Neurons, Synapses, Neurons, Synapses, and Signalingand SignalingOverviewOverviewNeural SignalsNeural SignalsOrganization of Nervous SystemsOrganization of Nervous Systems

Functions of Nervous Functions of Nervous SystemsSystems

Sensory inputSensory input-conduction of signals -conduction of signals from receptors to integration centers from receptors to integration centers (CNS)(CNS)

IntegrationIntegration-interpretation of signals and -interpretation of signals and generation of proper responsegeneration of proper response

Motor outputMotor output-signals to effector cells -signals to effector cells (muscles, glands), carry out response(muscles, glands), carry out response

Composition of Nervous Composition of Nervous SystemSystem

NeuronsNeurons cells specialized for transmitting electrical and cells specialized for transmitting electrical and

chemical signalschemical signals Composed of cell body, dendrites, and axonsComposed of cell body, dendrites, and axons Three types: sensory neurons, interneurons, Three types: sensory neurons, interneurons,

and motor neuronsand motor neurons Arranged into circuits of two or more kinds of Arranged into circuits of two or more kinds of

neurons: convergent, divergent, and neurons: convergent, divergent, and reverberating circuitsreverberating circuits

Composition of Nervous Composition of Nervous System con’t.System con’t.

Supporting Cells (glial cells)Supporting Cells (glial cells) Structurally reinforce, protect, insulate Structurally reinforce, protect, insulate

and assist neuronsand assist neurons Do not conduct impulsesDo not conduct impulses Outnumber neurons 10- to 50- foldOutnumber neurons 10- to 50- fold Types: astrocytes (blood-brain barrier), Types: astrocytes (blood-brain barrier),

oligodendrocytes, Schwann cells oligodendrocytes, Schwann cells

The Nature of Neural The Nature of Neural SignalsSignals

Signal transmission along a length of a Signal transmission along a length of a neuron depends on voltages created by neuron depends on voltages created by ionic fluxes across neuron plasma ionic fluxes across neuron plasma membranesmembranes

All cells have an electrical membrane All cells have an electrical membrane potential: range from –50 to –100 mV; potential: range from –50 to –100 mV; outside the cell is 0, inside is negatively outside the cell is 0, inside is negatively chargedcharged

Neurons have about –70mV at restNeurons have about –70mV at rest

Ion Concentrations:Ion Concentrations:

[Na+] 10x greater outside (main cation [Na+] 10x greater outside (main cation outside)outside)

[K+] 30x greater inside (principal cation [K+] 30x greater inside (principal cation inside)inside)

[Cl-] 12x greater outside[Cl-] 12x greater outside Other anions-100x insideOther anions-100x insideSelective permeability of the membrane Selective permeability of the membrane

maintains these differencesmaintains these differences

Action Potential-Nerve Action Potential-Nerve ImpulseImpulse

Four phasesFour phases Resting phase-Resting phase-no channels openno channels open Depolarizing phaseDepolarizing phase-brief reversal of polarity, -brief reversal of polarity,

Na+ gates openNa+ gates openNa+ in, result: Na+ in, result: -70mV to +30 mV-70mV to +30 mV Repolarizing phaseRepolarizing phase-Na+ gates close, K+ gates -Na+ gates close, K+ gates

open, K+ out, +30 mV to –70 mVopen, K+ out, +30 mV to –70 mV Undershoot phase (refractory periodUndershoot phase (refractory period)-K+ and )-K+ and

Na+ are returned to proper concentrations, Na+ are returned to proper concentrations, neuron insensitive to depolarizing stimulineuron insensitive to depolarizing stimuli

Action PotentialsAction Potentials

““all-or-none”all-or-none” require a minimum require a minimum stimulus or threshold stimulus or threshold

Self-propagatingSelf-propagating Saltatory conductionSaltatory conduction-action potential -action potential

“jumps” from one node of Ranvier to the “jumps” from one node of Ranvier to the next, skipping the myelinated regions of next, skipping the myelinated regions of the membranethe membrane

SynapsesSynapses

Tiny gap between terminal of an axon Tiny gap between terminal of an axon and beginning of a dendrite on the next and beginning of a dendrite on the next neuronneuron

Presynaptic cell-transmitting cellPresynaptic cell-transmitting cell Postsynaptic cell-receiving cellPostsynaptic cell-receiving cell Two types of synapses: electrical and Two types of synapses: electrical and

chemicalchemical

Electrical SynapsesElectrical Synapses

Electrical synapseElectrical synapse Less commonLess common Travel through gap Travel through gap

junctionsjunctions Example-giant Example-giant

neuron in neuron in crustaceanscrustaceans

Chemical SynapsesChemical Synapses

Separated by synaptic cleftSeparated by synaptic cleft CaCa2+ 2+ enters cell stimulating cytoskeleton to enters cell stimulating cytoskeleton to

move vesicles to the surface of the axonal move vesicles to the surface of the axonal knobknob

Neurotransmitter from the vesicle is released Neurotransmitter from the vesicle is released into the cleftinto the cleft

Neurotransmitter binds to receptor sites on the Neurotransmitter binds to receptor sites on the postsynaptic membrane, reestablishing the postsynaptic membrane, reestablishing the action potentialaction potential

Web sites and VideosWeb sites and Videos

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

http://people.eku.edu/ritchisong/301notes2.htm