9.14 - Brain Structure and its OriginsSpring 2005 Massachusetts Institute of Technology Instructor: Professor Gerald Schneider

From end of last class:

Division of the midbrain into two functionallydistinct regions, “limbic” and “somatic”

1. Somatic: Connected to the somatic sensory and motor systems

2. Limbic: Connected to the autonomic nervous system and the closely associated “limbic” forebrain system

These functionally distinct regions continue rostrally into the ‘tweenbrain.

Somatic regions

Limbic regions

A sketch of the central nervous system and its origins

G. E. Schneider 2005 Part 5: Differentiation of the brain vesicles

MIT 9.14 Class 1111(session 12)(session 12)

Forebrain of mammals with comparative studies relevant to its evolution

The forebrain (prosencephalon)

• General picture and subdivisions (color coded) – Hypothalamus (and epithalamus): related to “limbic”

system structures of the forebrain – Thalamus (and subthalamus): related to somatic

sensory and motor systems

• Origins and course of 2 major pathways: related to– somatic sensory & motor systems, and – limbic system

• Evidence concerning forebrain evolution

Rostral end of the thickening neural tube

DVR

Forebrain

Midbrain

Hindbrain

DVR = Dorsal Ventricular Ridge

LG = Lateral Ganglionic Eminence

MG = Medial Ganglionic Eminence

Thickened Ventricular Layer

Figure removed due to copyright reasons.Please see:Allman, and John Morgan. Evolving Brains. New York, NY: Scientific American Library.Distributed by W.H. Freeman and Co., 1999. ISBN: 0716750767.

Evolution of telencephalon based on expression patterns of regulatory genes during development. (Based on work of Anibal Smith Fernandez et al., figure from Allman textbook)

(prosencephalon

(

Forebrain ) :

endbrain telencephalon)

&

‘tweenbrain(diencephalon)

Diencephalon: Hypothalamus and epithalamus:

• Visceral inputs • Connections with endbrain and midbrain:

“Limbic system" connections • Gating of ascending pathways through

‘tweenbrain

Diencephalon:Thalamus and subthalamus

(= dorsal thalamus & ventral thalamus)

• Somatic inputs from the lemniscal pathways

• Connections from midbrain tectum and tegmentum (somatic parts of midbrain)

Somatic regions

Limbic regions

The forebrain (prosencephalon):

• General picture and subdivisions – Hypothalamus (and epithalamus) – Thalamus (and subthalamus)

• Origins and course of 2 major pathways

‘Tween-

Endbrain

Fibers of medial lemniscus to VP, & from Cb to VA, VL

Olfactory cortex

brain

and

‘Tweenbrain (diencephalon)

medial Fibers of

lemniscus to VP, & from Cb to VA, VL

Fibers of “Lateral Forebrain Bundle”

Endbrain (telencephalon)

Olfactory cortex

Origins and course of 2 major pathways:

• Lateral forebrain bundle • Medial forebrain bundle– Corpus striatum outputs – Olfactory cortex

– Limbic cortex – Neocortical white matter – Subcortical limbic – Internal capsule-Cerebral structures: amygdala, basal

peduncle-Pyramidal tract forebrain – lateral hypothalamic area – limbic midbrain areas

Forebrain:

‘tweenbrain:

Æ

Corticospinal tract Pyramidal tract

Cerebral peduncles (includes corticopontine)

Cortical white matter Internal capsule

endbrain &

the lateral forebrain

bundle

MFB

Somatic regions

Limbic regions

‘Tween­brain and Endbrain limbic & MFB

The neocortex is involved in both major systems

A schematic summary

Some Major Endbrain Connections

Dorsal striatum

Neocortex

Limbic structures

Hypothalamus

BrainstemSpinal cord

This leaves out a great many details!

• For example, the ventral striatum is lumped together with “limbic” structures.

• Ventral striatum is critical in habit formation, and is probably the most primitive part of the corpus striatum in evolution.

• Reward and punishment mechanisms exist with a special role of ascending projections, e.g., from taste and pain systems.

