Download Neuroscience and Behavior

Neuroscience and Behavior
Most information in this presentation is taken directly from UCCP content,
unless otherwise noted.
Diagram of a Neuron
Diagram taken from: www.csun.edu/~cmm14283/PSY%20150/handouts/NeuronDiagram.doc
Definitions
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Neuron – Highly specialized cell that communicates information
in electrical and chemical form; a nerve cell.
Cell Body – Processes nutrients and provides energy for the
neuron to function; contains the cell’s nucleus; also called the
soma.
Dendrites – Multiple short fibers that extend from the neuron’s
cell body and receive information from other neurons or from
sensory receptor cells.
Axon – The long, fluid-filled tube that carries a neuron’s
messages to other body areas.
Myelin Sheath – A white, fatty covering wrapped around the
axons of some neurons that increases their communication
speed.
Synapse – The point of communication between two neurons.
Synaptic Gap – The tiny space between the axon terminal of
one neuron and the dendrite of an adjoining neuron.
(All definitions in this power point are from the Hockenbury text)
Definitions cont …
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Action Potential – A brief electrical impulse by which
information is transmitted along the axon of a
neuron.
Stimulus Threshold – The minimum level of
stimulation required to activate a particular neuron.
Resting Potential – State in which a neuron is
prepared to activate and communicate its message if
it receives sufficient stimulation.
All-Or-None Law – The principle that either a neuron
is sufficiently stimulated and an action potential
occurs or a neuron is not sufficiently stimulated and
an action potential does not occur.
Neurotransmitters – Chemical messengers
manufactured by a neuron.
Communication Between Neurons
While a nerve impulse is primarily
electrical, communication between
neurons is chemical. When a neuron
fires, chemicals called neurotransmitters
are released from the axon terminal
buttons into the synaptic cleft, the
space between two neurons over which
messages pass. Neurotransmitters are
chemicals that alter activity in neurons.
Communication Between Neurons
After the presynaptic neuron releases the
neurotransmitters into the synaptic cleft, they cross
the cleft and attach themselves to receptor sites on
the postsynaptic neuron, or receiving neuron.
Receptors sites are areas on the cell membrane that are
sensitive to neurotransmitters. Neurotransmitters
and receptor sites work sort of like a lock and key.
Each receptor site (lock) is designed to receive only
one type of neurotransmitter (key).
Once released, not all molecules of neurotransmitters
find their way into receptor sites of other neurons.
Neurotransmitter molecules that do not attach to
receptor sites are either broken down or reabsorbed
by the presynaptic neuron in a process called
reuptake.
Communication Between Neurons
Does the release of a neurotransmittor always trigger
an action potential in the next neuron? Not
necessarily.
Some neurotransmittors act to excite receiving neurons,
causing them to fire: Other neurotransmittors act to
inhibit receiving neurons, preventing them from
firing. If a receiving neuron receives several
“exciting” messages close in time, the neuron will
fire.
However, if the receiving neuron also receives
“inhibiting” messages, it may or may not fire. The
sum of these “exciting” and “inhibiting” messages
determine whether or not the receiving neuron will
fire.
How Neurons Transmit
Information
Neural impulses are messages that travel along
neurons. They travel somewhere between 2 (in nonmyelinated neurons) and 225 miles per hour (in
myelinated neurons). Messages can travel from your
toe to your brain in about 1/50 of a second.
Neurons carry information in one direction only: From
dendrites to cell body to axon to terminal buttons.
Messages are then transmitted from the terminals
buttons to the dendrites or cell body of another
neuron.
What makes a Neuron “fire”?
Neural impulses travel by using an electrochemical process. Chemical
changes take place within a neuron that cause an electric charge to
be transmitted along the neuron. The conduction of the neural
impulse along the length of a neuron is what is meant by “firing.”
Think of a neuron as a tiny biological battery. Ions, electrically
charged chemical molecules, are located in and around nerve cells.
Some of the ions have a positive charge; others have a negative
charge. The ions involved in firing a neuron are Sodium (Na +),
Chloride (Cl-) and Potassium (K+). Different numbers of these ions
exist inside and outside of the neuron. More chloride ions inside the
neuron create an overall negative electrical charge of 70 millivolts
(mv) inside the neuron relative to the outside. This state is called
the resting potential and the neuron is polarized.
What makes a Neuron “fire”?
Neurons, however, rarely get rest. They constantly receive
messages from sensory sources or other neurons that
alter its electrical charge. When these messages
stimulate the neuron, the permeability of the cell
membrane changes allowing Sodium ions (Na+) to enter
the cell. The inside of the cell becomes more positive, or
depolarized.
If the electrical charge inside the cell changes enough, it
reaches a threshold, or trigger point. A neuron’s
threshold is around -50 mv. When the neuron reaches
this threshold, the neuron fires a nerve impulse that
sweeps down the axon and causes the terminal buttons
to release a chemical called neurotransmittors. The firing
of a neuron is called an action potential.
What makes a Neuron “fire”?
An action potential is an all-or-nothing event; the neuron fires
completely or doesn’t fire at all and each time it fires, the impulse is
of the same strength. This is known as the all-or-none principle.
To help illustrate this point, think of a row of dominoes that are set on
end. Once you tip the first domino, all of the other dominos topple
in turn until the last domino falls. When an action potential is
triggered, the nerve impulse travels the complete length of the axon
until it reaches the terminal buttons.
Once the neuron fires, it enters a refractory period, when the neuron
cannot fire. The refractory period can be thought of as the time it
takes to place the dominoes upright again before they are again
“ready” to be toppled.
During the refractory period, potassium ions (K+) flow out of the axon
restoring the neuron’s negative charge and preparing the neuron to
fire again. The sodium ions (Na+) that entered the axon during the
action potential are pumped back out of the axon at a slower rate,
while the Potassiuim ions (K+) that exited are pumped back in. This
results in restoring the original resting potential.