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Neurons • Vary in size and structure, but have common features: 1. Cell Body 2. Dendrites 3. Axon • Mature neurons do not divide, but neural stem cells can divide and form neurons or neuroglial cells. 1. Cell Body • Contains normal cellular structures (golgi apparatus, mitochondria, cytoplasm, cell membrane, etc.) • Neurofibrils – fine threads that extend into the axon • Nissl bodies (chromatophilic substances) – Membranous sacs in the cytoplasm – Similar to rough ER – Ribosomes on Nissl bodies synthesize ______ 2. Dendrites • Usually short and highly branched (dendr = ?) • The main receptive surfaces for receiving communication from axons of other neurons 3. Axons • Arise from a slight elevation of the cell body, called the axonal hillock. • Conduct nerve impulses away from the cell body • Contains many mitochondria, microtubules, and neurofibrils • Originates as a single structure, but may have branches, especially at the end to interact with receptive surfaces of other cells PNS Axons • Enclosed in myelin sheaths composed of many Schwann cells • Myelin is a lipoprotein. • Neurilemma sheath surrounds the myelin sheath • Nodes of Ranvier – narrow gaps in the myelin sheath between the Schwann cells Classification of Neurons • Classification based on Structural differences: • Bipolar neurons • Unipolar neurons • Multipolar neurons • Classification based on Functional differences: • Sensory neurons (afferent neurons) • Interneurons (association or internuncial neurons) • Motor neurons (efferent neurons) Structural Differences • Sketch the neurons below. Notes on the next 3 slides: Structural Differences, cont….. 1. Bipolar: – 2 processes • • Axon Dendrite – Found in specialized parts of the eyes, nose, and ears Structural Differences, cont….. 2. Unipolar: – 1 process divides into 2 branches, which function as a single axon • • 1 branch (peripheral process) associated with dendrites Other branch (central process) enters brain or spinal cord Structural Differences, cont….. 3. Multipolar: – Many processes arising from cell body: • • 1 axon Many dendrites – Most neurons whose cell bodies lie in the brain or spinal cord are multipolar. • Direction of impulse is ALWAYS from dendrites to axon. Functional Differences 1. Sensory (afferent) neurons – From peripheral body parts to the brain or spinal cord – Have specialized receptor ends at the tips of their dendrites OR – Dendrites closely associated with receptor cells in the skin or sensory organs. – Most are unipolar, but some are bipolar. Functional Differences, cont….. 2. Interneurons (association or internuncial neurons) – Lie entirely in the brain or spinal cord – Multipolar and link other neurons – Transmit impulses from one part of the brain or spinal cord to another 3. Motor (efferent) neurons – Multipolar – Carry nerve impulses from brain or spinal cord to effectors – Stimulate muscles or glands