The Nervous System 201many as 1,000 presynaptic inputs, the interactions of EPSPs
and IPSPs can vary greatly.
In presynaptic inhibition, the amount of an excitatory neu-
rotransmitter released at the end of an axon is decreased by the
effects of a second neuron, whose axon makes a synapse with
the axon of the first neuron (an axoaxonic synapse). The neu-
rotransmitter exerting this presynaptic inhibition may be GABA
or excitatory neurotransmitters, such as ACh and glutamate.
Excitatory neurotransmitters can cause presynaptic inhibition
by producing depolarization of the axon terminals, leading to inac-
tivation of Ca^2 1 channels. This decreases the inflow of Ca^2 1 into
the axon terminals and thus inhibits the release of neurotransmit-
ter. The ability of the opiates to promote analgesia (reduce pain) is
an example of such presynaptic inhibition. By reducing Ca^2 1 flow
into axon terminals containing substance P, the opioids inhibit the
release of the neurotransmitter involved in pain transmission.
| CHECKPOINT
- Define spatial summation and temporal summation,
and explain their functional importance.
17a. Describe long-term potentiation, explain how it is
produced, and discuss its significance.
17b. Explain how postsynaptic inhibition is produced and
how IPSPs and EPSPs can interact.
17c. Describe the mechanism of presynaptic inhibition.
Denise is taking three different drugs that alter the physi-
ology of neurons in her brain. She is taking an MAO
inhibitor that increases the activity of all the monoamine
neurotransmitters (dopamine, norepinephrine, and sero-
tonin), as well as cocaine, which acts powerfully as a
triple reuptake inhibitor to promote excessive amounts
of these neurotransmitters in the synapse. In addition,
cocaine blocks Na^1 channels to cause potentially dan-
gerous systemic effects. Denise is also taking Xanax, a
benzodiazepine that promotes the inhibitory action of
GABA at its synapses. All of these drugs, particularly
cocaine, can have potentially serious effects, even when
taken separately. Given this, her concern that she may
eat pesticides on her vegetables (which act like nerve gas
to inhibit AChE and promote cholinergic transmission) is
misplaced, because she is unlikely to ingest a danger-
ous amount. Her seizure is most likely a result of brain
damage caused by her use of cocaine, a common occur-
rence. The intravenous benzodiazepines administered by
the paramedics helped relieve her seizure by promoting
the inhibitory effect of GABA on spinal motoneurons.
See the additional chapter 7 Clinical Investigations on
Myasthenia Gravis and Parkinson’s disease in the Connect
site for this text.Clinical Investigation SUMMARY
Summary
D. Neuroglial cells include Schwann cells and satellite cells in the
PNS; in the CNS they include the various types of glial cells:
oligodendrocytes, microglia, astrocytes, and ependymal cells.
1. Schwann cells form a sheath of Schwann, or
neurilemma, around axons of the PNS.
2. Some neurons are surrounded by successive wrappings
of supporting cell membrane called a myelin sheath.
This sheath is formed by Schwann cells in the PNS and
by oligodendrocytes in the CNS.
3. Astrocytes in the CNS may contribute to the blood-brain
barrier.- 2 Electrical Activity in Axons 172
A. The permeability of the axon membrane to Na^1 and K^1 is
regulated by gated ion channels. - At the resting membrane potential of 2 70 mV, the
membrane is relatively impermeable to Na^1 and only
slightly permeable to K^1. - The voltage-regulated Na^1 and K^1 channels open in
response to the stimulus of depolarization. - When the membrane is depolarized to a threshold
level, the Na^1 channels open first, followed quickly by
opening of the K^1 channels.
7.1 Neurons and Neuroglial Cells 163
A. The nervous system is divided into the central nervous sys-
tem (CNS) and the peripheral nervous system (PNS).
- The central nervous system includes the brain and spinal
cord, which contain nuclei and tracts. - The peripheral nervous system consists of nerves,
ganglia, and nerve plexuses.
B. A neuron consists of dendrites, a cell body, and an axon. - The cell body contains the nucleus, Nissl bodies,
neurofibrils, and other organelles. - Dendrites receive stimuli, and the axon conducts nerve
impulses away from the cell body.
C. A nerve is a collection of axons in the PNS. - A sensory, or afferent, neuron is pseudounipolar and
conducts impulses from sensory receptors into the
CNS. - A motor, or efferent, neuron is multipolar and conducts
impulses from the CNS to effector organs. - Interneurons, or association neurons, are located entirely
within the CNS. - Somatic motor nerves innervate skeletal muscle;
autonomic nerves innervate smooth muscle, cardiac
muscle, and glands.