Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

134 Self and the Phenomenon of Life

b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”

receiving neuron is a matter of mass action, depending on temporal and
spatial summation of all the post-synaptic potentials.
There are a handful of major neurotransmitters (see Table 7.1).
Acetylcholine causes muscle contraction, and mediates parasympathetic
activities in the body and internal organs, while norepinephrine medi-
ates sympathetic activities. (Epinephrine, a variant of norepinephrine,
also known as adrenaline, is a stress hormone produced by the adrenal
gland when triggered by sympathetic activation.) Inside the brain, both
agents play a role in learning and memory, and in maintenance of cog-
nitive function. Glutamic acid and gamma amino-butyric acid (GABA)
both function inside the brain, the former as an excitatory and the latter
an inhibitory neurotransmitter. Glutamic acid in particular is essential
for synaptic learning. Dopamine controls muscle tone by acting on basal
ganglia; it also mediates motivation-and-reward behavior. Over-activity
of dopamine is correlated with hallucinatory and other schizophrenic
symptoms. Serotonin is a mood regulator and is essential for sleep and
feeding. The minor neurotransmitters include: aspartic acid, taurine,
glycine, histamine, adenosine, nitric oxide, and carbon monoxide, the
latter two being gaseous transmitters.

Table 7.1. Major Neurotransmitters and their Functions

1. Acetylcholine (ACh): Causes skeletal muscle contraction; parasympathetic
   component of the autonomic nervous system; involved in cognitive function.


2. Norepinephrine (noradrenaline): Sympathetic component of the autonomic nervous
   system; promotes arousal, vigilance, and aggression; enhances memory.


3. Glutamic acid: Excitatory transmitter; essential for synaptic mechanism of learning
   and memory consolidation.


4. Gamma amino-butyric acid (GABA): Inhibitory transmitter; counters the excitatory
   effect of glutamic acid.


5. Dopamine: Controls muscle tone; involved in motivational and reward behavior;
   over activity leads to hallucination.


6. Serotonin: Responsible for mood stabilization; involved in sleep and feeding behavior.
   Note: Norepinephrine, dopamine and serotonin are biogenic amines, since they all contain an
   amino group. Glutamic acid and GABA are amino acids. Glutamic acid is a natural constituent of
   protein. For structures of the neurotransmitters, refer to Appendix A.