When nerve cells, or neurons, are stimulated they undergo chemical changes that produce tiny waves of electricity – nerve impulses. Information is conveyed throughout the nervous system as nerve impulses, or action potentials. The information carried depends on the location of the impulses in the central nervous system, as well as on their frequency.
When the sensory information, i.e. impulses, reach a junction labeled as a synapse, they trigger the release of chemicals called neurotransmitters. Molecules of the neurotransmitter pass through the synapse and stimulate the receiving neuron to fire an impulse of its own (i.e. to pass the sensory information between two neurons via the axons). This action can be registered as a wavelike movement of the ions (electrically charged particles).
Anatomy and Function of Brain Synapses
Brain synapses are functional connections between the neurons present in the central nervous system and peripheral nerves. A brain synapse consists of the following elements:
- Presynaptic membrane – The axon of the sending cell’s membrane
- Postsynaptic membrane – Membrane of receiving cell dendrite
- Synaptic vesicle – A package of neurotransmitter molecules that fuses with the cell membrane when an impulse arrives, releasing the molecules
- Neurotubule – A specialized microtubule that conveys synaptic vesicles from the cell body to the axon terminal
- Synaptic knob – Enlarged end of an axon terminal
- Synaptic cleft – A cleft (or space) between the presynaptic and postsynaptic endings.
Due to the fact that the action potential of the neuronal cells cannot cross the synaptic cleft between neurons, the nerve impulse is carried by neurotransmitters. Each branch of a neuron ends in a bulb-like structure called synaptic knob, which possesses synaptic vesicles. These structures contain the neurotransmitters.
The neurotransmitter chemicals are created by the presynaptic neuron – a cell that is in charge of sending the impulse. After that, it is stored in synaptic vesicles at the end of the neuron’s axon. The postsynaptic neuron has chemically reactive ion channels in its membrane, or neuroreceptors, with dedicated linking sites for the neurotransmitters.
As junctions between the neurons in the central nervous system, the main function of the brain synapses is to make sure that the flow of impulses is one-directional.
How Are Synapses Formed in the Brain?
We can never find our path if we don’t look around and explore. In a similar manner, as neuron axons grow, they use their thread-like fibers (called filopodia) in order to find the way to the corresponding brain center where they need to deliver the information gained from the sensory stimuli.
These filopodia use the sensory stimuli gained via the sensory organs of touch, taste, and smell, in order to guide the axon to the direction of the corresponding neuron that will lead its way to the brain center at the receiving end.
As contact is made with other neurons, the axon forms a synapse based on the signaling dynamics between the two neurons. If their polarities match, this neuronal bond becomes permanent. Otherwise, the synapse is withdrawn and the axon continues in another quest to find a matching neuron, i.e. a new way to pass its information.
In other words, a brain synapse is formed by membranes of a presynaptic neuron and a postsynaptic neuron, which may or may not be separated by the synaptic cleft. When the neurotransmitters diffuse across the synaptic cleft, they bind to the neuroreceptors in the postsynaptic membrane.
In this way, the channels are triggered to open. This action sets off the process of depolarization of the postsynaptic cell membrane, which may activate the action potential of the neuronal cell, provided the threshold is reached.
What Are the Types of Brain Synapses?
The transmission of sensory information within the brain is achieved by using a combination of chemicals and electricity. Hence, there are two basic types of brain synapses created in the central nervous system:
- Electrical synapses (or gap junction) – Their function is to synchronize electrical activity among neurons while allowing the action potential to pass directly from one membrane to the next. Electrical synapses are faster than the chemical synapses since in these synapses ions flow directly between the neuronal cells.
- Chemical synapses – Most of the brain synapses are chemical. They are a gap between two neurons through which the information passes chemically, in the form of neurotransmitter molecules. Chemical synapses play a vital role in the processes of thought and perception.
Furthermore, synapses can be classified by the type of neurotransmitters and neuroreceptors used in the process of transmission of the impulse. These include:
- Excitatory ion channel synapse – A small gap at the end of a neuron that increases the likelihood of the firing action potential of a cell, i.e. the activity from one neuron increases the probability of activity in a neighboring neuron by initiating an inhibitory postsynaptic cell.
- Inhibitory ion channel synapse – A small gap at the end of a neuron that decreases the likelihood of the firing action potential of a cell, i.e. the activity from one neuron reduces the probability of activity in a neighboring neuron by initiating an inhibitory postsynaptic cell.
- Non-channel synapse – A membrane-bound enzyme that can alter both the number and sensitivity of the ion channel receptors in the same cell. These brain synapses are involved in slow and long-lasting response processes, like learning and memory.
- Neuromuscular junction (or myoneural junction) – A kind of chemical synapse created between motor neurons and muscle cells. It allows the motor neuron to transmit a signal to the muscle fiber, thus causing contraction of the muscle.
What Is the Role of Brain Synapses in Memory and Learning?
Long-term memories are stored in the brain in the form of synapses. Namely, memory is formed when specific groups of neurons are reactivated in the brain, i.e. synaptic connections between neurons are strengthened, or there is a formation of completely new synapses.
This leads to the concept of synaptic plasticity, or the ability of the synapses to strengthen and change. In other words, it’s a biological process by which specific patterns of synaptic activity result in changes in synaptic strength.
Synaptic strength also changes according to the number of stimuli received during a learning process. These changes contribute to the processes of learning and memory by aiding the encoding and trancing of the information stored in the dedicated part of the brain.
To Sum Up
Brain synapses are a junction between two neurons through which the brain and spinal column transfer sensory information to the corresponding parts of the body controlled by individual nerves.
Synapses are key to the brain’s function, especially when it comes to memory. These neural networks and their modified synaptic connections can account for the cognitive basis of learning and memory, as well as for deterioration of the memory.
Finally, brain synapses integrate, select, and transmit the sensory information to the corresponding neurons, thus alleviating the brain from an overload of informational impulses which could lead to fatigue of the central nervous system.
Fun Facts
Did you know?
- Neurotransmitters may also actively inhibit a receiving neuron from firing (passing the sensory information between two neurons via the axons).
- The filopodia are small and membranous protrusions that extend beyond dendritic stretches of developing neurons.
- There are over 100 trillion synapses in the human brain!
- The axodendritic synapse is one of the most common ones. The axon of this type of synapse connects with a dendrite of the postsynaptic neuron.
- Serotonin is a neurotransmitter that helps us with memory and learning.
- In comparison to computer memory, the memory capacity of the human brain can vary from 1 to 1,000 terabytes!
- During childhood, and particularly during adolescence, a natural process of elimination of the no longer needed synapses (known as “synaptic pruning”) occurs in the brain. In this way, the brain itself increases the efficiency of the neural network.
- If a neuron is constantly stimulated, the synapse will not be able to renew its supply of transmitter fast enough to continue passing the impulse across the cleft.
- In order to improve our memory, it would be a good idea for us to strengthen our brain synapses first. This can be done by challenging our mind with puzzles, crosswords, learning foreign languages, exercising (in order to increase the oxygen flow in the brain), avoiding routine, eating a nutritious diet, as well as sleeping sufficiently.
- An autapse is a form of a ‘self-synapse’, i.e. connection of the axon with dendrites of the same neuron.