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Home , Corticopontine fibers, Hindbrain, Lateral lemniscus, Medial lemniscus, Tectospinal tract, Trigeminal lemniscus

A sketch of the central and its origins G. E. Schneider 2009 Part 5: Differentiation of the vesicles

MIT 9.14 Class 11

Differentiation of the brain vesicles: Developmental distortions; evolution of & ; midbrain organization The "distortions" in the basic organization of the , 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 . – 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 Location of the Cerebellum: late-developing in the rostral hindbrain

Cb Forms here a. b. Hindbrain (rhombencephalon) c. Midbrain (mesencephalon) d. ‘Tweenbrain () e. Endbrain Cb = Cerebellum (telencephalon) Growth of cerebellum and pons in rostral hindbrain, by migration of neuroblasts from the

Cerebellar cortex

Red arrows: migration of neuroblasts Black arrows: Pons course of from pontine gray to Cb cortex 1) The dorsal column – medial pathway. 2) The corticospinal tract.

Cb

1

pons

1. Dorsal columns 2. Nuclei of the dorsal columns nnuclucleueuss 3. 4. Ventrobasal nucleus of (n. ventralis posterior) 5. Thalamocortical in the “” 6. Corticofugal axons, including corticospinal components. Called “pyramidal tract” in hindbrain below pons. 7. Pons Human rostral hindbrain,

Figure removed due to copyright restrictions. with pons and cerebellum Æ a quantitative “distortion” of the basic plan 9.14 MIT Brain Structure & Its Origins

Why a midbrain? Notes on evolution, structure and functions Above the hindbrain

• We can get ideas about evolution of the midbrain and forebrain from the primitive chordates like Amphioxus. • We get additional ideas from comparative studies, especially from primitive vertebrates. Recent clues to chordate origins: Studies of Amphioxus (Branchiostoma)

9 Evidence that this creature has more than a spinal cord, despite superficial appearances 9 Quantitatively the CNS is largely hindbrain and cord, but more rostral parts of the are present. 9 Structural studies show specific components of a primitive midbrain and forebrain Amphioxus above the hindbrain

• Gene expression studies give evidence for midbrain, ‘tweenbrain and endbrain regions. • Morphological studies have revealed two inputs above the midbrain in addition to a light sensitive pigmented structure of the forebrain region. – Two at the rostral end of the organism could correspond to the olfactory and to the “terminal ” although studies have not proven this. (The terminal nerve, often ignored in mammals, innervates the nasal septum.) • Evidence of a pituitary-like region (but functions are not very clear) Why a midbrain, and a forebrain rostral to it? Probable explanations • The midbrain, together with primitive components of the forebrain, was a kind of rostral extension of the hindbrain that enabled visual and olfactory control over FAPs (like locomotion, orienting movements, emotional expressions), and that added more control by motivational states. • The midbrain received visual and olfactory inputs from ‘tweenbrain and endbrain, as well as sensory and other inputs from more caudal structures, including cerebellum. • What were the roles of ‘tweenbrain and endbrain in primitive chordates? Proposal: Olfactory and visual inputs to these structures influenced motivational states and actions via the midbrain. 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 (Note that the develops as an outpouching of the neural tube in the hypothalamic region.) – Diencephalic controls of sleep-waking physiology and behavior: 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: These functions influenced evolution of amygdala – Discriminating “good to consume”, “bad to consume”: in conjunction with taste inputs to forebrain – Detecting “good place”, “bad place”: Led to evolution of medial pallium (hippocampus area) – For these functions, learning was important. • These approach-avoidance 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 A structural consequence of the priority of escape behavior for survival (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 , 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 an already existing descending uncrossed pathway for escape behavior. The priority of escape behavior for survival had a major structural consequence in evolution (continued)

• Later, with evolution of optical imaging and some topographic organization, the orienting mechanisms of the tectum evolved. These required a re-crossing of the midline for greatest efficiency of connections, hence there evolved the crossed tectofugal pathways that we find in modern vertebrates, for orienting towards objects. • The evolution of crossed visual representation in the midbrain led to the crossed representation of the outer world in the forebrain, not only for visual, but for somatosensory and auditory inputs as well. a. Spinal cord b. Hindbrain (rhombencephalon) c. Midbrain (mesencephalon) d. ‘Tweenbrain (diencephalon) e. Endbrain (telencephalon)

Fig 11-1 The midbrain (mesencephalon)

