Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
425
11
calcium appears to mediate such effects, which may be the basis
of learning.
8.
Presynaptic inhibition is mediated by axoaxonal synapses that reduce
the amount of neurotransmitter released by the inhibited neuron.
Nervous System II; Topic: Synaptic Potentials and Cellular
Integration, pp. 1-10.
Neurotransmitters and Their Receptors
(pp. 414–421)
Classification of Neurotransmitters by Chemical Structure
(pp. 414–417
)
1.
Te major classes of neurotransmitters based on chemical
structure are acetylcholine, biogenic amines, amino acids,
peptides, purines, dissolved gases, and lipids.
Classification of Neurotransmitters by Function
(pp. 417–419)
2.
Functionally, neurotransmitters are classified as (1) inhibitory
or excitatory (or both) and (2) direct or indirect action. Direct-
acting neurotransmitters bind to and open ion channels. Indirect-
acting neurotransmitters act through second messengers.
Neuromodulators also act indirectly presynaptically or
postsynaptically to change synaptic strength.
Neurotransmitter Receptors
(pp. 420–421)
3.
Neurotransmitter receptors are either channel-linked receptors that
open ion channels, leading to fast changes in membrane potential,
or G protein–linked receptors that oversee slow synaptic responses
mediated by G proteins and intracellular second messengers.
Second messengers most o±en activate kinases, which in turn act
on ion channels or activate other proteins.
Nervous System II; Topic: Synaptic Transmission, pp. 6–15.
Basic Concepts of Neural Integration
(pp. 421–423)
Organization of Neurons: Neuronal Pools
(p. 421)
1.
CNS neurons are organized into several types of neuronal pools, each
with distinguishing patterns of synaptic connections called circuits.
Types of Circuits
(p. 421)
2.
Te four basic circuit types are diverging, converging,
reverberating, and parallel a±er-discharge.
Patterns of Neural Processing
(pp. 421–423)
3.
In serial processing, one neuron stimulates the next in sequence,
producing specific, predictable responses, as in spinal reflexes. A
reflex is a rapid, involuntary motor response to a stimulus.
4.
Reflexes are mediated over neural pathways called reflex arcs.
Te minimum number of elements in a reflex arc is five: receptor,
sensory neuron, integration center, motor neuron, and effector.
5.
In parallel processing, which underlies complex mental functions,
impulses travel along several pathways to different integration centers.
Developmental Aspects of Neurons
(pp. 423–424)
1.
Neuron development involves proliferation, migration, and the
formation of interconnections. Te formation of interconnections
involves axons finding their targets and forming synapses, and
the synthesis of specific neurotransmitters.
2.
Axon outgrowth and synapse formation are guided by other
neurons, glial cells, and chemicals (such as N-CAM and nerve
growth factor). Neurons that do not make appropriate synapses
die, and approximately two-thirds of neurons formed in the
embryo undergo programmed cell death.
7.
An action potential (AP), or nerve impulse, is a large, but brief,
depolarization signal (and polarity reversal) that underlies
long-distance neural communication. It is an all-or-none
phenomenon.
8.
In the AP graph, an AP begins and ends at resting membrane
potential. Depolarization to approximately
1
30 mV (inside
positive) is caused by Na
1
influx. Depolarization ends when Na
1
channels inactivate. Repolarization and hyperpolarization are
caused by K
1
efflux.
9.
If threshold is reached, an AP is generated. If not, depolarization
remains local.
10.
In nerve impulse propagation, each AP provides the depolarizing
stimulus for triggering an AP in the next membrane patch.
Regions that have just generated APs are refractory; for this
reason, the nerve impulse propagates in one direction only.
11.
APs are independent of stimulus strength: Strong stimuli cause APs
to be generated more frequently but not with greater amplitude.
12.
During the absolute refractory period, a neuron cannot respond
to another stimulus because it is already generating an AP.
During the relative refractory period, the neuron’s threshold is
elevated because repolarization is ongoing.
13.
In nonmyelinated fibers, APs are produced in a wave all along
the axon, that is, by continuous conduction. In myelinated fibers,
APs are generated only at myelin sheath gaps and are propagated
more rapidly by saltatory conduction.
Nervous System I; Topic: The Action Potential, pp. 1–18.
The Synapse
(pp. 407–414)
1.
A synapse is a functional junction between neurons. Te
information-transmitting neuron is the presynaptic neuron; the
information-receiving neuron is the postsynaptic neuron.
Electrical Synapses
(p. 407)
2.
Electrical synapses allow ions to flow directly from one neuron to
another; the cells are electrically coupled.
Chemical Synapses
(pp. 407–410)
3.
Chemical synapses are sites of neurotransmitter release and binding.
When the impulse reaches the presynaptic axon terminals, voltage-
gated Ca
2
1
channels open, and Ca
2
1
enters the cell and mediates
neurotransmitter release. Neurotransmitters diffuse across the
synaptic cle± and attach to postsynaptic membrane receptors,
opening ion channels. A±er binding, the neurotransmitters are
removed from the synapse by diffusion, enzymatic breakdown, or
reuptake into the presynaptic terminal or astrocytes.
Nervous System II; Topics: Anatomy Review, pp. 1–9,
Ion Channels, pp. 1–8, Synaptic Transmission, pp. 1–7.
Postsynaptic Potentials and Synaptic Integration
(pp. 410–414)
4.
Binding of neurotransmitter at excitatory chemical synapses
results in local graded potentials called EPSPs, caused by the
opening of channels that allow simultaneous passage of Na
1
and K
1
.
5.
Neurotransmitter binding at inhibitory chemical synapses results
in hyperpolarizations called IPSPs, caused by the opening of K
1
or Cl
2
channels. IPSPs drive the membrane potential farther
from threshold.
6.
EPSPs and IPSPs summate temporally and spatially. Te
membrane of the axon hillock acts as a neuronal integrator.
7.
Synaptic potentiation, which enhances the postsynaptic neuron’s
response, is produced by intense repeated stimulation. Ionic
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