BIOL 2457 1st Edition Lecture 24Outline of Last Lecture Muscle tissues Outline of Current LectureNervous Tissue Functions of the Nervous System Sensory function: to sense changes in the internal and external environment through sensory receptors.  Sensory (afferent) neurons serve this function. Integrative function: to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors.  Association or interneurons serve this function. Motor function: to respond to stimuli by initiating action.  Motor(efferent) neurons serve this function.Nervous System Divisions Central nervous system (CNS)  Consists of the brain and spinal cord Peripheral nervous system (PNS) Consists of cranial and spinal nerves that contain both sensory and motor fibers Connects CNS to muscles, glands & all sensory receptorsNeuron structure Perikaryon Neurofilaments, neurotubules, neurofibrils Axon hillock Soma Axon Collaterals with telodendriaHistology of the Nervous System: Neurons Functional units of the nervous system Have capacity to produce action potentials Electrical excitability Cell body Single nucleus with prominent nucleolus Nissl bodies (chromatophilic substance)  Rough ER & free ribosomes for protein synthesis Neurofilaments give cell shape and support Microtubules move material inside cell Lipofuscin pigment clumps (harmless aging) Cell processes = dendrites & axonsAxonal Transport Cell body is location for most protein synthesis Neurotransmitters & repair proteins  Axonal transport system moves substances  Slow axonal flow Movement in one direction only -- away from cell body Movement at 1-5 mm per day  Fast axonal flow Moves organelles & materials along surface of microtubules At 200-400 mm per day Transports in either direction For use or for recycling in cell bodyNeuroglia of the CNS Supportive cells of the nervous system Types CNS Astrocytes  Oligodendrocytes Microglia Ependymal cells PNS Schwann cells Satellite cellsMyelinated & Unmyelinated Axons Schwann cells myelinate (wrap around) axons in the PNS during fetal development Schwann cell cytoplasm & nucleus form outermost layer of neurolemma with inner portion being the myelin sheath Tube guides growing axons that are repairing themselvesSubdivisions of the PNS Somatic (voluntary) nervous system (SNS) Neurons from cutaneous and special sensory receptors to the CNS Motor neurons to skeletal muscle tissue Autonomic (involuntary) nervous systems Sensory neurons from visceral organs to CNS Motor neurons to smooth & cardiac muscle and glands Sympathetic division (speeds up heart rate) Parasympathetic division (slow down heart rate) Enteric nervous system (ENS) Involuntary sensory & motor neurons control GI tract Neurons function independently of ANS & CNSResting Membrane Potential Negative ions along inside of cell membrane & positive ions along outside Potential energy difference at rest is -70 mV  Cell is “polarized” Resting potential exists because Concentration of ions different inside & outside Extracellular fluid rich in Na+ and Cl- Cytosol full of K+, organic phosphate & amino acids Membrane permeability differs for Na+ and K+ 50-100 greater permeability for K+ Inward flow of Na+ can’t keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in Graded Potentials Small deviations from resting potential of -70mV Hyperpolarization = membrane has become more negative Depolarization = membrane has become more positive The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and localized. Graded potentials occur most often in the dendrites and cell body of a neuron.Generation of Action Potentials An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization).  During an action potential, voltage-gated Na+ and K+ channels open in sequence  According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same.  A stronger stimulus will not cause a larger impulse.Depolarizing Phase Chemical or mechanical stimulus causes a graded potential to reach at least -55mV or threshold Voltage-gated Na+ channels open & Na+ rushes into the cell In resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) When threshold (-55mV) is reached, both open & Na+ enters Inactivation gate closes again in few ten-thousandths of second Only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably (up to +30mV) Positive feedback processRepolarizing Phase When threshold potential of -55mV is reached, voltage-gated K+ channels open K+ channel opening is much slower than Na+ channelopening which causes depolarization When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase K+ channels close and the membrane potential returns to the resting potential of -70mVRefractory Period Period of time during which neuron can not generate another action potential Absolute refractory period Even very strong stimulus will not begin another AP Inactivated Na+ channels must return to the resting state before they can be reopened Large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible Relative refractory period A suprathreshold stimulus will be able to start an AP K+ channels are still open, but Na+ channels have