g. e. schneider 2009 part 2: steps to the central nervous system, from initial steps to advanced...
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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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