Synaptic Transmission in a Simple Reflex Circuit
Figure 6.1 |
One of the simplest behaviors mediated by the central nervous system is knee-jerk or stretch reflex. In response to a neurologist's hammer to the patella tendon, there is a reflex extension of the leg. This figure illustrates the neurocircuitry that controls that reflex response. The stretch to the patella tendon stretches the extensor muscle. More specifically, it stretches a group of specific receptors known as muscle spindle receptors or simply stretch receptors. The stretch elicits action potentials in the stretch receptors which then propagate over type 1A afferent fibers, the somata of which are located in the dorsal root ganglion. Processes of these sensory neurons then enter the spinal cord and make synaptic connections with two types of cells. First, a synaptic connection is formed with the extensor motor neuron located in the ventral horn of the spinal cord. As the result of synaptic activation of this motor neuron, action potentials are elicited in the motor neuron and propagate out the ventral roots, ultimately invading the terminal regions of the motor axon (i.e., the neuromuscular junction), causing release of acetylcholine, depolarization of the muscle cell, formation of an action potential in the muscle cell, and a subsequent contraction of the muscle.
The sensory neurons also make synaptic connections with another type of neuron in the spinal cord called an interneuron. Interneurons are so named because they are interposed between one type of neuron and another. The particular interneuron shown is an inhibitory interneuron. As a result of its activation through the process of synaptic transmission, action potentials are elicited in the interneuron. An action potential in the inhibitory neuron leads to the release of a chemical transmitter substance that inhibits the flexor motor neuron, thereby preventing an improper movement from occurring. This particular reflex is known as the monosynaptic stretch reflex because this reflex is mediated by a single excitatory synaptic relay in the central nervous system.
Synaptic Potentials
Figure 6.2 |
The figure at right illustrates how it is possible to experimentally examine some of the components of synaptic transmission in the reflex pathway that mediates the stretch reflex. Normally, the sensory neuron is activated by a stretch to the stretch receptor, but this process can be bypassed by injecting a depolarizing current into the sensory neuron. That stimulus initiates an action potential in the sensory neuron which leads to a change in the potential of the motor neuron. This potential is known as an excitatory postsynaptic potential (EPSP); excitatory because it tends to depolarize the cell, thereby tending to increase the probability of firing an action potential in the motor neuron and postsynaptic because it is a potential recorded on the postsynaptic side of the synapse.
The ionic mechanisms for the EPSP in the spinal motor neuron are essentially identical to the ionic mechanisms for the EPSP at the neuromuscular junction. Specifically, the transmitter substance diffuses across the synaptic cleft and binds to specific ionotropic receptors on the postsynaptic membrane, leading to a simultaneous increase in the sodium and potassium permeability (See Figure 4.10). The mechanisms for release are also identical to those at the neuromuscular junction. An action potential in the presynaptic terminal leads to the opening of voltage dependent Ca2+ channels, and the Ca2+ influx causes transmitter substance to be released.
Differences between the EPSP at the Skeletal Neuromuscular Junction and EPSPs in the CNS
There are two fundamental differences between the process of synaptic transmission at the sensorimotor synapse in the spinal cord and the process of synaptic transmission at the neuromuscular junction. First, transmitter substance released by the sensory neuron is not ACh but rather the amino acid glutamate. Indeed, there are many different transmitters in the central nervous system - up to 50 or more and the list grows every year. Fortunately, these 50 or more transmitter substances can be conveniently grouped into four basic categories: acetylcholine, monoamines, peptides, and the amino acids. Second, in contrast to the 50-mV amplitude of the synaptic potential at the neuromuscular junction, the amplitude of the synaptic potential in a spinal motor neuron, as a result of an action potential in a 1A afferent fiber, is only about 1 mV.
