Download NEURONS, SENSE ORGANS, AND NERVOUS SYSTEMS

NEURONS, SENSE
ORGANS, AND NERVOUS
SYSTEMS
CHAPTER 34
KEY CONCEPTS
• 34.1 Nervous Systems Are Composed of
Neurons and Glial Cells
• 34.2 Neurons Generate Electric Signals by
Controlling Ion Distributions
• 34.3 Neurons Communicate with Other
Cells at Synapses
• 34.4 Sensory Processes Provide Information
on an Animal’s External Environment and
Internal Status
• 34.5 Neurons Are Organized into Nervous
Systems
Quick intro video
NERVOUS SYSTEMS ARE COMPOSED
OF NEURONS AND GLIAL CELLS
• Animals need a way to transmit
signals at high speeds from place to
place within their bodies (e.g., to
avoid danger).
• Mammalian neurons transmit
signals at 20–100 meters per
second, similar to a banjo where
each finger pluck is like a nerve
impulse in brain and sent down the
spinal cord.
NERVOUS SYSTEMS ARE COMPOSED
OF NEURONS AND GLIAL CELLS
• Neurons are excitable cells, which means its cell membranes can generate and conduct signals
called impulses
• This is a key specialization, only muscles are neurons are ‘excitable’ cells in the body
– they can generate and transmit electrical signals, called action potentials.
• Cell membranes ordinarily have electrical polarity: the outside is more positive than the inside.
• An impulse, or action potential, is a state of reversed polarity.
NERVOUS SYSTEMS ARE COMPOSED
OF NEURONS AND GLIAL CELLS
• In an excitable cell, an action potential generated at one point propagates over the whole
membrane.
• The region of depolarization moves along the cell membrane, and the membrane is said to
“conduct” the impulse.
An impulse or action potential is a state in which the polarity
across the cell membrane is reversed.
The impulse or action
potential moves along the cell
membrane, in a process called
conduction or propagation
NEURONS ARE SPECIALIZED TO
PRODUCE ELECTRIC SIGNALS
• Neurons (nerve cells) are specially
adapted to generate electric signals, usually
in the form of action potentials.
• Neurons are like ‘land lines’ and they must
make contact with target cells and are very
diverse in structure. (fast and addressed)
NEURONS ARE SPECIALIZED TO
PRODUCE ELECTRIC SIGNALS
• Synapse: cell-to-cell
contact point specialized
for signal transmission
• A signal arrives at the
synapse by way of the
presynaptic cell and
leaves by way of the
postsynaptic cell.
• The postsynaptic cell can
be another neuron or it
can be a sensory cell or as
discussed in chapter 33, it
can be a muscle cell.
FOUR ANATOMICAL REGIONS
• Most neurons have four regions; dendrites,
cell body, an axon, and a set of presynaptic
axon terminals
• Dendrites (‘tree’)—carry signals to the
cell body (incoming)
• They are short processes, or extensions that
tend to branch from the cell body like twigs
on a shrub
• Cell body—contains nucleus and organelles
• The main job is to combine and integrate
the incoming signals quickly
FOUR ANATOMICAL REGIONS
• Axon—conducts action potentials away from the cell
body; can be very long
• For example, an axon can run from your spinal cord all the
way to your finger to coordinate a movement.
• Action potentials are generated at the cell body and move
out the axon
• Presynaptic axon terminals—make contact with
other cells
• These terminals make contact with other cells, neurons
sensory cells or muscles.
Dendrites are the major site
for synaptic input from other
neurons
The cell body and, especially, the axon
hillock – where the cell body
transitions to the axon – are often
major sites of integration of signals
The axon is the conduction
component of the neuron
propagating action potentials
over a long distance to the
axon terminal
At the presynaptic axon
terminals. The output of
the neuron can alter the
activities of other cells
CONCEPT 34.1 NERVOUS SYSTEMS ARE
COMPOSED OF NEURONS AND GLIAL
CELLS
• Presynaptic Axon Terminals
– A neuron is said to innervate the cells that the
axon terminals contact.
– Axons of many neurons often travel together in
bundles called nerves.
– Nerve refers only to axon bundles outside the brain
and spinal cord.
– In the brain or spinal cord they are called tracts.
