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1H06 Anatomy & Physiology 1H06: Nervous System (ANS, CNS, etc.) .docx

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McMaster University
Health Sciences
Alexander Ball

Nervous System Two main networks: 1. Central nervous system 2. Peripheral nervous system Central Nervous System  Brain and spinal cord  Processes sensory information  Source of thoughts, emotions, memories  CNS causes muscles to contract and glands to secrete Peripheral Nervous System  Nerves, ganglia, enteric plexuses, and sensory receptors  12 pairs of cranial nerves emerge from brain  31 pairs of spinal nerves emerge from spinal cord  Ganglia – small masses of nervous tissue consisting primarily of neuron cell bodies located outside of brain and spinal cord  Enteric plexuses – networks of neurons located in walls of organs of gastrointestinal tract  Divided into somatic, autonomic, and enteric (intestines) nervous systems Somatic nervous system  Sensory neurons that convey information from somatic receptors in head, body wall, and limbs and from receptors for vision, hearing, taste, and smell to CNS  Motor neurons conduct impulses from CNS to skeletal muscles only  Voluntary control Autonomic nervous system  Sensory neurons that convey information from autonomic sensory receptors, located in visceral organs such as stomach and lungs to CNS  Motor neurons conduct nerve impulses from CNS to smooth muscle, cardiac muscle, and glands.  Involuntary control  Motor part of ANS consists of sympathetic and parasympathetic divisions  Sympathetic – increase heart rate, parasympathetic neurons slow down Nervous System Functions 1. Sensory function – detect internal stimuli (blood pressure), carried into brain and spinal cord through cranial and spinal nerves 2. Integrative function – processes sensory information by analyzing and making decisions (integration) 3. Motor function – elicit appropriate motor response by activating effectors through cranial and spinal nerves Histology of Nervous Tissue  Nervous tissue composed of neurons and neuroglia  Neurons provide functions for sensing, thinking, remembering, muscle activity, regulating glandular secretions  Neuroglia are smaller but greatly outnumber neurons, and nourish, support, and protect neurons, and maintain interstitial fluid that bathes them  Neuroglia continue to divide throughout individual’s lifetime, unlike neurons Neurons  Electrical excitability (respond to stimulus and convert to action potential)  Stimulus – change in environment strong enough to initiate action potential  Action potential – electrical signal that travels along surface of membrane of neuron Parts of a Neuron  Composed of cell body, dendrites, and axon  Cell body – contains nucleus surrounded by cytoplasm that includes lysosomes, mitochondria, and Golgi complex  Neurofibrils – composed of bundles of intermediate filaments that provide cell shape and support  Microtubules – assist in moving materials between cell body and axon  Lipofuscin – pigment that occurs as clumps of yellowish brown granules in cytoplasm  Nerve fiber – neuronal process that emerges from cell body of neuron  Dendrite – receiving or input portions of neuron  Axon – propagates nerve impulses toward another neuron, muscle fiber, or gland cell  Initial segment – part of axon closest to axon hillock  Trigger zone – junction of axon hillock and initial segment where nerve impulses arise  Axoplasm – cytoplasm of axon  Axolemma – plasma membrane that surrounds axoplasm  Axon collaterals – side branches branch off axon  Axon terminals – many fine processes, end of axon and collaterals  Synapse – site of communication between 2 neurons or between neuron and effector cell  Neurotransmitter – released from synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell Two transport systems: needed to carry materials fromcell body to terminals 1. Slow axonal transport a. Moves materials about 1-5mm/day b. Conveys axoplasm only from cell body towards axon terminals 2. Fast axonal transport a. Moves 200-400mm/day b. Uses proteins that function as motors to move materials c. Moves materials in both directions – away from and toward cell body d. Anterograde (forward) direction moves organells and synaptic vesicles from cell body to axon terminal e. Retrograde (backward) direction moves membrane vesicles from axon terminals to cell body to be degraded or recycled. Classification of Neurons Structural Classification  Classified by nuber of processes extending from cell body  Some neurons named for histologist who first discovered them or for aspect of shape or appearance (Purkinje cells, pyramidal cells) 1. Multipolar neurons – several dendrites and one axon. Most neurons in brain and spinal cord are this type, as well as all motor neurons 2. Bipolar neurons – one main dendrite and one axon found in retina of eye, inner ear, and smelling area of brain 3. Unipolar neurons – denderites and one axon fused together to form continuous process that emerges from cell body. Also called pseudounipolar neurons Functional Classification 1. Sensory or afferent – contain either sensory receptors at distal ends or located just after sensory receptors that are separate cells. Most are unipolar 2. Motor or efferent – convey action potentials away from CNS to effectors in PNS through cranial or spinal nerves 3. Interneurons or association neurons – mainly located within CNS between sensory and motor neurons. They integrate incoming response and elicit motor response by activating appropriate motor neurons Neuroglia of CNS Astrocytes  Star-shaped, many processes, largest and most numerous  Two types: protoplasmic and fibrous  Protoplasmic: many short branching processes found in gray matter  Fibrous: many long unbranched processes located mainly in white matter  Contain filaments that give strength, enabling them to support neurons  Processes wrapped around blood capillaries isolate neurons of CNS from harmful substances in blood by secretion of chemicals that mai
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