A sketch of the central nervous system and its origins G E Schneider 2009 Part 2 Steps to the central nervous system from initial steps to advanced chordates MIT 9 14 Class 4 Elaboration of the neural tube in evolution Evolution of multi cellular organisms Suggestions based on phylogenetic comparisons TOPICS sessions 3 4 Behaviors most fundamental for survival Intercellular conduction in Ctenophores and Coelenterates suggestions about evolution of the nervous system A generalized conception of the CNS The body plan of primitive chordates as suggested by Amphioxus Branchiostoma Elaboration of the neural tube in evolution Phylum Coelenterata now often called Cnidaria Animals with radial symmetry and stinging tentacles includes free swimming medusa forms like jellyfish and sessile polyp forms like corals and sea anemones Evolution of Brain 1 With head receptors and forward locomotion there evolved an increasing sophistication of sensorimotor abilities Sensory analyzing mechanisms Connected to inputs from cranial nerves Associated motor apparatus For directing the receptors orienting movements For controlling alterations in posture and locomotion under guidance from these receptors Crucial background maintenance of stability of the internal mileau Structures that accomplished those functions New sensory apparatus at rostral end of the tube Hindbrain midbrain and forebrain mechanisms connected to head receptors are added to primitive spinal somatosensory mechanisms Added motor control Hindbrain midbrain Control of mouth eyes ears head turning added to basic spinal hindbrain control of the body Forebrain vesicle evolves with Olfactory visual inputs Endocrine visceral control Evolution of Brain 2 Expansion of hindbrain Sensory analysers somatosensory face taste vestibular auditory electroreceptive Action pattern programs triggered by specific sensory patterns Evidence from comparative neuroanatomy Functional demands result in expansion of CNS components Some particularly dramatic examples have been found in studies of fish Pictures from CL Herrick who together with his brother CJ Herrick helped establish the field of comparative neurology Functional adaptations cause expansions in CNS illustrations from comparative anatomy from C L Herrick Brain of a fresh water mooneye Brain of a freshwater buffalo fish huge vagal lobe receives input from specialized palatal organ Brain of a catfish facial lobe and vagal lobe for processing taste inputs through two different cranial nerves Catfish 7th cranial nerve distribution re taste senses explains facial lobe Hyodon tergisus fresh water Mooneye Note the size and shape of the hindbrain The hindbrain includes the medulla oblongata and the cerebellar region between the two black arrows Olfactory Bulb Olfactory Stalk Primitive Endbrain Midbrain Cerebellum Medulla Oblongata Hindbrain Figure by MIT OpenCourseWare Endbrain Carpiodes tumidus buffalofish Midbrain Cerebellum Vagal Lobe Figure by MIT OpenCourseWare The vagal lobe of the hindbrain is huge It receives and processes taste input from a specialized palatal organ Pilodictis olivaris catfish Olfactory Stalk Primitive Endbrain Midbrain Cerebellum Facial Lobe The vagal lobe is enlarged although less than in the buffalo fish An enlarged facial lobe is also evident It receives taste inputs from all over the body surface Vagal Lobe Figure by MIT OpenCourseWare Amiurus melas the small catfish 7th cranial nerve facial nerve innervates taste buds in skin of entire body Figure by MIT OpenCourseWare Evolution of Brain 3 Concurrent with Evolution 2 Expansion of forebrain because of adaptive value of olfactory sense for approach avoidance functions feeding mating predator avoidance predation Note connections shown are actually on both sides This was the first expansion of the forebrain Outputs links to locomotion through the corpus stratum were most critical These links were via the midbrain Evolution of Brain 3 Concurrent with Evolution 2 Expansion of forebrain because of adaptive value of olfactory sense for approach avoidance functions feeding mating predator avoidance predation Note connections shown are actually on both sides These connections were plastic They could be strengthened or weakened depending on experience Outputs links to locomotion through the corpus stratum were most critical The links were via the midbrain Expansions continued Functional demands result in progressive changes in the neural tube to include 9 Sensory analyzing mechanisms 9 Corresponding motor apparatus Correlation centers Elaboration of complex programs for goaldirected activities Systems for modulating other brain systems in response to visceral and social needs Systems for anticipating events planning actions Correlation centers A network between the sensory analyzers and the motor apparatus For maintaining stability in space by integration of vestibular visual and tactile information For avoidance and approach movements guided by olfaction as well as other senses Example Evolution of structures to improve head and body orientation Midbrain tectum and pretectal cell groups Medial hindbrain reticular formation Medial cerebellar structures Olfactory Bulb Note the positions of the midbrain tectum and the cerebellum in Hyodon tergisus the fresh water Mooneye Olfactory Stalk Primitive Endbrain Midbrain Cerebellum Medulla Oblongata Figure by MIT OpenCourseWare Evolution of Brain 4 Expansion of midbrain with evolution of distance receptor senses visual and auditory receptors with advantages over olfaction for speed and sensory acuity for early warning and for anticipation of events Motor side turning of head and eyes with modulation by motivational states including those triggered by olfactory sense Note Neurons and pathways shown on one side of the brain are usually the same on the other side I often draw them on only one side to make the drawing simpler Why do sensory pathways decusssate This is a question that has given rise to various speculations but there have been no firm answers Next I present briefly a suggestion that will be developed later into an hypothesis that is more convincing than others that have been proposed Thinking about the evolution of these correlation centers Hypothesis Very early in the evolution of the midbrain and forebrain before the hemispheres appeared visual inputs from lateral eyes projected bilaterally but then in evolution became crossed This resulted in later evolution of decussations of non