NERVOUS SYSTEMS: Parkinson’s Disease, Critical Windows in Brain Dev...

Hello, dear readers. Thank you all for the consistent support. Today, I will like to conclude my series on NERVOUS SYSTEM by starting with the discussion of the Parkinson’s Disease in relationship to a special part of the human brain.

Parkinson’s disease

Parkinson’s disease is a disease in which the death of a small number of cells in the basal ganglia leads to an inability to select and initiate patterns of movement. The exact symptoms vary, but generally Parkinson’s patients have an increased rigidity in their muscles, usually accompanied by a tremor. This leads to slowness of movement, poor balance and speech problems. This shows the importance of the basal ganglia – without them, the body is incapable of normal movement even though most of the motor system is intact.

^{Pharmacological treatment of Parkinson's disease. Luigi Albert Maria, CC BY-SA 3.0}

Quite what triggers the onset of Parkinson’s is still not clear. It appears to be due to a genetic predisposition and an as yet unidentified environmental trigger. What is known is that the underlying cause is an inability to produce dopamine, a neurotransmitter that has a number of functions, including enabling us to move smoothly and normally.

Some relief from Parkinson’s has been achieved by treatment with levadopa, a precursor that is transformed into active dopamine in the brain. Dopamine reduces the symptoms – often spectacularly so – but is not a cure. Trials involving transplants of dopamine-producing cells from fetal or animal sources also show promise, but it is still early days.

Electroencephalograms and brain stem death

In 1929, the German scientist Berger discovered that the brain produces electrical activity that can be measured by electrodes placed on the scalp. The machine that measures brain waves produces a trace encephalogram, or EEG. It was discovered that the patterns of brain waves recorded on the EEG change in different situations, particularly with levels of consciousness. The four basic types of brain waves are alpha, beta, theta and delta. The absence of any electrical activity from the brain of a patient indicates brain stem death. The absence of breathing movements and electrical brain activity is the clinical definition of death. EEG traces tell us little about how the brain works, but they are useful for locating and diagnosing the various types of epilepsy, sleep disorders and brain tumours.

^{Human EEG with prominent resting state activity – alpha-rhythm. Left: EEG traces (horizontal – time in seconds; vertical – amplitudes, scale 100 μV). Right: power spectra of shown signals (vertical lines – 10 and 20 Hz, scale is linear). Alpha-rhythm consists of sinusoidal-like waves with frequencies in 8–12 Hz range (11 Hz in this case) more prominent in posterior sites. Alpha range is red at power spectrum graph. Andrii Cherninskyi, CC BY-SA 4.0}

Critical windows in brain development

We know that the way we develop is largely due to our genes, but it is becoming increasingly apparent that our nervous system does not develop properly without the right sensory stimulation at the right time.

There are critical windows or critical periods when a young animal must have sensory or motor input if ‘normal’ development is to take place. During a critical window, neural connections are made between sense organs and vital areas of the brain. These critical periods exist in virtually all species, from fruit flies to humans. If a skill is not acquired during a critical period, the acquisition of that skill in later life will be much harder, if not impossible.

For example, kittens are born with their eyes shut and if they do not open at the right time, permanent blindness results. There is nothing wrong with the eyes themselves, but the visual cortex where the incoming information is processed fails to develop properly.

Key work in this area came from the experiments of David Hubel and Torsten Nils Wiesel, who won the 1981 Nobel Prize for Physiology for their work. They found that if a kitten is deprived of light in one eye during the critical period, there is only partial development of the visual cortex. They can only see with one eye, and although they can perceive detail well, there is very poor judgement of distance. Electrical studies of the visual cortex showed that only about one-seventh of the visual cortex develops, compared with full development following input from two eyes.

To further refine their research, kittens were placed in striped tubes (where the stripes were horizontal for some kittens and vertical for others) or were fitted with goggles that presented vertical stripes to one eye and horizontal stripes to the other.
When tested a few months later, after removal of the striped tubes and goggles, the kittens seemed to be blind to stripes with the opposite orientation to those they saw during rearing. Most of the cortical cells of the cats reared with horizontal stimuli subsequently responded strongly to horizontal stimuli and hardly at all to vertical stimuli. The opposite was true of the vertical stimuli-reared cats.

