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CHAPTER 13

Terms in this set (19)

What are the two types of synapses?
1. Excitatory Synapses
2. Inhibitory Synapses
How do you determine what type of synapse?
by the type of ion channels in the postsynaptic cell activated by the transmitter
Transmitters are usually
inhibitory or excitatory
Neurons that release glutamate
typically act on receptors that produce excitation
Neurons that release GABA or glycine
act on receptors that produce inhibition
Explain the experimental setup for recording from cells in the (stretch reflex circuit) knee jerk
A single presynaptic axon can be stimulated by inserting a current-passing electrode into the sensory neuron cell body. An action potential in the sensory neuron stimulated in this way triggers a small EPSP in the extensor motor neuron.

Alternatively, the whole afferent nerve from the quadriceps can be stimulated electrically with an extracellular electrode. The excitation of many afferent neurons through the extracellular electrode generates a synaptic potential (dashed trace) large enough to initiate an action potential (red trace).

Lower panel: The setup for eliciting and measuring inhibitory potentials in the flexor motor neuron.
- Intracellular stimulation of a single inhibitory interneuron receiving input from the quadriceps pathway produces a small inhibitory (hyperpolarizing) postsynaptic potential (IPSP) in the flexor motor neuron (black trace). Extracellular stimulation recruits numerous inhibitory neurons and generates a larger IPSP (red trace). (Action potentials in the sensory neuron and interneuron appear smaller because they were recorded at lower amplification than those in the motor neuron
What are the two most common morphological types of synapses in the central nervous system
Gray type I and type II.
What type of synapse is Gray Type 1.
Excitatory
What type of synapse is type II
Inhibitory
Difference between Gray type I and type II synapses.
Differences include the shape of vesicles, the prominence of presynaptic densities, total area of the active zone, width of the synaptic cleft, and presence of a dense basement membrane. Type I synapses typically contact specialized dendritic projections, called spines, and less commonly contact the shafts of dendrites. Type II synapses contact the cell body (axosomatic), dendritic shaft (axodendritic), axon initial segment (axo-axonic), and presynaptic terminals of another neuron (
What is the major excitatory transmitter in the brain and spinal cord?
Glutamate
Glutamate receptors can be divided into two broad categories:
1. ionotropic receptor
2. metabotropic receptor
Describe the ionotopic glutamate receptor
directly gate ion channels permeable to cations. The AMPA and kainate types bind the glutamate agonists AMPA or kainate, respectively; these receptors contain a channel that is permeable to Na+ and K+ . The NMDA receptor binds the glutamate agonist NMDA; it contains a channel permeable to Ca2+, K+ , and Na+ . It has binding sites for glutamate, glycine, Zn2+, phencyclidine (PCP, or angel dust), MK801 (an experimental drug), and Mg2+, each of which regulates the functioning of the channel differently.
Describe the metabotropic glutamate receptor
Binding of glutamate (Glu) to metabotropic glutamate receptors indirectly gates ion channels by activating a GTP-binding protein (G protein), which in turn interacts with effector molecules that alter metabolic and ion channel activity
Early and late components of synaptic current
The drug APV selectively binds to and blocks the NMDA receptor. Shown here is the excitatory postsynaptic current (EPSC) before and during application of 50 μM APV at three different membrane potentials. The difference between the traces (blue region) represents the contribution of the NMDA receptor-channel to the EPSC. The current that remains in the presence of APV is the contribution of the AMPA receptor channels. At −80 mV, there is no current through the NMDA receptor-channels because of pronounced Mg2+ block (see Figure 13-8). At −40 mV, a small late inward current through NMDA receptor-channels is evident. At +20 mV, the late component is more prominent and has reversed to become an outward current. The time 25 ms after the peak of the synaptic current (dashed line) is used for the calculations of late current in part
Current-voltage relationship of the synaptic current
The postsynaptic currents through the NMDA and AMPA receptor-channels differ in their dependence on the membrane potential. The current through the AMPA receptor-channels contributes to the early phase of the synaptic current (filled triangles). The early phase is measured at the peak of the synaptic current and plotted here as a function of membrane potential. The current through the NMDA receptor-channels contributes to the late phase of the synaptic current (filled circles). The late phase is measured 25 ms after the peak of the synaptic current, a time at which the AMPA receptor component has decayed almost to zero (see part A). Note that the AMPA receptor-channels behave as simple resistors; current and voltage have a linear relationship. In contrast, current through the NMDA receptor-channels is nonlinear and increases as the membrane is depolarized from −80 to −40 mV, owing to progressive relief of the Mg2+ block. The reversal potential of both receptor-channel types is at 0 mV. The components of the synaptic current in the presence of 50 μm APV are indicated by the unfilled circles and triangles. Note how APV blocks the late (NMDA receptor) component of the EPSC but not the early (AMPA receptor) component
Figure 13-13 shows
A synaptic potential arising in a dendrite can generate an action potential at the axon initial segment
Describe the flow of a synaptic potential
An excitatory synaptic potential originating in the dendrites decreases with distance as it propagates passively to the soma. Nevertheless, an action potential can be initiated at the trigger zone (the axon initial segment) because the density of the Na+ channels in this region is high and thus the threshold for an action potential is low.
Compare the threshold for initiation of the action potential at different sites in the neuron (corresponding to drawing A).
An action potential is generated when the amplitude of the synaptic potential exceeds the threshold. The dashed line shows the decay of the synaptic potential if no action potential is generated at the axon initial segment
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