202 Chapter 7
B. The opening of voltage-regulated channels produces an
action potential.
- The opening of Na^1 channels in response to
depolarization allows Na^1 to diffuse into the axon,
thus further depolarizing the membrane in a positive
feedback fashion. - The inward diffusion of Na^1 causes a reversal of the
membrane potential from 2 70 mV to 1 30 mV. - The opening of K^1 channels and outward diffusion of
K^1 causes the reestablishment of the resting membrane
potential. This is called repolarization. - Action potentials are all-or-none events.
- The refractory periods of an axon membrane prevent
action potentials from running together. - Stronger stimuli produce action potentials with greater
frequency.
C. One action potential serves as the depolarization stimulus for
production of the next action potential in the axon. - In unmyelinated axons, action potentials are produced
fractions of a micrometer apart. - In myelinated axons, action potentials are produced
only at the nodes of Ranvier. This saltatory conduction
is faster than conduction in an unmyelinated nerve fiber.
7.3 The Synapse 180
A. Gap junctions are electrical synapses found in cardiac mus-
cle, smooth muscle, and some regions of the brain.
B. In chemical synapses, neurotransmitters are packaged in syn-
aptic vesicles and released by exocytosis into the synaptic cleft.
The neurotransmitter can be called the ligand of the
receptor.
Binding of the neurotransmitter to the receptor causes
the opening of chemically regulated gates of ion
channels.
4 Acetylcholine as a Neurotransmitter 184
A. There are two subtypes of ACh receptors: nicotinic and
muscarinic.
- Nicotinic receptors enclose membrane channels and
open when ACh binds to the receptor. This causes
a depolarization called an excitatory postsynaptic
potential (EPSP). - The binding of ACh to muscarinic receptors opens ion
channels indirectly, through the action of G-proteins.
This can cause either an EPSP or a hyperpolarization
called an inhibitory postsynaptic potential (IPSP). - After ACh acts at the synapse, it is inactivated by the
enzyme acetylcholinesterase (AChE).
B. EPSPs are graded and capable of summation. They decrease
in amplitude as they are conducted.
C. ACh is used in the PNS as the neurotransmitter of somatic
motor neurons, which stimulate skeletal muscles to contract,
and by some autonomic neurons.
D. ACh in the CNS produces EPSPs at synapses in the den-
drites or cell body. These EPSPs travel to the axon hillock,
stimulate opening of voltage-regulated channels, and gener-
ate action potentials in the axon.- 5 Monoamines as Neurotransmitters 191
A. Monoamines include serotonin, dopamine, norepinephrine,
and epinephrine. The last three are included in the sub-
category known as catecholamines. - These neurotransmitters are inactivated after being
released, primarily by reuptake into the presynaptic
nerve endings. - Catecholamines may activate adenylate cyclase in the
postsynaptic cell, which catalyzes the formation of
cyclic AMP.
B. Dopaminergic neurons (those that use dopamine as a neurotrans-
mitter) are implicated in the development of Parkinson’s disease
and schizophrenia. Norepinephrine is used as a neurotransmitter
by sympathetic neurons in the PNS and by some neurons in
the CNS.
7.6 Other Neurotransmitters 194
A. The amino acids glutamate and aspartate are excitatory in
the CNS.
1. The subclass of glutamate receptor designated as
NMDA receptors are implicated in learning and
memory.
2. The amino acids glycine and GABA are inhibitory. They
produce hyperpolarizations, causing IPSPs by opening
Cl^2 channels.
B. Numerous polypeptides function as neurotransmitters,
including the endogenous opioids.
C. Nitric oxide functions as both a local tissue regulator and a
neurotransmitter in the PNS and CNS. It promotes smooth
muscle relaxation and is implicated in memory.
D. Endocannabinoids are lipids that appear to function as retro-
grade neurotransmitters: they are released from the postsynap-
tic neuron, diffuse back to the presynaptic neuron, and inhibit
the release of neurotransmitters by the presynaptic neuron.7.7 Synaptic Integration 199
A. Spatial and temporal summation of EPSPs allows a depolar-
ization of sufficient magnitude to cause the stimulation of
action potentials in the postsynaptic neuron.
1. IPSPs and EPSPs from different synaptic inputs can
summate.
2. The production of IPSPs is called postsynaptic
inhibition.
B. Long-term potentiation is a process that improves synaptic
transmission as a result of the use of the synaptic pathway.
This process thus may be a mechanism for learning.
C. Long-term depression is a process similar to long-term
potentiation, but it causes depressed activity in a synapse.