Some of the presynaptic terminals on any particular motoneuron are the endings of sensory axons (known as group Ia fibres) from muscle spindles in the muscle which the motoneuron innervates. Sketching the muscle excites these axons, which may then excite the motoneurons supplying the muscle so that it contracts. This system is known as a monosynaptic reflex (Fig. 8.2). The knee-jerk reflex is a familiar example.
The postsynaptic responses of motoneurons can be observed by means of a microelectrode inserted into the soma. Stimulation of the group Ia fibres mV
Fig. 8.3. Excitatory postsynaptic potentials (EPSPs) recorded from a cat spinal motoneuron in response to stimuli of increasing intensity (from a to c) applied to the group Ia afferent fibres from the muscle. From Coombs, Eccles and Fatt (1955a).
which synapse with a particular motoneuron produces brief depolarizations, as is shown in Fig. 8.3. These responses are called excitatory postsynaptic potentials, or EPSPs. Their form is similar to that of the end-plate potential in a curarized skeletal muscle fibre: there is a fairly rapid rising phase followed by a slower return to the resting potential.
Each EPSP is the result of action potentials in a number of presynaptic fibres. With low intensity stimulation applied to the nerve from the muscle, only a few of the group Ia fibres are excited and the EPSP is correspondingly small. As we increase the stimulus intensity, more and more of the group Ia fibres are excited and the EPSP correspondingly grows in size. Thus the responses produced by activity at different synapses on the same motoneuron can add together. This phenomenon is known as spatial summation. If a second EPSP is elicited before the first one has died away, the net depolarization will be enhanced as the second EPSP adds to the first. This is known as temporal summation.
A large EPSP will be sufficient to cross the threshold for production of an action potential. This then propagates along the axon out to the periphery, where it ultimately produces contraction of the muscle fibres innervated by the axon.
The membrane potential of a motoneuron can be altered by inserting a special double-barrelled microelectrode into it and passing current down one barrel while the other is used to record the membrane potential. When the membrane potential is progressively depolarized, the EPSP decreases in size and eventually becomes reversed in sign. The reversal potential is about 0 mV. This suggests that the EPSP is produced by a change in ionic conductance which is independent of membrane potential, just as is the end-plate potential in muscle. The ions involved are probably sodium and potassium, just as they are in the end-plate potential.
These similarities between the EPSP and the end-plate potential, together with the existence of synaptic vesicles in the presynaptic terminals, suggest that the EPSP is produced by a neurotransmitter released from the group Ia terminals. There is good evidence, especially from spinal neurons grown in cell culture, that the transmitter is glutamate and that it acts by binding to glutamate receptors on the postsynaptic membrane. These receptors have intrinsic ion channels that open when the receptor binds glutamate.
Glutamate receptors have been cloned so that some deductions can be made about their structure. The subunits have three membrane-crossing segments and one (M2) that dips into and out of the membrane from the cytoplasmic side. This is rather different from the situation in the nicotinic acetylcholine receptor, and indeed there is no homology between the sequences of the two receptors. A whole channel is made up of four or perhaps five subunits, presumably surrounding a central pore.
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