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26/07/12 Cranial Nerves, source: training.seer.cancer.gov Nervous System Overview BIOL241 - Lecture 12a 1 26/07/12 Topics • • • • Divisions of the NS: CNS and PNS Structure and types of neurons Synapses Structure and function of glia in the CNS and PNS The Nervous System • Includes all neural tissue in the body 2 26/07/12 General Overview Neural Tissue • Contains 2 kinds of cells: – neurons: • cells that send and receive signals – neuroglia (glial cells): • cells that support and protect neurons 3 26/07/12 Organs of the Nervous System • Brain and spinal cord • Sensory receptors of sense organs (eyes, ears, etc.) • Nerves connect nervous system with other systems INTERCONNECTEDNESS Anatomical Divisions of the Nervous System 1. Central nervous system (CNS) 2. Peripheral nervous system (PNS) 4 26/07/12 Nervous System CNS PNS Soma7c NS Autonomic NS Sympathe7c Parasympathe7c The Central Nervous System (CNS) • Consists of the spinal cord and brain • Contain neural tissue, connective tissues, and blood vessels • INTERCONNECTEDNESS 5 26/07/12 Functions of the CNS • Are to process and coordinate: – sensory data: • from inside and outside body (“inputs”) – motor commands: • control activities of peripheral organs (e.g., skeletal muscles) [“controls’] – higher functions of brain: • intelligence, memory, learning, emotion (“outputs”) The Peripheral Nervous System (PNS) • Includes all neural tissue outside the CNS (nerves that control muscles and glands) 6 26/07/12 Functions of the PNS 1. Deliver sensory information to the CNS 2. Carry motor commands to peripheral tissues and systems Nerves • Also called peripheral nerves: – bundles of axons with connective tissues and blood vessels – carry sensory information and motor commands in PNS: • cranial nerves—connect to brain • spinal nerves—attach to spinal cord 7 26/07/12 Functional Divisions of the PNS • Afferent division: – carries sensory information – from PNS sensory receptors to CNS • Efferent division: – carries motor commands – from CNS to PNS muscles and glands SAME: Sensory is Afferent, Motor is Efferent PNS divisions • Somatic nervous system (SNS) • Autonomic nervous system (ANS) • More “layers” 8 26/07/12 The Somatic Nervous System (SNS) • Controls skeletal muscle contractions: – voluntary muscle contractions – involuntary muscle contractions (reflexes) The Autonomic Nervous System (ANS) • Controls subconscious actions: – contractions of smooth muscle and cardiac muscle – glandular secretions 9 26/07/12 Divisions of the ANS • Sympathetic division: – has a stimulating effect • Parasympathetic division: – has a relaxing effect Nervous System CNS PNS Soma7c NS Autonomic NS Sympathe7c Parasympathe7c 10 26/07/12 Neurons • The basic functional units of the nervous system The Structure of Neurons Figure 12–1 11 26/07/12 Major Organelles of the Cell Body • • • • Large nucleus and nucleolus Cytoplasm (perikaryon) Mitochondria (produce energy) RER and ribosomes (produce neurotransmitters) • Cytoskeleton • Nissl Bodies: RER and ribosomes Dendrites • Highly branched • Dendritic spines: – many fine processes – receive information from other neurons – 80–90% of neuron surface area 12 26/07/12 The Axon • Are long • Carries electrical signal (action potential) to target • Axon structure is critical to function Structures of the Axon • Axon hillock: – thick section of cell body – attaches to initial segment • Initial segment: – attaches to axon hillock • Collaterals: – branches of a single axon • Synaptic terminals with knobs: – tips of axon (knobs at ends) 13 26/07/12 Movie Neurophysiology: Neuron Structure The Synapse Figure 12–2 14 26/07/12 Synapse • Synapse: Area where a neuron communicates with another cell • Presynaptic cell: – neuron that sends message • Postsynaptic cell: – cell that receives message • Synaptic cleft The small gap