Fundamentals of the Nervous System and Nervous Tissue
calcium appears to mediate such eﬀects, which may be the basis
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
Classiﬁcation of Neurotransmitters by Chemical Structure
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
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 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
Organization of Neurons: Neuronal Pools
CNS neurons are organized into several types of neuronal pools, each
with distinguishing patterns of synaptic connections called circuits.
Types of Circuits
Te four basic circuit types are diverging, converging,
reverberating, and parallel a±er-discharge.
Patterns of Neural Processing
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.
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.
In parallel processing, which underlies complex mental functions,
impulses travel along several pathways to diﬀerent integration centers.
Developmental Aspects of Neurons
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.
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.
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
In the AP graph, an AP begins and ends at resting membrane
potential. Depolarization to approximately
30 mV (inside
positive) is caused by Na
inﬂux. Depolarization ends when Na
channels inactivate. Repolarization and hyperpolarization are
caused by K
If threshold is reached, an AP is generated. If not, depolarization
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.
APs are independent of stimulus strength: Strong stimuli cause APs
to be generated more frequently but not with greater amplitude.
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.
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.
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 allow ions to ﬂow directly from one neuron to
another; the cells are electrically coupled.
Chemical synapses are sites of neurotransmitter release and binding.
When the impulse reaches the presynaptic axon terminals, voltage-
channels open, and Ca
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
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
Neurotransmitter binding at inhibitory chemical synapses results
in hyperpolarizations called IPSPs, caused by the opening of K
channels. IPSPs drive the membrane potential farther
EPSPs and IPSPs summate temporally and spatially. Te
membrane of the axon hillock acts as a neuronal integrator.
Synaptic potentiation, which enhances the postsynaptic neuron’s
response, is produced by intense repeated stimulation. Ionic