What are Synaptic Knobs?

The synaptic knobs (synaptic terminals) are the ends of the neuron that are associated with the signaling of the neuronal impulses. The neurotransmitters enclosed in the vesicle fuse with these synaptic terminals and the chemical within the vesicle is released. The chemical interacts with the postsynaptic end and induces a change in the membrane potential.

The Function of the Synaptic Knob

• A neuron discharges the neurotransmitters into the region between two neurons, called the synaptic cleft. The neurotransmitters are chemical messengers that bind to specific receptors and activate or deactivate a neuron/cell.
• The neurotransmitters are stored in the secretory vesicles and are discharged into the synaptic cleft (or synaptic junction) by a process called exocytosis. The space between the neuron and a muscle fiber is known asthe neuromuscular junction.
• When the neurotransmitters are released into the synaptic cleft, they bind with their suitable receptors present on the membrane of the postsynaptic neuron.
• The neurotransmitters are discharged from the synaptic end by numerous potential events. The released chemicals either undergo enzymatic degradation or are carried by transporters.
• The process of neurotransmitter release is initiated by an electrochemical excitation known as the action potential, which travels from the dendrites to the axon terminal of the presynaptic neuron.
• The electrical depolarization takes place at the presynaptic membrane which triggers the opening of certain channels that facilitate the movement of calcium ions into the presynaptic neuron.
• The calcium ions move through these channels into the presynaptic neuron. The continuous influx of calcium ions increases the concentration of these ions inside the presynaptic neuron.
• The elevated calcium ion concentration initiates several calcium-sensitive proteins associated with the secretory vesicles. These proteins facilitate the fusion of the secretory vesicle with the presynaptic membrane. As a result, the vesicles open, and the neurotransmitter is dumped into the synaptic cleft.
• The neurotransmitters diffuse into the synaptic cleft, where some of them escape while others bind to the chemical receptors present on the postsynaptic membrane.
• The synaptic process is found to influence the role of the postsynaptic cell. Soon, the neurotransmitter molecules detach from the receptors and are reabsorbed by the presynaptic cell. These neurotransmitters are further repacked to ensure the availability of these chemical messengers during the next electrical impulse.

Structure of the Synaptic Knob

The synaptic knobs mediate the functional connection found between the neurons or neurons and other cells in the body. The synapses are concerned with the connection of the axons and the dendrites of the adjacent neurons. The synaptic knobs regulate the passage of information from the presynaptic cell to the postsynaptic cell. The axon terminal found in the synaptic knobs is the region of the axon compared to large neurotransmitters, which are found packed in the form of vesicles. These vesicles attach to the presynaptic plasma membrane at particular regions called active zones during exocytosis. The region that lies opposite to the presynaptic cell is the postsynaptic cell which possesses the receptors for the neurotransmitters. 

The interlinked proteins found in the postsynaptic membrane are known as postsynaptic receptors. The protease found in the postsynaptic density is concerned with the anchoring of the neurotransmitter to its receptor as well as detecting the modulation of the activity in the respective receptor. The synapses are described to be symmetric or asymmetric, where the asymmetric synapses are identified to be rounded vesicles found in the presynaptic cell and the postsynaptic density. The asymmetric synapses are excitatory and are associated with the activation of the postsynaptic cell whereas the symmetric synapses are inhibitory and inhibit the activity of the postsynaptic cell. The small volume of the cleft manages the concentration of the neurotransmitter. The autopsy is a type of synapse that develops from the connection between the axons and dendrites of the same neuron.

Discharge of Neurotransmitter from Synaptic Knob

The arrival of nerve impulses initiates the release of a neurotransmitter through cellular secretion. The arriving action potential can generate an influx of calcium ions by voltage-dependent ion channels.

Binding to the Receptor

The receptors located in the opposite direction of the presynaptic membrane initiate the binding of the neurotransmitter molecules. The binding opens the ligand-gated ion channels located on the postsynaptic cell membrane, resulting in the movement of ions in or out of the cell. The ionic movement across the postsynaptic membrane alters the voltage of the postsynaptic neuron, known as the postsynaptic potential. The postsynaptic potential generates depolarizing currents and hyperpolarizing currents in the neuron. The type of ion channel associated with the signaling determines the inhibitory or excitatory nature of the receptor.

The adjoined neurotransmitter must be detached from the site of the receptor for the continuous flow of impulses in the neurons. The thermally induced oscillations can move the neurotransmitter from the attached site. Several enzymes are associated with disconnecting the neurotransmitters from the receptor molecules. Certain pumps called the reuptake pumps to move the neurotransmitters back to the presynaptic axon terminal for re-releasing and reprocessing the neurotransmitter in the next impulse. This reuptake process decreases the continuous signaling by the neuron. Thus, the neurotransmitters undergo a recycling process.

The desensitization of the receptor may take place as the postsynaptic receptors lower the response towards the neurotransmitters. The strength of the synaptic knob connection diminishes with an ongoing action potential in a neuron and gives rise to a condition known as the frequency dependence of the synapses. This property is exploited by the action of the nervous system.

Contexts and Applications

This topic is a part of the syllabus for Bachelors of Medicine and Bachelor of Surgery (MBBS).

Related Concepts

Synaptic plasticity, volume transmission, acclimatization