Download Neurons and Nervous Tissue

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
C – Il corpo umano
Neurons and
Nervous Tissue
Neurons and Nervous Tissue
• What cells are unique to the nervous
system?
• How do neurons generate and
conduct signals?
• How do neurons communicate with
other cells?
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Nervous systems have two categories
of cells:
• neurons generate and propagate
electrical signals, called action
potentials;
• glial cells provide support and
maintain extracellular environment.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Neurons are organized into networks.
Afferent neurons carry information into the
system.
Sensory neurons convert input into action
potentials.
Efferent neurons carry commands to
effectors.
Interneurons store information and help
with communication in the system.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Networks vary in complexity.
Nerve net: simple network of neurons.
Ganglia: neurons organized into
clusters, sometimes in pairs.
Brain: the largest pair of ganglia.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Central nervous
system (CNS) –
consists of cells found
in brain and spinal cord.
Peripheral nervous
system (PNS) –
neurons and support
cells found outside the
CNS.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Neurons pass information at
synapses:
1.the presynaptic neuron sends the
message;
2.the postsynaptic neuron receives
the message.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Most neurons have four regions:
• cell body contains the nucleus and
organelles;
• dendrites bring information to the
cell body;
• axon carries information away from
the cell body;
• axon terminal forms synapse at tip
of axon.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Neurons
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Glial cells, or glia, outnumber
neurons in the human brain.
Glia do not transmit electrical signals
but have several functions:
• support during development;
• supply nutrients;
• maintain extracellular environment;
• insulate axons.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Oligodendrocytes produce myelin
and insulate axons in the CNS.
Schwann cells insulate axons in the
PNS.
Astrocytes contribute to the blood–
brain barrier, which protects the
brain.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Wrapping up an axon
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Oligodendrocytes produce myelin
and insulate axons in the CNS.
Schwann cells insulate axons in the
PNS.
Astrocytes contribute to the blood–
brain barrier, which protects the
brain.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials are the result of ions
moving across the plasma membrane.
Ions move according to differences in
concentration gradients and electrical
charge.
Membrane potential is the electric potential
across the membrane.
Resting potential is the membrane
potential of a resting neuron.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage causes electric current as ions
to move across cell membranes.
Major ions in neurons:
• Sodium (Na+)
• Potassium (K+)
• Calcium (Ca2+)
• Chloride (Cl–)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membrane potentials are measured
with electrodes.
The resting potential of an axon is
–60 to –70 millivolts (mV).
The inside of the cell is negative at
rest. An action potential allows
positive ions to flow in briefly, making
the inside of the cell more positive.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Measuring the resting potential
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The plasma membrane contains ion
channels and ion pumps that create
the resting and action potentials.
The sodium–potassium pump uses
ATP to move Na+ ions from inside the
cell and exchanges them for K+ from
outside the cell.
This establishes concentration
gradients for Na+ and K+.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion pumps and channels (part 1)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion pumps and channels (part 2)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion channels in the membrane are
selective and allow some ions to
pass more easily.
The direction and size of the
movement of ions depends on the
concentration gradient and the
voltage difference of the membrane.
These two forces acting on an ion are
its electrochemical gradient.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Potassium channels are open in the
resting membrane and are highly
permeable to K+ ions.
K+ ions diffuse out of the cell along the
concentration gradient and leave
behind negative charges within the
cell.
K+ ions diffuse back into the cell
because of the negative electrical
potential.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 1)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 2)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 3)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 4)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials are sudden, large
changes in membrane potential.
Voltage-gated Na+ and K+ channels
are responsible for action potentials.
If a cell body is depolarized, voltagegated Na+ channels open and Na+
rushes into the axon. The influx of
positive ions causes more
depolarization.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
A threshold is reached at 5–10 mV above
resting potential.The influx of Na+ is not
offset by the outward movement of K+.
Many voltage-gated Na+ channels then
open, the membrane potential becomes
positive, and an action potential occurs.
The axon returns to resting potential as
voltage-gated Na+ channels close and
voltage-gated K+ channels open.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 1)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 2)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 3)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage-gated Na+ channels have a
refractory period during which they
cannot open.
Na+ channels have two gates:
• an activation gate is closed at rest but
opens quickly at threshold;
• an inactivation gate is open at rest and
closes at threshold but responds more
slowly, the gate reopens 1–2 milliseconds
later than the activation gate closes.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage-gated K+ channels contribute
to the refractory period by remaining
open.
The efflux of K+ ions makes the
membrane potential less negative
than the resting potential for a brief
period.
The dip after an action potential is
called hyperpolarization.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
An action potential is an all-or-none
event because voltage-gated Na+
channels have a positive feedback
mechanism that ensures the
maximum value of the action
potential.
An action potential is selfregenerating because it spreads to
adjacent membrane regions.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials travel along axons (part 1)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials travel along axons (part 2)
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Myelination by glial cells increases the
conduction velocity of axons.
The nodes of Ranvier are regularly
spaced gaps where the axon is not
covered by myelin.
Action potentials are generated at the
nodes and the positive current flows
down the inside of the axon.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
When the positive current reaches the
next node, the membrane is
depolarized and another axon
potential is generated.
Action potentials appear to jump from
node to node, a form of propagation
called saltatory conduction.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Saltatory action potentials
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Neurons communicate with other
neurons or target cells at synapses.
In a chemical synapse chemicals
from a presynaptic cell induce
changes in a postsynaptic cell.
In an electrical synapse the action
potential spreads directly to the
postsynaptic cell.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The neuromuscular junction is a
chemical synapse between motor
neurons and skeletal muscle cells.
The motor neuron releases
acetylcholine (ACh) from its axon
terminals.
The postsynaptic membrane of the
muscle cell is the motor end plate.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The synaptic cleft is the space
between the presynaptic and
postsynaptic membranes.
An action potential causes release of
the neurotransmitter ACh when
voltage-gated Ca2+ channels open
and Ca2+ enters the axon terminal.
Vesicles release ACh into the synaptic
cleft.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Chemical synaptic transmission begins with the arrival
of an action potential
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The postsynaptic membrane responds
to ACh.
ACh diffuses across the cleft and
binds to ACh receptors on the motor
end plate.
These receptors allow Na+ and K+ to
flow through and the increase in Na+
depolarizes the membrane.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The acetylcholine receptor is a chemically gated channel
Neurons and Nervous Tissue - How do neurons communicate with other cells?
The synapses between motor neurons
and muscle cells are excitatory. ACh
always causes depolarization.
Other synapses can be inhibitory, if
the postsynaptic response is
hyperpolarization.
A neuron has many synapses and may
receive many different chemical
messages.
Neurons and Nervous Tissue - How do neurons communicate with other cells?
The postsynaptic cell must sum the
excitatory and inhibitory input.
Summation takes place at the part of
the cell body at the base of the axon.
Spatial summation adds up
messages at different synaptic sites.
Temporal summation adds up
potentials generated at the same site,
over time.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The postsynaptic neuron sums information