Chapter 7: The Structures Of The Nervous System
Chapter 7: The Structures Of The Nervous System NEUROSC 3000 - 020
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NEUROSC 3000 - 020
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This 17 page Class Notes was uploaded by Victoria Gonzalez on Wednesday November 4, 2015. The Class Notes belongs to NEUROSC 3000 - 020 at Ohio State University taught by Robert Boyd in Summer 2015. Since its upload, it has received 15 views. For similar materials see Introduction to Neuroscience in Neuroscience at Ohio State University.
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1 Chapter 7: The Structures Of The Nervous System Victoria Gonzalez Learning Objectives: Understand basic anatomical terms Know basic brain anatomy Understand imaging techniques used in neuroscience Understand basic elements of embryonic development of the nervous system Know the structure and function of divisions of the CNS (telencephalon, diencephalon, etc.) 1. Anatomical terms (humans) a. Anterior/rostral: towards forehead b. Posterior/caudal: towards the back c. Dorsal: top of head d. Ventral: towards belly (front) e. Midline: line running down the middle of the nervous system f. Medial: towards the midline g. Lateral: away from the midline h. Ipsilateral: same side i. Contralateral: opposite sides 2. Planes of section a. Sagittal plane: splits the brain into equal left and right halves b. Horizontal/transverse plane: cut is parallel to the ground; divides the brain into dorsal and ventral parts c. Coronal plane: perpendicular to the ground (and to sagittal plane); splits brain into anterior and posterior parts 2 3 3. The central nervous system: brain and spinal cord a. Brain consists of the cerebrum, the cerebellum, and the spinal cord i. Cerebrum: cortex of the brain 1. Has two hemispheres working contralaterally: right side of cerebrum receives input from left side of body ii. Cerebellum: controls movement 1. Contains as many neurons as the cerebrum 2. Has many connections to cerebrum and spinal cord 3. Functions ipsilaterally: right side of cerebellum controls right side of body iii. Brain stem: regulates body temperature, breathing, and consciousness 1. Most primitive part, but essential for life 2. A relay center that connects the spine and brain b. Spinal cord: encased in the vertebral column i. Spinal nerves are part of the peripheral nervous system (PNS) ii. Spinal nerves attach to spinal cord by two branches: 1. Dorsal root: sensory neurons a. Afferent: carry information to the CNS 2. Ventral root: motor neurons a. Efferent: carry information away from CNS 4. Peripheral nervous system a. Somatic (voluntary) i. Spinal nerves that innervate muscles, skin, and joints ii. Soma (cell bodies) are in CNS, axons are in PNS iii. Sensory neurons enter spine by dorsal roots; cell bodies are located in the dorsal root ganglia b. Visceral (involuntary) i. Also called autonomic nervous system (ANS) ii. Controls sensory and motor (smooth muscle) functions iii. Organs, glands, blood vessels 4 5. Cranial nerves a. 12 pairs; numbered by Galen anterior to posterior b. Exit the brain stem and innervate the head c. Some are part of CNS, some are part of PNS (both somatic & visceral) 6. Meninges: three layers that cover and protect the CNS a. Dura mater: outermost layer i. Forms a hard, inelastic layer around the brain and spinal cord b. Arachnoid membrane: look and consistency of a spider web i. Subarachnoid space: filled with cerebrospinal fluid (CSF) c. Pia mater: thin membrane that adheres close to the brain i. Blood vessels run along the pia mater 7. Ventricular system a. The brain has 4 ventricles filled with CSF b. CSF is produced in the choroid plexus (on the walls of the ventricles) c. CSF circulates to the subarachnoid space (between arachnoid and pia) 5 d. CSF is absorbed into the bloodstream via the arachnoid villi e. Hydrocephalus: excess CSF build up in ventricles i. Sometimes babies are born with this; since their skull is soft, their head expands and does not cause brain damage ii. In adults, skull is hard and cannot expand causing intra cranial pressure and brain damage or death iii. Treatment involves draining the ventricles 8. Brain imaging techniques a. Computed Tomography (CT scan): x-rays with a large amount of computer analysis to construct a 3D image of a slice of the brain b. Magnetic Resonance Imaging (MRI): excite protons to high energy state with electromagnetic waves and measure the frequencies emitted; frequency is proportional to the size of the field i. Replaced MRI; more detailed image without x- irradiation ii. Makes a detailed image of the whole brain; any orientation iii. Protons are spinning a specific tilt normally, when disrupted by magnetic fields they change the spin and tilt, when they return to their original state they produce radio signals detected by the MRI machine, images are made c. Diffusion tensor imaging (DTI): enables visualization of large bundles of axons in the brain by measuring water diffusion d. Functional brain