Transcript Nervous System Structure and Function Pt 1

Nervous System Structure and
Function Pt 1
Nervous System Function
• The nervous system controls and
coordinates functions throughout the
body, and responds to external and
internal stimuli. Irritability
• The basic unit of structure and
function in the nervous system are
specialized cells called neurons.
Neurons
• Neurons are specialized cells that can
transmit electrical signals called
impulses.
• Impulses are the messages carried by
the nervous system.
• Neurons can be classified into three types
–Motor neurons
–Interneurons
–Sensory neurons
Neuron Structure
Axon Terminals
Node
Neuron Structure
• Cell body (soma): The largest part of a
typical neuron which contains the nucleus
and much of the cytoplasm.
• Dendrites: Short branching extensions
from the cell body. Carry impulses from
the environment or from other neurons
toward the cell body.
Neuron Structure
• Axon: Long fiber that carries impulses
away from the cell body.
• Axon Terminals: Small swellings at the
ends of axons.
• Myelin Sheath: Insulating membrane
surrounding the axon of a neuron.
Contains gaps called nodes that speed
up the transmission of impulses.
Types of Neurons
1. Sensory Neurons: Carry impulses from
the sense organs to the spinal cord and
brain.
2. Motor Neurons: Carry impulses from
the brain and spinal cord to the muscles
and glands.
3. Interneurons: Connect sensory and
motor neurons and carry impulses
between them.
Sensory and Motor Neurons
Label each neuron
A
B
C
The Nerve Impulse
• The production of a nerve impulse can be
compared to the flow of electricity through a
wire.
• The transmission of electricity depends on the
movement of negatively charged electrons.
• The production of nerve impulses depends on
the movement of positively charged ions
across the cell membrane.
• The cell membrane is the primary structure
involved in the production of an impulse.
Resting Potential
• The distribution of sodium (Na +) and
Potassium (K+) ions inside and outside
of a neuron is shown in the following
diagram.
• There are more potassium (K +) ions in
the cytoplasm of the neuron than in the
fluid outside of the cell.
• There are more sodium (Na +) ions in
the fluid outside of the cell than inside
the neuron.
Resting Potential
Resting Potential
• Because both sodium and potassium ions
can diffuse across the cell membrane, the
unequal distribution of these ions must be
maintained by active transport.
• Proteins in the cell membrane actively
pump sodium ions out of the neuron and
actively pump potassium ions into the
neuron.
Sodium and Potassium Pumps
Actively Transport Na+ and K+
Across the Cell Membrane
Resting Potential
Sodium Potassium Pump Requires
ATP
Resting Potential
• As a result of active transport (K+ in,
Na+ out) and diffusion (K+ out, Na+ in), a
negative charge builds up on the inside
of the membrane and a positive charge
builds up on the outside of the
membrane.
• The difference in electrical charge across
the cell membrane of a resting neuron is
called is resting potential.
• A neuron has a resting potential of about
-70 millivolts (mV)
The Moving Nerve Impulse
The Moving Nerve Impulse
• In most animals, the axons and dendrites
of neurons are clustered into bundles of
fibers called nerves.
• A nerve impulse is similar to the ripple
caused when a pebble is dropped into a
pond.
• The ripple is caused by the up and down
movement of water. The impulse is
caused by the movement of ions across
the cell membrane.
The Moving Nerve Impulse
• A nerve impulse begins when a neuron is
stimulated by another neuron or its
environment.
• The nerve impulse travels along the axon,
away from the cell body and toward the
axon terminals.
• The cell membrane contains thousands of
protein channels. Generally the sodium
channels are closed.
The Moving Nerve Impulse
• At the leading edge of an impulse, sodium
channels open allowing sodium ions to flow into
the cell.
• This flow of positive ions causes a temporary
change in the charges on the cell membrane.
• The inside of the membrane gains a positive
charge and the outside of the membrane gains
a negative charge.
• This reversal of charges across the membrane
along the length of an axon is called an action
potential.
Action Potentials
• A neuron has an action potential of about
+30 mV.
• As the impulse passes through the axon,
potassium channels open allowing K+
ions to flow out of the cell.
• The resting potential is now reestablished
with the negative charge inside the
membrane and the positive charge
outside the membrane.
Action Potential
• An action potential is caused by positive ions
moving in and then out of the neuron at a
certain spot on the neuron membrane.
• An action potential is initiated by a stimulus
above a certain intensity or threshold.
• Not all stimuli initiate an action potential. The
stimulus could be a pin prick, light, heat, sound
or an electrical disturbance in another part of
the neuron.
• Action potential is an all or nothing
mechanism, just like a mousetrap or stack of
dominoes.
Action Potential: Depolarization
• Depolarization
• A stimulus causes a gate in the Na+
Channel to open. Since there is a
high concentration of Na+ outside,
Na+ diffuses into the neuron. The
electrical potential changes to ~ +40
mV.
Action Potential - Repolarization
• Repolarization
• Depolarization causes the K+
Channel gate to immediately open.
K+ diffuses out of the neuron. This
reestablishes the initial electrical
potential of ~-60 mV.
Action Potential – Refractory Period
• Refractory Period
• During this time (~ 1 msec), the Na+
and K+ Channels cannot be opened
by a stimulus.
• The Na+/K+ Pump actively pumps
Na+ out of the neuron and K+ into the
neuron. This reestablishes the initial
ion distribution of the resting neuron.
Summary of Action Potential
Action Potential
• This single action potential acts as a stimulus to
neighboring proteins and initiates an action
potential in another part of the neuron.
• Ultimately a wave of action potentials travels
from the dendrites all the way to the axon
terminals.
• At the axon terminal, the electrical impulse is
converted to a chemical signal that stimulates a
neighboring neuron. These chemical signals
are called neurotransmitters.
Propagation of Action Potential
Dendrite End
Axon Terminal
End
The Synapse
• Synapse: Small space where a neuron
can transfer an impulse to another cell.
• It is a small gap that separates the axon
terminal from the dendrites of the next
neuron.
• The axon terminals contain tiny sacs filled
with neurotransmitters.
• Neurotransmitters: Chemicals used by a
neuron to transmit an impulse to another
cell.
The Synapse
• When an action potential arrives at an
axon terminal, the sacs release the
neurotransmitters into the synapse.
• The neurotransmitter molecules diffuse
across the synapse from one neuron to
the next stimulating an impulse or action
potential in the neighboring cell.
• Dopamine, seratonin, and acetylcholine
are all neurotransmitters.
The Synapse
The Synapse
Cool sites for animations
http://www.blackwellpublishing.com/matthews/nmj.html
(synapse and neurotransmitters)
http://www.blackwellpublishing.com/matthews/channel.html
(Action potentials)