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Neural Tissue Parts list Overview: Nervous system functions 1. Sensory input (info travels “in” along afferent pathways) – Sensory neurons 2. Integration (information is processed) – Spinal cord and brain 3. Motor output (info results in a response, travels along efferent pathways) – Stimulation of skeletal muscle, smooth muscle, cardiac muscle, & glands Sensory-Motor Pathways Overview Neural tissue: specialized cells • Neurons • Supporting cells (non-excitable cells) or neuroglia; outnumber neurons 10 to 1 • Blood vessels • Connective tissue Neurons • Specialized cells that communicate with other cells via changes in the membrane potential and synaptic connections • Characteristics: – Extremely long-lived (> 100yrs) – Amitotic – Extremely high metabolic rate Typical Neuron Neurons have special names for cell structures • Cell body: contains the nucleus • Perikaryon: cytoplasm around the nucleus • Nissl bodies are localized RER and ribosomes Typical motor neuron Synapse of a motor neuron Classified by structure Classified by function (sensory or motor) Neuroglia are supporting cells Neuroglia are supporting cells Wandering police force CNS Regulating/recycling/maintaining Circulate CSF PNS myelinate axons Ependymal cells make and help circulate CSF Schwann cells in PNS Nerve regeneration is assisted by Schwann cells Neurophysiology Overview Neurons are excitable cells • Have we talked about a model in another cell for propagation of an impulse? • How does the chemical composition of the cell membrane promote a separation of ions? • Which ions are in greater concentration inside the cell? Outside the cell? Electrochemical gradient • A charge difference exists, like between poles of a battery. • Intracellular: high concentration of K+ ions & negatively charged proteins • Extracellular: high concentration of Na+ & Cl- ions Two gradients = • The sum of the chemical and electrical forces acting across the membrane = Electrochemical gradient (AKA membrane potential) • Size of the potential difference is measured in milliVolts (mV) • Neurons have a resting transmembrane potential of approx. -70mV Resting Membrane Potential Electrochemical gradient Ion diffusion in neurons • Ions are always diffusing, BUT can cross membrane quickly ONLY through channels • Types of channels – Passive or leak channels (ungated; always open) – Active or gated channels (always closed, unless…) Gated Channels • Chemically regulated: Respond to presence of particular chemicals • Voltage regulated: respond to changes in membrane potential • Mechanically regulated: respond to physical deformation Have we seen examples of these channels in other tissue? Gated Channels chemical electrical mechanical If gated channels open… • What happens to the transmembrane potential? Graded Potential • A change in the transmembrane potential that does not travel far from the area of stimulation • Definitions – Depolarization: shift towards 0mV – Repolarization: shift towards the resting membrane potential (~-60mV) – Hyperpolarization: membrane becomes more negative Graded Potential Graded Potential Graded Potential Action Potential • Propagated changes in the transmembrane potential that affect an entire excitable membrane and results in release of a neurotransmitter • ALL OR NOTHING response. Action Potential Action Potential AP Propagation