The nervous system relies on rapid, precise communication between billions of neurons to coordinate all bodily functions. This communication occurs at specialized junctions called synapses, which are the fundamental points of information transfer. The vast majority of these are chemical synapses, where an electrical signal is converted into a chemical message. These chemical messengers, known as neurotransmitters, bridge the physical gap between two nerve cells, enabling all neural functions.
Anatomy of the Synapse
A chemical synapse is structured around three components that form the communication site. The signal originates in the presynaptic terminal, the enlarged end of the sending neuron’s axon, which stores and releases chemical messengers.
Separating the sending and receiving neurons is the synaptic cleft, a minute gap generally between 20 and 50 nanometers wide. This space prevents the electrical signal from passing directly, necessitating the use of neurotransmitters.
The final component is the postsynaptic terminal, a specialized region of the receiving neuron. Its membrane is densely packed with specialized protein structures called receptors, which recognize and bind to specific neurotransmitters. This binding determines the ultimate effect on the receiving neuron.
The Neurotransmitter Life Cycle
Chemical messengers must be manufactured and prepared before communication can occur. Small-molecule transmitters, such as acetylcholine or dopamine, are synthesized locally within the presynaptic terminal. Precursor molecules are taken up, and enzymes catalyze the final synthesis reactions.
Once synthesized, these neurotransmitters are packaged and concentrated into small, membrane-bound sacs called synaptic vesicles. This energy-dependent packaging ensures the molecules are ready for quick release. Small-molecule transmitters are stored in clear-core vesicles, approximately 40 to 60 nanometers in diameter.
Larger peptide neurotransmitters (3 to 36 amino acids) follow a different path. They are synthesized in the neuron’s cell body, requiring the nucleus and endoplasmic reticulum. These neuropeptides are packaged into larger sacs, called large dense-core vesicles, and transported down the axon to the terminal via fast axonal transport.
Step-by-Step Synaptic Transmission
Synaptic communication begins when an electrical impulse, or action potential, reaches the presynaptic terminal. This causes the terminal membrane to depolarize, triggering the opening of voltage-gated calcium channels.
Since the concentration of calcium ions (\(\text{Ca}^{2+}\)) is much higher outside the neuron, the channel opening causes a rapid influx of \(\text{Ca}^{2+}\) into the terminal. This rise in internal \(\text{Ca}^{2+}\) concentration directly triggers neurotransmitter release.
The calcium ions bind to a protein sensor, initiating a molecular cascade involving the SNARE proteins. The SNARE complex facilitates the fusion of the vesicle membrane with the presynaptic cell membrane. This fusion allows stored neurotransmitters to be rapidly expelled into the synaptic cleft through exocytosis.
Once in the cleft, neurotransmitter molecules diffuse across the gap and bind specifically to postsynaptic receptors, causing a change in the receiving neuron. The signaling action is brief and must be swiftly terminated to allow for new signals. Termination is achieved through reuptake by the presynaptic terminal, enzymatic degradation within the cleft, or diffusion away from the synapse.
Excitatory and Inhibitory Signaling
Neurotransmitter action is defined by the type of receptor it activates, resulting in two main electrical changes: excitation or inhibition. An Excitatory Postsynaptic Potential (EPSP) is a temporary depolarization that makes the neuron more likely to generate an action potential. This occurs when a neurotransmitter binds to a receptor that opens ion channels permeable to positively charged ions, like sodium (\(\text{Na}^{+}\)). Glutamate is the major excitatory neurotransmitter in the brain.
Conversely, an Inhibitory Postsynaptic Potential (IPSP) makes the postsynaptic neuron less likely to fire. IPSPs often involve the opening of channels permeable to negatively charged ions, such as chloride (\(\text{Cl}^{-}\)), which stabilizes or hyperpolarizes the membrane. The primary inhibitory neurotransmitter in the central nervous system is gamma-aminobutyric acid (GABA).
A neuron’s decision to fire is determined by the summation of all incoming EPSPs and IPSPs it receives. If the combined excitatory signals outweigh the inhibitory ones, the neuron reaches its threshold and transmits an impulse. This dynamic balance allows the nervous system to perform complex computations.