Chapter 3

1. Chapter 3 Structure of the Nervous System

4. The major divisions of the nervous system

5. Meninges • The 3 layers of tissue that encase the CNS • Dura mater – outermost of meninges; tough & flexible • Arachnoid – middle layer; resembles spider’s web • Pia mater – innermost layer; clings to brain, very thin and delicate • Subarachnoid space – space b/t arachnoid and pia; filled with CSF • Cerebrospinal fluid (CSF) – clear fluid that fills the ventricular system and the subarachnoid space

7. Development of the CNS • Begins around 18 days after conception • Part of the ectoderm (outer layer) of the embryo thickens and forms a plate, which edges curl up and meet each other to form the neural tube (Fig. 3.7 in text) • The tube eventually completely closes (with a small area of space inside for ventricular system) and begins to form the 3 major parts of the brain: • Forebrain: • Lateral ventricle  Telencephalon  Cerebral cortex, basal ganglia, limbic system • 3rd ventricle  Diencephalon  Thalamus, hypothalamus • Midbrain: • Cerebral aqueduct  Mesencephalon  Tectum, tegmentum • Hindbrain: • 4th ventricle  Metencephalon  Cerebellum, pons • Myelencephalon  Medulla oblongata

8. Details of brain development • The cells that line the inside of the neural tube, the ventricular zone, give rise to the cells of the CNS • These cells divide and form into neurons and glia (founder cells) • The first phase of this division is called symmetrical division, because each cell splits into 2 identical new founder cells • The second phase is called asymmetrical division, because the divide into a new founder cell and a neuron, which migrates away (this lasts about 3 months) • The neurons migrate away from the center, and are guided to their places by radial glia • The end of cortical development occurs when the founder cells receives a signal for apoptosis (cell death) • Once neurons reach their destinations, they begin to form connections with each other, grow dendrites & axons

9. The Forebrain • Telencephalon • Cerebral hemispheres – the 2 major portions of the forebrain, divided into 2 halves; covered by the cerebral cortex • Subcortical region – located beneath cortical surface • Cerebral cortex • Convoluted with sulci (small grooves) fissures (large grooves) and gyri (bulges b/t adjacent sulci or fissures) • ~ 3mm thick • Lobes: • Frontal, Parietal, Occipital & Temporal

13. Diencephalon • Thalamus • 2 lobes connected by massa intermedia • Contains nuclei that project info to certain regions of the cortex (via projection fibers) and receive info from it • Nuclei of thalamus: • Lateral geniculate nucleus (LGN) – receive fibers from retina and projects to primary visual cortex • Medial geniculate nucleus (MGN) – receives fibers from auditory system and projects to primary auditory cortex • Ventrolateral nucleus – receives input from cerebellum and sends axons to primary motor cortex • Hypothalamus • Controls autonomic nervous system, pituitary glands, species-typical behaviors (e.g. the 4 F’s: fighting, fleeing, feeding, & mating) • Optic chiasm – an x-shaped connection between the optic nerves • Pituitary • Antierior pituitary – an endocrine gland whose secretions are controlled by hypothalamic hormones (which are secreted by neurosecretory cells); e.g. gonadotropic hormones, growth hormones • Posterior pituitary – contains terminal buttons of axons from hypothalamus that secrete hormones (e.g. oxytocin, controls milk let-down; vasopressin, conserves water reabsorption in kidneys)

14. Midbrain • Tectum • Dorsal part of midbrain • Includes: • Superior colliculi – protrusions on top of midbrain; part of visual system • Inferior colliculi – part of auditory system • Tegmentum • Ventral part of midbrain • Includes: • Periaqueductal gray – surrounds cerebral aqueduct; control species-typical behaviors • Reticular formation – located in central region of brain stem;, from medulla to diencephalon • Red nucleus – receives input from cerebellum and motor cortex and sends axons to motor neurons in SC • Substantia nigra – contains neurons that communicate with the caudate and putamen of the BG

15. Hindbrain • Metencephalon • Cerebellum • 2 hemispheres covered with cerebellar cortex; part of the motor system • Deep cerebellar nuclei – receive projections from the cerebellar cortex and project out of the cerebellum to other parts of the brain • Cerebellar peduncles – one of 3 bundles (superior, middle & inferior) o axons that attach each cerebellar hemisphere to the dorsal pons • Cerebellar damage impairs standing, walking, or performance of coordinated movements • Pons • Contains a portion of the reticular formation • Also a large nucleus that relays info from the cortex to the cerebellum • Myelencephalon • Medulla oblongata – contains part of RF, regulates cardiovascular system, respiration, and skeletal muscle tonus

16. The Spinal Cord • Primary function is to distribute motor fibers to the effector organs of the body (glands and muscles) and to collect somatosensory info to be passed onto the brain • Protected by vertebral column, composed of 24 individual vertebrae of the: • Cervical (neck region) • Thoracic (chest region) • Lumbar (lower back region) • And fused vertebrae composed of: • Sacral • Coccygeal • The SC passes through a hole in each of the vertebrae (spinal foramens) • The SC ends in a mass of spinal roots called the cauda equina (horse tail) • Dorsal root – spinal root containing incoming (afferent) sensory fibers • Ventral root – spinal root that contains outgoing (efferent) motor fibers

18. Peripheral Nervous System

19. Spinal nerves • Begin at the junction of the dorsal and ventral roots of the SC • They leave the vertebral column and travel to the muscles or sensory receptors they innervate • The cell bodies of all axons that bring sensory info into the brain & spinal cord are located outside the CNS (except for visual system) • These incoming axons are referred to as afferent axons • The cell bodies (unipolar neurons) that give rise to these axons reside in the dorsal root ganglia and send one linb to the SC and one to the sensory organ • Cell bodies that give rise to the ventral roots are located in the SC and project to muscles and glands (efferent axons)

20. Cranial Nerves! • 12 pairs of cranial nerves are attached to the ventral surface of the brain • Most of these are serve sensory and motor functions of the head and neck region • They are: • Olfactory • Optic • Oculomotor • Trochlear • Trigeminal • Abducens • Facial • Vestibulocochlear (auditory) • Glossopharyngeal • Vagus • Spinal Accessory • Hypoglossal • Oh, oh, oh, to touch and feel very good velvet, ah! • Vagus nerve – conveys efferent fibers of the parasympathetic division of the autonomic nervous system to organs

21. The Autonomic Nervous System • Regulates smooth muscle, cardiac muscle and glands • 2 branches: • Sympathetic branch • Involved in activities associated with expenditure of energy from reserves that are stored in the body (“fight or flight”) • e.g. increases blood flow to skeletal muscles, stimulates secretion of epinephrine, causes piloerection (“goose bumps”) • Cell bodies located in the gray matter of thoracic and lumbar regions of SC (preganglionic neurons) and exit via ventral roots • After joining spinal nerves, they branch off and pass into sympathetic ganglia forming a sympathetic ganglion chain • From there, these axons (now termed postganglionic neurons) project to target organs (e.g. kidney, stomach, etc.) • Parasympathetic branch • Supports activities that increase the body’s supply of stored energy • e.g. salivation, gastric and intestinal motility, etc. • Cell bodies for preganglionic axons located in the nuclei of some cranial nerves (3,7,9,10) and gray matter of sacral region of SC • Parasympathetic ganglia located very near target organs; thus postganglionic fibers are very short