• Next picture: the schematic summary is augmented

Some Major Endbrain Connections

Neocortex

Brainstem Spinal cord

striatumDorsal striatum Limbic structures

Hypothalamus

Ventral

Neuroanatomy review• Subdivisions of CNS; definitions of cell types

– Shapes of the neural tube at various levels • Sensory channels of conduction; dermatomes• Diaschisis: lesion-produced deafferentation results in

functional depression of neurons • Evolution of neocortex and major ascending and

descending pathways to and from it • Spinal cord structure; differences between levels • Propriospinal system • Autonomic N.S.: its components • Hindbrain organization; distortions of the basic plan• Cranial nerves: the 5th (trigeminal nerve)

Neuroanatomy review• Midbrain: tectum and the tegmentum; species

differences; outputs for three major types of movement • Diencephalon: two major and two other

functional/structural subdivisions • Telencephalon: the endbrain (cerebral hemispheres and

basal forebrain); origins of two major pathways for descending axons (Both contain some ascending axons also)

• Other major pathways in mammals:– Spinothalamic tract – Dorsal columns, connecting to the medial lemniscus pathway

to the ventrobasal nuc. of thalamus (VP) – Corticospinal & corticopontine pathways (the former to all

levels of CNS, the latter connecting to the cerebellum)

Notes on Evolution of ForebrainBriefly: 3 topics

• Neuromeres of the forebrain • Origins of neocortex • Non-cortical structures in amphibians, reptiles

and birds that are related to neocortex

Neuromeric models of embryonic mammalian brain

2) Puelles & Rubenstein,’93

3) Revision of the model

1) Embryonic brain with curved longitudinal axis

Gene expression data indicate existence of neuromeres also in the non-amniotes lamprey and zebrafish.

Diencephalon

Mesencephalon

Rhombencephalon

Eye StalkD

DD

D

R CV

DP

OB

MP

Str. P5

P6

P4

P3

P2P1

Mes.

1st

R1R2

R3R4

R5R6

R7

PaStr.

VP

LP

DPMP

P5

P4P3

P6

Telencephalon

OB

Figures by MIT OCW.

Figure removed due to copyright reasons.Please see:Allman, and John Morgan. Evolving Brains. New York, NY: Scientific American Library.Distributed by W.H. Freeman and Co., 1999. ISBN: 0716750767.

Evolution of telencephalon based on expression patterns of regulatory genes during development. (Based on work of Anibal Smith Fernandez et al., figure from Allman textbook)

A) Radial glia cells (black lines) in embryonic mammals and birds

Embryology and the Claustroamygdalar DVR Hypothesis

B) Transcription factor expression: Tbr-1 Tbr-1 and Emx-1 Dix-2

Figures by MIT OCW.

DPMP

LP

G

DVR

Str.

Str.

CA

Neo

Mouse Chick

Mammal Bird

Regulatory Gene Expression in Mice and Chicks

Radial Glia in Embryonic Mice and Chicks

Tbr-1 Tbr-1 and Emx-1 Dlx-2

VP

DP

MP

LPMG

LGVP

Migration of cells from the dorsal ventricular ridge and from ventricular layer to neocortex

A) Karten’s migrations B) migrationshypothetical Actual

LG, MG: Lateral, Medial Ganglionic Eminence Str.=Striatum; OC=Olfactory Cortex; N=Neocortex

eDVR

LG

MG

Str.

OC

N

eDVR

LG

MG

Str.

OC

N

Figure by MIT OCW.

Selected References

Slide 26: Striedter, Georg F. Principles of Brain Evolution. Sunderland, MA: Sinauer Associates, 2005, p. 82. ISBN: 0878938206. Slide 28: Striedter, Georg F. Principles of Brain Evolution. Sunderland, MA: Sinauer Associates, 2005, p. 278. ISBN: 0878938206. Slide 29: Striedter, Georg F. Principles of Brain Evolution. Sunderland, MA: Sinauer Associates, 2005, p. 280. ISBN: 0878938206.