Transmission from nerve to muscle resembles chemical synaptic transmission from one neuron to another. The neuromuscular junction, the specialized area where a motor nerve terminates on a skeletal muscle fiber, is the site of a stereotyped transmission process. The contacts between autonomic neurons and smooth and cardiac muscle are less specialized, and transmission in these locations is a more diffuse process.
Synaptic transmission: functional anatomy types of synapses
The anatomic structure of synapses varies considerably in the different parts of the mammalian nervous system. The ends of the presynaptic fibers are generally enlarged to form terminal boutons (synaptic knobs) In the cerebral and cerebellar cortex, endings are commonly located on dendrites and frequently on dendritic spines, which are small knobs projecting from dendrites. In some instances, the terminal branches of the axon of the presynaptic neuron form a basket or net around the soma of the postsynaptic cell (basket cells of the cerebellum and autonomic ganglia). In other locations, they intertwine with the dendrites of the postsynaptic cell (climbing fibers of the cerebellum) or end on the dendrites directly (apical dendrites of cortical pyramidal cells). Some end on axons of postsynaptic neurons (axoaxonal endings).
Temporal & spatial summation
Summation may be temporal or spatial. Temporal summation occurs if repeated afferent stimuli because new EPSPs before previous EPSPs have decayed. A longer time constant for the EPSP allows for a greater opportunity for summation. When activity is present in more than one synaptic knob at the same time, spatial summation occurs and activity in one synaptic knob summates with activity in another to approach the firing level. The EPSP is therefore not an all-or-none response but is proportionate in size to the strength of the afferent stimulus.
Slow postsynaptic potentials
In addition to the EPSPs and IPSPs described previously, slow EPSPs and IPSPs have been described in autonomic ganglia, cardiac and smooth muscle, and cortical neurons. These postsynaptic potentials have a latency of 100 to 500 ms and last several seconds. The slow EPSPs are generally due to decreases in K+ conductance, and the slow IPSPs are due to increases in K+ conductance.
Generation of the action potential in the postsynaptic neuron
The constant interplay of excitatory and inhibitory activity on the postsynaptic neuron produces a fluctuating membrane potential that is the algebraic sum of the hyperpolarizing and depolarizing activity. The soma of the neuron thus acts as a sort of integrator. When the 10 to 15 mV of depolarization sufficient to reach the firing level is attained, a propagated spike results.
However, the discharge of the neuron is slightly more complicated than this. In motor neurons, the portion of the cell with the lowest threshold for the production of a full-fledged action potential is the initial segment, the portion of the axon at and just beyond the axon hillock.
The function of the dendrites
For many years, the standard view has been that dendrites are simply the sites of current sources or sinks that electronically change the membrane potential at the initial segment; that is, they are merely extensions of the soma that expand the area available for integration. When the dendritic tree of a neuron is extensive and has multiple presynaptic knobs ending on it, there is room for a great interplay of inhibitory and excitatory activity.
Presynaptic terminals are separated from the postsynaptic structure by a synaptic cleft. The postsynaptic membrane contains many neurotransmitter receptors and usually, a postsynaptic thickening called the postsynaptic density.