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3/27 INFORMATION FLOW AND THE NEURON OR HOW NEURONS
WORK PART II
A. CELLS OF THE NERVOUS SYSTEM: GLIAL CELLS AND NEURONS
B. THE GENERATION OF AN ACTION POTENTIAL
DEPOLARIZE: Become more positive ( less negative)
HYPERPOLARIZE: Become more negative
A typical neuron is at - 70 mv at rest.
Threshold is -55 mv
The peak of the action potential is +35 mv.
1) The membrane of the resting neuron is polarized with the inside negative
relative to the outside.
2)The concentration of Na+ is much higher outside, and the concentration of
K+ is higher inside.
3) Stimulation causes the membrane to undergo a large but short-lived increase
in permeability to Na+ ions, which rush across the membrane into the cell. The
inward flux of Na+ is so great that for a moment the inside actually becomes
positively charged relative to the outside,
4) A fraction of a second later, the permeability of the membrane to Na+ returns
to normal, while its permeability to K+ increases greatly. The K+ ions now rush
out of the cell restoring the charge inside the cell to its original negative
state.
5) The impulse is propagated along the neuron because the cycle of changes at
each point depolarizes the membrane at the adjacent point, and initiates a similar
cycle of permeability changes there: this in turns starts the cycle further
along the axon and so on.
Voltage-gated channels are the key to understanding the generation of action
potentials.
SYNAPSE: Junction between two neurons
There are both electrical and chemical synapses between neurons.
We are going to focus chemical synapses. A neuron that terminates at a synapse
is called presynaptic and the neuron that begins in the synapse is post synaptic.
NEUROMUSCULAR JUNCTION: Junction between neuron and muscle
NEUROGLANDULAR JUNCTION: Junction between neuron and gland
SYNAPTIC CLEFT: 20nM space between pre and post synaptic cells
When the action potential in the presynaptic neuron reaches the synaptic terminal
region it is unable to jump the distance to the post synaptic cell an entirely
different mechanism is needed, CHEMICAL NEUROTRANSMITTERS.
These are diffusable transmitter chemicals released from thousand
of synaptic vesicles in the nerve terminal. Each vesicle contains
up to 10,000
molecules of transmitter.
(see figure 34.8 of your text)
When an action potential reaches the terminal special voltage gated calcium
channels are open. Calcium rushes into the nerve terminal causing synaptic vesicles
to fuse with the synaptic membrane and release their neurotransmitter. The neurotransmitter
diffuses across the synaptic cleft and binds to receptors on the post synaptic
membrane. the receptors are specific for a particular neurotransmitter. The
binding of the transmitter to the receptor opens ligand-gated channels allowing
a specific ion to enter the post synaptic membrane. This movement of ions changes
the post synaptic membrane potential. It becomes an input or receptor potential.
AN EXAMPLE
Acetylcholine is the neurotransmitter found at vertebrate neuromuscular junctions..
Two molecules must bind for a post synaptic receptor to activate a channel.
One vesicle can release 4,000 molecules..
Acetyl cholinesterase destroys the acetylcholine after it binds to a receptor.
Removal of transmitter is important. The system has to be reset. Many pesticides
or organophospates (nerve gas) inhibit cholinesterase, will cause paralysis
and death.
Transmitters open ligand gated channels on the post synaptic membrane. Ions
flow through the open channel and change the post synaptic potential. Post synaptic
potentials are graded. They can be either excitatory EPSP or inhibitory IPSP.
There are at least 60 different substances that act as
neurotransmitters
ACETYLCHOLINE : nerve- muscle; autonomic system and parts of the brain
NOREPINEPHRINE: parts of the brain dealing with emotions dreaming and arousal.
DOPAMINE: parts of the brain dealing with emotions and movement
SEROTONIN: spinal cord and the brain
GABA (gamma-aminobutyric acid): Spinal cord and the brain AN INHIBITORY TRANSMITTER
Synaptic malfunctions have been implicated in a variety
of neurological diseases. Synapses are also the site of action of many mind
altering drugs.
Example:
Diseases:
Depression linked to low levels of seretonin.
Dopamine thought to be involved in schizophrenia and Parkinson's disease .
Drugs:
Amphetamines increases norephinephrine in the brain.
Nicotine mimics the effects of acetylcholine.
Prozac inhibits the reabsorption of serotonin.
LSD acts in the serotonin system.
Valium works synergistically with GABA.
