action potentials - Zanichelli online per la scuola

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Transcript action potentials - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Neurons, Glia, and
Nervous Systems
What Cells Are Unique to the Nervous System?
Nervous systems have two types of cells:
Neurons, or nerve cells, are excitable—they
generate and transmit electrical signals, called
action potentials.
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
•Synapse
What Cells Are Unique to the Nervous System?
Glia, or glial cells, modulate neuron activity and
provide support.
In brain and spinal cord,
glia called
oligodendrocytes wrap
around neuron axons.
Glia called Schwann cells
wrap the axons of other
nerves.
Oligodendrocytes and
Schwann cells produce
myelin that covers axons.
What Cells Are Unique to the Nervous System?
Glia called astrocytes contribute to the blood–brain
barrier, which protects the brain from toxic
substances in the blood.
How Do Neurons Generate and Transmit Electrical Signals?
Membrane potential is the electrical charge difference
across the membrane.
Resting potential is the steady state membrane
potential of a neuron.
Voltage (electric potential difference): force that
causes charged particles to move between two points.
The resting potential of an axon is –60 to –70 millivolts
(mV).
How Do Neurons Generate and Transmit Electrical Signals?
The inside of the cell is negative at rest. A stimulus
that changes the permeability of the membrane allows
ions to move quickly across.
How Do Neurons Generate and Transmit Electrical Signals?
In solutions and across cell membranes, electric
current is carried by ions.
Major ions in neurons: sodium (Na+), potassium (K+),
calcium (Ca2+), chloride (Cl–).
Ion transporters and channels generate membrane
potentials.
The sodium–potassium pump moves Na+ to the
outside and K+ to the inside; requires energy;
establishes concentration gradients.
Ion Transporters and Channels
Sodium-potassium
pumps in all animal
cells create gradients
of Na+ and K+ across
the cell membrane.
Ion channels in the
membrane allow ions
to pass through.
How Do Neurons Generate and Transmit Electrical Signals?
Some ion channels are “gated”:
• Voltage-gated channels respond to change in
voltage across membrane
• Chemically-gated
channels depend on
specific molecules that bind
or alter the channel protein
• Mechanically-gated
channels respond to force
applied to membrane
How Do Neurons Generate and Transmit Electrical Signals?
If Na channels open suddenly, Na+
diffuses in and the inside of the
cell becomes less negative—
plasma membrane is depolarized.
If gated K+ channels open and K+
efflux, the membrane potential
becomes even more negative,
and the plasma membrane is
hyper-polarized.
How Do Neurons Generate and Transmit Electrical Signals?
When the membrane is depolarized about 5 to 10 mV
above resting potential, a threshold is reached.
A large number of sodium channels open and an
action potential is generated.
The Course of an Action Potential
Action Potentials Travel along Axons
How Do Neurons Generate and Transmit Electrical Signals?
The axon returns to resting potential as voltage-gated
Na+ channels close and voltage-gated K+ channels
open.
Voltage-gated Na+ channels cannot open during the
refractory period.
How Do Neurons Generate and Transmit Electrical Signals?
Action potentials travel faster in myelinated than in
non-myelinated axons.
The nodes of Ranvier are
regularly spaced gaps in
the myelin along an axon.
Action potentials are
generated at the nodes and
appear to jump from node
to node, a form of
propagation called
saltatory conduction.
How Do Neurons Generate and Transmit Electrical Signals?
Action potentials can travel over long distances with
no loss of signal.
An action potential is an all-or-none event—positive
feedback to voltage-gated Na+ channels ensures the
maximum action potential.
An action potential is self-regenerating because it
spreads to adjacent membrane regions.
How Do Neurons Communicate with Other Cells?
Neurons communicate with other neurons or target
cells at synapses.
Axons carry information as action potentials away from
the originating cell body (presynaptic cell) to the
receiving target cell (postsynaptic cell).
Chemical synapse: chemicals from a presynaptic cell
induce changes in a postsynaptic cell.
Electrical synapse: the action potential spreads
directly to the postsynaptic cell.
How Do Neurons Communicate with Other Cells?
Neuromuscular junctions are chemical synapses
between motor neurons and skeletal muscle cells.
The neurotransmitter
is acetylcholine
(ACh).
ACh diffuses across
the synaptic cleft to
the motor end plate
on the muscle cell.
How Do Neurons Communicate with Other Cells?
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 by
exocytosis, ACh
diffuses across the
cleft and binds to
receptors on the
motor end plate.
How Do Neurons Communicate with Other Cells?
Synapses between motor neurons and muscle cells
are excitatory. ACh always causes depolarization.
Other synapses can be inhibitory if the postsynaptic
response is hyper-polarization.
Neurons have many dendrites that can form synapses
with axons of other neurons.
The mix of excitatory and
inhibitory activity determines
whether the graded
membrane potential is more
positive or more negative
than resting.
How Do Neurons Communicate with Other Cells?
Excitatory and inhibitory postsynaptic potentials are
summed over space and over time.
Temporal summation
adds up potentials
generated at the same
site in a rapid sequence.
Spatial summation adds up
messages at different synaptic
sites.
