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+ 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]
• 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
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 [-]