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Brain, Mind, and Belief: The Quest for Truth. II. Brain Structure. If the brain were simple enough for us to understand,     we would be too simple-minded to understand it.          Anonymous. Brain Structure: Topics. Components of the Brain The cerebral cortex Neurons, axons, dendrites

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Ii brain structure

Brain, Mind, and Belief: The Quest for Truth

II. Brain Structure

If the brain were simple enough for us to understand,    

we would be too simple-minded to understand it.   

      Anonymous


Brain structure topics
Brain Structure: Topics

  • Components of the Brain

  • The cerebral cortex

  • Neurons, axons, dendrites

  • Synapses

  • Transmission of neural activity

  • Left brain and right brain

  • Front brain and back brain

  • Layers of cortex

  • Cortical columns

  • The essence of cortical geography


The nervous system
The nervous system

  • Central nervous system

    • Spinal cord

    • Brain

  • Peripheral nervous system

    • Motor and sensory neurons connected to the spinal cord


The brain
The brain

  • Medulla oblongata – Myelencephalon

  • Pons and Cerebellum – Metencephalon

  • Midbrain – Mesencephalon

  • Thalamus and hypothalamus – Diencephalon

  • Cerebral hemispheres – Telencephalon

    • Cerebral cortex

    • Basal ganglia

    • Basal forebrain nuclei

    • Amygdaloid nucleus

  • More..


The brain1
The brain

*Brain Stem

  • Medulla oblongata – Myelencephalon

  • Pons and Cerebellum – Metencephalon

  • Midbrain – Mesencephalon

  • Thalamus and hypothalamus – Diencephalon

  • Cerebral hemispheres – Telencephalon

Alternative partition:

Brain stem*

Cerebellum

Thalamus & hypothalamus

Cerebral hemispheres


The brain2
The brain

  • Medulla oblongata – Myelencephalon

  • Pons and Cerebellum – Metencephalon

  • Midbrain – Mesencephalon

  • Thalamus and hypothalamus – Diencephalon

  • Cerebral hemispheres – Telencephalon

    • Cerebral cortex

    • Basal ganglia

    • Basal forebrain nuclei

    • Amygdaloid nucleus


Thalamus and cortex
Thalamus and Cortex

  • The cortex is the area for

    • High-level information processing

    • Language

  • But the thalamus is also very important

    • Timing and coordination of cortical activity

    • Details not yet well understood

  • Metaphor:

    • The cortex is the orchestra

      • A very large orchestra

    • The thalamus is the conductor


Two hemispheres
Two hemispheres

Right

Left

Interhemispheric fissure (a.k.a. longitudinal fissure)



Major left hemisphere landmarks
Major Left Hemisphere landmarks

Central Sulcus

Sylvian fissure



Major landmarks and the four lobes
Major landmarks and the four lobes

Central Sulcus

Parietal

Lobe

Frontal

Lobe

Occipital

Lobe

Temporal

Lobe

Sylvian fissure


Primary motor and somatosensory areas
Primary motor and somatosensory areas

Primary Somato-

sensory Area

Central Sulcus

Primary

Motor Area

Sylvian fissure


Some terms
Some terms..

  • Fissures and sulci (the “grooves”)

    • Singular: sulcus – Plural: sulci

    • The major sulci are usually called fissures

      • Interhemispheric fissure

      • Sylvian fissure

      • Sometimes the term Rolandic fissure is used for the central sulcus

  • Gyri

    • Singular: gyrus – Plural: gyri


Alternatives terms for some fissures
Alternatives terms for some fissures

  • Interhemispheric fissure

    • Also known as Longitudinal fissure

  • Sylvian fissure

    • Also known as Lateral sulcus

  • Central sulcus

    • Also known as Rolandic fissure


Primary areas
Primary Areas

Primary Somato-

sensory Area

Primary

Motor Area

Primary Auditory

Area

Primary

Visual Area


Divisions of primary motor and somatic areas
Divisions of Primary Motor and Somatic Areas

