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

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 10

Hindbrain specializations; midbrain and its specializations

Sensory specializations, 5th cranial nerveand other hindbrain specializations

• Snake sensory pit (in pit vipers) for infrared radiation detection

• Rodent vibrissae, for sensing the space around the head: We will illustrate the brain representations.

• Recall other specializations of the hindbrain mentioned earlier: – For taste functions in some fishes – For electrosensory abilities in weakly electric fish – Cerebellar expansions in large animals with highly developed

manipulatory abilities

Rattlesnake trigeminal nerve: Innervation of a specialized distance sense

Question: What corresponding brain specializations can be found?

Figure by MIT OCW.

Sensory (Infrared) pit

Maxillary Branch

Mandibular Branch

Ophthalmic Branch

The evolution of changes in brain involve both size and architectural details

• Illustrated by the trigeminal system of moles and rodents – Relative size of central maps and sensory acuity

are correlated.* – Organizational specializations: the barrel fields

representing the vibrissae

* Studies of visual acuity and the visual cortex representation have shown this very well.

Somatosensory representation in mole neocortex

Figure removed due to copyright reasons.Please see:Figure 22-12 from Zigmond, Michael J., et al., eds. Fundamental Neuroscience.San Diego, CA: Academic Press, 1999, part III. ISBN: 0127808701. (Illustrations by Robert S. Woolley.)

Somatosensory representation in a mouse or rat, from whiskers to “barrels”

Figure removed due to copyright reasons.Please see:Figure 1 in Li, Y., R. S. Erzurumlu, C. Chen, S. Jhaveri, and S. Tonegawa."Whisker-related Neuronal Patterns Fail to Develop in the TrigeminalBrainstem Nuclei of NMDAR1 Knockout Mice." Cell 76, no. 3 (February 11, 1994): 427-37.

P4 rat neocortex, coronal section, DiI in VB

Figure removed due to copyright reasons.

Please see:

Jhaveri, S., R. S. Erzurumlu, and K. Crossin. "Barrel construction in rodent neocortex: role ofthalamic afferents versus extracellular matrix molecules." Proc Natl Acad Sci USA 88, no. 10

(May 15, 1991): 4489-93.

Similar case, tangential section

Figure removed due to copyright reasons.

Please see:

Jhaveri, S., R. S. Erzurumlu, and K. Crossin. "Barrel construction in rodent neocortex: role ofthalamic afferents versus extracellular matrix molecules." Proc Natl Acad Sci USA 88, no. 10

(May 15, 1991): 4489-93.

P5 rat barrel fields, AChE stain,tangential section

Figure removed due to copyright reasons.

Please see:

Jhaveri, S., R. S. Erzurumlu, and K. Crossin. "Barrel construction in rodent neocortex: role ofthalamic afferents versus extracellular matrix molecules." Proc Natl Acad Sci USA 88, no. 10

(May 15, 1991): 4489-93.

Rat barrel fields, Cytochrome Ctangential section

Figure removed due to copyright reasons.

Please see:

Jhaveri, S., R. S. Erzurumlu, and K. Crossin. "Barrel construction in rodent neocortex: role ofthalamic afferents versus extracellular matrix molecules." Proc Natl Acad Sci USA 88, no. 10

(May 15, 1991): 4489-93.

From whiskers to

barrelettesto

barreloidsto

barrels

Figure removed due to copyright reasons.Please see:Figure 1 in Li, Y., R. S. Erzurumlu, C. Chen, S. Jhaveri, and S. Tonegawa."Whisker-related Neuronal Patterns Fail to Develop in the TrigeminalBrainstem Nuclei of NMDAR1 Knockout Mice." Cell 76, no. 3 (February 11, 1994): 427-37.

The evolution of changes in brain: Many examples of sizeincreases and changes in architectural details within

hindbrain systems

• Cerebellum of electric fish • Cerebellum of mammals • Somatosensory system of rodents • Specializations of taste reception (7th, 9th, 10th n.)

– Ray-finned fishes: vagal lobes and facial lobes of hindbrain, with specialized receptors for bottom feeding

• Lateral line receptor systems:– Electroreception – Mechanoreception

Large variations in size of specific brain parts are examples of “mosaic evolution” (Striedter, 2005, ch 5)

The changes cause "distortions" in the basic organization of the hindbrain

• Variations in relative size of parts – Huge vagal lobe of the fresh-water buffalofish

[Review] – Vagal and facial lobes of the catfish [Review] – Electric fish have an enormous and specialized

cerebellum. [Review] – The cerebellum is very large in mammals, especially

in humans. • Cell migrations from the alar plate:

– Cerebellum – Pre-cerebellar cell groups – especially the cells of the

pons

Illustrations from C.J. Herrick

· Brain of a freshwater buffalo fish: – Huge "vagal lobe."

