A synaptic cleft is a gap between two neurons in a chemical synapse. It’s located between a presynaptic and a pre-junctional neuron, as well as between a postsynaptic and a post-junctional neuron.
The void can be found between a neuron and a non-neuronal cell, also (a muscle cell, glandular cell, or a sensory cell). Thanks to the void between the brain cells, the synaptic cleft controls and regulates the transmission of the nerve impulses between the cells.
What Is the Function of the Synaptic Cleft?
Apart from being anatomically labeled as a void space between two cells specialized in conducting nerve impulses and regulating their transmission, the synaptic cleft also functions as a junction that connects two or more neurons.
The synaptic clefts are also involved in the fusion and degradation of neurotransmitters. Namely, the synaptic clefts retain different enzymes. These enzymes participate in the degradation of the neurotransmitters that have been released through the neuron gap.
In this way, they break down the neurotransmitters, lowering their concentration, which slows down the transmission of the nerve impulse.
What Exactly Is a Brain Synapse?
The brain synapses are functional connections between the neurons present in the central nervous system and peripheral nerves. They integrate, select, and transmit the sensory information to the corresponding neurons.
In this way, they unburden the brain from an overload of informational impulses which could lead to fatigue of the central nervous system.
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 – An enlarged end of an axon terminal.
- Synaptic cleft – A cleft (or space) between the presynaptic and postsynaptic endings.
How Does a Brain Synapse Form?
A brain synapse consists of 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 Is a Depolarisation of the Neuron Cell?
The neurons receive signals from the sensory stimuli via their dendrites. After that, they transmit that information electrochemically. The outgoing signal continues its way to the other neuronal cells in the brain through the neuron’s axons, with the help of electrical and chemical signals.
The cell membrane of the neurons has specialized passageways that allow for the transport of matter from the inside to outside of the cell and vice versa. It allows some ions to pass through and blocks the passage of other ions.
The inside and outside of a neuronal cell will have different electrical charges (positive and negative) depending on what kinds of nutrients are in and around the cell. When this occurs, we are talking about the depolarization of the neuron cell.
Due to this occurrence, the inner surface of the membrane becomes less negative in relation to the outer surface (the neuronal cell becomes more positively charged, thus becoming less negative as a unit).
What Is a Chemical Synapse?
Most of the communication between the neurons, i.e. synapses, is chemical. This means that the neurons use chemical messengers to transmit the information from the sensory stimuli to the dedicated brain center for further processing.
For this reason, the action potential of the cell activates the presynaptic neuron to release the neurotransmitters. When released, the chemical molecules of the neurotransmitters bind to the postsynaptic cell’s receptor, receiving the signal that activates the action potential of the postsynaptic cell.
Why Do Synapses Have a Gap?
The synaptic gap is actually a channel through which the chemical impulses flow only in one direction to the neighboring neuron, i.e. from one neuron cell to another. At this ‘cell gate’, the impulses can be directed, diverted, branched out to several other cells, or blocked.
The gap between the synapses of the neuron cells, in fact, controls the strength of the signals received from the sensory stimuli.
What Is the Difference Between a Synapse and a Synaptic Cleft?
In the most simple terms, a synapse is a junction between two neurons, while the synaptic cleft is a gap between these two elements of the brain.
The synaptic cleft is a part of the synapse that supports the flow of the current that relays the information from one neuronal cell to another.
What Is the Structure of the Synaptic Cleft?
We can distinguish several types of a synaptic cleft based on the nerve fiber type that is involved in the formation of the synapse they belong to:
- Axo-axonic synapse – A type of synapse that is formed when one neuron projects its axon terminals onto the axons of another neuron’s axon.
- Axo-dendritic synapse – A type of synapse that is formed when one neuron projects its axon terminals onto the dendrites of another neuron.
- Axo-somatic synapse – A type of synapse that is formed when one neuron projects its axon terminals onto the body of another neuron.
- Dendro-dendritic synapse – A type of synapse that is formed between the dendrites of two neuronal cells.
What Are the Steps of Synaptic Transmission?
