Synaptic Knob

Synaptic knobs, also known as axon terminals or terminal boutons, are the tiny, rounded endings of axons where neurons form synapses with other neurons, muscle cells, or glands. These structures are crucial in the transmission of nerve impulses. 

1.0What is Synaptic Knob?

Synaptic knobs are specialized structures located at the end of an axon, housing synaptic vesicles filled with neurotransmitters. When an action potential (nerve impulse) reaches the synaptic knob, it initiates a sequence of events that result in the release of neurotransmitters into the synaptic cleft (the small gap between the synaptic knob and the target cell). These neurotransmitters then bind to receptors on the postsynaptic cell membrane, triggering a response in the target cell.

2.0Location of Synaptic Knob

• At the End of Axons: Synaptic knobs are located at the terminal ends of axons, which are the long, threadlike extensions of a nerve cell responsible for transmitting impulses from the cell body to other cells.
• Near Synapses: Synaptic knobs are positioned adjacent to the synaptic cleft, the small gap between the presynaptic neuron (the neuron sending the signal) and the postsynaptic neuron (the neuron receiving the signal), or another target cell such as a muscle or gland cell.

3.0Synaptic Transmission

• The information passes from one neuron to another through the synapse. So, the action potential reaches the end of the axon; it reaches the terminal boutons.
• The point where the terminal button from one neuron contacts the dendrite of another neuron is known as a synapse. 
• Information passes at these junctions formed with the next neuron. 
• Synapses occur at three places on the next neuron, mainly on the dendrites, soma or axons. 
• Thus, synapses are known as axodendritic, axosomatic, and axoaxonic synapses. 

Synapses can be categorized into two types: electrical synapses and chemical synapses.

1. Electrical Synapse 

• The membranes of pre- and postsynaptic neurons are in close proximity at electrical synapses, allowing electrical current to flow directly from one neuron to another. The transmission of impulses across electrical synapses resembles the conduction of impulses along a single axon, and it is always faster compared to chemical synapses. However, electrical synapses are relatively rare in the human body. This contrasts with the transmission of nerve impulses across chemical synapses.

2. Chemical Synapse 

• At a chemical synapse, the pre- and postsynaptic neuron membranes are separated by a fluid-filled gap known as the synaptic cleft. Neurotransmitters, the chemicals responsible for transmitting impulses, are stored in vesicles within the axon terminals. When an action potential reaches the axon terminal, it triggers these synaptic vesicles to move toward and fuse with the plasma membrane, releasing neurotransmitters into the synaptic cleft.
• These neurotransmitters bind to specific receptors on the postsynaptic membrane, leading to the opening of ion channels and the entry of ions. This ion movement can generate a new potential in the postsynaptic neuron, which can be either excitatory (EPSP) or inhibitory (IPSP).

4.0Structure of a Synapse

• The synapse is made of three structures. 
• These are the synaptic knob, the synaptic cleft and the plasma membrane of the postsynaptic neuron.
• The synaptic knob is a tiny bulge that is there when the terminal buttons end. 
• There are a number of vesicles or sacs in the bulge which contain numerous neurotransmitters.
• The synaptic cleft is the space between neurons . Information cannot pass directly from one neuron to the other. 
• The information is transmitted by converting the first neuron's electrical signal to a chemical signal that passes across the gap before it is converted back into an electrical signal in the second neuron. 
• Presynaptic terminals consist of synaptic vesicles ("fluid-filled sac"). 
• The chemical molecules, known as neurotransmitters, are released from the synaptic vesicles into the cleft and move about with the help of the extracellular fluid in the cleft. 
• The plasma membrane of the postsynaptic neuron is the membrane of the neuron where the information is going. 
• Certain receptors are present on this membrane where the neurotransmitter molecules come and attach themselves. 

Steps of Synaptic Transmission

• When the nerve impulse reaches the terminal buttons of the presynaptic neuron, the calcium ions move inside the membrane very rapidly.
• This causes the vesicles to move about in the synaptic knob and merge with the walls of the presynaptic neuron membrane. When this happens, then the neurotransmitters are released from the vesicles.
• Neurotransmitters move across the synaptic cleft and try to reach the plasma membrane of the postsynaptic neuron. 

