Transcript Neurotransmitters

Neurons and their
Actions
Get ready to sense,
receive, and react.
Things that make you go hmmm…
Average number of neurons in the
human brain= 100 billion
Average number of neurons in an
octopus brain= 300 billion
Rate of neuron growth during
development of a fetus (in the
womb)= 250,000 neurons/minute
Longest axon of a neuron= around 15
feet (Giraffe primary afferent axon
from toe to neck)
Velocity of a signal transmitted
through a neuron= 1.2 to 250
miles/hour
Neuron
A nerve cell; consists of many different parts.
Parts of a Neuron
Cell Body: Life support center of the neuron.
Dendrites: Branching extensions at the cell body.
Receive messages from other neurons.
Axon: Long single extension of a neuron, covered with
myelin [MY-uh-lin] sheath to insulate and speed up
messages through neurons.
Terminal Branches of axon: Branched endings of an
axon that transmit messages to other neurons.
Synapse
Synapse [SIN-aps] a junction between the axon
tip of the sending neuron and the dendrite or
cell body of the receiving neuron. This tiny gap
is called the synaptic gap or cleft.
Types of Neurons
Most communicate within the central
nervous system (CNS - brain & spinal
cord) with rare exceptions.
There are three major types of neurons
upon which information travels. The
information travels from the Sensory
Neurons to the Interneurons, and then
finally to the Motor Neurons.
Sensory Neurons
bring information from sensory receptors to the
central nervous system. Brings information
from the eyes, ears, etc., as well as from
within the body like the stomach.
Interneurons
neurons in the brain
and spinal cord that
serve as an
intermediary
between sensory
and motor neurons.
They carry info
around the brain for
processing.
Motor Neurons
carry the information from the CNS to
the appropriate muscles to carry out
behaviors.
Follow the Neuron Path
For example, if you hold your hand over a hot flame,
the information about "heat" travels from your hand on
the sensory neurons, to the Interneurons where it is
brought to the appropriate brain region to process the
information (now you know it is "hot") and make a
decision about a corresponding action (too hot, let's
move the hand). The information then travels on the
Motor Neurons from the brain to the hand so that your
muscles move the hand from the hot flame. See how
easy that is?
Neural Impulse (A Neuron Firing)
Defined as: the electrical and chemical
transmission of information from one neuron
to another.
It takes the same path all the time - it is a
process of conducting information from a
stimulus by the dendrite of one neuron and
carrying it through the axon and on to the
next neuron. Let's look at all the elements
that are involved in the neural impulse:
Change of the Neuron
inside the neuron, the ions are mostly negatively
charged. Outside the neuron, the ions are mostly
positively charged. In this state (with mostly negative
charge inside and positive charge on the outside) the
neuron is said to be Polarized
When a neuron fires (neuron impulse), that all
changes… Just watch!
Ions
Definition: [n] a particle that is
electrically charged (positive or negative);
an atom or molecule or group that has lost
or gained one or more electrons
We have positively (+) and negatively (-)
charged particles called ions.
Neural Impulse =
Sodium (Na+)
Potassium (K-)
Selectively Permeable Membrane
The outer membrane of the neuron is not
impermeable, but instead selectively
allows some ions to pass back and forth.
The way it selects is easy - it has pores
that are only so big. So, only very small
ions can fit through. Any large ions simply
can't pass through the small pores.
Lights, Stimulus, Action
The following slides
go through the stages
of a neuron firing, or a
neural impulse, with
other neurons or
muscle fibers.
You will want to write
down these steps as
a chain of events that
goes in a circular
motion.
Resting Potential
while the neuron is Polarized, it is in a
stable, negatively charged, inactive state
The charge is approx. -70 millivolts, and it
means that the neuron is ready to fire
(receive and send information).
Stimulus
eventually, some
stimulation occurs
(ex. hand to close to a
flame), and the
information is brought
into the body by a
sensory receptor and
brought to the
dendrites of a neuron.
Threshold
a dividing line that determines
if a stimulus is strong enough
to warrant action. If the
threshold is reached, an
action potential will occur.
For example, how hot does a
stove have to be for a person
to pull away. When you have
reached that level of
uncomfortable temperature,
then you have reached the
threshold.
All or None Law
all-or-none law - a
neural impulse will either
occur or not. There is no
in between. Once the
threshold is reached,
there is no going back,
the neural impulse will
begin and will go through
the complete cycle.
Action Potential
Once the stimulus reaches
a certain threshold the
neural membrane opens at
one area and allows the
positively charged ions to
rush in and the negative
ions to rush out. The
charge inside the neuron
then rises to approx. +40
mv. This only occurs for a
brief moment, but it is
enough to create a domino
effect.
Absolute Refractory
Period
after the action potential occurs, there is a brief
period during which the neuron is unable to have
another action potential. Then the charge inside
the neuron drops to about -90 mv (refractory
period) before restoring itself to normal.
Analogy: the absolute refractory period is like a
gun. After you shoot it, you must reload it before
you can shoot again. The charge dropping in
the neuron is its way of reloading.
Repolarization
the neuron tries to quickly restore
it's charge by pumping out the
positively charged ions and bringing
back the negative ones.
occurs fast enough to allow up to
1,000 action potentials per second.
The Firing of a Neuron
Resting
(Beginning & End)
Repolarization
(Neuron becomes
Negatively charged again.)
Stimulus
Threshold
(Neuron Fires =
Action Potential,
Neuron becomes
positively charged)
Absolute Refractory Period
(Neuron in unable to fire.)
Speed of an Action Potential
Neural impulses can travel from 10120
meters/sec, or 2-270 miles/hour.
The speed of light is 186,000 (1.86 X 105) miles
per second.
The speed of sound, at sea-level, is 968 ft/sec.
The speed of a garden snail is 0.03 mph.
What should you learn from this? Neurons are
slower than the energy neurons use, and faster
than anything that uses neurons to move.
Neuron Activity (10 minutes)
You have 8 minutes to organize yourselves as a
neuron.
You must all have a role as a part of the neuron
(more than one person can make one part of a
neuron).
After 8 minutes of preparation you must show
me through your movements a neuron firing.
Before you fire you must identify which part of
the neuron you represent.