Neurons Structure and Conduction of a Nerve Impulse.

NeuronsStructure 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 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

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+ 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 (+)

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 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]

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)

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 channelstriggering 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

Ca2+

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 [-]