A sketch of the central nervous system and its origins

G. E. Schneider 2009Part 5: Differentiation of the brain vesicles

MIT 9.14 Class 1212

Forebrain of mammals with comparative studies relevant to its evolution

The forebrain (prosencephalon) Topics

• Major subdivisions and overview of ‘tweenbrain – Hypothalamus and epithalamus: related to “limbic”

system structures of the forebrain – Thalamus and subthalamus: related more 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

Also: Review of neuroanatomy covered thus far

DVR

LG

MG

Thickened Ventricular Layer

Rostral end of the thickening neural tube: a few more descriptive terms

DVR = Dorsal Ventricular Ridge

LG = Lateral Ganglionic Eminence

MG = Medial Ganglionic Eminence

Hindbrain

Midbrain

Forebrain

Preview: Evidence on endbrain evolution

• Recent data has come from studies of expression patterns of regulatory genes, like the hox genes, in various species.

• Prior to these studies, cross-species comparisons were made using only morphological data (cytoarchitecture, connections patterns).

Homeobox gene expression: Emx-1 Dlx-1

s = septum

h = hyperstriatum

dvr = dorsal ventricular ridge

dc = dorsal cortex

st = striatum

am = amygdala & claustrum

Dorsal

Medial Lateral

Ventral

Archetypal Embryonic Stage

Turtle

dc

sst

dvr

Chick

st

dvr

hMouse

sst

cx

am

Frog

s

st

dc

Evolution of telencephalon based on expression patterns of regulatory genes during development--Based on work of Anibal Smith Fernandez et al., figure from Allman (2000), p 113.

Figure by MIT OpenCourseWare.

We will return to these pictures of the endbrain at the end of this section.

• First: Major features of the forebrain structure

• and then a pause for a brief review of major concepts in brain anatomy.

Forebrain (prosencephalon) :

endbrain (telencephalon)

&

‘tweenbrain (diencephalon)

The forebrain (prosencephalon)

• Major subdivisions and overview– Diencephalon

• Hypothalamus (and epithalamus)• Thalamus (and subthalamus)

– Telencephalon • Pallium

– Limbic – Non-limbic (neocortex)

• Corpus striatum – Ventral (limbic) – Dorsal (non-limbic)

• Origins and course of 2 major pathways

Diencephalon 1: Hypothalamus & epithalamus

• Visceral inputs • Connections with endbrain and midbrain:

“Limbic system" connections • Functions include gating of pathways

ascending through thalamus.

Diencephalon 2: Thalamus & 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):

• Major subdivisions and overview – Diencephalon

• Hypothalamus (and epithalamus) • Thalamus (and subthalamus)

– Telencephalon • Pallium

– Limbic cortex – Non-limbic cortex (neocortex)

• Corpus striatum & pallidum – Ventral – Dorsal

• Origins and course of 2 major pathways

Telencephalon: major structures – Neocortex

• “Primary” sensory and motor cortical areas • Unimodal association cortex • Heteromodal association cortex

– Limbic cortex • Olfactory cortex • Paleocortical & closely related structures

– Corpus striatum • Dorsal striatum (sometimes called neostriatum)• Ventral striatum structures including Olfactory Tubercle • Globus pallidus & ventral pallidum: output structures of

the striatum

The forebrain (prosencephalon):

• Major subdivisions and overview – Diencephalon

• Hypothalamus (and epithalamus) • Thalamus (and subthalamus)

– Telencephalon • Neocortex • Corpus striatum • Limbic endbrain

• Origins and course of 2 major pathways

‘Tween-brain

and

Endbrain

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

Olfactory cortex

Let’s look at these sections one at a time:

‘Tweenbrain (diencephalon)

Fibers of medial 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,to and from: – Corpus striatum – Neocortex, via:

• Neocortical white matter,

Outputs of neocortex viaInternal capsule-Cerebralpeduncle-Pyramidal tract

(See following slide)

• Medial forebrain bundle, to and from: – Olfactory cortex – Limbic cortex – Subcortical limbic

structures: amygdala, basal forebrain

– lateral hypothalamic area

– limbic midbrain areas

Forebrain: endbrain &

‘tweenbrain: the lateral forebrain

bundleCorticospinal tract Pyramidal tract

Cerebral peduncles (includes corticopontine)

Cortical white matter Æ Internal capsule

MFB

REVIEW

Somatic regions

“Limbic” regions

REVIEW:

‘Tween-brain and Endbrain limbic & MFB

Lateral forebrain bundle

Medial forebrain bundle

Lateral forebrain bundle

Medial forebrain bundle

The neocortex is involved in both major systems

A schematic summary

Hypothalamus

Some Major Endbrain Connections

Neocortex

Dorsal striatum

Brainstem Spinal cord

Limbic structures

This leaves out a great many details!

