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Functional Organization of Nervous Systems Maguire et al. 2000 http://www.pnas.org/cgi/reprint/97/8/4398 Figure 4 For surgical planning, a series of simple tasks (e.g., auditory, visual, motor, and language tasks) may be performed in a sequence to identify critical functional areas. These may then be superimposed on high-resolution anatomic images. J Clin Invest. 2003 July 1; 112(1): 4–9. Practice Does Make Perfect! Overview of the Nervous System One of the body’s homeostatic control systems Contains sensors, integrating centers, and output pathways More interneurons in a pathway ? greater number of interconnections and ability to integrate information Cnidarians Most nervous systems are organized into three functional divisions Afferent sensory, integrating, and efferent motor Cnidarians are an exception Nervous system is an interconnected web or nerve net Neurons carry action potentials in both directions Neurons are not only sensory or only motor Organism can still perform complex behaviors Nervous System Terms Bilaterally symmetrical – right and left side are mirror images Cephalization – sense organs and nervous integrating centers are concentrated at the anterior end Ganglia – groups of neuronal cell bodies Nuclei – groups of neuronal cell bodies within the brain Brain – an integrating center made up of clusters of nuclei Tracts – bundles of many axons within the CNS Nerve – a bundle of many axons outside of the CNS Structure of a Nerve Bundles of myelinated and unmyelinated axons enclosed in several layers of connective tissue Endoneurium – wraps each axon Perineurium – wraps a bundle (fascicle) of axons Epineurium – wraps the entire nerve Mixed nerves – contain both afferent and efferent neurons Each neuron is either afferent (sensory) or efferent (motor) Nervous Systems Across Animal Groups Cephalization occurs in most animals and is more apparent in more complex nervous systems Cnidarians and Echinoderms are exceptions in that they lack cephalization Organisms with more complex nervous systems have more neurons; and therefore, more synapses Increased numbers of synapses ? more integration of information ? more complex behaviors Since memories are stored in synapses, a complex nervous system provides greater potential for learning The Vertebrate Central Nervous System High degree of Cephalization Unique in having a hollow dorsal nerve cord (spinal cord) How evolve from invertebrate? Is still segmented! Part of the nervous system is encased within cartilage or bone CNS – brain and spinal cord Part of the nervous system extends to the periphery of the body PNS – nerves outside of the CNS Cranial and Spinal Nerves (PNS) Cranial nerves Exit directly from skull 13 pairs (labeled with roman numerals) Some afferent, some efferent, some mixed Spinal nerves Branch from spinal cord Enter and exit between adjacent vertebrae Named based on region of vertebral column from which they emerge Cervical, thoracic, lumbar, sacral, and coccygeal Mixed nerves Segmented Ancestor? Gray and White Matter Brain and spinal cord contain two types of tissue Gray matter – neuronal cell bodies White matter – tracts of axons and their myelin sheaths Spinal cord: white matter on surface, gray matter inside Cerebral cortex: gray matter on surface, white matter inside The CNS Is Isolated and Protected Meninges Layers of connective tissue that surround brain and spinal cord Fish have one Mammals have three Cerebral spinal fluid (CSF) Fills spaces within the CNS and acts as shock absorber Blood-brain barrier Tight junctions in brain capillaries limit passage of solutes from blood into the CSF The Vertebrate Brain Extension of the spinal cord Nerve tracts extend between brain and spinal cord Has several cavities called ventricles containing CSF Three main regions Rhombencephalon (hindbrain) Reflexes and involuntary behaviors Mesencephalon (midbrain) Coordination of sensory information Relay center in mammals Prosencephalon (forebrain) Integration of sensory information Regulation of body temperature, reproduction, eating, emotion Learning and memory in mammals Hindbrain and Midbrain Three regions Pons – located above medulla Pathway between medulla, cerebellum, and forebrain Controls alertness, initiates sleep and dreaming Cerebellum – two hemispheres at back of brain Responsible for motor coordination Contains half of the neurons in the brain Medulla oblongata – located at top of spinal cord Regulates breathing, heart rate, diameter of blood vessels, blood pressure Contains pathways between spinal cord and brain Primary center for coordinating and initiating behavioral responses in fish and amphibians Size and function reduced in mammals Relay center between spinal cord and forebrain Sometimes grouped with the pons and medulla and termed the brainstem Forebrain (Prosencephalon) Involved in processing and integrating sensory information, and in coordinating behavior Greatly enlarged in birds and mammals Main regions: Cerebrum