9Why 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 – The incentives for approach & avoidance: and pleasure – Moods (of major adaptive significance) and related emotional expressions – Visceral sensory inputs in addition to other inputs • (SC) or “optic tectum” – With deeper multimodal layers below the visual layers – For escape behavior and for orienting behavior • (IC) & Nuclei of – Auditory relays to SC and to forebrain • : limb control; grasping Outputs of midbrain for motor control • Three major systems for control of multipurpose action patterns: 1) Mibrain Locomotor Area (MLA) 2) , from deep tectal layers 3) , from red nucleus • By these means, the midbrain controls 3 types of body movements critical for survival: 1) Locomotion: • Approach & avoidance; • Exploring/ foraging/ seeking behavior 2) Orienting 3) Limb movements for exploring, reaching and grasping. 1) The midbrain locomotor area

• Defined by electrophysiological studies; found in caudal midbrain • Ancient origins, crucial for approach and avoidance • Inputs from the primitive corpus – These inputs from the endbrain were originally for olfactory control of locomotor functions The other basic types of movement crucial for survival

2) Orienting towards/ away via midbrain tectum 3) Reaching, grasping via red nucleus Midbrain neurons projecting to spinal cord and hindbrain for motor control

Superior Colliculus (optic tectum)

Red nucleus (n. ruber) Tectospinal tract Rubrospinal tract Sensory systems in, and passing through, the midbrain

Fibers from retina to Superior Colliculus

Brachium of Inferior Colliculus (auditory pathway to thalamus, also to SC)

Oculomotor nucleus (somatosensory; some fibers terminate in SC)

Medial lemniscus : contains Red corticospinal + , + cortex to hindbrain fibers nucleus (n. ruber) Tectospinal tract Rubrospinal tract Superior Colliculus

Brachium of Inferior Colliculus (auditory pathway, midbrain to thalamus)

Oculomotor nucleus Spinothalamic tract (some fibers terminate in SC)

Medial lemniscus Cerebral peduncle: contains Red corticospinal + corticopontine fibers, + cortex to hindbrain fibers nucleus (n. ruber) Tectospinal tract Rubrospinal tract Remember: The midbrain was the link between forebrain and the motor system in primitive chordates. We have omitted another major midbrain system that provided a crucial link between the forebrain and the control of actions: the Central Gray Area, or Periaquaductal Gray Area. Moods and motivational states:

Superior Colliculus Central gray area

(CGA) Brachium of Inferior Colliculus (auditory pathway, midbrain to thalamus)

Oculomotor nucleus Spinothalamic tract (some fibers terminate in SC)

Medial lemniscus Cerebral peduncle: contains Red corticospinal + corticopontine fibers, + cortex to hindbrain fibers nucleus (n. ruber) Tectospinal tract Rubrospinal tract

Ventral Tegmental Area (VTA)

Connections to the CGA, also called the Periaquaductal Gray (PAG), and to the VTA enabled control of or influence on moods/motivations crucial for survival: defensive, aggressive, sexual. Activation of these areas is accompanied by feelings of pain (CGA) or pleasure (VTA). For your reference: a few details: The midbrain “correlation centers” • Superior colliculus (SC): • Inferior colliculus: “optic tectum” – Auditory inputs – Relay to thalamus –Visualinpu ts to surface layers – Auditory, Somatosensory • Multimodal regions: inputs to deeper layers – Deeper layers of the SC – Functions: – • Including the MLA • Novelty detection – Central gray area • Head & eye orientation • Anti-predator responses • Red nucleus: – Modulators: corpus striatum, – Sensorimotor control of diffuse projection systems limbs, especially distal muscles 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, myelin stained section

Figure by MIT OpenCourseWare. (Sections are not drawn to the same scale)

Rodent

Human

Tree Shrew Fig 11-3 (Squirrel is similar) Long axons passing through the midbrain

• Ascending visceral sensory: “Dorsal longitudinal fasciculus” • Ascending Somatosensory • Spinothalamic tract, with spinotectal axons terminating • Medial lemniscus • fibers are found medially in this group of axons. Thus, the medial-lateral topography of somatosensory axons is reversed from spinal cord. • Ascending cerebellar output to forebrain • Corticopontine and corticospinal axons of the cerebral peduncle Long axons passing through the midbrain

Fibers from retina to Superior Colliculus

Brachium of Inferior Colliculus (auditory pathway to thalamus, also to SC)

Oculomotor nucleus Spinothalamic tract (somatosensory; some fibers terminate in SC)

Medial lemniscus* Cerebral peduncle: contains Red corticospinal + corticopontine fibers, nucleus + cortex to hindbrain fibers (n. ruber) Tectospinal tract Rubrospinal tract * The trigeminal lemniscus joins the medial 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 functionally distinct regions, “limbic”and “somatic”

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

2. Limbic: Connected to the and the closely associated “limbic” forebrain system Somatic regions

Limbic regions

These functionally distinct regions continue rostrally into the ‘tweenbrain. Fig 11-4 MIT OpenCourseWare http://ocw.mit.edu

9.14 Brain Structure and Its Origins Spring 2009

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