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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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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.   


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

  • Central nervous system

    • Spinal cord

    • Brain

  • Peripheral nervous system

    • Motor and sensory neurons connected to the spinal cord

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 brain

*Brain Stem

  • Medulla oblongata – Myelencephalon

  • Pons and Cerebellum – Metencephalon

  • Midbrain – Mesencephalon

  • Thalamus and hypothalamus – Diencephalon

  • Cerebral hemispheres – Telencephalon

Alternative partition:

Brain stem*


Thalamus & hypothalamus

Cerebral hemispheres

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

  • 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



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

Corpus Callosum Connects Hemispheres

Corpus Callosum

Major Left Hemisphere landmarks

Central Sulcus

Sylvian fissure

The Sylvian Fissure opened up (it’s huge)

Major landmarks and the four lobes

Central Sulcus









Sylvian fissure

Primary motor and somatosensory areas

Primary Somato-

sensory Area

Central Sulcus


Motor Area

Sylvian fissure

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

  • Interhemispheric fissure

    • Also known as Longitudinal fissure

  • Sylvian fissure

    • Also known as Lateral sulcus

  • Central sulcus

    • Also known as Rolandic fissure

Primary Areas

Primary Somato-

sensory Area


Motor Area

Primary Auditory



Visual Area

Divisions of Primary Motor and Somatic Areas

Primary Somato-

sensory Area



Motor Area






Primary Auditory



Visual Area

Higher level motor areas

Primary Somato-

sensory Area

Actions per-

Formed by leg




by hand






by mouth



Primary Auditory



Visual Area

Video of basic cortical anatomy


From Medical Legal Art (2009)

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

  • 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)

Gray matter and white matter (coronal section)





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

  • 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

  • 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

    • 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

      • Seven times the distance from the Earth to the moon

Big lesson: Connectivity rules!

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

Types of cortical neurons

Pyramidal neurons

Microelectronic probe

About 70% of cortical neurons are of this type

Structure of pyramidal neuron

Apical dendrite

Cell body




  • 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


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



Myelin (and other features)


Axon terminal

Node of Ranvier


Schwann cell

Myelin sheath


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

  • 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

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

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

Gray matter and white matter (coronal section)





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 Somato-

sensory Area


Motor Area

All other areas are secondary, association,

or executive areas


Visual Area

Primary Auditory


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

Sequence of development in the cortex

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

Analytical thinking



Heightened contrast


Right Hemisphere

Holistic thinking



Fuzzy boundaries

Hunches, intuition

Left hemisphere vs. right hemisphere

Corpus Callosum(revealed by excision of top of RH)

Corpus Callosum

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

  • 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)

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

  • An example of the misapplied metaphor

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


Exact, literal

Rapid calculation

Rapid sorting

Rapid searching

Faultless memory

Do what they are told



Flexible, fault tolerant

Slow processing



Adaptability, plasticity

Self-driven activity


Self-driven learning

Computers and Brains: Different Structures, Different Skills

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

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

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

  • 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

Apical dendrite

Basal dendrites


Interconnections of pyramidal neurons

Input from distant cells

Input from neighboring columns

Output to distant cells

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?

  • 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

T h a n k s f o r y o u r a t t e n t i o n !

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