A sketch of the central nervous system and its origins

G. E. Schneider 2009Part 2: Steps to the central nervous system,

from initial steps to advanced chordates

MIT 9.14 Class 4

Elaboration of the neural tube in evolution

Evolution of multi-cellular organisms: Suggestions based on phylogenetic comparisons

TOPICS, sessions 3 & 4: √ Behaviors most fundamental for survival √ Intercellular conduction in Ctenophores and

Coelenterates: suggestions about evolution of the nervous system

√ A generalized conception of the CNS √ The body plan of primitive chordates, as

suggested by Amphioxus (Branchiostoma) – Elaboration of the neural tube in evolution

Phylum Coelenterata: now often called Cnidaria. Animals with radial symmetry and stinging tentacles; includes free-swimming medusa forms like jellyfish, and sessile polyp forms like corals and sea anemones.

Evolution of Brain 1

With head receptors and forward locomotionthere evolved an increasing sophistication of

sensorimotor abilities

• Sensory analyzing mechanisms – Connected to inputs from cranial nerves

• Associated motor apparatus – For directing the receptors (orienting movements)– For controlling alterations in posture and

locomotion under guidance from these receptors. • Crucial background: maintenance of stability of

the internal mileau

Structures that accomplished those functions

• New sensory apparatus at rostral end of the tube – Hindbrain, midbrain and forebrain mechanisms connected

to head receptors are added to primitive spinal somatosensory mechanisms.

• Added motor control – Hindbrain & midbrain: Control of mouth, eyes, ears, head

turning, added to basic spinal & hindbrain control of the body

• Forebrain vesicle evolves, with: – Olfactory & visual inputs – Endocrine & visceral control

Evolution of Brain 2

Expansion of hindbrain: ¾ Sensory analysers: somatosensory (face), taste,

¾ Action pattern programs vestibular, auditory, electroreceptive.

triggered by specific sensory patterns

Evidence from comparative neuroanatomy

• Functional demands result in expansion of CNS components

• Some particularly dramatic examples have been found in studies of fish – Pictures from CL Herrick, who together with

his brother, CJ Herrick, helped establish the field of comparative neurology

Functional adaptations cause expansions in CNS: illustrations from comparative anatomy

(from C. L. Herrick):

• Brain of a fresh water mooneye. • Brain of a freshwater buffalo fish:

– huge "vagal lobe“ (receives input from specialized palatal organ)

• Brain of a catfish: – "facial lobe" and "vagal lobe“ (for processing taste

inputs through two different cranial nerves)

• Catfish 7th cranial nerve distribution, re: – taste senses (explains facial lobe)

Olfactory Bulb

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Medulla Oblongata

Hyodontergisus

(fresh water Mooneye)

Note the size and shape of the hindbrain. (The hindbrain includes the medulla oblongata and the cerebellar region, between the two black arrows. Figure by MIT OpenCourseWare.

Hindbrain

Carpiodestumidus

(buffalofish)

Endbrain

Midbrain

Cerebellum

Vagal Lobe

of thlobe”The “vagalhindbrain is huge. It receives and processetaste input from a specialized palatal org

e

s

an.Figure by MIT OpenCourseWare.

Pilodictis olivaris (catfish)

The vagal lobe is enlarged, although less than in the buffalo fish. An enlarged “facial lobe” is also evident. It receives taste inputs from all over the body surface.

Figure by MIT OpenCourseWare.

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Facial Lobe

Vagal Lobe

Amiurus melas (the small catfish): 7th cranial nerve (facial nerve) innervates taste buds in skin of entire body

Figure by MIT OpenCourseWare.

Evolution of Brain 3 Expansion of (Concurrent with “Evolution 2”)

This was the first expansion of the forebrain

Note: connections shown are actually on both sides

forebrain because of adaptive value of olfactory sense for approach & avoidance functions (feeding, mating, predator avoidance, predation). ……………………..Outputs: links to locomotion through the corpus stratum were most critical. These links were via the midbrain.

