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Topics Nervous System Overview 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 General Overview Lecture 12a. • • • • • Includes all neural tissue in the body Neural Tissue • Contains 2 kinds of cells: – neurons: • cells that send and receive signals – neuroglia (glial cells): • cells that support and protect neurons Organs of the Nervous System • Brain and spinal cord • Sensory receptors of sense organs (eyes, ears, etc.) • Nerves connect nervous system with other systems 1 Anatomical Divisions of the Nervous System 1. Central nervous system (CNS) 2. Peripheral nervous system (PNS) Nervous System CNS PNS Somatic NS Autonomic NS Sympathetic The Central Nervous System (CNS) • Consists of the spinal cord and brain • Contain neural tissue, connective tissues, and blood vessels Parasympathetic Functions of the CNS • Are to process and coordinate: – sensory data: • from inside and outside body – motor commands: • control activities of peripheral organs (e.g., skeletal muscles) – higher functions of brain: • intelligence, memory, learning, emotion The Peripheral Nervous System (PNS) • Includes all neural tissue outside the CNS (nerves that control muscles and glands) Functions of the PNS 1. Deliver sensory information to the CNS 2. Carry motor commands to peripheral tissues and systems 2 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 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) The Autonomic Nervous System (ANS) • Controls subconscious actions: – contractions of smooth muscle and cardiac muscle – glandular secretions The Somatic Nervous System (SNS) • Controls skeletal muscle contractions: – voluntary muscle contractions – involuntary muscle contractions (reflexes) Divisions of the ANS • Sympathetic division: – has a stimulating effect • Parasympathetic division: – has a relaxing effect 3 Nervous System CNS Neurons • The basic functional units of the nervous system PNS Somatic NS Autonomic NS Sympathetic Parasympathetic Major Organelles of the Cell Body The Structure of Neurons • • • • Large nucleus and nucleolus Cytoplasm (perikaryon) Mitochondria (produce energy) RER and ribosomes (produce neurotransmitters) • Cytoskeleton • Nissl Bodies: RER and ribosomes Figure 12–1 Dendrites • Highly branched • Dendritic spines: – many fine processes – receive information from other neurons – 80–90% of neuron surface area The Axon • Are long • Carries electrical signal (action potential) to target • Axon structure is critical to function 4 Structures of the Axon Movie • Axon hillock: Neurophysiology: Neuron Structure – 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) Synapse The 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 Figure 12–2 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 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 5 Unipolar Neurons Multipolar Neurons • Have very long axons • Dendrites fused to axon • Cell body to 1 side • Often have long axons • Multiple dendrites, 1 axon Figure 12–3 (3 of 4) 3 Functional Classifications of Neurons • Sensory neurons: – afferent neurons of PNS • Motor neurons: – efferent neurons of PNS Figure 12–3 (4 of 4) Sensory (Afferent) Neurons • Monitor internal environment (visceral sensory neurons) • Monitor effects of external environment (somatic sensory neurons) • Interneurons: – association neurons 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 6 Neuroglia are supporting cells Neuroglia • Half the volume of the nervous system • Many types of neuroglia in CNS and PNS Neuroglia of the Central Nervous System 4 Types of Neuroglia in the CNS 1. 2. 3. 4. Ependymal cells Astrocytes Microglia Oligodendrocytes Figure 12–4 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 2. Astrocytes • Maintain blood–brain barrier (isolates CNS) • Create 3-dimensional framework for CNS • Repair damaged neural tissue • Guide neuron development • Control interstitial environment 7 3.Microglia • Migrate through neural tissue • Clean up cellular debris, waste products, and pathogens • Not of neural origin; related to macrophages (like osteoclasts) Myelination • Increases speed of action potentials • Myelin insulates myelinated axons • Makes nerves appear white 4. Oligodendrocytes • Processes contact other neuron cell bodies • Wrap around axons to form myelin sheaths Nodes and Internodes • Internodes: – myelinated segments of axon • Nodes: – also called nodes of Ranvier – gaps between internodes – where axons may branch White Matter and Gray Matter • White matter: – regions of CNS with many myelinated nerves 2 Neuroglia of the Peripheral Nervous System 1. Satellite cells (amphicytes) 2. Schwann cells (neurilemmacytes) • Gray matter: – unmyelinated areas of CNS 8 1. Satellite Cells Ganglia vs Nuclei • Surround ganglia • Regulate environment around neuron • Masses of neuron cell bodies – Called ganglia in the PNS and are surrounded by satellite cells – Called nuclei in the CNS Schwann Cells 1. Schwann Cells • Form myelin sheath around peripheral axons (nerves) • 1 Schwann cell sheaths 1 segment of axon: – many Schwann cells sheath entire axon Figure 12–5a Summary Schwann Cells • • • • Divisions of the NS: CNS and PNS Structure and types of neurons Synapses Structure and function of glia in the CNS and PNS Figure 12–5b 9