A sketch of the central nervous system and its origins G. E. Schneider 2009 Part 5: Differentiation of the brain vesicles MIT 9.14 Class 11 Differentiation of the brain vesicles: Developmental distortions; evolution of midbrain & forebrain; midbrain organization 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 Location of the Cerebellum: late-developing in the rostral hindbrain Cb Forms here a. Spinal cord b. Hindbrain (rhombencephalon) c. Midbrain (mesencephalon) d. ‘Tweenbrain (diencephalon) e. Endbrain Cb = Cerebellum (telencephalon) Growth of cerebellum and pons in rostral hindbrain, by migration of neuroblasts from the rhombic lip Cerebellar cortex Red arrows: migration of neuroblasts Black arrows: Pons course of axons from pontine gray to Cb cortex 1) The dorsal column – medial lemniscus pathway. 2) The corticospinal tract. Cb 1 pons 1. Dorsal columns 2. Nuclei of the dorsal columns nnuclucleueuss 3. Medial lemniscus 4. Ventrobasal nucleus of thalamus (n. ventralis posterior) 5. Thalamocortical axon in the “internal capsule” 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 neural tube 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 nerves at the rostral end of the organism could correspond to the olfactory and to the “terminal nerve” 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 hypothalamus (Note that the retina develops as an outpouching of the neural tube in the hypothalamic region.) – Diencephalic controls of sleep-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: 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 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 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 Ventral Tegmental Area – The incentives for approach & avoidance: Pain and pleasure – Moods (of major adaptive significance) and related emotional expressions – Visceral sensory inputs in addition to other inputs • Superior Colliculus (SC) or “optic tectum” – With deeper multimodal layers below the visual layers – For escape behavior and for orienting behavior • Inferior Colliculus (IC) & Nuclei of Lateral Lemniscus – Auditory relays to SC and to forebrain • Red nucleus: limb control; grasping Outputs of midbrain for motor control • Three major systems for control of multipurpose action patterns: 1) Mibrain Locomotor Area (MLA) 2) Tectospinal tract, from deep tectal layers 3) Rubrospinal tract, 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 tegmentum • Ancient origins, crucial for approach and avoidance • Inputs from the primitive corpus striatum – 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 Spinothalamic tract (somatosensory; 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 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