Introduction to Neurons

Types, Structure, OperationCortical Columns

Ling 411 – 02

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 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

About 70% of cortical neurons are of this type

Microelectronic probe

Structure of pyramidal neuron

Apical dendrite

Cell body

Axon

Myelin

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 neuro- transmitter

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

Myelin

Cell body

Axon hillock

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:

113,000Mountcastle 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 NeuroscienceHarvard 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 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

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 Somato-sensory Area Primary

Motor Area

Primary AuditoryArea

PrimaryVisual Area

All other areas are secondary, association,or executive areas

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