GLIAL CELLS
• Glial cells support, nourish, and insulate neurons
• Glial cells, or glia or neuroglia is the second major type of
cell in the nervous system
– 50% of the mammalian brain are glial cells
– There are several distinct types of glial cells that have distinct
roles.
• They are not excitable or produce action potentials
however, they have several functions:
• Help orient neurons toward their target cells during
embryonic development
• Provide metabolic support for neurons
• Help regulate composition of extracellular fluids and perform
immune functions
• Assist signal transmission across synapses
GLIAL CELLS
• In vertebrates typically not in invertebrates,
certain glial cells are insulated by a lipid-rich
cell membrane, similar to the insulators of a
power cord.
• In brain and spinal cord, this glial wrap
around the axons is called
Oligodendrocytes
• Schwann cells insulate axons in nerves
outside of these areas.
• The glial membranes form a electrically
nonconductive sheath called myelin.
• Myelin-coated axons are white matter and
areas of cell bodies are gray matter.
Multiple layers of Schwann
cell membrane insulate the
axon
GLIAL CELLS
N E U R O N S G E N E R AT E
ELECTRIC SIGNALS BY
CONTROLLING ION
DISTRIBUTION
34.2
INTRODUCTION
• 1 min video
• One feature common to all nervous systems
is that they encode and transmit information
in the form of action potentials.
• Some basic electrical principles help us
understand how neurons produce action
potentials and other electrical signals.
ACTION POTENTIALS - TERMINOLOGY
• Current: flow of electric charges from place to place; in cells, current is based on flow of ions
such as Na+
• Voltage, or electrical potential difference exists if positive charges are concentrated in one
place and negative charges are concentrated in a different place.
• Voltages produce currents because opposite charges attract and will move toward one another
if given a chance.
ACTION POTENTIALS
• No voltage differences exist within open solutions such as the intracellular fluids.
• Voltage differences exist only across membranes such as the cell membrane.
Within a few nanometers of the
membrane on either side, net positive
and negative charge concentrations may
accumulate
Farther away, in the bulk solution
on either side, the net charge is
zero
ACTION POTENTIALS
• The voltage across a membrane is called membrane potential and is easily measured.
• Resting neuron: membrane potential is the resting potential, typically –60 to –70 millivolts
(mV)
– Negative sign means the inside of the cell is electrically negative relative to the outside.
2…and connected with a
wire to an amplifier
3. The voltage difference
between the electrode
placed inside the axon
and a reference
electrode outside the
axon is detected…
4… and this small potential
difference is displayed on a
computer screen.
1. An electrode
made from a glass
tube (pulled to a
sharp tip and open
at the end) is filled
with electrically
conducting
solution….
5, In an unstimulated neuron, a
potential difference is about -65mV
is observed between outside and
inside. This is the resting potential
ACTION POTENTIALS
• Membrane potential can change rapidly, and only relatively small numbers of positive charges
need to move through the membrane for this change of membrane potential to occur.
• Composition of the bulk solutions (the intra- and extracellular fluids) does not change.
• Animated tutorial 34.1 – good textbook link
SODIUM-POTASSIUM PUMP SETS UP
CONCENTRATION GRADIENTS
• Ion redistribution occurs through
membrane channel proteins and ion
transporters in the membrane.
• Sodium–potassium pump—uses
energy from ATP to move 3 Na+ ions to
the outside and 2 K+ to the inside;
establishes concentration gradients of
these ions
SODIUM-POTASSIUM PUMP SETS UP
CONCENTRATION GRADIENTS
• Potassium channels are open in
the resting membrane.
• K+ ions diffuse out of the cell
through leak channels and
leave behind negative charges
within the cell.
• K+ ions diffuse back into the
cell because of the negative
electrical potential.
• At this equilibrium point, there
is no net movement of K+;
called the equilibrium
potential of K+.
SODIUM-POTASSIUM PUMP SETS UP
CONCENTRATION GRADIENTS
• Diffusion of ions is controlled by concentration effect and electrical effect. When they
are equal, electrochemical equilibrium is reached.
Crash Course –
Nervous System 2
Video on Action
Potentials and Sodium
Potassium Pump
NO!!!!
• Electrochemical equilibrium is called the equilibrium potential of the ion, calculated by the
Nernst equation:
Eion
ion outside
RT

 2.3
log
zF
 ion inside
GATED ION CHANNELS CAN ALTER
MEMBRANE POTENTIAL
• Most ion channels are “gated”—they open and close under certain conditions. Most are
closed in a resting neuron, which is why K+ leak channels determine resting membrane
potential.
