Download NEUROPHYSIOLOGY

NEUROPHYSIOLOGY
OBJECTIVES:
 Describe resting membrane potential.
 Explain how action potentials are generated
and propagated along nerves.
 Explain how an impulse is transmitted from
one nerve to another.
 Clinical Application: Influence of various
drugs on nerve conduction.
Review of Neuron Structure
ACTION POTENTIAL
(Nerve Impulse)
 When a neuron is adequately stimulated, an
electrical impulse is conducted along the
length of its axon.
 The response is always the same, regardless
of the source or type of stimulus.
 Analogy: Nerves serve as roadways for
electrochemical signals, and myelin as their
pavement.
paved road vs gravel road
REST POTENTIAL:
The Polarized State
 The surface of a cell membrane is usually
electrically charged or polarized with respect
to the inside
 This is due to an unequal distribution of + and
– ions between sides of the membrane
At Rest Potential:
 More sodium is outside the cell than inside
 More potassium is inside the cell than outside
 Simple diffusion: K+ moves out
Na+ moves in
Every millisecond, more + charges leave the
cell by diffusion than enter it. So the outside
gains a slight + charge and the inside a
negative charge
The voltage gated Na+ and K+ channels are
closed
Role of Na+/K+ Pump during Rest
Potential
 At the same time the Na+/K+ pump uses ATP
to transport Na+ and K+ in opposite
directions to maintain this difference
 It ejects 3 Na+ out and only 2 K+ in
 This stabilizes rest potential
Generation of an Action Potential
 Potential Change: Typically an environmental change
(temperature, light, pressure, chemicals, etc..) affects
the resting membrane potential.
 Depolarization occurs and is followed by
repolarization.
1. Na channels open (-55mV)-Na diffuses indepolarizing the membrane (close at +30mV)
2. K channels open and Na channels close (+30mV)K diffuses out- repolarizing the membrane
3. Na+/K+ pump “revs up” to restore rest potential
Anatomy & Physiology Revealed
 Nervous System
 Topic: physiology
 Animation: Action Potential Propagation
 Nervous System
 Topic: physiology
 Animation: Action Potential Generation
Impulse Conduction
Synaptic Potentials
PRESYNAPTIC
NEURON
POST SYNAPTIC
NEURON
Synaptic Transmission
 1. Action potential reaches




the synaptic knob
2. Knob membrane becomes
permeable to Ca++ and it
diffuses inward
3. Synaptic vesicles fuse
with the knob membrane
4. Vesicles release
neurotransmitters into the
synaptic cleft
5. Neurotransmitters bind to
receptors on the
postsynaptic membrane
causing the channels to
open and allow sodium to
leak in-thus setting up the
action potential.
Anatomy Revealed
 Nervous System
 Topic: Physiology
 Animation: Chemical Synapse
After release: Neurotransmitters
 1. are decomposed by enzymes in the
synaptic cleft
or
 2. reuptake by the synaptic knob that
released it (or by nearby neurons)
 This action prevents continuous stimulation of
the postsynaptic neuron.
Effect of Drugs on
Synaptic Transmission
 1. can alter re-uptake
Ex: Cocaine & amphetamines
When present in the brain they block the ability of the
presynaptic neuron to transport the excess dopamine
neurotransmitter from the synaptic cleft. The
dopamine builds up and continues to excite the
postsynaptic neuron. The result leads to feeling of
euphoria
http://science.education.nih.gov/supplements/nih2/addic
tion/activities/lesson3_cocaine.htm
 2. They can increase the amount of
neurotransmitter that is released by binding to
the presynaptic neuron.
Ex: heroin & nicotine excite the dopamine
containing neurons in a particular area of the
brain. They produce more action potentials
 3. Can slow down the release of a
neurotransmitter
Ex. Alcohol
Slows down the release of acetylcholine
which is the neurotransmitter released at the
neuromuscular junction. Speech becomes
slurred, reaction time slows-muscles cannot
fire as quickly.
 4. Some drugs decrease membrane
permeability to sodium
Ex. Local anesthetic drugs (like dentist uses)
It interrupts impulses from passing through
the affected region and reaching the brain,
preventing sensations of touch and pain.
 5. Some drugs can increase the effectiveness
of the sodium potassium pump.
Ex. During epileptic seizures
neurotransmitters become exhausted
Dilantin is a drug that causes more sodium
ions out of neurons to stabilize membrane
thresholds against too rapid stimulation
 6. Some drugs mimic neurotransmitters.
Ex. Morphine mimics the brains endorphins
and enkephalins which are neurotransmitters
that inhibit transmission (pain relievers).