Synaptic transmission is the process allowing neurons to communicate, forming the basis for every thought, sensation, and action. This process moves information throughout the brain and to the rest of the body. A message is passed from a sending neuron to a receiving target, such as another neuron, a muscle, or a gland cell.
The Two Main Types of Synapses
The nervous system uses two distinct strategies for communication at synapses: electrical and chemical. Electrical synapses are less common in humans and are specialized for speed. In this arrangement, two neurons are physically connected by protein channels called gap junctions. These channels allow electrical current in the form of ions to flow directly from one cell to the next, making transmission nearly instantaneous. These synapses are useful for synchronizing the activity of a group of neurons, such as those that regulate rhythmic breathing.
The most common form of communication occurs at chemical synapses. Unlike electrical synapses, these are not physically connected but are separated by a fluid-filled space called the synaptic cleft. To bridge this gap, the sending neuron releases chemical messengers called neurotransmitters. This process introduces a slight delay but offers greater flexibility, allowing signals to be modified, amplified, or inverted.
The Chemical Transmission Process
The sequence at a chemical synapse begins when an electrical signal, an action potential, travels down the axon of the sending (presynaptic) neuron and arrives at its terminal. This impulse depolarizes the terminal membrane, triggering the opening of voltage-gated calcium channels. The opening of these channels allows calcium ions (Ca²⁺) to rush into the presynaptic terminal.
This influx of calcium triggers the next step. Inside the axon terminal are small, membrane-bound sacs called synaptic vesicles, each filled with thousands of neurotransmitter molecules. The rise in intracellular calcium causes these vesicles to move and fuse with the presynaptic membrane at specialized regions called active zones. This fusion allows the vesicles to release their contents into the synaptic cleft in a process called exocytosis.
Once released, neurotransmitter molecules diffuse across the synaptic cleft. On the membrane of the receiving (postsynaptic) neuron are receptor proteins. Each neurotransmitter has a specific receptor it can bind to, functioning much like a key fitting into a lock. This binding event causes the receptor protein to change shape, which opens associated ion channels in the postsynaptic membrane.
The opening of these channels allows ions to flow into or out of the postsynaptic neuron, altering its electrical potential. For the signal to be terminated, the neurotransmitter must be removed from the synaptic cleft. This occurs when the neurotransmitter is broken down by enzymes, reabsorbed into the presynaptic neuron via reuptake, or diffuses away.
Key Messengers and Their Receptors
Neurotransmitters are the chemical messengers that facilitate communication, and there are more than 40 different types in the human nervous system. They are grouped by their effect on the receiving neuron. Excitatory neurotransmitters increase the likelihood that the postsynaptic neuron will generate an action potential. The most abundant excitatory messenger in the central nervous system is glutamate, which has a widespread role in cognitive functions like learning and memory.
Conversely, inhibitory neurotransmitters decrease the chance that the receiving neuron will fire. Gamma-aminobutyric acid, known as GABA, is the primary inhibitory neurotransmitter in the brain. It acts as a calming agent, regulating neuronal excitability throughout the nervous system to prevent issues like anxiety and seizures. The balance between the excitatory effects of glutamate and the inhibitory effects of GABA is important for stable brain function.
Other well-known neurotransmitters modulate brain activity in more specific circuits. Dopamine is heavily involved in the brain’s reward pathways, motivation, and the control of voluntary movement. Serotonin is another modulator, regulating mood, sleep patterns, and appetite. The function of each neurotransmitter is determined by the receptor it binds to, ensuring the correct message is delivered to the correct target cell.
Modulation and Importance in Brain Function
Chemical synapses can change in strength over time through a process called synaptic plasticity. This adaptability is the cellular mechanism behind learning and memory. When we learn new information, the synaptic connections involved in that neural circuit can become stronger and more efficient, a process called long-term potentiation (LTP). This strengthening means the presynaptic neuron becomes better at sending the signal, and the postsynaptic neuron becomes more responsive to it.
This potentiation can be achieved through various changes, such as increasing the amount of neurotransmitter released or adding more receptors to the postsynaptic membrane. Repeated stimulation of a synapse can lead to these lasting modifications, making it easier for the signal to cross the next time. This enhanced communication is what allows a memory to be stored and recalled more easily.
Conversely, synapses can also weaken through a process called long-term depression (LTD). This is equally important for refining neural circuits and forgetting information that is no longer relevant. This dynamic ability to remodel connections gives the brain its capacity to adapt to new experiences, acquire skills, and store vast amounts of information.
External Influences on Synaptic Communication
The chemical balance at the synapse can be altered by external substances, which modify signaling for therapeutic or recreational purposes. Caffeine exerts its stimulating effect by interfering with a neuromodulator called adenosine. Adenosine normally binds to its receptors and promotes relaxation; caffeine’s molecular structure is similar enough that it can block these receptors, preventing the calming signal and leading to increased alertness.
Many medications are designed to target synaptic processes. Selective serotonin reuptake inhibitors (SSRIs), a common class of antidepressants, work by blocking the reuptake of serotonin from the synaptic cleft. By inhibiting the transporter proteins that normally recycle serotonin, SSRIs increase the concentration of the neurotransmitter in the synapse. This enhances its effect on the postsynaptic neuron and helps to regulate mood.
The disruption of synaptic transmission is also a feature of several neurological disorders. Parkinson’s disease is characterized by the progressive loss of dopamine-producing neurons in an area of the brain called the substantia nigra. This depletion of dopamine disrupts the neural circuits responsible for controlling movement, leading to motor symptoms such as tremors, rigidity, and slowness of movement.