Synapses

Introduction

The term synapse refers to the small gap formed between the neuron cells through which the information gained from sensory stimuli and the organs of sense (skin, eyes, nose, ears, mouth) is relayed to the processing brain areas. Information passes from one neuron to another at this particular point of the synapse. 

 

The synapses produce fast responses to the sensory stimuli via chemical or electrical impulses. In this way, they prevent the cortical structures from becoming overburdened by the information-carrying impulses, which would result in dysfunction of the central nervous system.

 

 

What Are the Main Types of Synapses?

 

There are two main types of synapses in our body:

• Electrical synapses

• Chemical synapses

What Are Electrical Synapses and How Do They Function?

When the sensory stimuli signal is directly transmitted from one neuron cell to another, we refer to it as an electrical synapse. This type of synapse provides the free flow of ions, creating the action potential between the neuron cells at the gap junctions. 

 

When the membranes of two cells connect, the cell’s pores create a gap between them that allows only certain types of neurotransmitters and types of molecules and ions to pass through. Due to the direct flow of the information gained from the neuron cells’ sensory stimuli, the synapses allow a bi-directional relay of information. 

What Are Chemical Synapses and How Do They Function?

Most of the synapses that occur in our bodies are chemical ones. Unlike the electrical synapses, the chemical synapses require a neurotransmitter to relay the information gained from the sensory stimuli from one neuron cell to another.  

 

Namely, when the action potential triggers a neurotransmitter at the axon terminal, they spread over the synaptic cleft and bind to the receptors located on the postsynaptic neuron cell’s membrane. This, in turn, changes the potential of the cell’s membrane. 

The Path of an Electrical or Chemical Impulse Through a Neuron

When the neuron receives an information-carrying impulse, it must relay it to the neighboring neuron cell, at the same time making sure that the impulse stays on the right path to the dedicated processing areas of the brain. This is achieved via the filopodia, i.e., the fibers branching from the neuron cell’s axons. These thread-like fibers follow the brain center’s corresponding neuronal route, where they need to deliver their information received from our environment. 

 

Next, the neural impulse occurs in the neuron cell’s dendrite and flows through its body. From here, it’s relayed via the axon of the neuron as an electric or chemical impulse. When the two cells connect, they create a synapse. 

 

This bond between two neuron cells becomes permanent, provided the polarity of their membranes matches. If that’s not the case, the synapse will be broken, and the neuron cell’s axon will continue to search for the adequate neuron cell to transmit its information. 

What Are the Other Types of Synapses?

According to the type of neurotransmitters and neuroreceptors that support the process of transmission of information between the neuron cells, we can distinguish the following types of synapses:

• Excitatory ion channel synapses,

• Inhibitory ion channel synapses,

• Non-channel synapses,

• Neuromuscular junctions.

Excitatory Ion Channel Synapses

The excitatory ion channel synapses have sodium channeled neuroreceptors. The action potential at the small gap between the neuron cells increases the probability of action in the postsynaptic neuron cell because the ion channels provide an unobstructed flow of sodium into the cell. This, in turn, creates a positive action potential of the cell. 

Inhibitory Ion Channel Synapses

When the synaptic gap decreases the cell’s probability to fire an action potential of the neuron, it’s referred to as an inhibitory ion channel synapse. Inhibitory ion channel synapses have chloride channeled neuroreceptors. When the negative ions enter the cell body through the open chloride channels, they inhibit the next cell’s impulse that relays the information. 

Non-Channel Synapses

This type of synapses has a membrane-bound enzyme instead of receptors that have channels for the chemical and electrical impulses. When the neurotransmitter activates these synapses, they produce neuro-transmitting chemicals within the cell, and by that, change the neuron cell’s metabolism. The non-channel synapses mostly occur during long-lasting responses of the brain, such as memory and learning.

Neuromuscular Junctions

Like the name itself implies, the neuromuscular junction is the type of synapse formed between the motor neurons and the muscle neuron cells. When the motor neuron relays the sensory information to the muscles, they receive a brain signal to contract. 

 

This junction is supported by the acetylcholine, a neurotransmitter which always has an excitatory influence in the synapse. 

What Are the Main Elements of a Synapse?

The functional synaptic connections between the neuron cells in the brain are comprised of the following elements:

• Presynaptic nerve ending,

• Postsynaptic nerve ending,

• Synaptic cleft,

• Synaptic vesicle,

• Synaptic knob,

• Neurotubule,

• Dendrites.

Presynaptic Nerve Ending

The presynaptic nerve ending comprises the following elements:

• Neurotransmitters,

• Mitochondria,

• Various cell organelles. 

Postsynaptic Nerve Ending

The postsynaptic nerve ending is comprised of the following elements

• Receptor sites dedicated for the neurotransmitters,

• A synaptic cleft. 

Synaptic Cleft

The gap between the presynaptic and the postsynaptic neuron is labeled as a synaptic gap. It enables the flow of information in the form of electrical or chemical impulses between two neuron cells. It also directs the corresponding neurons’ impulses that distribute the information to a dedicated processing area of the cerebral cortex. 

