The following statements describe the action of dopamine, a neurotransmittter. D

Question

The following statements describe the action of dopamine, a neurotransmittter. Determine the correct order of the statements. The dopamine-containing vesicles travel to the cell membrane and fuse with it, releasing the dopamine into the synaptic cleft. Vesicles store dopamine in the presynaptic neuron. Dopamine binds to receptors on the presynaptic neuron. Transport proteins actively transport dopamine across the synaptic cleft. Dopamine binds to receptors on the postsynaptic neuron, generating a nerve impulse in the neuron. Dopamine diffuses across the synapse. Dopamine is synthesized from epinephrine in the postsynaptic neuron. Dopamine is synthesized from L-DOPA in the presynaptic neuron. A nerve impulse travels to the presynaptic neuron. Dopamine is removed from the synaptic cleft by reuptake into the presynaptic neuron, ending the signal. Write the numbers of the statements in the correct order, beginning with the synthesis of dopamine. If a statement is incorrect, do not include it in your answer.

Explanation / Answer

8, 9, 3, 2, 1, 6, 10

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Within the midbrain, the substantia nigra and the ventral tegmental area synthesize the most dopamine, which is then transmitted along four major neuronal pathways. This dopamine serves as a neurotransmitter, acting on dopamine-specific receptors in various parts of the brain.

Dopaminergic pathways consist of bundles of neuron fibers that dispatch the neurohormone, sending signals from one part of the brain to the next. In each of the four tracts, dopamine elicits a different function.

The adrenal glands and kidneys produce the remainder of the neurotransmitter for participation in the sympathetic activation of the alpha and beta receptors within the body. The stimulation of these receptors is what invokes the "fight-or-flight" response. This aspect of dopamine function has great clinical significance in the treatment of shock and heart failure.

Transmitting dopamine from the substantia nigra to the striatum, the nigrostriatal pathway is involved motor function and sensation. According to the Lundbeck Institute, damage to this tract is associated with Parkinson's disease and other Parkinson's-related disorders.

Any lesions within this tract reduce the transmission of dopamine to the striatum. This reduction produces tremors, cog-wheel rigidity, uncontrollable spasms, decreased purposeful movement, blunted affect, and restless leg syndrome.

The tuberoinfundibular pathway governs neuronal control of the release of dopamine from the hypothalamus into the anterior pituitary gland. The dopamine transversing here is referred to as Prolactin-inhibiting factor, because it acts on the pituitary gland to suppress the secretion of prolactin.

Disturbances of this tract cause hyperprolactinemia, which is an abnormal level of prolactin in the bloodstream. Aberrant increases of prolactin can result in amenorrhea, painful intercourse, problems with ovulation and decreased libido in women. In men, milky discharge from nipples, erectile dysfunction and enlargement of the breasts can occur.

The mesocortical pathway, which controls motivation and emotion, sends dopamine to the frontal cortex. Reduction of the neurotransmitter along these fibers may result in the "negative symptoms" of schizophrenia, or those manifestations that are known to be "withdrawn," such as flat expression, lack of emotion, apathy and poor social functioning.

The mesolimbic pathway extends to the nucleus accumbens--the pleasure center of the brain. Dopamine functions here by stimulating reward and emotional behavior. Yet, dopaminergic increases in this region may result in hallucinations and delusions.

For instance, the dopamine receptors in this area are targeted by antipsychotic drugs, such as haloperidol. These drugs act by blocking the receptors in order to reduce the "positive symptoms," or the outward manifestations, of schizophrenia, which are hearing voices and seeing hallucinations.

According to the University of Texas, addictive behaviors are also associated with this pathway. Cocaine, nicotine and amphetamines stimulate a spastic production of dopamine along these neurons. For instance, cocaine increases dopamine in this region by promoting its reuptake at the neurosynapse. On the other hand, amphetamines promote the release of the neurotransmitter in the presynpatic neurons.

Functioning as catecholamine, dopamine takes an active role in the treatment of low blood pressure in situations of shock and acute heart failure. Proper doses of dopamine stimulate the alpha and beta receptors within the heart and blood vessels.

Thus, the narrowing of blood vessels, increased cardiac output and increased heart rate occur in response. This dopaminergic action protects the heart wall when blood volume resuscitation is achieved. According to "Harrison's Principles of Internal Medicine," dopamine is considered most useful in the treatment of patients who have lower cardiac output and poor tissue perfusion.

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