that separates the presynaptic membrane and the postsynaptic membrane • Area of terminal containing synaptic vesicles filled with neurotransmitters Neurotransmitters • Chemical messengers (like ACh) • Released at presynaptic membrane • Affect receptors of postsynaptic membrane • Broken down by enzymes and, or, taken up into presynaptic cell • Are reassembled at synaptic knob 15 26/07/12 Soma • What does soma mean? Major Structural Classifications of Neurons • Unipolar neurons: – found in sensory neurons of PNS • Multipolar neurons: – common in the CNS – include all skeletal muscle motor neurons – cell body (soma) – short, branched dendrites – long, single axon 16 26/07/12 Unipolar Neurons • Have very long axons • Dendrites fused to axon • Cell body to 1 side Figure 12–3 (3 of 4) Multipolar Neurons • Often have long axons • Multiple dendrites, 1 axon Figure 12–3 (4 of 4) 17 26/07/12 3 Functional Classifications of Neurons • Sensory neurons: – afferent neurons of PNS • Motor neurons: – efferent neurons of PNS • Interneurons: – association neurons Sensory (Afferent) Neurons • Monitor internal environment (visceral sensory neurons) • Monitor effects of external environment (somatic sensory neurons) 18 26/07/12 Motor (Efferent) Neurons • Signals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into: – preganglionic fibers – postganglionic fibers Interneurons • Most are located in brain, spinal cord, and autonomic ganglia: – between sensory and motor neurons • Are responsible for: – distribution of sensory information – coordination of motor activity • Are involved in higher functions: – memory, planning, learning 19 26/07/12 Neuroglia are supporting cells Neuroglia • Half the volume of the nervous system • Many types of neuroglia in CNS and PNS 20 26/07/12 Volume • What is volume? Neuroglia of the Central Nervous System Figure 12–4 21 26/07/12 4 Types of Neuroglia in the CNS 1. 2. 3. 4. Ependymal cells Astrocytes Microglia Oligodendrocytes 1. Ependymal Cells • Form epithelium called ependyma • Line central canal of spinal cord and ventricles of brain: – secrete cerebrospinal fluid (CSF) – have cilia or microvilli that circulate CSF – monitor CSF – contain stem cells for repair 22 26/07/12 Blood-Brain Barrier • What is this? 2. Astrocytes • Maintain blood–brain barrier (isolates CNS) • Create 3-dimensional framework for CNS • Repair damaged neural tissue • Guide neuron development • Control interstitial environment 23 26/07/12 3. Microglia • Migrate through neural tissue • Clean up cellular debris, waste products, and pathogens • Not of neural origin; related to macrophages (like osteoclasts) 4. Oligodendrocytes • Processes contact other neuron cell bodies • Wrap around axons to form myelin sheaths 24 26/07/12 Myelination • Increases speed of action potentials • Myelin insulates myelinated axons • Makes nerves appear white Nodes and Internodes • Internodes: – myelinated segments of axon • Nodes: – also called nodes of Ranvier – gaps between internodes – where axons may branch 25 26/07/12 White Matter and Gray Matter • White matter: – regions of CNS with many myelinated nerves • Gray matter: – unmyelinated areas of CNS 2 Neuroglia of the Peripheral Nervous System 1. Satellite cells (amphicytes) 2. Schwann cells (neurilemmacytes) 26 26/07/12 1. Satellite Cells • Surround ganglia • Regulate environment around neuron Ganglia vs Nuclei • Masses of neuron cell bodies – Called ganglia in the PNS and are surrounded by satellite cells – Called nuclei in the CNS 27 26/07/12 1. Schwann Cells • Form myelin sheath around peripheral axons (nerves) • 1 Schwann cell sheaths 1 segment of axon: – many Schwann cells sheath entire axon Schwann Cells Figure 12–5a 28 26/07/12 Schwann Cells Figure 12–5b “Summary” • • • • Divisions of the NS: CNS and PNS Structure and types of neurons Synapses Structure and function of glia in the CNS and PNS 29