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Neurons Structure and Conduction of a Nerve Impulse.

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1 Neurons Structure and Conduction of a Nerve Impulse

2 Two coordinating systems which respond to environmental stimuli Nervous System & Endocrine (hormone) System Begin with Nervous System (data processing system) 3 interconnected functions  input / integration / output

3 Basic Organization Sensory Input triggered by stimuli –conduction of signals to processing center Integration –interpretation of sensory signals within processing centers Motor output –conduction of signals to effector cells (i.e. muscles, gland cells) sensory receptor (sensory input)  integration  (motor output)  effector

4 Neuron Dendrite - conducts “signal” toward the cell body -- [input zone] –often short, numerous & highly branched –signal comes from sensory cell or neighboring neuron Axon - usually a single fiber -- [conducting zone] –conducts signal away from cell body to another neuron or effector cell Axon Ending –a cluster of branches (100’s to 1000’s) –each with a bulblike synaptic knob –relays signal to next neuron / effector cell

5 Generation - Conduction of Neural Impulses Dependent on concentration gradients of Na + & K + –Na + 14x greater outside –K + 28x greater inside Membrane permeability –lipid bilayer bars passage of K + & Na + ions –protein channels and pumps regulate passage of K + & Na + at rest more K + move out than Na + move in K + ions diffuse out leave behind excess negative charge Sodium-potassium pump –Na + out - K + in (more Na+ out than K + in –contributes to loss of (+)

6 Overview of Neural Impulse

7 Maintenance of negative charge within neuron –resting membrane potential about -70 millivolts –[5% voltage of AA battery] Dissolved organic molecules [negative charge] kept inside Na + - K + balance

8 Stimulus causes opening of Na + gates & closing of K + gates - Threshold [~ +30 mV] –all - or - nothing response Action potential localized electrical event Changes permeability of region immediately ahead –changes in K + & Na + gates –domino effect –propagation of signal Intensity of stimuli (i.e. pinch vs. punch) = number of neurons firing Speed on impulse based on diameter of axon & amount of myelination [wire for internet]

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10 Myelin Sheath Resembles chain of beads Prevents ions from flowing through membranes Na + channels highly concentrated at nodes Allows signal to travel faster because impulse “jumps” from node of Ranvier to node of Ranvier (with myelin sheath (225 mph / without 11 mph) MS  destruction of mylin sheath by own immune system (progressive loss of signal conduction, muscle control & brain function)

11 Neurons Communicate at Synapses Electrical [no synapse] –common in heart & digestive tract - maintains steady, rhythmic contraction –All cells in effector contain receptor proteins for neurotransmitters Chemical - skeletal muscles & CNS –presence of gap (SYNAPTIC CLEFT) which prevents action potential from moving directly to receiving neuron –ACTION POTENTIAL (electrical) converted to CHEMICAL SIGNAL at synapse (molecules of neurotransmitter) then generate ACTION POTENTIAL (electrical) in receiving neuron

12 Overview of Transmission of Nerve Impulse Action potential  synaptic knob  opening of Ca + channels  neurotransmitter vesicles fuse with membrane  release of neurotransmitter into synaptic cleft  binding of neurotransmitter to protein receptor molecules on receiving neuron membrane  opening of ion channels  triggering of new action potential Neurotransmitter is broken down by enzymes & ion channels close -- effect brief and precise

13 Nerve Impulse Presynaptic neuron Vesicles [Calcium channels] Synaptic cleft Postsynaptic neuron Neurotransmitter receptor

14 Nerve Impulse Action potential  synaptic knob  opening of Ca + channels  neurotransmitter vesicles fuse with membrane  release of neurotransmitter into synaptic cleft Ca 2+

15 Nerve Impulse Action potential  neurotransmitter vesicles fuse with membrane  release of neurotransmitter into synaptic cleft

16 Action potential  binding of neurotransmitter to protein receptor molecules on receiving neuron membrane  opening of sodium channels  triggering of new action potential

17 Neurotransmitters Catecholamine Neurotransmitters – Derived from amino acid tyrosine Dopamine [Parkinson’s], norepinephrine, epinephrine Amine Neurotransmitters – acetylcholine, histamine, serotonin Amino Acids – aspartic acid, GABA, glutamic acid, glycine Polypeptides – Include many which also function as hormones – endorphins

18 Transmission of signals based on MULTIPLE STIMULI –combined excitatory & inhibitory neurons Inhibition in Pre-synaptic neuron –Ca + channels blocked stops release of neurotransmitter Inhibition in Post-synaptic neuron –opens Cl- channels makes interior more [-] increase permeability of K + ions –makes interior more [-]


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