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Lesson 34 Lesson Outline: Nervous System Structure and Function Central Nervous System Brain Embryonic Origin Phylogenetic Origin/Differences Form and Function Telencephalon Diencephalon Mesencephalon Metencephalon Mylencephalon Objectives: References: Chapter 16 : 387-428 Reading for Next Lesson: All Chapters - Begin to Review for Final Exam!!!! Nervous System Structure and Function Central Nervous System Brain Embryonic Origin The neural tube, neural crest and notochord form just as we have previously discussed. Formation of prosencephalon mesencephalon and rhombencephalon (or forebrain, midbrain and hindbrain). At the anterior end of the neural tube, prechordal mesoderm induces the swelling of the prosencephalon, which in turn induces the formation of the mesencephalon and rhombencephalon. These three regions subsequently differentiate to form the Telencephalon Diencephalon, Mesencephalon, Metencephalon and Mylencephalon. As the brain forms, the hollow centre of the neural tube becomes the ventricles of the brain, which fill with the cerebrospinal fluid (CSF). Formation of Meninges The meninges are the membrane that surrounds the brain. It is derived in part from the neural crest. In fishes there is a single membrane, the meninx. In terrestrial vertebrates there is further development of the meninges into two and subsequently three layers (the pia mater, arachnoid and dura mater). Formation of CSF The CSF is the fluid that flows through the ventricles of the brain, the central canal of the spinal cord and between the brain and the meninges The fluid is produced by filtration from arterial blood in the choroid plexus, and is reabsorbed ultimately by venous sinuses. Because of the filtration, it lacks blood cells and other large molecules. It has some nutritive function but in terrestrial vertebrates it also serves to cushion and protect the nervous system. Phylogenetic Origin/Differences The extent to which the different parts of the brain are developed in different vertebrate groups, including the sharks, reflects the relative importance of different sensory and motor systems in the lifestyles of the species. The sizes of the sensory nuclei, motor nuclei and integrating areas increase/decrease with the demands for information processing required by a particular habitat or mode of life. Form and Function Telencephalon (Cerebral Hemispheres and Olfactory lobes) Reception of olfactory information is one important role of the telencephalon. Formation of Olfactory Organs The olfactory system begins as a pair of olfactory placodes, thickenings of ectoderm that invaginate toward the neural tube. The lateral walls of each placode form the olfactory epithelium lining the nasal passages. Olfactory sensory cells differentiate within the epithelium and send axons to meet the forming telencephalon. These axons induce the telencephalon to produce the olfactory bulb that is connected to the rest of the telencephalon by the olfactory tract. The term olfactory nerve applies only to the very short axons from the olfactory sensory cells. The remainder of the telencephalon is composed of a dorsal pallium and a ventral subpallium, which make up the cerebrum. These areas receive olfactory, auditory, lateral line, somatosensory and visceral inputs relayed from other sites in the brain. They process this information, integrate it and transmit responses to lower areas of the brainstem. Thus, they indirectly control locomotion and other motor functions. Diencephalon (Hypothalamus, Thalamus) The diencephalon consists of three regions, the epithalamus, the hypothalamus, and the thalamus. The epithalamus includes the pineal gland which serves as a third eye for monitoring photoperiod and regulating both skin colour and biological rhythms. The hypothalamus in the floor of the diencephalon houses a collection of nuclei that regulate homeostasis, processes pertaining to temperature, water balance, appetite, metabolism, blood pressure, sexual behaviour alertness and some aspects of emotional behaviour. It controls the pituitary gland that is situated immediately below it. Formation of the Pituitary The pituitary has two embryonic sources, the infundibulum that is an outgrowth of the floor of the diencephalon and Rathke's pouch, which is a diverticulum from the stomodeum. These two structures fuse and the infundibulum retains its connection to the brain and becomes the neurohypophysis while Rathke's pouch becomes pinched off from the stomnodeum and becomes the adenohypophysis The thalamus receives sensory input indirectly from all parts of the body. Almost all sensory inputs ultimately project here (except the olfactory tracts). This area integrates the information and relays it to the cerebral cortex. It receives direct input from the eyes. Formation of Eyes The eye is a composite structure formed from neural ectoderm, ectoderm and mesenchyme. The optic vesicles first appear as paired outgrowths of the diencephalon. As the optic vesicles approach the overlying ectoderm, it thickens and becomes the optic placode. This invaginates and forms the lens primordium. It pinches off and settles into the optic cup, which is the extension of the optic vesicle. Mesenchyme surrounding the developing eye condenses to produce the coats of the eye. Thus, the outer ectoderm gives rise to the eyelid, cornea and lens. The optic vesicle forms the iris and the retina, which retains its connection to the brain as the optic stalk. This carries the axons that project to the optic areas in the optic area of the diencephalon and the optic tectum of the midbrain. This stalk or tract is the optic or second cranial nerve. The mesenchyme forms the choroid, and sclera. Mesencephalon (Tectum and Tegmentum) The roof of the midbrain is the tectum, which receives sensory information while the floor is the tegmentum, which initiates motor output. The tectum is divided into an optic tectum, receiving visual information, and a torus semicircularis, receiving vestibular and lateral line input. In higher vertebrates, it also receives auditory input. Thus, the optic tectum receives direct input from the eyes as does the thalamus of the diencephalon. While this information is integrated with other inputs into a pattern of sensation in the thalamus that project to the cerebrum, it is integrated into a spatial map in the optic tectum. The tectum also receives indirect input from the lateral line system, the vestibular apparatus, the cerebellum and the olfactory system. The tegmentum is an integrating site and contains nuclei for two of the cranial nerves that control the movements of the eye. Metencephalon (Pons and Cerebellum) The pons is a swelling in the floor of the rostral hindbrain, which is an important crossroads for the flow of information, both up and down the brain. It houses the nuclei for cranial nerves V-VII. The cerebellum is also derived from the rostral hindbrain. This area modifies motor output to help maintain equilibrium and to refine motor activity. . It is relatively large in sharks, receiving extensive input from the lateral line sensory system. Mylencephalon (Medulla) The medulla oblongata has three major functions: 1) It contains centres for visceral auditory and proprioceptive functions including respiration, circulation and digestion. 2) It is the major route through which ascending and descending fibre tracks must run between the spinal cord and higher brain centres. 3) It houses the primary nuclei of many of the cranial nerves (VII through X in sharks, VII to XII in mammals). Formation of Otic Capsules The vestibular system is a balancing organ that arises as part of the lateral line system. It forms from the otic placodes, which sink inwards and differentiate to form hair cells, which are modified neuromast cells. These form the sensory cells in the semicircular canals that respond to rotation or angular acceleration. They also form the sensory cells in the two chambers at the base of the semicircular canals, the sacculus and utirculus (the sensory cells are the macula and otoliths), which respond to changes in orientation. Together these form the vestibular apparatus, which conveys information about movement and orientation. Remember- in fishes this is an organ of equilibrium. “Hearing” is accomplished by the lateral line system.