Evolution of Brain 3(Concurrent with “Evolution 2”)

Note: connections shown are actually These connections were plastic: on both sides They could be strengthened or

weakened, depending on experience.

Expansion of forebrain because of adaptive value of olfactory sense for approach & avoidance functions (feeding, mating, predator avoidance, predation). …………………….. Outputs: links to locomotion through the corpus stratum were most critical. The links were via the midbrain.

Expansions, continued:Functional demands result in progressive

changes in the neural tube, to include:

9 Sensory analyzing mechanisms

9 Corresponding motor apparatus • “Correlation centers”

Elaboration of complex programs for goal-• directed activities

• Systems for modulating other brain systems in response to visceral and social needs

• Systems for anticipating events & planning actions

"Correlation centers"

• A network between the sensory analyzers and the motor apparatus: – For maintaining stability in space by integration

of vestibular, visual and tactile information – For avoidance and approach movements,

guided by olfaction as well as other senses • Example: Evolution of structures to

improve head and body orientation– Midbrain tectum and pretectal cell groups – Medial hindbrain reticular formation – Medial cerebellar structures

Note the positions of the midbrain tectum and the cerebellum in Hyodon tergisus (the fresh water Mooneye)

Olfactory Bulb

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Medulla Oblongata

Figure by MIT OpenCourseWare.

Note: Neurons and pathways shown on one side of the brain are usually the same on the other side. I often draw

Evolution of Brain 4 Expansion of midbrain with evolution of distance-receptor senses: visual and auditory, receptors with advantages over olfaction for speed and sensory acuity, for early warning and for anticipation of events. …………………………….. Motor side: turning of head and eyes with modulation by motivational states, including those triggered by olfactory sense.

them on only one side to make the drawing simpler.

Why do sensory pathways decusssate?

• This is a question that has given rise to various speculations, but there have been no firm answers.

• Next, I present briefly a suggestion that will be developed later into an hypothesis that is more convincing than others that have been proposed.

Thinking about the evolution of these correlation centers

• Hypothesis: Very early in the evolution of the midbrain and forebrain, before the hemispheres appeared, visual inputs from lateral eyes projected bilaterally but then in evolution became crossed. This resulted in later evolution of decussations of non-visual pathways.

Why did the axons from the lateral eyes become crossed? Because it was more adaptive – supporting the better survival of the organism: Crossed pathways were able to reach crucial output mechanisms most quickly. These mechanisms must have controlled rapid escape/avoidance movements. We will argue this again later when we study somatosensory connections in the hindbrain, and later when we study the visual system.

Evolution of Brain 5a (introduction): non-olfactory systems invade the ‘tweenbrain and endbrain in pre-mammalian vertebrates.

Thalamic axons carrying visual, somatosensory & auditory information reached the corpus striatum and the dorsal cortex. These

(These systems took advantage of the plastic links in the striatum…)

Second expansion of the forebrain

connections resulted in pronounced expansions of the endbrain.

Functional demands result in progressive changes in the neural tube, to include: 9 Sensory analyzing mechanisms 9 Corresponding motor apparatus 9 “Correlation centers” –e.g., the midbrain tectum • With more complex sensory analysis there was

also an elaboration of complex programs for goal-directed activities

• Systems for modulating other brain systems in response to visceral and social needs

• Systems for anticipating events & planning actions

Elaboration of complex programs for goal-directed activities

• “Fixed action patterns" (FAPs) – Not the same as reflexes, but controlled by motivational states – “Fixed” in the genes – Examples from Ethology (Instinctive movements)

• Modifications of FAPs; learning of new action patterns• Correlated structural elaborations

– Hindbrain and spinal-cord control of movement patterns with motivational control, and some sensory control, via midbrain & forebrain

– Elaborations that were particularly prominent in mammals: Mechanisms for reward-driven learning and habit formation, using corpus striatum and neocortex linked to motivation systems