This has implications for children who have injured an eye, and have it covered with a bandage or patch. If the injury occurs during the critical window, the visual cortex will not develop properly and vision will be impaired.

The ethics of animal research

Few subjects in science cause so much fierce debate as that of animal rights. Was it right for Hubel and Wiesel to experiment on kittens and make them partially sighted for life? Was it right for Banting and Best to experiment on dogs giving them diabetes and ultimately killing them?

Most people believe that humans have certain rights, but it is very difficult to apply these ideals to animals. We can’t do medical research on people without their consent, but it is obviously not possible for animals to give their consent. It makes more sense to consider animal welfare rather than animal rights. Most people think that animals should have clean water, food, exercise/stimulation and access to veterinary care.

Some people think that to use animals in experiments is being speciesist (in the same sense as racist or sexist). They argue that by experimenting we are automatically assuming that human life is more valuable than that of any other species. Others argue that it is very difficult to draw the line, and that if the argument is taken to ts logical conclusion, we shouldn’t ever kill pests like rats or even locusts.

How much do animals suffer?

There is a general consensus among scientists that humane treatment should be given to those organisms whose nervous system is advanced enough to appreciate it. The argument is that pulling the legs off a daddy-long-legs or boiling a lobster (which, for all its size, is no more sophisticated than an insect) is not inflicting pain because these animals do not have a nervous system advanced enough to perceive pain and suffering.

No country in the European Union is allowed to experiment on vertebrates in medical experiments if non-vertebrate alternatives exist. If there is no alternative – as would be the case with the kittens – animals can be used provided that strict guidelines are followed. Many drugs – sleeping pills, for example, can only be tested using an intact, conscious mammal.

Scientists commonly take a utilitarian, or ‘greater good’, approach, meaning that the right course of action is the one that leads to the greatest overall benefit, i.e. the least suffering and loss of life in the long-term. A utilitarian framework is in place today, allowing a certain amount of animal experimentation provided that the overall benefits are expected to be significantly greater than the expected suffering and or loss of life.

Epilepsy and the two sides of the brain

Epilepsy is a common disorder of the brain, Symptoms range in severity from a mild loss of concentration, known as an absence or petit mal, to full-blown convulsive fits (grand mal) in which the subject blacks out and falls to the floor. These can be dangerous if the sufferer lashes out – injuring himself and others – or bites his own tongue.

^{A bite to the tip of the tongue due to a seizure. James Heilman, MD, CC BY-SA 3.0}

The underlying cause of epilepsy is random, uncontrolled activity of some cells in the brain. This chaotic activity In both sensory and motor nerves causes patients to see and hear a variety of strange things, such as flashing lights and bells, while muscles jerk uncontrollably.

A diagnosis of epilepsy can be confirmed and studied using an EEG machine.It can be used to show a trace from a person during an epileptic seizure: you can compare it with the normal readings.

Epilepsy can often be controlled successfully by drugs. However, in extreme cases, the condition is treated by brain surgery, and one such operation has given a fascinating insight into the workings of the brain. The cerebral hemispheres have been described as two separate brains, and in order to work effectively as a whole, the two halves must communicate.

The bridge between the two halves is known as the corpus callosum. Neuroscientists discovered that the corpus callosum was involved in the spread of epileptic seizures. In a seemingly drastic operation, surgeons sever most of the corpus callosum. This often causes the seizures to be less intense and dangerous. However, there are other amazing consequences. Initially, subjects appear to be perfectly normal: they can talk and read, and have no problems in recognizing the world around them. However, if they close their right eye, and was given a familiar object such as a comb, they cannot put a name to it. Open the other eye, however, and ‘Ah, it's a comb!’

