action_potential.html: 48_13ActionPotGenerat.jpg
action_potential1.html: 48_13ActionPotGenerat1.jpg
Resting state
The Na+ and K+ channels
are closed, and the membrane’s resting potential is maintained.
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Depolarizarion
A stimulus opens some Na+ channels.
Na+ influx depolarizes the membrane.
If the depolarizarion reaches the threshold, it triggers an action potential.
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Rising phase of the action potential
Depolarization opens most Na+ channels,
while the K+ channels remain closed.
Na+ influx makes the inside of the membrane positive with respect to the outside.
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Falling phase of the action potential
Most Na+ channels close and K+ channels open, permitting K+ efflux
which again makes the inside of the cell negative.
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Undershoot
Some K+ ions continue to diffuse through open channels, resulting in undershoot
before the membrane returns to its resting potential.
autonomic.html: 48_22ANSorganization.jpg
axon.html: 48_14ActionPotPropagation.jpg
Conduction of an action potential.
An action potential is generated as Na+ flows in at one location.
The depolarization spreads to the neighboring region of the membrane, initiating
an action potential there.
The original region repolarizes as K+
flows out.
The depolarization-repolarization process is
repeated as the action potential is propagated down the axon.
brain.html: 48_23BrainDevelopment.jpg
cerebral.html: 48_27HumanCerebralCortex.jpg
The human cerebral cortex.
On each lobe, primary sensory areas receive a specific type of sensory information,
association areas integrate the information from various parts of the brain and make complex behavior
and learning possible.
cerebrum.html: 48_26HumanCerebrum.jpg
The human cerebrum.
The largest part of the brain is the cerebral cortex,
with a convoluted surface called the neocortex.
This is where sensory information is analyzed, motor commands are issued, and language is generated.
The left hemisphere is more adept at language, math, and logical operations; while
the right hemisphere is stronger at pattern recognition, nonverbal thinking, and emotional processing.
cranial_nerves.html: Cranial_nerves.jpg
depolarization.html: 48_12GradedActionPotent.jpg
| Graded potentials and an action potential in a neuron. | ||||
|---|---|---|---|---|
| Hyperpolarizations produced by 2 stimuli that increase membrane permeability to K+. The larger stimulus produces a larger hyperpolarization. | Depolarizations produced by 2 stimuli that increase membrane permeability to Na+. The larger stimulus produces a larger depolarization. | Action potential triggered by a depolarization that reaches the threshold. | ||
gradients.html: 48_10NeuronIonicGradients_L.jpg
Ionic gradients across the plasma membrane of a mammalian neuron.
The concentrations of Na+ and Cl- are higher in the extracellular fluid than in the
cytosol
.
The reverse is true for K+. The resting potential is about -70 mV.
information_processing.html: 48_03InfoProcessing.jpg
Nervous systems
process information in three stages by specialized populations of neurons:
sensory input, integration, and motor output.
knee-jerk.html: 48_04KneeJerkReflex.jpg
The knee–jerk reflex.
Tapping the tendon stretches the quadriceps (extensor) muscle.
Sensory neurons convey the information to the spinal cord, sending signals to motor neurons that supply the quadriceps, causing it to contract and jerk the lower leg up.
The sensory neurons also communicate with interneurons which inhibit motor neurons that supply the hamstring (flexor) muscle, preventing it from contracting,
Myelin sheaths of neuron axons give them a whitish appearance; regions of the CNS rich in axons are called white matter.
Axons that reach from the spinal cord to muscles in the feet may be over a meter long.
language.html: 48_01LanguageMRI.jpg
A functional magnetic resonance image of brain areas activated during language processing. The fMRI
measures changes in the magnetic resonance of oxygenated hemoglobin as blood flows to the active areas.
myelin.html: 48_15SaltatoryConduction.jpg
Saltatory conduction.
Voltage–gated Na+ and K+ channels are concentrated at gaps in the
myelin sheath called nodes of Ranvier.
The depolarizing current at one node spreads along the interior of the axon to the next node,
causing the action potential to jump from node to node as it travels along the axon.
nervous-arthropod.html: 48_02NervousSystDiversityE.jpg
Arthropods
have a brain and ventral nerve cord containing clusters of neurons called ganglia in segments.
A peripheral nervous system (PNS) connects the CNS with the rest of the body.
nervous-chordate.html: 48_02NervousSystDiversityH.jpg
Chordates
have a CNS consists of the brain and the spinal cord supported by a notochord.
nervous-cnidarian.html: 48_02NervousSystDiversityA.jpg
Cnidarians
have radially symmetrical bodies whose contraction is controlled by neurons arranged in a
diffuse nerve net.
nervous-echinoderm.html: 48_02NervousSystDiversityB.jpg
Echinoderms
exhibit radial symmetry as adults and have a nerve net in each arm,
connected by radial nerves to a central nerve ring.
nervous-flatworm.html: 48_02NervousSystDiversityC.jpg
Flatworms
have bilaterally symmetrical bodies with cephalization.
A simple central nervous system (CNS) is composed of a cluster of neurons in a brain
and primitive nerve cords.
nervous.html: 48_19VertNervousSystem_L.jpg
The vertebrate nervous system.
The central nervous system (CNS) consists of the brain and spinal cord.
Cranial nerves
, spinal nerves, and ganglia
outside the CNS make up the peripheral nervous system (PNS).
neuron.html: 48_05NeuronStructure.jpg
Structure of a vertebrate neuron.
Arising from the cell body are two types of extensions: numerous dendrites and a single axon.
Dendrites receive signals from other neurons.
An axon may be enclosed by a myelin sheath amd transmits signals to other cells
(neurons or effector cells)
via small gaps called synapses.
neuron_diversity.html: 48_06NeuronDiversity.jpg
| Structural diversity of vertebrate neurons. | ||||
|---|---|---|---|---|
| The cell body is connected only to the axon, which conveys signals from the dendrites to synapses at the bottom. | An interneuron has about 100,000 synapses on its highly branched dendrites. | A motor newron synapses with muscle cells. | ||
peripheral.html: 48_21PNSorganization.jpg
Functional hierarchy of the vertebrate peripheral nervous system.
The somatic nervous system regulates skeletal muscles by
voluntary (conscious) control or by reflexes.
The autonomic nervous system regulates smooth and cardiac muscles and many organs
by involuntary control
.
schwann.html: 48_08SchwannMyelin.jpg
Schwann cells and the myelin sheath.
Schwann cells wrap their membranes around axons, forming layers of myelin.
These membranes are mostly lipid and provide electrical insulation for the axon.
Gaps between Schwann cells are called nodes of Ranvier.
summation.html: 48_18Summation.jpg
| Summation of postsynaptic potentials. | ||||||
|---|---|---|---|---|---|---|
| Subthreshold, no summation. A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron. | Temporal summation. Temporal summation occurs when two PSPs are produced in rapid succession. | Spatial summation. Spatial summation occurs when different synapses produce PSPs on the same postsynaptic neuron simultaneously. | Spatial summation of EPSP and IPSP. An IPSP can counter the effect of an EPSP. | |||
synapse.html: 48_17ChemicalSynapse.jpg
A chemical synapse.