Transcript Chapter 3
Chapter 3 Structure of the Nervous System The major divisions of the nervous system CNS PNS Brain Nerves Spinal cord Peripheral ganglia 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 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 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 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 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) 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 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 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 Peripheral Nervous System Somatic Nervous System Autonomic Nervous System Spinal nerves Sympathetic branch Afferents from sense organs Efferents to muscles Spinal nerves (from thoracic & lumbar) Sympathetic ganglia Cranial nerves Parasympathtic branch Afferents from sense organs Efferents to muscles Cranial nerves (3rd, 7th, 9th, & 10th) Spinal nerves (from sacral region) Parasympathetic ganglia (adjacent to target organs) 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) 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: I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII. – – 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 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