Primary Somato-

sensory Area

Leg

Primary

Motor Area

Trunk

Arm

Hand

Fingers

Mouth

Primary Auditory

Area

Primary

Visual Area


Higher level motor areas
Higher level motor areas

Primary Somato-

sensory Area

Actions per-

Formed by leg

Leg

Actions

performed

by hand

Trunk

Arm

Hand

Actions

performed

by mouth

Fingers

Mouth

Primary Auditory

Area

Primary

Visual Area


Video of basic cortical anatomy
Video of basic cortical anatomy

http://www.youtube.com/watch?v=HVGlfcP3ATI&NR=1&feature=fvwp

From Medical Legal Art (2009)


The brain operates by means of connections
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


The cerebral cortex is a very large network
The cerebral cortex is a very large network

  • Made up of interconnected neurons

  • Very large

  • Dynamic

    • Changes take place in connection strengths

  • Every neuron is connected (directly or indirectly) to every other neuron

    • Therefore, all of the information in it has the form of a network

      • The information is in the connectivity

      • (stay tuned for further details)



Some brain quantities
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


Cortical neurons
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


Cellular communication how to communicate with other cells
Cellular Communication: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
Santiago Ramon y Cajal

  • 1852-1934

  • Spanish neuroscientist

    • “The father of modern neuroscience”

  • Used microscope to examine brain tissue

    • Was skilled at drawing

    • Many of his drawings are still used today in teaching neuroscience

  • Nobel Prize in Medicine, 1906



Some quantities relating to neurons
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
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
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
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

      • Seven times the distance from the Earth to the moon

Big lesson: Connectivity rules!


Types of cortical neurons
Types of cortical neurons

  • Cells with excitatory output connections (spiny)

    • Pyramidal cells (about 70% of all cortical neurons)

    • Spiny stellate cells

  • Cells with inhibitory output connections (non-spiny)

    • Large basket cells (two subtypes)

    • Columnar basket cells

    • Double bouquet cells

    • Chandelier cells

    • Others



Pyramidal neurons
Pyramidal neurons

Microelectronic probe

About 70% of cortical neurons are of this type


Structure of pyramidal neuron
Structure of pyramidal neuron

Apical dendrite

Cell body

Axon

Myelin


Synapses
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!)



Release of neurotransmitter
Release of neurotransmitter

Presynaptic terminal releases neurotransmitter


Video of synaptic transmission
Video of Synaptic Transmission

http://www.youtube.com/watch?v=HXx9qlJetSU&feature=related

By Jokerwe


Connections to other neurons
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
Inhibitory connections

Axosomatic

Axoaxonal


Myelin and other features
Myelin (and other features)

Dendrite

Axon terminal

Node of Ranvier

Soma

Schwann cell

Myelin sheath

Nucleus


Integration of neural inputs
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



Spread of activation
Spread of activation

  • Activation moves across links

  • At the small scale

    • from neuron to neuron

  • At larger scale, across multiple links

    • In vision

      • From retina to conceptual area of cortex

    • In speech production,

      • from meanings to their expression

    • For a listener,

      • From expression to meaning


Another kind of neurotransmitter
Another kind of neurotransmitter

Released into interneural space, has global effect – e.g. serotonin, dopamine


Events in short time periods
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
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
Three views of the gray matter

Different stains show different features


Layers of the cortex
Layers of the Cortex

From top to bottom, about 3 mm


Long distance cortico cortical connections
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



The 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
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 and other areas

Primary Somato-

sensory Area

Primary

Motor Area

All other areas are secondary, association,

or executive areas

Primary

Visual Area

Primary Auditory

Area


The cortical network has a hierarchical structure
The cortical network has a hierarchical structure

  • 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



Major anatomical functional dichotomies
Major anatomical-functional dichotomies

  • Left hemisphere vs. Right hemisphere

    • Left

      • Analytical, linguistic, digital

      • Maintains existing beliefs

    • Right

      • Metaphorical, artistic, analog

      • Open to new data and ideas

  • Front (anterior) vs. Back (posterior)

    • Front

      • Action and planning of action

      • Process oriented

    • Back

      • Perception

      • Perceptual integration

      • Object oriented


Left hemisphere vs right hemisphere

Left hemisphere

Analytical thinking

Exact

Digital

Heightened contrast

Proof

Right Hemisphere

Holistic thinking

Metaphorical

Analog

Fuzzy boundaries

Hunches, intuition

Left hemisphere vs. right hemisphere


Corpus callosum revealed by excision of top of rh
Corpus Callosum(revealed by excision of top of RH)

