Anatomy & Physiology: The Synapse

The Synapse

A synapse, or synaptic cleft, is the gap that separates adjacent neurons or a neuron and a muscle. Transmission of an impulse across a synapse, from presynaptic cell to postsynaptic cell, may be electrical or chemical. In electrical synapses, the action potential travels along the membranes of gap junctions, small tubes of cytoplasm along the membranes of gap junctions, small tubes of cytoplasm that allow the transfer of ions between adjacent cells. In chemical synapses, action potentials are transferred across the synapse by the diffusion of chemicals, as follows:

Calcium (Ca²⁺) gates open. When an action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open that allow Ca²⁺ to enter.
Synaptic vesicles release neurotransmitter. The influx of Ca²⁺ into the terminal end of the axon causes synaptic vesicles to merge with the presynaptic membrane, releasing a neurotransmitter into the synaptic cleft.
Neurotransmitter binds with postsynaptic receptors. The neurotransmitter diffuses across the synaptic cleft and binds with specialized protein receptors on the postsynaptic membrane. Different proteins are receptors for different neurotransmitters.
The postsynaptic membrane is excited or inhibited. Depending upon the kind of neurotransmitter and the kind of membrane receptor, there are two possible outcomes for the postsynaptic membrane, both of which are graded potentials.
- If positive ion gates open (which allow more Na⁺ and Ca²⁺ to enter than K⁺ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.
- If K⁺ or chlorine ion (Cl⁻) gates open (allowing K⁺ to exit or Cl⁻ to enter), the membrane becomes more polarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.
The neurotransmitter is degraded and recycled. After the neurotransmitter binds to the postsynaptic membrane receptors, it is either transported back to and reabsorbed by the secreting neuron, or it is broken down by enzymes in the synaptic cleft. For example, the common neurotransmitter acetylcholine is broken down by cholinesterase. Reabsorbed and degraded neurotransmitters are recycled by the presynaptic cell.

Here are some of the common neurotransmitters and the kinds of activity they generate:

Acetylcholine (ACh) is commonly secreted at neuromuscular junctions, the gaps between motor neurons and muscle cells, where it stimulates muscles to contract (by opening gated positive ion channels). At other kinds of junctions, it typically produces an inhibitory postsynaptic potential.
Epinephrine, norepinephrine (NE), dopamine, and serotonin are derived from amino acids and are secreted mostly between neurons of the CNS.
Gamma aminobutyric acid (GABA) is usually an inhibitory neurotransmitter (opening gated Cl⁻ channels) among neurons in the brain.