The synaptic transmission of the chemical impulses in the brain is a process developed in 5 stages:
- Stage 1 and 2 – Neurotransmitter Synthesis and Neurotransmitter Packaging – At this stage, the electrical signal cannot cross the gap between two neurons. Thus, the presynaptic neuron transforms the electrical signal into a chemical message, i.e. a neurotransmitter.
- Stage 3 – Release – The chemicals are launched into the synaptic cleft so they can reach the other neuron cell. The neurotransmitters flow to the gap between the neurons in small groups in order to open the calcium channels in the membrane.
After they’re open, the membranes of the small groups of neurons synthesize right at the synaptic cleft, and the neurotransmitters flow into the synaptic cleft.
- Stage 4 – Neurotransmitter Binding – When the other neuron cell receives the chemical signal, it translates this signal back into an electrical impulse by the receptors located in the cell’s membrane. Then, the electric impulse flows until it reaches the postsynaptic membrane.
- Step 5 – Stopping the Chemical Signal – Once the electric impulse reaches the postsynaptic cleft, the gap needs to be cleared fast. In fact, the signal must be stopped so that the synaptic cleft can receive new neurotransmitters.
For this reason, some of the neurotransmitters are returned to the presynaptic terminal, some are completely stopped, while some are absorbed by the postsynaptic terminal.
How Does the Synaptic Cleft Support Nerve Impulse Transmission?
Once the nerve cells are stimulated, they undergo chemical changes that produce tiny waves of electricity, i.e. nerve impulses. Next, the sensory information (in the form of impulses) reaches a junction labeled as a brain synapse and triggers the release of the neurotransmitters.
The 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).
Which Neurological Disorders Are Associated With the Alteration of the Synaptic Cleft?
The neurological disorders caused by an alteration of the synaptic cleft are labeled as synaptopathies. Such are:
- Fragile X Syndrome: Mental Retardation
- Autism Spectrum Disorder
- Addiction
- Long‐term depression
- Temporal lobe epilepsy
- Epilepsy
- Hyperekplexia
- Down syndrome
Which Drugs Are Activated in Their Full Potential at the Synaptic Gap?
All drugs change the way the neurons communicate, as well as the chemistry in the human brain. For example, we’re already too familiar with the effects of caffeine and alcohol on heart rate. They rapidly increase the fluctuation of blood through the arteries, and also change the behavior of the individual.
The synaptic cleft is the anatomical point of action of the following drugs:
- Nicotine
- Curare drug
- Alcohol
- Caffeine
- Strychnine
- Morphine
- Acetylcholinesterase inhibitors
- Methamphetamine
- Heroin
Some of the aforementioned drugs activate the receptors located in the cells’ membranes, while others block them.
However, they all alter the chemistry of the brain, resulting in changes in behavior leading to various neurological conditions, etc.
To Sum Up
Synapses are the basic structural elements of the neurons in the brain. They allow the information gained from the sensory stimuli to flow in a coordinated manner throughout the central nervous system.
The synaptic gap is a crucial anatomical point in the process of transmission of that information for the fine-tuning of our reactions to the world that surrounds us, the control of a variety of body functions, for the storing and consolidation of memories, as well as for the regulation of our emotions.
Thus, optimal synaptic communication is of vital importance for the proper functioning of the brain, because the alteration of the synapse between the neurons can result in severe neurological disorders.
Fun Facts
Did you know?
- Synaptic plasticity refers to 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.
- 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 oxygen flow in the brain), avoiding routine, eating a nutritious diet, as well as sleeping sufficiently.
- 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.
- There is a special type of chemical substances that become a part of the biomolecules in order to block the biological response of the receptors called antagonist drugs or blockers.
- Without the neurotransmitters, our brain would lose the ability to make use of the chemical molecules which are of vital function for the body.
- The synapse will not be able to renew its supply of neurotransmitters fast enough to continue passing the impulse across the cleft if a neuron is constantly stimulated.
- There are more than 200 distinctive neurotransmitters identified in the human brain. The most important ones both for the brain and body are serotonin, dopamine, glutamate, and norepinephrine.
- The neurotransmitter glutamate activates a large number of receptors within the postsynaptic membrane of the synapse.
- The typical width of a synaptic cleft measures about 0.02 micron.