imaging: measures blood flow and metabolism; neurons that are active demand more glucose and oxygen 6 i. Positron Emission Tomography (PET scan): uses a positron emitting isotope that is taken up by cells ii. Functional MRI (fMRI): observes the oxyhemoglobin to deoxyhemoglobin ratio 1. Has a better resolution and is faster than PET 7 9. Formation of the neural tube from an embryo a. Gastrulation: single layered blastula is reorganized into a flat disk with three layers: i. Endoderm: gives rise to internal organs (inside) ii. Mesoderm: gives rise to muscle and skeleton (middle) iii. Ectoderm: gives rise to the nervous system and skin (outside) b. Neurulation: transformation of the neural plate into the neural tube i. Neural plate: part of the ectoderm that gives rise to the nervous system ii. After 3 weeks of gestation (pregnancy), a neural groove forms in the neural plate running rostral to caudal iii. Walls of the groove (neural folds) fuse to form a neural tube 1. At embryonic day 22 2. The CNS is derived from the walls of the neural tube iv. Part of the ectoderm pinches off to the sides of the neural tube and becomes the neural crest 1. PNS cells come from the neural crest v. The mesoderm forms bulges on the sides of the neural tube called somites 1. Somites for 33 vertebra and associated skeletal muscles vi. Defects in neurulation: 1. Failure of the neural tube to close can cause birth defects (1 out of 500 births) 2. Anencephaly: skull and forebrain degenerate because anterior neural tube doesn’t close (fatal) 3. Spina bifida: posterior neural tube doesn’t close (usually not fatal) 8 10. Differentiation: the process by which structures become more complex and functionally specialized during development a. 3 primary vesicles form at the rostral end of the neural tube: the brain is derived from these i. Prosencephalon (forebrain): rostral-most vesicle ii. Mesencephalon (midbrain) iii. Rhombencephalon (hindbrain): connects with the caudal neural tube to give rise to the spinal cord 11. Forebrain differentiation a. Secondary vesicles sprout from prosencephalon i. Optic vesicles: form the optic nerves 1. Grow and invaginate to form the optic stalks and the optic stalks 2. Become optic nerves and retina 3. The retinas and optic nerves are part of the brain, not the PNS ii. Telencephalic vesicles: cerebral hemispheres 1. Telencephalon: the telecephalic vesicles 2. The telencephalic vesicles grow and envelop the diencephalon 9 3. Olfactory bulbs arise from telencephalon 4. The cells of the telencephalon walls divide, differentiate, and become several different structures 5. White matter develops carrying axons to and from neurons of the diencephalon b. Diencephalon: the central structure that remains after the secondary vesicles have sprouted off c. Ventral-medial surfaces of the hemispheres fuse with the lateral surfaces of the diencephalon d. Ventricles i. Lateral ventricles: in the telencephalon ii. Third ventricle: in the diencephalon e. Telencephalon forms cerebral cortex and basal cortex f. Diencephalon becomes thalamus and hypothalamus g. Forebrain neurons extend axons in 3 major systems: i. Cortical white matter: contains axons that run to and from neurons in the cerebral cortex ii. Corpus callosum: forms an axonal bridge that connects the two hemispheres iii. Internal capsule: links the cortex with the thalamus iv. Tract: axons with the same origin and destination v. Bundle: axons that run together but do not have the same origin or destination 12. Forebrain structure-function a. The forebrain is responsible for many higher functions: cognition, perception, voluntary action b. Most important part of the forebrain: cerebral cortex c. The thalamus is an important relay center of senses i. Thalamic neurons send axons to cortex via internal capsules ii. Axons of internal capsules carry information to the cortex on the contralateral side of the body 1. Sensory on right side of the body; left thalamus responds; left internal capsule relays information to left side of the brain 10 d. The cortex communicated with the brainstem via internal capsules i. Some connections extend past the brainstem to the spinal cord e. Basal ganglia in the basal telencephalon; controls movement f. Hypothalamus: controls many “old” and basic functions, ANS (involuntary nervous system), controls hormones in the pituitary, regulates body temperature 11 13. Midbrain differentiation a. The midbrain does not change a lot after the neural tube is formed b. The dorsal (top) surface of the mesencephalon becomes the tectum c. The bottom of the midbrain becomes the tegmentum i. Cerebral aqueduct forms in the middle of the tegmentum 1. Good landmark for identifying the midbrain 14. Midbrain structure-function a. Midbrain serves as a pathway for information between the forebrain and the spinal cord b. Midbrain is involved in sensory systems and movement c. The tectum differentiates into two structures: i. Superior colliculus (optic tectum): receives input from the eye and controls eye movements ii. Inferior colliculus: relays information from ears to thalamus d. Tegmentum: controls movement, pain, pleasure, mood, consciousness 15. Differentiation of the hindbrain a. Metencephalon: rostral hindbrain; becomes cerebellum and pons b. Myelencephalon: caudal hindbrain; becomes medulla oblongata c. Fourth ventricle forms; continuous with the cerebral aqueduct of the midbrain d. Medullary pyramids form: bundles of axons in a triangular shape connected to the spinal cord 12 13 16. Hindbrain structure-function a. Hindbrain is a relay center between forebrain and spinal cord b. Cerebellum: receives a large input from the spinal cord and pons i. Responsible for coordinated movements c. Pons: 90% of descending axons passing through the midbrain synapse here i. Major switchboard connecting the cortex to the cerebellum d. Medulla: involved in sensory (auditory, taste, touch) and motor functions (tongue movements) e. Medullary pyramids: axons that bypass the pons enter here i. Axon bundles running through the medullary pyramids make up the corticospinal tract f. Pyramidal decussation: near where the medulla joins the spinal cord, pyramidal tracts cross from one side to the other i. Explains contralateral processing 17. Differentiation of the spinal cord a. Gray matter (neurons): i. Dorsal horn: upper part of “butterfly” ii. Intermediate zone iii. Ventral horn: lower part of “butterfly” b. White matter (axons): i. Dorsal column ii. Lateral column iii. Ventral column 14 18. Spinal cord structure-function a. Grey matter: i. Dorsal horn: receives sensory input from dorsal root (afferent) ii. Ventral horn: projects to ventral roots onto muscles (efferent) iii. Intermediate zone: interneurons, coordinate sensory and brain information to form output b. White matter: i. Dorsal columns: carry sensory information to medulla (ipsilaterally) 1. Neurons in the medulla cross and connect to thalamus on contralateral side ii. Lateral columns: axons from descending corticospinal tract which crossed at pyramidal decussation; they innervate the intermediate zone (interneurons) and the ventral horn which controls voluntary movement c. Most tracts in the spinal cord are one-way 19. Genesis of neurons in 3 stages: proliferation, migration, and differentiation a. Cell proliferation i. Early in development the ventricle walls are made of 2 layers: 1. Ventricular zone: lines the inside of each vesicle 2. Marginal zone: on the outside facing the pia mater ii. Process: 1. A cell in the ventricular zone extends a process that reaches upwards towards the pia mater 2. The nucleus of the cell migrates upward from the ventricular surface toward the pial surface 3. The cell’s DNA is copied 15 4. The nucleus, containing two complete copies of the genome, settles back to the ventricular surface 5. The cell retracts its arm from the pial surface 6. The cell divides in two iii. Radial glial cells: the dividing cells; they give rise to all neurons and astrocytes of the cerebral cortex 1. Multipotent stem cells: assume different destinies iv. Neocortical neurons are mostly made before birth, but a small amount is still made in adult life (only in a small part of brain) b. Determination of cell fate i. The fate of the daughter cell is determined by: 1. Age of precursor cell 2. Position within ventricular zone 3. Environment at the time of division ii. Proliferation of pyramidal neurons and astrocytes: in the ventricular zone of the telencephalon iii. Inhibitory interneurons (GABA) and oligodendrocytes are generated in the ventricular zone of the ventral telencephalon; cells must migrate laterally 16 20. Development of cortex a. Neuronal cells differentiate first b. Astrocyte differentiation c. Oligodendrocytes differentiate last 21. Rat brain vs. human brain a. Similarities i. Telencephalon is rostral (front) to diencephalon ii. Diencephalon surrounds the third ventricle iii. Midbrain surrounds the cerebral aqueduct iv. Fourth ventricle is surrounded by the pons, medulla, and cerebellum b. Differences i. Sulci (grooves) and gyri (bumps) ii. Surface area iii. Human olfactory bulb is small 22. Three types of cortex: a. Hippocampus: only one layer; is medial to lateral ventricles i. For learning and memory b. Olfactory cortex: two cell layers; posterior to olfactory bulb c. Neocortex (cortex): only in mammals; “new” part of cerebral cortex i. Responsible for voluntary movement, vision, hearing, and somatic sensation 23. Cerebral cortex a. Systems for learning, speech, sensations, cognition, perceptions, voluntary movement 17 b. Neuronal cell bodies are arranged in layers c. The most superficial layer has no neurons (layer 1, molecular layer) i. Pyramidal cells extend to layer 1 d. Areas of neocortex i. Brodmann made a cytoarchitectural map where areas with a common structure were given a number 1. Thought that different areas had different functions but never proved it 2. Some of his predictions were right: 17 is vision, 4 is motor ii. Types of cortex: 1. Primary sensory 2. Secondary sensory 3. Motor 4. Association areas: in temporal and frontal lobes