Synaptic Vesicle

These small membraneous sacs have a significant role in the process of neurotransmission. Namely, the synaptic vesicles transmit the neurotransmitters from the neuron cell body to the target cell’s presynaptic membrane where the neurotransmitters are released. 

 

With the help of kinesin (a type of motor protein), the synaptic vesicles flow along the axon terminal until they reach the nerve terminal. 

Synaptic Knob

The axon terminals have enlarged structures at their ends, labeled as synaptic knobs. This is when the calcium molecules enter the neuron cell and trigger the release of neurotransmitters in the synaptic vesicles. 

Neurotubule

The neurotubules support synaptic vesicle, RNA, and organelle transportation, from the neuron cell body to the axon terminal. 

Dendrites

The tree-like structures that branch out of the neuron cell body are labeled as dendrites. They receive signals from the sensory stimuli at the synaptic junctions. These signals can generate an electrical impulse (excitatory function), or they can prevent the neuron from firing the electrical information-carrying impulse (inhibitory function). 

How Is the Impulse Transmitted Across the Synaptic Cleft?

What Are Neurotransmitters?

Neurotransmitters are chemical molecules that relay information from the neuron cells’ sensory stimuli because the cells’ action potential cannot pass through the synaptic cleft. 

How Many Types of Neurotransmitters Are There?

There are two classes of neuro-transmitting molecules in the cerebral cortex:

• Small molecule neurotransmitters (Aminoacids, Acetylcholine, Gasotransmitters, Biogenic amines, Monoamines, Trace amines, Nitric oxide, Peptides, Carbon monoxide); 
• Neuropeptides (Adrenomedullin, Arthropod CHH/MIH/ GIH/VIH hormone, Bradykinin, Calcitonin, Cystatin, Glucagon, Myomodulin, Myosuppressin, Natriuretic peptide, Somatotropin/Prolactin, Vasopressin/Oxytocin, etc.)

What Are the 7 Major Neurotransmitters?

The most active and essential neurotransmitters in our brain are the following:

• Acetylcholine – This neurotransmitter can be found at the neuromuscular junction. It’s released by the neurons located in the peripheral nervous system and a smaller number of neurons located in the central nervous system. As an inhibitory neurotransmitter, it influences the heart rate by slowing it down, while as an excitatory neurotransmitter, it supports the contraction of muscles. 

• Dopamine – Released by a small number of neurons in the brain, dopamine reacts to stimulants that our brain translates topleasure such as food, music, etc. Together with the hormone serotonin, it’s also known as the happiness hormone. 

• Gamma-aminobutyric acid (GABA) – Being an inhibitory neurotransmitter, the GABA molecules inhibit the target neuron cell’s activity. They participate in the regulation of anxiety, in the control of the motor neuron function, and the regulation of fear. 

• Glutamate – The brain and the spinal cord are the main locations of this neurotransmitter. With its excitatory function, it supports the learning processes and formation of short-term and long-term memory. Lack of glutamate can lead to destruction, i.e., death of the brain cells. 

• Histamine – This organic compound supports the immune system. It controls and regulates the inflammatory processes. Histamine also alerts the body to the presence of an allergic substance. 

• Norepinephrine – This neurotransmitter is also labeled as a stress-hormone since its release is triggered by a stressful event. For this reason, norepinephrine prepares both the brain and the body for defense by activating the ‘fight-or-flight reaction.’ 

• Serotonin – The serotonin neurotransmitter participates in the cognitive processes, memory, and hormonal activity, as well as appetite, the regulation and control of the sleep-wake cycle, the regulation of the body’s temperature, emotions, mood, etc. 

What Is the Difference Between Neurotransmitters and Neuropeptides?

Both neurotransmitters and neuropeptides participate in the transmission of the information-carrying impulse between the neurons.  While neurotransmitters are small and fast-acting molecules that are released individually after the action potential at the synaptic cleft, neuropeptides are large and slow-acting molecules that trigger a prolonged response.

 

Neuropeptides are always released to the synaptic cleft along with molecules of another neurotransmitter.  Also, their target site is still different from their original location. 

What Is the Main Function of the Synapses?

The synapses in the brain are responsible for transmitting the information via electrical or chemical impulses in only one direction. If it weren’t for the synapses, the sensory input could be passed on to neurons that do not lead the information to the brain’s corresponding processing area. This could cause the brain to misinterpret some of the received data from our environment and give us a wrong signal. 

Fun Facts

Did you know?

1. Action potentials are unable to pass through the synaptic cleft. 
2. A single neuron cell body can have several thousands of synapses formed on its dendrites. 
3. The neuromuscular junction is also labeled as a myoneural junction. 
4. The axodendritic synapse is the most commonly occurring type of synapse in the human brain.
5. Electrical synapses are faster than chemical synapses when it comes to transmitting the impulse to the target neuron cell.
6. The speed of the impulse, carrying information from the sensory stimuli, is 0.5 ms while passing through a synapse. 
7. Glutamate is the most commonly found neurotransmitter in the brain. 
8. A presynaptic neuron refers to the neuron cell that relays the impulse, which contains information, to another neuron cell. 
9. A postsynaptic neuron receives information via its neuroreceptors.
10.  The neuromuscular junction is the most secure synapse in the brain.