• E.g., in the diagram, 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

Dorsal striatum Ventral Limbic structures Thal striatum

Hypothalamus

Brainstem

Neocortex

Spinal cord

“Striatum” (dorsal & ventral) includes the output structures—the pallidum (dorsal, ventral)

Check your knowledge: 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 causes a

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

descending pathways to it 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 continued

• Midbrain: tectum and tegmentum; species differences; outputs for three major types of movement

• Diencephalon: two major and two additional subdivisions (functional/structural)

• Telencephalon: the endbrain (cerebral hemispheres and basal forebrain); origins of two major pathways for descending axons (Both contain some ascending axons also.)

• Some major axonal pathways in mammals: – Spinoreticular, trigeminoreticular tracts (mostly ipsilateral)– Spinothalamic tract; longest axons to ventrobasal nuc. of

thalamus (VB = VPM and VPL) – Dorsal columns, connecting to the medial lemniscus pathway,

which projects to the ventrobasal nuc. of thalamus – Corticospinal & corticopontine pathways (the former to all

levels of CNS, the latter connecting to the cerebellum via pons)

Notes on Evolution of Forebrain Briefly: 3 topics

• Neuromeres of the forebrain – Segmentation rostral to rhombomeres and

mesomere

• Origins of neocortex – Structural studies give evidence that non-cortical

structures in amphibians, reptiles and birds are related to neocortex of mammals

– Gene expression studies have clarified the phylogenetic relationships

nalian braimmayonic m embrs ofodelmmeric euroN2 8 p.,)5020(r etdeir StFrom

Figures by MIT OpenCourseWare.

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

Structural studies give evidence that non-cortical structures in amphibians, reptiles and birds are related to neocortex of mammals

• Each sensory system connects to thalamic cell groups, which project mainly to the neocortex of mammals

• In birds, also in reptiles and amphibians, thalamus contains unimodal cell groups that project to structures that appear to bestriatum (studies by Karten and others using, originally, Nautamethods for silver-staining of degenerating axons).

• A novel proposal was put forth by Harvey Karten when he was with Nauta at MIT: Embryonic cells migrate from a striatal location to neocortex. Thus, many neocortical cells in mammals are directly homologous to striatal cells in birds (more specifically, cells of the ectostriatum). – This has recently been tested, with surprising results.

Homeobox gene expression: Emx-1 Dlx-1

Evolution of telencephalon based on expression patterns of regulatory genes during development. Based on work of Anibal Smith Fernandez et al., figure from Allman (2000), p 113.

Dorsal

Medial Lateral

Ventral

Archetypal Embryonic Stage

Turtle

dc

sst

dvr

Chick

st

dvr

hMouse

sst

cx

am

Frog

s

st

dc

Figure by MIT OpenCourseWare.

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 Dlx-2

From Striedter (2005), p.278

Figures by MIT OpenCourseWare.

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 to neocortex includes cells from striatal area

arch reser ed in latfountions graimActual B) grations iml acitehtpohy Karten’s )A

eDVR = embryonic Dorsal Ventricular Ridge

eDVR

LG

MG

Str.

OC

N

eDVR

LG

MG

Str.

OC

N

LG, MG: Lateral, Medial Ganglionic Eminence From Striedter (2005), p. 280 Str.=Striatum; OC=Olfactory Cortex; N=Neocortex

Figures by MIT OpenCourseWare.

Next:

A major aspect of nervous system differentiation at the cellular level: The growth, plasticity and regeneration of axons.

MIT OpenCourseWare http://ocw.mit.edu

9.14 Brain Structure and Its Origins Spring 2009

For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.