Corpus Callosum


Cerebral dominance for language
Cerebral dominance for language

  • Linguistic abilities are subserved by the left hemisphere in about 97% of people

    • 99% of right-handed people

    • A majority of left-handers

  • But this is just a first approximation


The role of rh in semantics
The Role of RH in semantics

  • Conceptual information, even for a single item, is complex

    • Therefore, widely distributed

    • A network

    • Occupies both hemispheres

  • RH information is more connotative

    • LH information more exact


Faulty thinking in cognitive science among some but not all practioners
Faulty thinking in cognitive science(among some but not all practioners)

  • The brain (likewise the mind) is like the computer

  • An example of the misapplied metaphor


The cortex is a network entirely different structure than that of computers
The Cortex is a NetworkEntirely different structure than that of computers

  • Connectivity as key property of brain structure

  • Symbol-manipulation is the key property of computers

  • The cortex operates by means of connections

    • Transmission of activation along neural pathways

    • Changes in connection strengths


Computers and brains different structures different skills

Computers

Exact, literal

Rapid calculation

Rapid sorting

Rapid searching

Faultless memory

Do what they are told

Predictable

Brains

Flexible, fault tolerant

Slow processing

Association

Intuition

Adaptability, plasticity

Self-driven activity

Unpredictable

Self-driven learning

Computers and Brains: Different Structures, Different Skills


Things that brains but not computers can do
Things that brains but not computers can do

  • Acquire information to varying degrees

    • “Entrenchment”

    • How does it work?

      • Variable connection strength

      • Connections get stronger with repeated use

  • Perform at varying skill levels

    • Degrees of alertness, attentiveness

    • Variation in reaction time

    • Mechanisms:

      • Global neurotransmitters (next slide)

      • Variation in blood flow

      • Variation in available nutrients

      • Presence or absence of fatigue

      • Presence or absence of intoxication


Global neurotransmitters
Global neurotransmitters

Released into interneural space, has global effect – e.g. serotonin, dopamine


Neuronal structure and function connectivity
Neuronal Structure and Function:Connectivity

  • White matter: it’s all connections

    • Far more voluminous than gray matter

    • Cortico-cortical connections

      • The fibers are axons of pyramidal neurons

      • They are all excitatory

    • White since the fibers are coated with myelin

      • Myelin: glial cells

  • There are also grey matter connections

    • Unmyelinated

    • Local

    • Horizontal, through gray matter

    • Excitatory and inhibitory


Pyramidal neurons and their connections
Pyramidal neurons and their connections

  • Connecting fibers

    • Dendrites (input): length 2mm or less

    • Axons (output): length up to 10 cm

  • Synapses

    • Afferent synapses: up to 50,000

      • From distant and nearby sources

        • Distant – to apical dendrite

        • Local – to basal dendrites or cell body

    • Efferent synapses: up to 50,000

      • On distant and nearby destinations

        • Distant – main axon, through white matter

        • Local – collateral axons, through gray matter


Connecting fibers of pyramidal neurons
Connecting fibers of pyramidal neurons

Apical dendrite

Basal dendrites

Axon


Interconnections of pyramidal neurons
Interconnections of pyramidal neurons

Input from distant cells

Input from neighboring columns

Output to distant cells


Neuronal structure and function connectivity1
Neuronal Structure and Function:Connectivity

  • Synapses of a typical pyramidal neuron:

    • Incoming (afferent) – 50,000 (5 x 104)

    • Outgoing (efferent) – 50,000

  • Number of synapses in cortex:

    • 28 billion neurons (Mountcastle’s estimate)

      • i.e., 28 x 109

  • Synapses in the cortex (do the math)

    • 5 x 104 x 28 x 109 = 140 x 1013 = 1.4 x 1015

    • Approximately 1,400,000,000,000,000

    • i.e., over 1 quadrillion


How does all this complex structure work
How does all this complex structure work?

  • A structure sui generis – quite unlike computers and in fact unlike anything else we have ever known

  • Extraordinarily complex

    • Billions of neurons

    • Trillions of interconnections

  • How can we make sense of it?

  • Stay tuned:

    • Next week: Brain function



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