How Do Neurons Communicate with Other Cells?
The three main neurotransmitters in the brain are
amino acids:
• Glutamate—excitatory
• Glycine—inhibitory
• γ-aminobutyric acid (GABA)—inhibitory
• monoamines—includes dopamine, norepinephrine,
and serotonin.
How Is the Mammalian Nervous System Organized?
Vertebrate nervous systems:
• Central nervous system (CNS): brain and spinal
cord—the sites of most information processing,
storage, and retrieval
• Peripheral nervous
system (PNS): nerves
that connect the CNS to
all tissues and sensors
of the body.
How Is the Mammalian Nervous System Organized?
The CNS develops from the neural tube of an embryo.
• The anterior part
develops into the brain
• The rest becomes the
spinal cord
How Is the Mammalian Nervous System Organized?
Gray matter is rich in neural cell bodies; white matter
contains myelinated axons.
Afferent (sensory) axons in a spinal nerve enter the
spinal cord through the dorsal root; efferent (motor)
axons leave through the ventral root.
An anatomically distinct group of CNS neurons is a
nucleus.
Brainstem nuclei are involved in keeping higher brain
areas awake or allowing them to sleep.
How Is the Mammalian Nervous System Organized?
The diencephalon consists of the thalamus and
hypothalamus.
The thalamus communicates sensory information to
the cerebral cortex, the hypothalamus regulates many
homeostatic functions.
The telencephalon (cerebrum) consists of left and
right cerebral hemispheres.
The cerebral cortex (outer layer) has a large surface
area; it is folded and these foldings, or convolutions,
allow the large surface of the cortex to fit in the skull.
How Is the Mammalian Nervous System Organized?
The limbic system is responsible for instincts, longterm memory formation, physiological drives such as
hunger and thirst, and emotions such as fear.
• Amygdala—involved in
fear and fear memory
• Hippocampus—transfers
short-term memory to longterm memory
How Is the Mammalian Nervous System Organized?
The cerebral cortex is divided into lobes:
• Temporal
• Frontal
• Parietal
• Occipital
How Is the Mammalian Nervous System Organized?
Different regions of the cerebral cortex have specific
functions.
Association cortex: many areas that integrate or
associate sensory information or memories; higherorder information processing.
The left hemisphere of the brain controls the right side
of the body; the right hemisphere controls the left side,
except in the head.
The two hemispheres are not symmetrical with respect
to all functions, e.g., language abilities reside in the left
hemisphere.
How Is the Mammalian Nervous System Organized?
Temporal lobe:
• Receives and processes auditory information
• Association areas involve identifying and naming
objects
Agnosias—inability to identify objects
Damage in certain areas cause inability to recognize
faces.
How Is the Mammalian Nervous System Organized?
Frontal Lobe:
• Central sulcus—divides frontal
and parietal lobes
• Primary motor cortex is located
in front of the central sulcus;
controls muscles in specific body
areas
Parietal lobe: primary
somatosensory motor
cortex—just behind the central
sulcus.
Receives touch and pressure
information from the thalamus.
How Is the Mammalian Nervous System Organized?
The midbrain, medulla, and pons are known as the
brainstem.
All information traveling between the spinal cord and
higher brain areas must pass through the brainstem.
Medulla and pons control physiological functions,
such as breathing.
Cerebellum coordinates muscle activity and
maintaining balance.
How Is the Mammalian Nervous System Organized?
The spinal cord:
• Conducts information to and from the brain
• Integrates information coming from the PNS and
issues motor commands (e.g., knee-jerk reflex)
• Complex motor programs also exist in the spinal
cords of many vertebrates
How Is the Mammalian Nervous System Organized?
Nerve: a bundle of axons that carries information.
Some axons in a nerve may be carrying information to
the CNS (Afferent), while others in the same nerve
are carrying information from the CNS to the body’s
organs and glands (Efferent).
The brainstem regulates many autonomic functions
(involuntary physiological functions).
It has 12 paired cranial nerves, including the
olfactory, optic, and auditory nerves.
How Is Information Processed by Neural Networks?
Spinal reflex:
conversion of
afferent to efferent
information in the
spinal cord without
participation of the
brain.
It is a monosynaptic
reflex—only one
synapse between
the afferent and
efferent neurons.
How Is Information Processed by Neural Networks?
Autonomic Nervous
System (ANS)—
output pathways of
the CNS that control
involuntary functions.
Two divisions work in
opposition:
sympathetic and
parasympathetic.
One causes increase
in an activity, the
other decreases it.
What Are the Major Diseases?
Some diseases affect myelin and impair conduction of
action potentials.
Multiple sclerosis is an autoimmune disease;
antibodies to proteins in myelin in the brain and spinal
cord are produced.
How Are Neurons and Glia Organized into Information-Processing
Systems?
Nervous systems vary in complexity.
Cnidarians have simple networks of neurons called nerve nets.
There is little or no integration or processing of signals.
More complex animals must process and integrate larger
amounts of information. Neurons are organized into clusters
called ganglia.
In bilaterally symmetrical animals, the ganglia may be
enlarged at the anterior end to form a brain.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved