LOCAL ANESTHETICS
novocaine
/ lidocaine used to block pain
block opening of voltage
gated Na+ channels
SALTATORY CONDUCTION
Myelinated axons
Myelin sheath acts as an
insulator
Blocks ionic currents across
the membrane
SALTATORY CONDUCTION
Nodes of
Ranvier
Interrupt myelin sheath
High density of voltage-gated
Na+ channels
Membrane depolarization can
occur
Currents
carried by Na+ and K+ can flow across the plasma membrane
Currents carried by ions through extracellular fluid around
myelin sheath
Current flows across membrane only at nodes
Impulse appears to leap from
node to node
Current
travels faster than in continuous conduction in fibers of equal
diameter
SPEED OF IMPULSE
1) diameter of
fibers
2) presence or absence of myelin sheath
3) temperature
ex. Pain reduced by localized
cooling of nerve
FIBER DIAMETER
Largest
diameter called A fibers
All myelinated
Speed 12-130 m/sec (27-280
mph)
Touch, pressure,
propriception, heat, cold, skeletal muscle motor nerves
Exist where quick ractions
are critical
IMPULSE SPEED
large
diameter axons can transmit up to 2500 impulses/sec
small diameter axons can transmit only 250 impulses/sec
normal rate is 10-1000 impulses/sec
NEUROTRANSMITTERS
Neurotransmitters cause either
1)
excitatory potentials
2) inhibitory potentials
EPSP
Excitatory Postsynaptic Potential
Membrane depolarizes
Result from
opening of chemically gated cation channels
Allow Na+, K+, Ca++ to pass
into the neuron
Na+ in flow is
grater than Ca++ inflow or K+ outflow
Electrical and concentration
gradients promote inflow
IPSP
Inhibitory Postsynaptic Potential
Membrane hyperpolarizes
Increases
membrane potential by making inside more negative
Generation of nerve impulse
more difficult
Often result
from opening chemically gated Cl- or K+ channels
Inside becomes more negative
by Cl- inflow or increased K+ outflow
REMOVAL OF
NEUROTRANSMITTER
1) diffusion
2) enzymatic
degradation
ex. acetylcholine
3) cellular
uptake
REMOVAL OF
NEUROTRANSMITTER
active
transport of neurotransmitter back into the neuron that released
them (recycling)
cocaine produces intense
pleasurable euphoria because it blocks transporters for the
reuptake of dopamine
allows dopamine to linger in
synthetic cleft
excessively stimulating
certain brain regions
