Neurons Structure and Conduction of a Nerve Impulse

Neurons Structure and Conduction of a Nerve Impulse

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

Basic Organization sensory receptor (sensory input) integration (motor output) effector • 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)

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

Generation - Conduction of Neural Impulses • Dependent on concentration gradients of Na+ & K+ – Na+ 14 x greater outside – K+ 28 x 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 (+)

Overview of Neural Impulse

• 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

• Stimulus causes opening of Na+ gates & closing of K+ gates • Threshold [~ +30 m. V] – 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]

• 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) Myelin Sheath

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

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

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

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

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

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

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

• 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 [-]