Thalamus – integrates sensory information Epithalamus – hunger, thirst and melatonin secretion Hypothalamus Figure 7.10 Cerebrum and Hypothalamus Outer layer is the cortex Divided into two cerebral hemispheres Left side controls right side of body Right side controls left side of body Neurons pass between the two sides via the corpus callosum Located at base of forebrain Under the thalamus Helps maintain homeostasis Body temperature, thirst, hunger, reproduction, etc. Interacts with ANS Regulates secretion of pituitary hormones Limbic System “emotional brain” Network of connected structures that lie between the cortex and the rest of the brain Influences emotions, motivation, memory Includes hypothalamus and other parts Amygdala – aggression and fear responses Hippocampus – converts short-term memory to long-term memory Olfactory bulbs – sense of smell Cortex (Isocortex in mammals) Integrates and interprets sensory information and initiates voluntary movements Has taken over many of the midbrain functions of lower vertebrates Necessary for cognition and higher brain functions More folded in more advanced mammals Gyri (singular: gyrus) = folds; Sulci (singular: sulcus) = grooves Cortical Topology/Functionality Figure 8.5 Each region of the cortex corresponds to a specific part of the body that it controls by motor output, or from which it receives sensory input Size of the brain region devoted to different parts of the body varies widely (Functional regions) Figure 7.16 Peripheral Nervous System Divisions Autonomic Nervous System “Involuntary nervous system” Involved in maintaining homeostasis Branches Sympathetic Most active during periods of stress or physical activity “Fight-or-flight” system Parasympathetic Most active during periods of rest “Resting and digesting” system Enteric Independent of other two systems Innervates organs of alimentary canal Maintaining Homeostasis Balancing sympathetic and parasympathetic branches Three mechanisms: Dual innervation Most organs receive input from both systems Antagonistic action One system stimulates while the other inhibits Basal tone Even under resting conditions autonomic neurons carry APs Figure 7.17 Dual Innervation Table 7.3 Antagonistic Action Similarities in Autonomic Pathways Two neurons in series Preganglionic May synapse with many postganglionic neurons and intrinsic neurons Postganglionic Neurotransmitter is released at effector organ from varicosities pre- and postganglionic neurons synapse in the autonomic ganglia Differences in Autonomic Pathways Preganglionic cell body location S – thoracic and lumbar regions of spinal cord PS – hindbrain and sacral region of spinal cord Ganglia location S – chain that runs close to spinal cord PS – chain that runs close to the effector Number of postganglionic neurons that synapse with a single preganglionic neuron S – ten or more PS – three or fewer Neurotransmitter released at the effector organ Figure 7.20 Only Sympathetic Innervation Some effectors receive only sympathetic innervation Adrenal medulla Collection of modified postganglionic neurons Sweat glands Arrector pili muscles in skin Kidneys Most blood vessels Figure 7.21 Regulation of the Autonomic System Set of neurons in brainstem Figure 7.22 Autonomic Reflex Arcs Most autonomic changes occur via simple neural circuits that do not involve conscious centers of the brain Somatic Motor Pathways “Voluntary nervous system” Control of skeletal muscles Usually under conscious control Cerebrum Somatic Motor Pathway Characteristics Control only one type of effector, skeletal muscle Cell bodies of motor neurons are located in the CNS Monosynaptic Axons are very long, and extend all the way to the muscle Axon splits into a cluster of axon terminals at the neuromuscular junction Release the neurotransmitter acetylcholine Synaptic cleft between the motor neuron and the muscle is very narrow Effect on the muscle cell always excitatory For example, causes depolarization and contraction Animal Behaviors Three categories: Reflex behaviors Involuntary and simple Convergence (retina) versus divergence (ANS) Rhythmic behaviors Underlie locomotion, breathing, and the function of the heart Governed by pattern generators Groups of neurons that produce self-sustaining, rhythmic depolarizations Voluntary behaviors Most complex and diverse Consciously planned and coordinated by brain Learning and Memory Most animals can learn and form memories due to the plasticity of the nervous system Learning Process of acquiring new information Memory Retention and retrieval of information Plasticity Changes in synaptic and neuronal function in response to stimuli Memory in Mammals Hippocampus involved in long-term memory, but memories are “stored” in cerebrum Memories are “stored” by increasing the efficiency of the synapse between two neurons Long-term potentiation (LTP) – repetitive stimulation of hippocampal tissue leads to an increase in the response of the postsynaptic neuron Table 7.4 Sympathetic vs. Parasympathetic Systems