• Voltage-gated channels open or close in response to changes in membrane potential
• Stretch-gated channels respond to tension applied to cell membrane
• Ligand-gated channels open or close when a specific chemical (ligand) binds to the channel
protein.
GATED ION CHANNELS CAN ALTER
MEMBRANE POTENTIAL
• Opening and closing gated channels can alter membrane potential.
• If Na+ channels open, Na+ diffuses into the neuron because it is more concentrated outside the
cell, and the cell membrane is more negative on the inside.
• When membrane becomes less negative on the inside, the membrane is depolarized.
• The membrane is hyperpolarized if the charge on the inside becomes more negative.
• Nerve Impulse explained
• Depolarization and hyperpolarization explained
CHANGES IN MEMBRANE POTENTIAL
CAN BE GRADED OR ALL-OR-NONE
• Two types of membrane potentials can occur: graded or all-or-none.
• Graded membrane potentials are changes from the resting potential that are less than the
threshold of –50 mV.
– Graded means any value of the membrane potential is possible
– Caused by various ion channels opening or closing
CHANGES IN MEMBRANE POTENTIAL
CAN BE GRADED OR ALL-OR-NONE
• Graded membrane potentials spread only a short distance.
CHANGES IN MEMBRANE POTENTIAL
CAN BE GRADED OR ALL-OR-NONE
• If neuron depolarizes to the –50
mV threshold, an all-or-none
event occurs: an action
potential is generated.
• Action potentials are not graded
(always the same size) and do not
become smaller, they stay the
same in size as they propagate
along the cell membrane.
CHANGES IN MEMBRANE POTENTIAL
CAN BE GRADED OR ALL-OR-NONE
• Graded changes can give rise to all-or-none changes by being summed together; provides a
mechanism for integrating signals.
• A key area for this integration is the axon hillock, where action potentials are most often
generated.
• Graded changes resulting from multiple signals reaching the dendrites, spread to the axon
hillock, where all the depolarizations and hyperpolarizations sum.
CONCEPT 34.2 NEURONS GENERATE
ELECTRIC SIGNALS BY CONTROLLING ION
DISTRIBUTIONS
• An action potential (nerve impulse) is a rapid,
large change in membrane potential that reverses
membrane polarity.
• The membrane depolarizes from –65 mV at rest
to about +40 mV (depolarization).
• It is localized and brief but is propagated with no
loss of size—an action potential at one location
causes currents to flow that depolarize
neighboring regions.
CONCEPT 34.2 NEURONS GENERATE
ELECTRIC SIGNALS BY CONTROLLING ION
DISTRIBUTIONS
• When membrane potential reaches threshold, many voltage-gated Na+ channels open quickly,
and Na+ rushes into the axon.
• The influx of positive ions causes more depolarization, and an action potential occurs.
PRODUCTION OF AN ACTION
POTENTIAL (PART 1)
CONCEPT 34.2 NEURONS GENERATE
ELECTRIC SIGNALS BY CONTROLLING ION
DISTRIBUTIONS
• The axon quickly returns to resting potential:
• Voltage-gated Na+ channels close
• Voltage-gated K+ channels open slowly and stay open longer—K+ moves out
FIGURE 34.7 PRODUCTION OF AN
ACTION POTENTIAL (PART 2)
PRODUCTION OF AN ACTION
POTENTIAL (PART 3)
POSITIVE FEEDBACK
• Positive feedback during depolarization:
– When the membrane is partially
depolarized, some Na+ channels open; as
Na+ starts to diffuse into the cell, more
depolarization occurs, opening more
channels.
– This continues until all voltage-gated Na+
channels open and maximum
depolarization occurs.
ACTION POTENTIALS
• Action potential travels in
only one direction:
– After the action potential,
Na+ channels cannot open
again for a brief period
(refractory period) and
cannot depolarize.
– Thus the action potential
can only propagate in the
direction of the axon
terminals.
ACTION POTENTIALS
• Action potentials travel faster in
larger diameter axons.
• Myelination by glial cells also
increases speed of action potentials.
• The nodes of Ranvier are gaps
where the axon is not covered by
myelin.
• Action potentials are generated only
at the nodes and jump from node to
node (saltatory conduction).