Transcript Ling411-02-Neurons - OWL-Space

Ling 411 – 02
Introduction to Neurons
Types, Structure, Operation
Cortical Columns
Connectivity
Video of basic cortical anatomy
http://www.youtube.com/watch?v=HVGlfcP3ATI&NR=1&feature=fvwp
From Medical Legal Art (2009)
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
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That is, about 400 sq inches
 Weight of cortex –
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•
Range: 1,130 – 1,610 grams
Average: 1,370 grams
 Brain mass nears adult size by age six yrs
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Female brain grows faster than male during 1st 4 yrs
 Thickness of cortex – (inf. from Mountcastle 1998)
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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
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
Santiago Ramon y Cajal
 1852-1934
 Spanish neuroscientist
•
“The father of modern neuroscience”
 Used microscope to examine
brain tissue
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•
Was skilled at drawing
Many of his drawings are still used
today in teaching neuroscience
 Nobel Prize in Medicine, 1906
View of the cortex by Ramon y Cahal
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
Video of Synaptic Transmission
http://www.youtube.com/watch?v=HXx9qlJetSU&feature=related
By Jokerwe
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
Myelin (and other features)
Dendrite
Axon terminal
Soma
Node of Ranvier
Schwann cell
Myelin sheath
Nucleus
Integration of neural inputs
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
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
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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
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•
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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
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
Some long-distance fiber bundles
(schematic)
Arcuate Fasciculus
(From: www.rice.edu/langbrain)
Coronal Section
Grey
matter
White
matter
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
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
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 ‘higherlevel’ 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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