Unit 4 Lecture 3 Notes I Nerve Regeneration 1 Mature neurons cannot regenerate so severe damage close to the cell back can cause neuron to die i May kill neurons with connections to injured neuron due to lack of stimulation 2 Injured peripheral axons regenerate i Proximal axon end seals off and swells up ii Organelles and materials accumulate iii Wallerian Degeneration distal axon begins to degenerate iv If reconnected the axon regains its function 3 In CNS regeneration is almost never successful i Microglia and astrocytes phagocytize debris ii Oligodendrocytes surrounding fibers will die no channels glial scare II Transmission of Information tissue blocks path 1 Path from receptor modified dendritic end to effector Information sent from receptor across afferent neuron i ii To synapse with association neuron iii To another association neuron OR efferent neuron iv Moves across efferent neuron to effector muscle or gland 2 Neurons respond to stimulation electrical impulse along axon i Impulse is the same regardless of the source or type 3 Resting membrane potential i More K inside cell and more Na outside cell movement through channels in response to electrical and chemical gradient membrane more permeable to K ii Sodium potassium pump maintains potential difference across membrane to generate nerve impulse info from one neuron to next iii Depolarization of Membrane 1 Reduction in membrane potential inside becomes less negative and ions move to cause depolarization iv Hyperpolarization of membrane increase membrane potential more negative v Local or Graded Potentials 1 Short lived local changes in membrane potential 2 Magnitude varies stronger stimulus greater change 3 Local changes cause flow of current to adjacent regions set up different potential difference in adjacent regions 4 Local currents die with increasing distance important in initiating action potential Action Potentials how neurons communicate III 1 Brief large depolarization of plasma membrane that requires threshold stimulus i Threshold stimulus causes depolarization of membrane to trigger zone of axon hillock 55 to 50 mv becomes all or none ii 1 Spike Potential and overshoot to 30mv potential will become less and less negative 2 After stimulus depolarization is driven by influx of Na ions i Na ions enter membrane potential less negative voltage gated Na channel opens Na comes in ii Called Hodgkin Cycle iii Spike Potential occurs iv Repolarization RMP Na gates close and K gates open to move toward 3 After hyperpolarization undershoot i Potassium gates open longer so more leaves Na diffusion stops ii Interior becomes more negative than RMP and Na K pump helps restore RMP 4 Latent Period 5 Absolute Refractory time when no stimulus will cause depolarization Na time between stimulus and depolarization gates open guarantees rest interval so neuron is not continuously fired depolarization after hyper polarization stage Na gates closed and K gates open time larger than normal stimulus will cause 6 Relative Refractory IV Propagation conduction of impulse 1 Stimulus creates local currents graded potential that s spread along membrane of dendrites and cell body toward the axon hillock i Motor Neurons Action Potential generated at axon hillock ii Unmyelinated fibers Action Potential takes place over limited area of axon 2 Process i Stimulus ii Inward Na causes region to become less negative eventually positive iii Na ions move toward axon area still Negative iv This sets up a local current that depolarizes next section of axon v Impulses travel in one direction axon refractory behind the leading edge of depolarization 3 New Action Potential is created at each depolarization point different from conduction of electrical current V Saltatory Conduction propagation 1 Occurs in Myelinated axons i Action potential occurs at Nodes of Ranvier ii Channels for Na and K all crowded at bare part of axon nodes iii Ionic current flows through extracellular fluid and axoplasm current jumps from node to node 1 Occurs faster than unmyelinated fibers iv Speed depends of fiber diameter and presence of myelin 1 Type A myelinated fibers 1 20 microns in diameter 5 150 meters per second FASTEST a Motor to skeletal muscles b Sensory fibers for muscle stretch and localized pain 2 Type B finely myelinated up to 3 microns in diameter 3 15 3 Type C unmyelinated 3 5 microns in diameter 0 6 2 meters meters per second a Visceral nerve fibers per second SLOWEST a Poorly localized pain VI Synapse region of functional but not actual contact nerve impulses cross 1 Axondendritic between axon and dendrite 2 Axosomatic between axon and cell body 3 Axoaxonic between axon and axon 2 Electrical synapses relatively few gap junctions bridges between neurons i Multiple neurons synchronize response ii Like pacemakers in single unit smooth muscles iii In brain regions responsible for stereotyped movements 3 Chemical Synapses synaptic neuron in telodendron of axon i Synaptic knob contain synaptic vesicles that hold neurotransmitter and large numbers of mitochondria ii Synaptic cleft fluid filled space 1millionth inch across that does not have direct contact with neurons 1 Neurotransmitter NT diffuses across synaptic cleft iii Postsynaptic neuron or effector cell 1 Dendrites 2 Cell body 3 Effector muscle or gland a Postsynaptic membrane with NT receptors b Protein specialized to react with NT c Different NT may effect postsynaptic neuron 4 Functional types of Synapses i Excitation NT causes partial depolarization membrane potential of postsynaptic membrane is decreased ii Excitatory Postsynaptic Potential EPSP iii Synaptic resistance to firing is reduced facilitation 5 Inhibition i NT causes partial hyperpolarization Sometimes Chloride gates open Cl enters Membrane potentials increases ii Called an Inhibitory Postsynaptic Potential IPSP iii Resistance to firing is increased VII Neurotransmitter over 100 kinds 1 Chemical compounds that diffuse across aynaptic cleft 2 Cause depolarization impulse transmitted or hyperpolarization transmission blocked 3 NTs SYNTHESIZED in cytoplasm of cell bod or in synaptic knob and STORED in synaptic vesicles 4 Different types of NT transmit different information i Acetylcholine Ach 1 Ach NT for neuromuscular junction causes sarcolemma to depolarize for MUSCLE CONTRACTION In ANS synapses and in CNS 2 ii Biogenic Amines or Monoamines 1 Catecholamines norepinephrine NorE Epinephrine Dopamine Indolamines Seratonin and Histamine 2 3 Widely