· Brain of a catfish: – “Facial lobe" and "vagal lobe".

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

Carpiodes tumidus (buffalofish)has a specialized palatal organ for filtering the water for food; it is innervated by the vagus nerve.

Figure by MIT OCW.

Endbrain

Midbrain

Cerebellum

Vagal Lobe

Pilodictisolivaris(catfish) has taste receptors all over its body innervated by the facial nerve (7th

cranial nerve)

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Facial Lobe

Vagal Lobe

Figure by MIT OCW.

7Amiurus melas (the small catfish):

th cranial nerve (facial nerve) innervating taste buds in skin of entire body

Figure by MIT OCW.

The "distortions" in the basic organization of the hindbrain, continued

• Variations in relative size of parts – Huge vagal lobe of the fresh-water buffalofish – Vagal and facial lobes of the catfish – Electric fish have an enormous and

specialized cerebellum.– The cerebellum is very large in mammals,

especially in humans. • Cell migrations from the alar plate cause major

distortions in large mammals – Migration into the cerebellum – Migration to pre-cerebellar cell groups – especially the

cells of the pons

Cb Forms here

Cb = Cerebellum

Location of the late-developing Cerebellum, in the rostral hindbrain

Growth of cerebellum and pons in rostral hindbrain,by migration of neuroblasts from the rhombic lip

Pons

Cerebellar cortex

Red arrows: migration of neuroblasts

Black arrows: course of axons from pontine gray to Cb cortex

Specimen slide removed due to copyright reasons.

Human rostral hindbrain, with pons and cerebellum Æ a quantitative “distortion” of the basic plan

Above the hindbrain

• We can get ideas about evolution of the midbrain and forebrain from the primitive chordates, especially Amphioxus

• We get additional ideas from primitive vertebrates

Recent clues to chordate origins: Studies of Amphioxux

• Evidence that this creature has more than a spinal cord, despite superficial appearances

• Quantitatively the CNS is largely hindbrain and cord, but more rostral parts of the neural tube are present.

• Structural studies show specific components of a primitive midbrain and forebrain

Amphioxus above the hindbrain

• Gene expression studies give evidence for a midbrain, ‘tweenbrain and endbrain.

• Morphological studies show two inputs above the midbrain: optic and a more rostral nerve that could correspond to the olfactory or to the “terminal nerve”. (The terminal nerve innervates the nasal septum.)

• Evidence of an infundibulum (but functions not clear)

Why a midbrain, and a forebrain rostral to it?

• The midbrain, together with primitive components of the forebrain, was a kind of rostral extension of the hindbrain that enabled visual and olfactorycontrol over FAPs (like locomotion, orienting, emotional expression), and that added more control by motivational states.

• The midbrain received visual and other inputs from‘tweenbrain and endbrain, as well as sensory and cerebellar inputs from more caudal structures.

• What were the roles of ‘tweenbrain and endbrain in primitive chordates?

Primitive vision• Early role of optic input to the ’tweenbrain: Control

of daily cycles of activity, with entrainment of the endogenous clock by the day-night cycle – Pineal eye – Retinal input to hypothalamus (The retina develops as an

evagination of the neural tube in the hypothalamic region.)

– Diencephalic controls of sleep and waking physiology and behavior: epithalamus and anterior hypothalamus

• Various cyclic motivational states/behaviors are influenced by the biological clock and regulated by ‘tweenbrain: foraging and feeding, drinking, nesting, etc.

Primitive olfaction• Olfaction was and is an important controller of

behavioral state – Detecting sexual and individual identity: Influenced

evolution of amygdala – Detecting “good place”, “bad place”: Led to evolution

of medial pallium (hippocampus area) – Discriminating “good to consume”, “bad to consume”:

in conjunction with taste inputs to forebrain – Note the importance of learning

• These functions required links from endbrain and ‘tweenbrain to more caudal structures. The main links were in the midbrain. – Locomotion via the midbrain locomotor area (MLA) – Escape from predator threat via the midbrain tectum – Orienting towards food or mild novelty via the

midbrain tectum

The priority of escape behavior for survival led to a major structural consequence (review of hypothesis previously introduced)

• Optic inputs were very useful for triggering rapid escape from predators or potential predators, especially when they could give info about location.

• Escape mechanisms had already evolved in the somatosensory system, and were present in hindbrain.

• The most rapid route from the visual world to the mechanisms for turning away required a crossed projection to the midbrain. This was in order to engage a descending uncrossed pathway for escape.

