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Chapter 12 Nervous System Cells Introduction • The function of the nervous system, along with the endocrine system, is to communicate – Controls and integrates many different functions performed by different organs, tissues, and cells – Communication, Control, Integration, Homeostasis,Survival • The nervous system is made up of the – Brain – Spinal cord – Nerves Organization of the Nervous System • Organized to… – Detect changes in internal and external environments – Evaluate the information – Initiate an appropriate response • Central nervous system (CNS) – Structural and functional center of entire nervous system – Consists of the brain and the spinal cord – Integrates sensory information, evaluates it, and initiates an outgoing response • Peripheral nervous system (PNS) – Nerves that lie in “outer regions” of nervous system – Cranial nerves—originate from brain – Spinal nerves—originate from spinal cord Organization of the Nervous System • Divisions of the nervous system – Afferent division - incoming sensory pathways – Efferent division - outgoing motor pathways • Somatic Nervous System (SNS) – skeletal muscles – Somatic motor division (efferent) – Somatic sensory division (afferent) – Somatic integration centers • Autonomic Nervous System (ANS) – smooth/cardiac muscles, glands – Autonomous of voluntary control – Sympathetic division – “fight or flight” (efferent) • Middle portions of the spinal cord – Parasympathetic division – “rest and repair” (efferent) • Brain and lower portions of spinal cord – Visceral sensory division (afferent) – Autonomic integration centers Cells of the Nervous System • Glia – Support the functions of neurons – 900 billion in the human nervous system – Ability to do cell division throughout adulthood – Susceptible to abnormalities…cancer – Five types • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Schwann cells Cells of the Nervous System • Astrocytes – Star-shaped – Largest – Most numerous type – Cell extensions connect to both neurons and capillaries – Transfer nutrients from the blood to the neurons – Constitute the blood-brain barrier (BBB) Cells of the Nervous System • Microglia – Small, usually stationary – In inflamed brain tissue, they enlarge, move about, and carry on phagocytosis • Ependymal cells – Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS – Some produce fluid; others aid in circulation of fluid Cells of the Nervous System • Oligodendrocytes – Smaller than astrocytes with fewer processes – Hold nerve fibers together and produce the myelin sheath Cells of the Nervous System • Schwann cells – Found only in PNS – Support nerve fibers and form myelin sheaths (Figure 12-4) – Gaps in the myelin sheath are called nodes of Ranvier – Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowth – Satellite cells are Schwann cells that cover and support cell bodies in the PNS Cells of the Nervous System • Neurons – – – – Excitable cells that initiate and conduct impulses Components of neurons 100 billion in the brain (10% of nervous system cells) Structure • Cell body (perikaryon) – Nucleus, cytoplasm, mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum (Nissl bodies) • Dendrites – Receive the stimuli and initiate nerve signals – Conduct nerve signals to the cell body • Axon – – – – – A single process extending from the axon hillock Axon collaterals (side branches) Sometimes covered by a fatty layer called a myelin sheath Conducts nerve impulses away from the cell body of the neuron Distal tips of axons are telodendria, each of which terminates in a synaptic knob – Thicker diameter = faster impulse Cells of the Nervous System • Classification of neurons – Structural classification—classified according to number of processes extending from cell body • Multipolar – One axon and several dendrites • Bipolar – Only one axon and one dendrite – Least numerous kind of neuron • Unipolar (pseudounipolar) – One process comes off neuron cell body but divides almost immediately into two fibers » Central fiber (toward CNS) » Peripheral fiber (away from CNS) Cells of the Nervous System • Classification of neurons – Functional classification • Afferent (sensory) neurons – Conduct impulses to spinal cord or brain • Efferent (motor) neurons – Conduct impulses away from spinal cord or brain toward muscles or glandular tissue • Interneurons – Entirely within CNS Cells of the Nervous System • Reflex arc – A signal conduction route to and from the CNS • Electrical signal beginning in receptors and ending in effectors – Three-neuron arc • Most common • Consists of afferent neurons, interneurons, and efferent neurons • Afferent neurons—conduct impulses to the CNS from the receptors • Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue) – Two-neuron arc • Simplest form • Consists of afferent and efferent neurons Cells of the Nervous System • Reflex arc (cont.) – Synapse • Where nerve signals are transmitted from one neuron to another • Located at the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron • Electrical and chemical synapses (more on this later) – Ipsilateral reflex arc • Receptors and effectors located on the same side of body – Contralateral reflex arc • Receptors and effectors located on opposite sides of the body – Many signals fail to be conducted across a synapse – Not all signals that terminate in effectors start in receptors Nerves and Tracts • Nerves – Bundles of peripheral nerve fibers held together by several layers of connective tissue • Endoneurium—delicate layer of fibrous connective tissue surrounding each nerve fiber • Perineurium—connective tissue holding together fascicles (bundles of fibers) • Epineurium—fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve • Tracts – Within the CNS, bundles of nerve fibers are called tracts rather than nerves Nerves and Tracts • White matter – PNS—myelinated nerves – CNS—myelinated tracts • Gray matter – Made up of cell bodies and unmyelinated fibers – CNS—referred to as nuclei – PNS—referred to as ganglia • Mixed nerves – Contain sensory and motor neurons – Sensory nerves—predominantly sensory neurons – Motor nerves—predominantly motor neurons