The same happens with words. If a word such as ‘TIGER’ is looked at it with the left eye only, the patient can’t read it. If they open the right eye, they can read the word immediately. This is because the left eye supplies information to the right side of the brain, and that is not where the language centre is situated. The right eye supplies information to the left side of the brain, to the language-processing neurons.

From studies of split-brain patients, and other studies, it appears that different sides of the brain have different functions. The left hemisphere contains the language centre, and the three Rs – reading, writing and ‘rithmetic. The right side, in contrast, is responsible for our imagination and sense of humour. It can also appreciate form, geometry and music. It cannot, however, put words to things. If the right hemisphere needs a word, it has to put in a call to the left side, via the corpus callosum.

Split-brain patients do not experience the symptoms tor ever. Within a few months the right hemisphere develops more language skills and can function on its own. It has even been suggested that split-brain patients could read two books simultaneously, one with each eye!

THE PERIPHERAL NERVOUS SYSTEM

The nerves of the peripheral nervous system behave like major road systems, carrying traffic in and out of the central nervous system. Afferent nerves, also called sensory nerves, carry information from sensory receptors into the CNS. Efferent nerves, also called motor nerves, carry information from the CNS out to effector organs. The efferent system can be further subdivided into the somatic and autonomic systems. These differ in their function, rather than their structure or position in the body. Afferent means ‘incoming’ while efferent means ‘outgoing’. You can refer to ‘afferent nerves’ or ‘efferent blood vessels’, for example.

The somatic nervous system

The somatic nervous system contains both afferent and efferent nerves. It receives and processes information from receptors in the skin, voluntary muscles, tendons, joints, eyes, tongue, nose and ears, giving an organism the sensations of touch, pain, heat, cold, balance, sight, taste, smell and sound. It also controls voluntary actions such as the movement of arms and legs.

^{A diagram of the human nervous system. Medium69, Jmarchn , CC BY-SA 4.0}

The autonomic nervous system

The autonomic nervous system (ANS) consists of two sets of involuntary nerves that generally act antagonistically – these are the sympathetic and parasympathetic systems and they have opposing effects. The system is entirely motor, made up of efferent nerves only. It does not carry sensory information: feedback from muscles and glands travels via the somatic system.

The ANS controls basic ‘housekeeping’ functions such as heart rate, breathing, digestion and blood flow. Heart rate, for example, can increase or decrease. So, while the sympathetic system increases heart rate, the parasympathetic system lowers it. Generally, the sympathetic system has a stimulatory effect, and prepares the body for action, while the parasympathetic system returns body functions to normal.

Normally, the activity of both systems is balanced. But if the body is stressed, then the ‘fight or flight’ reactions of the sympathetic nervous system take over, causing an increase in heart rate, faster breathing, an increase in blood pressure and an increase in blood sugar level. This makes the body ready for sudden strenuous activity. When the emergency is over, the parasympathetic system takes over. It decreases the heart and breathing rates and diverts blood supply back to housekeeping’ activities such as digestion and food absorption. The actions of the parasympathetic nervous system have been described as ‘feed or breed’ because parasympathetic stimulation leads to increased blood flow and peristalsis in the intestines, and sexual responses such as gaining an erection.

The autonomic system was originally thought to be independent of the rest of the nervous system, hence the term autonomic, meaning ‘on its own’ or ‘self governing’. Now we appreciate that it is not autonomous, but is regulated by areas within the central nervous system, including the hypothalamus, cerebral cortex and the medulla oblongata.

Do we have any conscious control over the autonomic nervous system?

The answer is yes, in some cases. As children we became potty trained when we learned conscious control over the muscular valves (sphincters) in our bladder and rectum. There are people who have learnt conscious control over Some other autonomic functions; those who are adept at advanced meditation and yoga techniques can voluntarily lower their heartbeat.

And with this, I’ve come to the end of my interesting series of posts on Nervous system. I hope you enjoyed it.

NERVOUS SYSTEMS: Parkinson’s Disease, Critical Windows in Brain Development, Epilepsy and The Peripheral Nervous System.