmission of activation (sensory neuron)
Kandel 28
27
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

28
Another kind of neurotransmitter
Released into interneural space, has global
effect e.g. serotonin, dopamine
29
Events in short time periods

• Duration of one action potential about 1 ms
• Frequency of action potentials 1100 per sec
• Rate of transmission of action potential
• 1100 mm per ms
• Faster for myelinated axons
• Faster for thicker axons
• Synaptic delay ½ 1 ms

30
Traveling the pathways of the brain

• Neuron-to-neuron time in a chain (rough estimate)
• Neuron 1 fires _at_ 100 Hz
• Time for activation to reach ends of axon
• 10 mm _at_ 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

31
Three views of the gray matter
Different stains show different features
32
Layers of the Cortex
From top to bottom, about 3 mm
33
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

34
Cortical Columns
A graphic model, not an anatomical diagram From
M. vanLandingham, unpublished
35
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)

36
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
37
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
38
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)

39
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!
40
Quotation from Mountcastle
My general hypothesis is that the minicolumn is
the smallest processing unit of the
neocortex. (165)
Vernon Mountcastle, Perceptual Neuroscience Harvar
d University Press, 1998
41
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

42
Arcuate Fasciculus
(From www.rice.edu/langbrain)
43
Some long-distance fiber bundles(schematic)

44
Coronal Section
Grey matter White matter
45
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

46
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

47
Major features of cortical anatomy

• Each hemisphere appears to be a three-dimensional
  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

48
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

49
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)

50
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

51
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
52
Sequence of development in the cortex
53
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

54
end