• Later, with improved topographic organization, the orienting mechanisms of the tectum evolved. These required a re­crossing of the midline, hence there are crossed tectofugal pathways for orienting towards objects.

• The evolution of crossed visual representation in the midbrain led to the crossed representation of the outer world in the endbrain, not only for visual, but for somatosensory and auditory inputs as well.

Midbrain

The midbrain (mesencephalon)

• Why a midbrain? • The "correlation centers" • Motor outputs • Species comparisons • Connections with forebrain • Long axon tracts passing through

The midbrain "correlation centers“(see the pictures on brain evolution)

• Midbrain locomotor area: for approach & avoidance

• Central gray area and ventral tegmental area: visceral sensory, pain and pleasure, emotional expression (with non-visceral inputs as well)

• Superior colliculus (SC) or “optic tectum” with deeper multimodal layers: for escape and for orienting behavior

• Lateral lemniscus nuclei and the inferior colliculus: auditory relays to tectum and to forebrain

• Red nucleus: limb control

Outputs of midbrain for motor control• Origins of the

– Locomotor commands from the MLA – Tectospinal tract, from deep tectal layers – Rubrospinal tract, from red nucleus

• By these means, the midbrain controlsbody movements critical for survival:

1) Locomotion: • Approach & avoidance; • Exploring/ foraging/ seeking behavior

2) Orienting 3) Limb movements for exploring, reaching and

grasping.

Midbrain neurons projecting to spinal cord And hindbrain for motor control

Fibers from retina to superior colliculus

Auditory fibers, passing from inferior colliculus to medial geniculate body of thalamus Oculomotor

nucleus Spinothalamic tract (some fibers terminate in SC)

Medial lemniscus Cerebral peduncle: contains corticospinal +

Red corticopontine fibers, + cortex to hindbrain nucleus (n. ruber) Tectospinal tract Remember Shmoo 1: the midbrain Rubrospinal tract

was the link between forebrain and the motor system

The midbrain locomotor area

• Defined by electrophysiological studies; found in the caudal midbrain tegmentum

• Ancient origins, crucial for approach and avoidance

• Inputs from the primitive corpus striatum: Olfactory control of locomotor functions

The other basic types of movement crucial for survival

• Orienting towards/ away via midbraintectum

• Reaching, grasping via red nucleus

The midbrain "correlation centers“(for your reference: a few details)

• Superior colliculus (SC): • Inferior colliculus: “optic tectum” – Auditory inputs,

– Visual inputs to surface layers – Relay to thalamus

– Aud., SS inputs to deeper • Multimodal regions: layers – Deeper layers of the SC

– Functions: – Reticular formation • Novelty detection – Central gray area • Head & eye orientation • Anti-predator responses • Red nucleus:

– Modulators: corpus striatum, – Sensorimotor control of diffuse projection systems limbs

The midbrain (mesencephalon)

• Why a midbrain? • The "correlation centers" • Motor outputs • Species comparisons • Connections with forebrain • Long axon tracts passing through

Midbrain:Species comparisons

An exercise in topology: size distortions (another example of “mosaic evolution”, as opposed to “concerted evolution”)

– Huge optic tectum in tree shrews and squirrels –cf. birds’ optic lobes

– Smaller optic tectum in rats and humans

Human midbrain,

Figure removed due to copyright reasons.myelin stained section

(Sections are not drawn to the same scale)

Rodent

Human

Tree Shrew (Squirrel is similar)

Long axons passing through the midbrain

• Ascending visceral sensory: Dorsal longitudinal fasciculus

• Ascending Somatosensory • Spinothalamic tract and trigeminal lemniscus • Medial lemniscus

• Ascending cerebellar output to forebrain • Corticopontine and corticospinal axons of

the cerebral peduncle

Long axons passing through midbrain:

Fibers from retina to superior colliculus

Auditory fibers, passing from inferior colliculus to medial geniculate body of thalamus

Oculomotor nucleus Spinothalamic tract (some fibers terminate in SC)

Medial lemniscus*Cerebral peduncle: contains corticospinal +

Red corticopontine fibers, + cortex to hindbrain nucleus (n. ruber) Tectospinal tract * The trigeminal lemniscus joins the medial

Rubrospinal tract lemniscus, forming the medial-most axons of this collection of fibers traversing the midbrain and terminating in the posterior part of the ventral nucleus of the thalamus.

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

Selected References

Slide 3: Butler, Ann B., and William Hodos. Comparative Vertebrate Neuroanatomy: Evolution and Adaptation. New York, NY: Wiley-Liss, 1996. ISBN: 0471888893.