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Neural tissue.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Neural tissue An Overview of the Nervous System Nervous system overview • Nervous system • Provides swift, brief responses to stimuli • Endocrine system • Adjusts metabolic operations and directs long-term changes • Nervous system includes • All the neural tissue of the body • Basic unit = neuron Divisions of the Nervous system • CNS (Central Nervous system) • Brain and spinal cord • PNS (Peripheral Nervous system) • Neural tissue outside CNS • Afferent division brings sensory information from receptors • Efferent division carries motor commands to effectors • Efferent division includes somatic nervous system and autonomic nervous system Neurons Neuron structure • Perikaryon • Neurofilaments, neurotubules, neurofibrils • Axon hillock • Soma • Axon • Collaterals with telodendria Synapse • Site of intercellular communication • Neurotransmitters released from synaptic knob of presynaptic neuron Neuron classification • Anatomical • Anaxonic • Unipolar • Bipolar • Multipolar Functional • Sensory neurons • deliver information from exteroceptors, interoceptors, or proprioceptors • Motor neurons • Form the efferent division of the PNS • Interneurons (association neurons) • Located entirely within the CNS • Distribute sensory input and coordinate motor output Neuroglia Neuroglia of the Central Nervous System • Four types of neuroglia in the CNS • Ependymal cells • Related to cerebrospinal fluid • Astrocytes • Largest and most numerous • Oligodendrocytes • Myelination of CNS axons • Microglia • Phagocytic cells Neuroglia of the Peripheral Nervous System • Two types of neuroglia in the PNS • Satellite cells • Surround neuron cell bodies within ganglia • Schwann cells • Ensheath axons in the PNS Neurophysiology: Ions and Electrical Signals The transmembrane potential • Electrochemical gradient • Sum of all chemical and electrical forces acting across the cell membrane • Sodium-potassium exchange pump stabilizes resting potential at ~70 mV Changes in the transmembrane potential • Membrane contains • Passive (leak) channels that are always open • Active (gated) channels that open and close in response to stimuli Three types of active channels • Chemically regulated channels • Voltage-regulated channels • Mechanically regulated channels Graded potential • A change in potential that decreases with distance • Localized depolarization or hyperpolarization Action Potential • Appears when region of excitable membrane depolarizes to threshold • Steps involved • Membrane depolarization and sodium channel activation • Sodium channel inactivation • Potassium channel activation • Return to normal permeability Characteristics of action potentials • Generation of action potential follows all-or-none principle • Refractory period lasts from time action potential begins until normal resting potential returns • Continuous propagation • spread of action potential across entire membrane in series of small steps • salutatory propagation •
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