Functional demands result in progressive changes in the neural tube, to include:

9Sensory analyzing mechanisms 9Corresponding motor apparatus 9“Correlation centers” –e.g., the midbrain tectum 9 Elaboration of complex programs for goal-directed

activities • Systems for modulating other brain systems in

response to visceral and social needs • Systems for anticipating events & planning actions

Systems for modulating other brain systems in response to visceral and social needs:

Motivational control• Cyclic and episodic control of motivation: present in

the most primitive animals, but elaborated with forebrain expansion

• Elaboration of goal hierarchies, both built-in and learned

• Communication of needs & desires (emotions, feelings)

• Structures that achieve this: – Hypothalamus, epithalamus, & associated midbrain &

hindbrain systems – Evolution of “limbic forebrain”, connected to these systems

Functional demands result in progressive changes in the neural tube, to include: 9 Sensory analyzing mechanisms 9 Corresponding motor apparatus 9 “Correlation centers” 9 Elaboration of complex programs for goal-

directed activities 9 Systems for modulating other brain systems in

response to visceral and social needs • Systems for anticipating events & planning

actions (cognitive systems)

Evolution of cognitive systems of the brain

• Sensory side: images that simulate objects & eventsMotor side: planning of and preparing for actions– These are non-reflex functions involving memory and

internal representations of the external world. • Evolution of structures that accomplish these

functions: forebrain, especially in the neocortex of the endbrain

With evolution of these cognitive functions, the endbrain expanded further.

• This was a third major expansion of the forebrain

• What structures in the endbrain expanded the most?

What structures in the endbrain expanded the most?

• Expansion of the neocortex, especially the so-called "association cortex"

• Also the parts of the corpus striatum and the cerebellum closely connected to those areas of neocortex.

Third forebrain expansion Evolution of Brain 5b: The expansion of the endbrain, dominated by the expanding area of the neocortex in mammals. Correlated with this was an expansion of the cerebellar hemispheres, and also the “neostriatum”

Also note: The advantages of control of fine movements, especially with evolution of distal appendages with capacity for manipulation, resulted in evolution of motor cortex as well as the cerebellar hemispheres.

How much expansion has occurred?

• Data have been collected on total size of the brains of many species of animals.

• Relative brain size can be seen by plotting brain weight vs. body weight.

Vertebrate brain-body scaling (Striedter, 2005, based on Jerison, 1973)

Note the use of log-log coordinates

Figure by MIT OpenCourseWare.

Brain & body weights: animal groups

Similar to fig. 4.3 in Striedter (2005).

Which dimension indicates relative brain size?

Figure by MIT OpenCourseWare.

Mammalian brain-body scaling (Striedter, 2005, based on several earlier studies)

Shrew

PocketMouse

Microbat

Chipmunk

European Mole

Hedgehog

Tenrec

Galago

Talapoin Monkey

Rabbit

Cebus MonkeyBaboon

Beaver

Chimpanzee

Warthog

Bison

HumanHarbor Porpoise

Beluga Whale

Sperm WhaleElephant

Blue Whale

Walrus

Hippopotamus

Rhinoceros

10010-2

10-1

100

101

102

103

104

105

101 102 10 3 104 105 106 107 108

Body Weight (g)

Bra

in W

eigh

t (g)

Figure by MIT OpenCourseWare.

Brain & body weights in mammals

See also fig. 4.1 in Striedter (2005)

MAMMALS

ElephantPorpoise

BlueWhale

Chimpanzee

Lion

Mole

Vampire Bat

Rat

Opossum

Wolf

BaboonAustralopithecus

Gorilla

Modern human

BR

AIN

WEI

GH

T (g

)

10,0005000

1000500

10050

10.05.0

1.00.5

0.10.05

0.010.001 0.01 10.1 10 100 1000 10,000 1000,000

BODY WEIGHT (kg)

Figure by MIT OpenCourseWare.

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9.14 Brain Structure and Its Origins Spring 2009

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