• There, they bind with the receptors placed on the membrane of the postsynaptic neuron.
• This causes a local postsynaptic potential. The excitatory neurotransmitters cause the sodium ions to come inside the membrane much faster than the potassium ions moving out from the membrane.
• This state is known as excitatory postsynaptic potential (EPSP). 
• Once the threshold point of EPSP is reached, the action potential is initiated in the postsynaptic membrane. 
• The inhibitory neurotransmitters open the potassium channels, causing the potassium ions to move inside.
• This makes the membrane much more negative than in the resting position.
• This temporary hyper-polarization state is known as inhibitory postsynaptic potential (IPSP). 
• Neurotransmitters that do not bind to the receptors are then sent back to the synaptic knob.
• There, they are either taken back into the synaptic vesicles to be used again in a process known as reuptake or degraded using the synaptic enzymes. 
• In this way, the synapse is cleared for the next release of neurotransmitters. 

5.0Function of Synapse

• Synapse plays an important part in the functioning of the nervous system.  
• It helps connect the neurons via synapses and hence sends information that mediates behavioral responses. 
• If there is any dysfunction in the synaptic activity, then it may lead to a change in behavior and cause depression, schizophrenia, etc.  
• Synapse makes sure that impulses traveling across neurons should follow one direction only. 
• Since transmitters are present only in the presynaptic membrane, receptor molecules can be found only in the postsynaptic membrane. 
• Due to this, impulses travel in one direction only. Synapses help integrate the impulses traveling down from different neurons.  
• It helps filter out unwanted and unnecessary stimuli. In order to cross a synaptic cleft, an impulse must reach an action potential of +40mV. If an impulse is weak, i.e., less than +40mV, it will not be able to generate enough neurotransmitters, and thus, no communication will occur between neurons. 
• As a result, our body will not react to such stimuli, which helps filter out unnecessary stimuli.

6.0Neurotransmitters

• Neurotransmitter is a chemical found in the synaptic vesicles and, when released, has an effect on the next cell. 
• As the name suggests, it is inside a neuron and transmits a message. When neurons fire, neurotransmitters are released from their terminal buttons.
• Neurotransmitters are classified into three classes of small-molecule neurotransmitters: amino acids, monoamines, and acetylcholine. 
• There is a fourth group in this category known as unconventional neurotransmitters. 
• One group of large-molecule neurotransmitters is neuropeptides. 
• Neurotransmitters most often produce either excitation or inhibition. 
• But a few neurotransmitters produce excitation in one situation and inhibition in the other. 
• There are various neurotransmitters, including excitatory neurotransmitters such as acetylcholine (ACh), catecholamines, Glutamate, histamine, serotonin, and some neuropeptides. 

1. Acetylcholine

• ACh was the first neurotransmitter identified. 
• ACh plays an important role in neuromuscular function, sleep regulation, learning, and memory. 
• It also stimulates the skeletal muscles to contract but slows contraction in heart muscles. 

2. Gamma- Aminobutyric Acid (GABA)

• The inhibitory neurotransmitters include Gamma-Aminobutyric Acid (GABA), glycine, and some peptides. 
• Amine neurotransmitters are responsible for emotions, control of motor actions, etc. Monoamines are like dopamine, nor-epinephrine, epinephrine, melatonin and serotonin. Epinephrine and norepinephrine are involved in motor functions. 
• Any imbalance in the presence of the neurotransmitter GABA may also predispose conditions for stroke when Glutamate destroys certain neurons. 
• GABA is a major neurotransmitter with an inhibitory effect. 
• It helps reduce anxiety. 

3. Dopamine

• Dopamine (DA) is found in the brain. 
• It helps to maintain body balance. 
• When it is deficient, it leads to tremors and overstimulation of the muscles, which is responsible for Parkinsonism. If too much DA is released, it may be a cause of Schizophrenia. 
• Dopamine (DA) may have both excitatory and inhibitory effects depending on the synapse being affected. It is involved in regulating mood, emotions, sleep and appetite. Amino acids are the most common neurotransmitters involved in protein synthesis. 

4. Glutamate

• Glutamate is the major neurotransmitter with an excitatory effect. 
• An excess of Glutamate may result in overactivation and neuronal damage. Neuropeptides, called endorphins, have pain-reducing effects on the body. 
• The preceding account of neurotransmitters reflects on how the chemical substance may have an excitatory or inhibitory effect.