Neurons and the Nervous System
Introduction
Humans have several senses that
allow for detailed interactions with their surrounding environments. Once a
stimulus is sensed and perceived, an action is taken in response to the
stimulus. Thus, the most basic way to describe behavior is simply a stimulus
(S) followed by a response (R). This S>R relationship has been extensively
studied at various levels of brain function. In order for this relationship to
function effectively, precise integration of sensory inputs with motor systems
in the brain must occur. Further, the outgoing motor signals must be able to be
accurately converted into their intended muscle movements. Changes in any of
the above factors due to alterations in the internal or external environment of
the body, sensory system damage, deficits in perception, or the presence of
movement disorders can significantly affect performance and the ability of the
brain to function properly.
Auditory Awareness is Dependent on
Environment
The ability to detect and properly
interpret sensations is dependent in part on the environment. For example,
sound travels four times faster in water than in air because water is denser
than air and is thus better able to transmit sound vibrations. Because humans
localize sound in part by using the difference in time required for an acoustic
vibration to reach one ear versus the other, it is very difficult for humans to
localize sound underwater, since the faster transmission speed results in a
time difference between the ears that is too small for humans to distinguish
(Shupak et al., 2005). For scuba divers this can present problems. When divers
hear a sound, unless well trained to know otherwise, they typically look up,
even if the sound is coming from one side.
Cetaceous mammals such as dolphins
and whales, however, are extremely well adapted to the underwater environment,
even using intricate acoustic cues to determine their location and the movement
of prey (Tyack & Clark, 2000). Their ability to discriminate sounds is much
greater than that of most terrestrial mammals (Thompson & Herman, 1975).
Olfaction Can Be Subtle but Often
Important
Seeing tears often evokes feelings
of concern. But the smell of tears has a different effect. An interesting study
by Gelstein and colleagues (2011), showed that the smell of female tears did
not elicit empathy, but did decrease sexual arousal and testosterone levels.
Olfaction can also help identify a genetically compatible mate.
Some research findings show that our
brain may react differently to the two putative pheromones compared with common
odors and suggest a link between sexual orientation and hypothalamic neuronal
processes (Savic, Berglund, & Lindström, 2005). The authors noted that subtle
olfactory responses can also provide information about sexual response. Other
tested common odors showed similar activation in all groups of subjects and
engaged only the olfactory centers of the brain. These types of subtle, indeed
often subconscious, responses show the importance of olfaction in many of our
perceptions, even though individuals may not be consciously aware of the
effect.
Hallucinations Are Distorted
Perceptions
Events such as drug use, sensory
deprivation, disease, or injury can cause aberrations in sensory perception.
These aberrations often come in the form of hallucinations, which are generated
by the mind rather than by actual sensory stimuli. They may be seen, heard,
felt, smelled, or tasted. It is possible to induce multisensory hallucinations
through as little as 15 minutes of sensory deprivation. However, few scientists
are studying sensory deprivation today. The common spice, nutmeg, also has
psychoactive effects and can produce visual and time distortions and paranoid
ideation; it contains myristicin, a psychoactive substance related to ketamine
(Shulgin, Sargent, & Naranjo, 1967; Weil, 1966). Hallucinations can also
occur in other disorders, including schizophrenia, major depression, and
alcohol or drug withdrawal (Blazer, Stefens, & Busse, 2004; Rabins,
Lyketsos, & Steele, 2006).
Synesthesia Also Occurs Naturally
Synesthesia is a condition in which
someone perceives a stimulus, such as a sound, through more than one sensory
pathway. It occurs naturally, and synesthetes are typically not aware that
their experiences are unusual (van Campen, 2007). Although sometimes called a
neurological condition, synesthesia generally does not interfere with normal
daily functioning. Indeed, most synesthetes report that their experiences are neutral,
or even pleasant. In other words, like other sensory differences among humans,
such as color blindness or perfect pitch, synesthesia is simply a difference in
perceptual experience (Sagiv & Robertson, 2005).
Movement Disorders: A Disruption in
Sensory-Motor Output
There are many movement disorders.
While the most studied is Parkinson’s disease (PD), various tremors, dystonia,
and tics are other common disorders of movement. All movement disorders are due
in part to genetic risk. Movement disorders can be divided into two groups:
those that result in too much movement and those that result in slowness or too
little movement (Pinel, 2007).
Parkinson’s Disease
Parkinson’s disease is a slowly
progressive disorder that affects movement, muscle control, and balance. Part
of the disease process develops as cells are destroyed in certain parts of the
brain stem, particularly in thesubstantia nigra. Nerve cells in thesubstantia
nigra send out fibers to tissue located in both sides of the brain. There
the cells release essential neurotransmitters that help control movement and
coordination.
Catatonia
Catatonia is a state of neurogenic
motor immobility and behavioral abnormality manifested by stupor. Catatonic
patients will sometimes hold rigid poses for hours and will ignore any external
stimuli (Fink & Taylor, 2003). It can occur alone, but is associated with
psychiatric conditions such as schizophrenia, bipolar disorder, post-traumatic
stress disorder, depression, and other mental disorders, as well as drug abuse
(Deuschle & Lederbogen, 2001; Rosebush & Mazurek, 1996).
Artificial Brain Stimulation
Chronic recurrent thalamic
stimulation has been effective in alleviating a variety of movement disorders.
Abnormal diencephalic electrical discharges have been observed and thought to
be associated, in some way, with either generating or sustaining the movement
abnormalities. The beneficial effects are ascribed to an electrophysiologic,
functional ablation of the discharging systems. This interpretation is based on
the observation that the diencephalic discharges are attenuated by the applied
stimulation and that the beneficial effects are reversible even after several
months of applied therapeutic stimulation (Andy, 1983).
Memory in Muscles
The repeated use of certain muscles,
such as in athletics and in music, is another way muscle tissues and the brain
interact. When a movement is repeated over time, a long-term muscle memory is
created, allowing it to be performed without conscious effort. This decreases
the need for attention to the task, which can improve the efficiency of sensory
motor, muscle, and memory systems (Krakauer & Shadmehr, 2006).
Conclusion
While the human brain is highly
dependent on visual sensations, other senses play major roles in sensing and responding
to the environment. The ability of the brain to sense environmental cues is
dependent on the nature of the environment. If a person has to operate within
an environment distinct from that in which the brain developed, performance is
decreased. The brain sometimes responds to an environmental stimulus through
more than one sensory pathway. Synesthetes naturally sense a stimulus through
more than one sensory pathway. Movement disorders disrupt the sensory-motor
networks in the brain and interfere with the ability to respond accurately to
stimuli. Advances are being made in stimulating certain brain areas in order to
overcome these impairments. It is interesting to note that repeated use of
muscles in learning and rehearsing sensory-motor actions can cause a type of
memory to occur that allows a person to perform a task automatically.
Sensory-motor systems involve all of the senses and cause a myriad of
responses, some of which are obvious, while others are subtle. Whether
consciously or subconsciously, the brain is constantly sensing and responding
to the surrounding environment.
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