Which mimics the effects of a neurotransmitter molecule

The terminal buttons contain synaptic vesicles that house neurotransmitters , the chemical messengers of the nervous system. Axons range in length from a fraction of an inch to several feet. In some axons, glial cells form a fatty substance known as the myelin sheath , which coats the axon and acts as an insulator, increasing the speed at which the signal travels.

The myelin sheath is crucial for the normal operation of the neurons within the nervous system: the loss of the insulation it provides can be detrimental to normal function. Multiple sclerosis MS , an autoimmune disorder, involves a large-scale loss of the myelin sheath on axons throughout the nervous system. The resulting interference in the electrical signal prevents the quick transmittal of information by neurons and can lead to a number of symptoms, such as dizziness, fatigue, loss of motor control, and sexual dysfunction.

While some treatments may help to modify the course of the disease and manage certain symptoms, there is currently no known cure for multiple sclerosis. In healthy individuals, the neuronal signal moves rapidly down the axon to the terminal buttons, where synaptic vesicles release neurotransmitters into the synapse. The synapse is a very small space between two neurons and is an important site where communication between neurons occurs.

Once neurotransmitters are released into the synapse, they travel across the small space and bind with corresponding receptors on the dendrite of an adjacent neuron. The neurotransmitter and the receptor have what is referred to as a lock-and-key relationship—specific neurotransmitters fit specific receptors similar to how a key fits a lock. The neurotransmitter binds to any receptor that it fits. Each vesicle contains about 10, neurotransmitter molecules.

We begin at the neuronal membrane. The neuron exists in a fluid environment—it is surrounded by extracellular fluid and contains intracellular fluid i. The neuronal membrane keeps these two fluids separate—a critical role because the electrical signal that passes through the neuron depends on the intra- and extracellular fluids being electrically different.

This difference in charge across the membrane, called the membrane potential , provides energy for the signal. The electrical charge of the fluids is caused by charged molecules ions dissolved in the fluid. The semipermeable nature of the neuronal membrane somewhat restricts the movement of these charged molecules, and, as a result, some of the charged particles tend to become more concentrated either inside or outside the cell.

Like a rubber band stretched out and waiting to spring into action, ions line up on either side of the cell membrane, ready to rush across the membrane when the neuron goes active and the membrane opens its gates i. Ions in high-concentration areas are ready to move to low-concentration areas, and positive ions are ready to move to areas with a negative charge. In addition, the inside of the cell is slightly negatively charged compared to the outside. This provides an additional force on sodium, causing it to move into the cell.

Other molecules, such as chloride ions yellow circles and negatively charged proteins brown squares , help contribute to a positive net charge in the extracellular fluid and a negative net charge in the intracellular fluid. From this resting potential state, the neuron receives a signal and its state changes abruptly. With this influx of positive ions, the internal charge of the cell becomes more positive.

If that charge reaches a certain level, called the threshold of excitation , the neuron becomes active and the action potential begins. At the peak of the spike, the sodium gates close and the potassium gates open. As positively charged potassium ions leave, the cell quickly begins repolarization. At first, it hyperpolarizes, becoming slightly more negative than the resting potential, and then it levels off, returning to the resting potential. During the action potential, the electrical charge across the membrane changes dramatically.

This positive spike constitutes the action potential : the electrical signal that typically moves from the cell body down the axon to the axon terminals. The electrical signal moves down the axon like a wave; at each point, some of the sodium ions that enter the cell diffuse to the next section of the axon, raising the charge past the threshold of excitation and triggering a new influx of sodium ions.

The action potential moves all the way down the axon to the terminal buttons. The action potential is an all-or-none phenomenon. In simple terms, this means that an incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation. There is no in-between, and there is no turning off an action potential once it starts.

Think of it like sending an email or a text message. You can think about sending it all you want, but the message is not sent until you hit the send button.

