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4 Overview of the Nervous System Dendrites Dendritic spines (gemmules) Rough endoplasmic reticulum (Nissl substance) Ribosomes Mitochondrion Nucleus Axon Nucleolus Axon hillock Initial segment of axon Neurotubules PR SA O M PE PL R E TY C O O F N E TE L N SE T V - N IE O R T FI N AL Golgi apparatus Lysosome Cell body (soma) Axosomatic synapse Glial (astrocyte) process Axodendritic synapse 1.1 Brain Imaging: MRI (Magnetic Resonance Imaging)—Coronal and Horizontal T1—Weighted Images Neuronal structure reflects the functional characteristics of the individual neuron. Incoming information projects to a neuron mainly through axonal terminations on the cell body and dendrites. These synapses are isolated and protected by astrocytic processes. The dendrites usually provide the greatest surface area of the neuron. Some protrusions from dendritic branches (dendritic spines) are sites of specific axo-dendritic synapses. Each specific neuronal type has a characteristic dendritic branching pattern, called the dendritic tree, or dendritic arborizations. The neuronal cell body varies from a few micrometers (m) in diamater to more than 100 m. The neuronal cytoplasm contains extensive rough endoplasmic reticulum (rough ER), reflecting the massive amount of protein synthesis necessary to maintain the neuron and its processes. The Golgi apparatus is involved in packaging potential signal molecules for transport and release. Large numbers of mitochondria are necessary to meet the huge energy demands of neurons, particularly related to maintenance of ion pumps and membrane potentials. Each neuron has a single (or occasionally no) axon. The cell body tapers to the axon at the axon hillock, followed by the initial segment of the axon, containing the Na+ channels, the first site where action potentials are initiated,. The axon extends for a variable (up to one meter or more) distance from the cell body, and if greater than 1-2 m in diameter are insulated by sheaths of myelin provided by oligodendroglia in the CNS or Schwann cells in the PNS. An axon may branch into more than 500,000 axon terminals, and may terminate in either a highly localized and circumscribed zone (e.g. primary somatosensory axon projections for fine discriminative touch), or may branch to many disparate regions of the brain (e.g. noradrenergic axonal projections of the locus coeruleus). Neurons whose axons terminate at a distance from its cell body and dendritic tree are called macroneurons or Golgi type I neurons, and neurons whose axons terminate locally, close to its cell body and dendritic tree are called microneurons, Golgi type II neurons, local circuit neurons, or interneurons. There is no “typical” neuron, as each type of neuron has its own specialization. However, pyramidal cells or lower motor neurons often are used to portray the “typical” neuron. CLINICAL POINT Neurons require extraordinary metabolic resources to sustain their functional integrity, particularly related to the maintenance of membrane potentials for the initiation and propagation of action potentials. Neurons require aerobic metabolism for the generation of ATP, and have virtually no ATP reserve. This requires continuous delivery of glucose and oxygen to the brain, generally in the range of 15–20% of the body’s resources, a disproportional consumption of resources. During starvation, when glucose availability is limited, the brain can shift gradually to use of beta-hydroxybutyrate and acetoacetate as an energy source for neuronal metabolism; however, this is not an acute process and is not available to buffer acute hypoglycemic episodes. An ischemic episode of even 5 minutes, from a heart attack or an ischemic stroke, can lead to permanent damage in some neuronal populations, such as pyramidal cells in the CA1 region of the hippocampus, and with longer ischemia, widespread neuronal death. Because neurons are postmitotic cells, except for a small subset of interneurons, dead neurons are not replaced. One additional consequence of the post- Advance Sample Chapter -- NOT FINAL PRODUCT Neurons and Their Properties 13 Schematic of synaptic endings Dendrite Axon il Axon Initia hillock segme Initial segment Node Dendrites Axon Myelin sheath of presynaptic neurons ermi knobs) ating Numerous boutons (synaptic a m to n neurons ron an terminating ts dendri e ofnpresynaptic on a motor neuron and its dendrites Neurofilaments PR SA O M PE PL R E TY C O O F N E TE L N SE T V - N IE O R T FI N AL Enlarged section of bouton Neurotubules Axon (axoplasm) Axolemma Mitochondria Glial process Synaptic vesicles Synaptic cleft Presynaptic membrane (densely staining) Postsynaptic membrane (densely staining) Postsynaptic cell 1.9 Synaptic Morphology Synapses are specialized sites where neurons communicate with each other and with effector or target cells. The upper figure shows a typical neuron that receives numerous synaptic contacts on its cell body and associated dendrites, derived from both myelinated and unmyelinated axons. Incoming myelinated axons lose their myelin sheaths, exhibit extensive branching, and terminate as synaptic boutons (terminals) on the target (in this example, motor) neuron. The lower figure shows an enlargement of an axo-somatic terminal. Chemical neurotransmitters are packaged in synaptic vesicles. When an action potential invades the terminal region, depolarization triggers Ca2+ influx into the terminal, causing numerous synaptic vesicles to fuse with the presynaptic membrane, releasing their packets of neurotransmitter into the synaptic cleft. The neurotransmitter can bind to receptors on the postsynaptic membrane, resulting in graded excitatory or inhibitory postsynaptic potentials, or in neuromodulatory effects on intracellular signaling systems in the target cell. There is sometimes a mismatch between the site of release of a neurotransmitter and the location of target