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THE NERVE OF IT ALL COMMUNICATION CONTROL AND INTEGRATION COURTESY OF BRAIN, SPINAL CORD (CNS) AND NERVES (PNS) KINDS OF CELLS • NEURONS – Classified by direction of impulse • Afferent (sensory) – transmit signal to spinal cord • Efferent (motor) – transmit signal away from spinal cord or brain to glands or muscles • Interneurons – lie within CNS; transmit from afferent to efferent CELLS, cont’d – Can also be classified by # of processes • Multipolar – (CNS and neurons that carry AWAY from CNS) one axon, several dendrites (brain and spinal cord) • Bipolar – (receptors) one axon, one dendrite (retina, inner ear, olfactory pathways) • Unipolar – (to CNS) sensory neurons CELLS, cont’d • Large myelinated neurons (eg. Those that innervate muscles) conduct AP up to 100 m/sec • Small unmyelinated neurons (visceral organs) ~.5 to 1 m/sec More on Neurons • CELL BODIES (gray matter) – In CNS = nuclei – Outside CNS = ganglia • PROCESSES (white matter) – In CNS = tracts – Outside CNS = nerves Epineurium – tough and cordlike; perineurium – around fascicles; endoneurium – delicate CT CELLS, cont’d • Neuroglia – CT cells – Astrocytes – largest and most numerous; form sheaths around blood capillaries of brain – Microglia – small phagocytes – Oligodendrocytes – lay down myelin sheath around brain and spinal cord neurons (CNS) – Schwann – form neurolemma and myelin sheath only on neurons OUTSIDE of CNS = PNS – Ependymal – line central canal and ventricles; help produce and circulate CSF in choroid plexus Dendrite – receivers - take impulse to soma Cell body = soma Axon – away from soma Nissl bodies Myelin sheath Neurilemma – Sheath of Schwann – only in PNS Myelin sheath – segmented wrapping around an axon – concentric layers Saltatory conduction WHY? NEURAL PATHWAYS • Stimulus -> dendrites of sensory neuron -> soma -> axon -> axon terminus -> release neurotransmitters -> synapse -> dendrites of interneuron -> interneuron -> interneuron releases neurotransmitters across synapse to motor neuron -> effect ( contraction or secretion) ACTION POTENTIAL • An electrical gradient exists between inner and outer membrane • At rest, neuron is –70mV inside membrane; courtesy of Na+/K+ pump (3Na+ out/2K+ in) • Membrane is polarized • Stimulus – Na+ rushes into cells – membrane is depolarized – reverse polarization • Inside cell membrane is now +30mV • This current flows down membrane to axon terminals • Lasts for a millisecond, then repolarization • Na+ gates close and membrane is impermeable to Na+ again, but very permeable to K+; K+ exits • Absolute refractory period = neuron resists stimulation REFLEX ARC • Route traveled by nervous impulses • Afferent neuron -> interneuron in CNS -> efferent neuron • Vesicles in synaptic knob have ~10,000 molecules of neurotransmitters NEUROTRANSMITTERS 1. Acetylcholine – autonomic NS and brain - neuromuscular junctions – excite muscles – inactivated by cholinesterase - flea collars, pest strips, nerve gas contain cholinesterase inhibitors 2. Norepinephrine – sympathetic NS; regulates activity of of visceral organs and controls certain brain functions 3. Dopamine – Brain - motor functions; too little – muscle rigidity, tremors, forward gait (Parkinson’s ); too much – mental disorders (schizophrenia) 4. Serotonin – CNS - mental functions – wakefulness and sleep and other Circadian rhythms 5. GABA – inhibitory 6. Glycine – inhibitory in spinal cord Net effect is what determines if a neuron will fire Neuropharmacological agents • Exogenous substances that can affect neuron excitability by either mimicking or blocking action of neurotransmitter • Curare = blocks Ach receptors • Theobromine, caffeine increase excitability by affecting membrane perm. to Ca++ • Strychnine and tetanus toxin inhibit inhibitory neurotransmitters and cause convulsions • Ether = inc K+ perm causing hyperpol. Making membrane less responsive to stimulation • Valium enhances inhibitory effects of GABA • LSD mimics serotonin and binds to its receptors – effects are quite different