When neurotransmitters are accepted by the receptors on the receiving neurons, their effect may be either excitatory i. Furthermore, if the receiving neuron is able to accept more than one neurotransmitter, it will be influenced by the excitatory and inhibitory processes of each. If the excitatory effects of the neurotransmitters are greater than the inhibitory influences of the neurotransmitters, the neuron moves closer to its firing threshold; if it reaches the threshold, the action potential and the process of transferring information through the neuron begins.

Neurotransmitters that are not accepted by the receptor sites must be removed from the synapse in order for the next potential stimulation of the neuron to happen. This process occurs in part through the breaking down of the neurotransmitters by enzymes, and in part through reuptake , a process in which neurotransmitters that are in the synapse are reabsorbed into the transmitting terminal buttons, ready to again be released after the neuron fires.

More than chemical substances produced in the body have been identified as neurotransmitters, and these substances have a wide and profound effect on emotion, cognition, and behaviour.

Neurotransmitters regulate our appetite, our memory, our emotions, as well as our muscle action and movement. And as you can see in Table 4. Drugs that we might ingest — either for medical reasons or recreationally — can act like neurotransmitters to influence our thoughts, feelings, and behaviour. An agonist is a drug that has chemical properties similar to a particular neurotransmitter and thus mimics the effects of the neurotransmitter.

When an agonist is ingested, it binds to the receptor sites in the dendrites to excite the neuron, acting as if more of the neurotransmitter had been present. As an example, cocaine is an agonist for the neurotransmitter dopamine. Because dopamine produces feelings of pleasure when it is released by neurons, cocaine creates similar feelings when it is ingested.

An antagonist is a drug that reduces or stops the normal effects of a neurotransmitter. When an antagonist is ingested, it binds to the receptor sites in the dendrite, thereby blocking the neurotransmitter. As an example, the poison curare is an antagonist for the neurotransmitter acetylcholine.

When the poison enters the brain, it binds to the dendrites, stops communication among the neurons, and usually causes death. Still other drugs work by blocking the reuptake of the neurotransmitter itself — when reuptake is reduced by the drug, more neurotransmitter remains in the synapse, increasing its action.

Skip to content Chapter 4. Brains, Bodies, and Behaviour. Learning Objectives Describe the structure and functions of the neuron. Draw a diagram of the pathways of communication within and between neurons. List three of the major neurotransmitters and describe their functions. Key Takeaways The central nervous system CNS is the collection of neurons that make up the brain and the spinal cord. Neurons are specialized cells, found in the nervous system, which transmit information.

Neurons contain a dendrite, a soma, and an axon. Some axons are covered with a fatty substance known as the myelin sheath, which surrounds the axon, acting as an insulator and allowing faster transmission of the electrical signal. Later in the 's dopamine achieved the same status. Also in the 's and 's, four amino acids glutamate, aspartate, GABA, and glycine became contenders in the race to identify neurotransmitters. These four candidates were distinct from those previously discovered because of their widespread biochemical applications.

Another distinct category of neurotransmitters emerged in the 's, the neuropeptides Myers, These substances were found localized in regions of the nervous system; some of them functioning as neurohormones in the hypothalamus and pituitary gland. Study of the neuropeptides has been prolific. Over a quarter of a million experiments have been published on this category of neurotransmitter since they were first identified as active in the nervous system in In this period of time forty-two different peptides have been studied with significant frequency.

This research peaked in , and since has settled at a level similar to that existing for other categories of neurotransmitters. The list of substances that affect the electrical potential across the postsynaptic membrane is extensive and growing as the research focuses on this exciting topic. The following is a list of such substances currently under study in the laboratory and clinic.

Most of the substances listed have satisfied the criteria to be called full-fledge neurotransmitters, whereas many of the peptides and substances in the OTHERS category are of putative neurotransmitter or neurohormone status:.

Cooper, J. The Biochemical Basis of Neuropharmacology. New York: Oxford University Press.