neurons possessing receptors for the neurotransmitter (can be immediately adjacent, or at a distance). Many nerve terminals can release multiple neurotransmitters, regulated by gene activation and by the frequency and duration of axonal activity. Some nerve terminals possess presynaptic receptors for their released neurotransmitter(s). Activation of these presynaptic receptors regulates neurotransmitter release. Some nerve terminals also possess high-affinity uptake carriers for transport of the neurotransmitter (e.g. dopamine, norepinephrine, serotonin) back into the nerve terminal for repackaging and reuse. Advance Sample Chapter -- NOT FINAL PRODUCT 72 Overview of the Nervous System Origin and Spread of Seizures Normal firing pattern of cortical neurons Thalamus E – – + P P + Recurrent inhibitory circuit Recurr Single stimulus I + P + E+ + P + + + – – – Recurrent excitatory circuit Recurr PR SA O M PE PL R E TY C O O F N E TE L N SE T V - N IE O R T FI N AL – + + i – Ceretral cortex – – P + + Action potentials (nonsynchronous) ction p (E) en Normal activation of cortical neurons (P) modulated by excitatory E and inhibitory (l) feedback circuits E Substantia nigra Corpus striatum Excitatory pathways between cerebral cortex and thalamus u r at modulated by tonic midbrain inhibitory stimuli Epileptic firing pattern of cortical neurons – + P Depolarization ↑ field potential – E + + + Depressed inhibition P + + Cortex + + + High frequency I + Repetitive stimuli – – Depolarization ↑ extracellular K+ – P + P + + + E + Thalamus Substantia nigra Corpus striatum + E + – – Increased excitation P + – Burst firing action potentials (hypersynchronous) Repetitive cortical activation potentiates excitatory transmission and depresses inhibitory transmission, creating self-perpetuating excitatory circuit (burst) and facilitating excitation (recruitment) of neighboring neurons. 2.2 Normal Electrical Firing Patterns of Cortical Neurons and the Origin and Spread of Seizures The collective electrical activity of the cerebral cortex can be monitored by electroencephalography. Normal cortical electrical activity reflects the summation of excitatory and inhibitory actions, modulated through feedback circuits. Thalamic inputs Cortical bursts to corpus striatum and thalamus block inhibitory projections and create selfperpetuating feedback circuit. th r r i w ISBN Autho F g. # can driveDoelectrical um #nt nam excitability; the to the cortex can (if ne midbrain ded) F lt n 0OK Cor Initials provide inhibitory control over this process. Repetitive cortical Fi D D 5 activation can inhibition, enhance excitatory feedback -2/ dampen 022-NB86 eps Date I ti l OK ev o Artist/ recruit R Dat excitatory circuits, and repetitive in adjacent CE's revi w 07/25/07 circuitry 4/C 2/C xx Bpu/df Da e OK Corr Initials cortical neurons. These self-perpetuating excitatory feedback Che k f rev sion G aphic Wor d Illust a ion Stud o • St L circuits can initiate and spread seizure activity. 4/C X 2/C xxx B W Advance Sample Chapter -- NOT FINAL PRODUCT 74 Overview of the Nervous System Cingulate gyrus Precentral sulcus Central (rolandic) sulcus Cingulate sulcus Paracentral lobule Medial frontal gyrus Corpus callosum Sulcus of corpus callosum Precuneus Fornix Superior sagittal sinus Choroid plexus of 3rd ventricle Septum pellucidum Interventricular foramen (of Monro) Parietoccipital sulcus Interthalamic adhesion Anterior commissure Hypothalamic sulcus Subcallosal (parolfactory) area Paraterminal gyrus Gyrus rectus Lamina terminalis Optic recess Optic chiasm Tuber cinereum Mammillary body PR SA O M PE PL R E TY C O O F N E TE L N SE T V - N IE O R T FI N AL Thalamus Habenular commissure Calcarine sulcus Lingual gyrus Calcarine cortex (lower bank) Pineal gland Straight sinus (in tentorium cerebelli) Great cerebral vein (of Galen) Posterior (epithalamic) commissure Superior and inferior colliculi Cerebellum A P Pituitary gland (anterior and posterior) Midbrain Pons Medulla oblongata 3.1 Anatomy of the Medial (Midsagittal) Surface of the Brain In Situ The entire extent of the neuraxis, from the spino-medullary junction through the brain stem, diencephalon, and telencephalon, is visible in mid-sagittal section. The corpus callosum, a major commissural fiber bundle interconnecting the two hemispheres, is a landmark separating the cerebral cortex above from the thalamus, fornix, and subcortical forebrain below. The ventricular system, including the interventricular foramen (of Munro), the third ventricle (diencephalon), the cerebral aqueduct (midbrain), and the fourth ventricle (pons and medulla), is visible in midsagittal view. This subarachnoid fluid system provides internal and external protection to the brain and also may serve as a fluid transport system for important regulatory molecules. The thalamus serves as a gateway to the cortex. Stria medullaris of thalamus Cuneus Calcarine cortex (upper bank) Cerebral aqueduct (of Sylvius) Superior medullary velum 4th ventricle and choroid plexus Inferior medullary velum The hypothalamic proximity to the median eminence (tuber cinereum) and the pituitary gland reflects the important role of the hypothalamus in regulating neuroendocrine function. A midsagittal view also reveals the midbrain colliculi, sometimes called the visual (superior) and auditory (inferior), tecta. CLINICAL POINT The foramina of the skull are tightly confined openings for the passage of nerves and blood vessels. Under normal circumstances, there is enough room for comfortable passage of these structures without traction or pressure. However, with the presence of a tumor at a foramen, the passing structures can be compressed or damaged. A tumor at the internal acoustic meatus can result in ipsilateral facial and vestibuloacoustic nerve damage, and a tumor at the jugular foramen can result in damage to the glossopharyngeal, vagus, and spinal accessory nerve. Advance Sample Chapter -- NOT FINAL PRODUCT