ˈak.son ˈtɚmɪnəl An axon terminal is any of the button-like endings of axons through which axons make synaptic contacts with other nerve cells or with effector cells. At the axon terminal, synaptic vesicles containing neurotransmitters are docked. Upon activation by a graded potential or by an action potential of the presynaptic neuron, the cell allows the entry of calcium ions. This triggers a cascade reaction resulting in the synaptic vesicles fusing to the membrane of the axon terminal. The synaptic vesicle proteins that are activated by the calcium ions form fusion pores through which the neurotransmitters can exit. The neurotransmitters exert their effects on the target cell within a limited span of time. Soon after, the neurotransmitters are absorbed back to the presynaptic neuron or degraded metabolically by enzymes. The axon terminal is therefore essential in cell to cell communication. It is crucial in providing a means for neurotransmitters to exit the neuron and relay signals to the target cell. An axon terminal refers to the axon endings that are somewhat enlarged and often club- or button-shaped. Axon terminals are that part of a nerve cell that make synaptic connections with another nerve cell or with an effector cell (e.g. muscle cell or gland cell). EtymologyThe term axon came from the Ancient Greek ἄξων, meaning “áxōn” or “axis”. The term terminal is from Latin terminalis, which pertain to a boundary or to an end. It came from Latin terminus, meaning “a limit” or “an end”. Synonyms: axon terminal bouton; bouton terminaux; pieds terminaux; synaptic bouton; synaptic knob; synaptic ending; synaptic terminal; terminal bouton. Neuronal structure
This is an illustration of a neuron or a nerve cell with parts indicated: soma, dendrites, and axon. The distal portion of the axon is the axon terminal. A neuron (also called a nerve cell) is an excitable cell in the body of higher animals, including humans. The cell can be distinguished from other cell types in having distinctive parts, such as soma, dendrites and axons.
Axon terminal and synapseAn axon terminal contains various neurotransmitters that are released at the small gap between two communicating neurons. This gap is called a synapse. The neuron that sends nerve impulses by releasing neurotransmitters via the axon terminal at the synapse is called a presynaptic neuron. In contrast, the neuron that receives the impulse is called a postsynaptic neuron. The synapse that serves as the junction between two neurons may be of two types: a chemical synapse or an electrical synapse.
Synaptic activity
A schematic diagram of a typical chemical synapse. Notice that the synaptic vesicles fuse to the membrane at the axon terminal of the presynaptic neuron. This is triggered by an influx of calcium ions. The neurotransmitters are released into the synaptic cleft (synapse) by exocytosis. They bind to the receptors on the surface of the dendrite of the postsynaptic neuron. Neurons transmit nerve impulses by means of electrochemical signals and neurotransmitters. Neurotransmitters belong to a group of chemicals that are released on cue. Neurotransmitters are stored in synaptic vesicles. These vesicles are located in the axon terminal of a presynaptic neuron of the central or peripheral nervous system. There are many types of neurotransmitters and they may be either excitatory or inhibitory. Examples of neurotransmitters are acetylcholine, noradrenaline, adrenaline, dopamine, glycine, y aminobutyrate, glutamic acid, substance P, encephalins, endorphins, and serotonin. These chemicals are responsible for relaying signals from a neuron to the target cell across a synapse. They are kept inside the synaptic vesicles. The vesicles then move down the axon and finally to the axon terminal where they will be clustered near the plasma membrane. The transmission of nerve impulses begins with a graded electrical potential or with an action potential traveling along the membrane of the presynaptic neuron until the synapse. Electrical depolarization of the membrane at the synapse will lead to increased permeability to calcium ions. An influx of calcium ions activates calcium-sensitive proteins into releasing the neurotransmitters into the synaptic cleft. These neurotransmitters bind to the receptors of the target cell. If the target cell is another neuron, the neurotransmitters bind to the postsynaptic receptors on the dendritic membrane of the postsynaptic neuron. The neurotransmitters may act as either excitatory or inhibitory. An example is when a neuron receives more excitation than inhibition from the neurons connected to it, the neuron will be activated to generate a new action potential at its axon hillock to signal the release of neurotransmitters that will relay the impulse to the new target cell (e.g. another neuron). The neurotransmitters in the synaptic cleft are often available for only a short period of time. Those that did not bind to the post-synaptic receptors and therefore have not been used for synaptic transmission, their possible fates are as follows: (1) reuptake by the presynaptic neuron, (2) metabolic degradation by enzymes. ImportanceThe axon terminal is the part of the axon that releases the neurotransmitters that relay signals across a synapse. For example, in a neuromuscular junction, the axon terminal releases neurotransmitters to relay nerve impulses from the neuron to the target cell, which may be another neuron, a muscle cell, or a gland cell. Acetylcholine, for example, is the neurotransmitter that is released by a neuron to stimulate a muscle cell. If the axon terminal fails to release acetylcholine, there will be no chemical to activate the target muscle. As such, this could lead to paralysis. Apart from acetylcholine, other major neurotransmitters that the axon terminal provides a way across the synapse are dopamine, norepinephrine, epinephrine, serotonin, oxytocin, somatostatin, etc. These chemicals are important chemical regulators of many biological activities. Without an axon terminal to mediate their release from the neuron, these chemicals, similar to acetylcholine, will not be able to exert their crucial role. Thus, the communication between cells will be obstructed by a dysfunctional axon terminal. Apart from that, the reuptake of unused neurotransmitters from the synaptic cleft back to the presynaptic neuron would be affected if there is no functional axon terminal. Try to answer the quiz below to check what you have learned so far about axon terminals. References© Biology Online. Content provided and moderated by Biology Online Editors |