Excitatory postsynaptic potential
From Free net encyclopedia
In neuroscience, an excitatory postsynaptic potential (EPSP) is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. They are the opposite of inhibitory postsynaptic potentials (IPSPs), which usually result from the flow of negative ions into the cell. A postsynaptic potential is defined as excitatory if it makes it easier for the neuron to fire an action potential. EPSPs can also result from a decrease in outgoing positive charges, while IPSPs are sometimes caused by an increase in positive charge outflow. The flow of ions that causes an EPSP is an excitatory postsynaptic current (EPSC).
EPSPs, like IPSPs, are additive. When multiple EPSPs occur on a single patch of postsynaptic membrane, their combined effect is the sum of the individual EPSPs. Larger EPSPs result in greater membrane depolarization and thus increase the likelihood that the postsynaptic cell reaches the threshold for firing an action potential.
Contents |
Overview
EPSPs in living cells are caused chemically. When an active presynaptic cell releases neurotransmitters into the synapse, some of them bind to receptors on the postsynaptic cell. Many of these receptors contain an ion channel capable of passing positively-charged ions either into or out of the cell (such receptors are called ionotropic receptors). At excitatory synapses, the ion channel typically allows sodium into the cell, generating an excitatory postsynaptic current. This depolarizing current causes an increase in membrane potential, the EPSP.
Excitatory molecules
The neurotransmitter most often associated with EPSPs is the amino acid glutamate. Its ubiquity at excitatory synapses has led to it being called the excitatory neurotransmitter. In the neuromuscular junction, EPSPs (called end-plate potentials, EPP) are mediated by the neurotransmitter acetylcholine. However, classifying neurotransmitters as such is technically incorrect, as there are several other synaptic factors that help determine a neurotransmitter's excitatory or inhibitory effects.
Spontaneous EPSPs
The release of neurotransmitter vesicles from the presynaptic cell is probabilistic. In fact, even without stimulation of the presynaptic cell, vesicles will occasionally be released into the synapse, generating EPSPs. Bernard Katz pioneered the study of these spontaneous EPSPs (often called miniature end-plate potentialsTemplate:Ref) in 1951, revealing the quantal nature of synaptic transmission.
Notes
- Template:Note Functionally, EPSPs and miniature end-plate potentials (mEPPs) are identical. The name end-plate potential is used since Katz' studies were performed on the neuromuscular junction, the muscle fiber component of which is commonly called the motor end-plate.