Pathology of the Nervous System

By Dr Ananya Mandal, MD

The nervous system is affected by numerous drugs and diseases. These are very important in general practice and also in awareness against drug abuse.

Drugs acting on the nervous system

Several drugs are used to change the mood and/or emotional state of the user.  These drugs importantly either promote or decrease the action of a particular neurotransmitter – the chemical messengers that transmit impulses between neurons.

How do these drugs act on the nervous system?

These drugs act by 5 basic ways:

  • drug stimulate the release of neurotransmitter and enhances action
  • drugs combines with neurotransmitters preventing their breakdown and enhancing their action
  • drugs mimics neurotransmitter and enhance their action
  • drugs block the release of neurotransmitter and decrease their action
  • drugs blocks receptors so neurotransmitters are unable to act

Examples of drugs that act on the nervous system

  • Caffeine is a stimulant that is found in coffee and acts by blocking the action of adenosine, a chemical that inhibits the release of neurotransmitters.
  • Nicotine in tobacco enhances the action of acetylcholine and acts as a stimulant.
  • Drugs like Thorazine affect cognitive or thinking processes.
  • Amphetamines are similar to noradrenalin (NA) and stimulate the release of NA and dopamine in brain. 
  • Cocaine blocks the uptake of dopamine so it is present in the synaptic cleft longer.  Use of cocaine thus leads to hallucinations and other neurological effects.
  • Methamphetamine (Ice) acts similarly as cocaine.
  • Marijuana (Cannabis sativa) acts on the neurotransmitter serotonin.
  • Depressant effects are seen with tranquillizers like barbiturates and benzodiazepines as well as with alcohol. These act by action of the inhibitory transmitter GABA. 

Diseases of the nervous system

There are numerous disorders of the nervous system. Some may affect the brain and spinal cord while some affect the peripheral nervous system.

Diseases of the Central Nervous System

  • Infections

These include meningitis and encephalitis. These may be bacterial or viral origin and may often be life threatening.

Polio is another infection of the central nervous system. It involves inflammation of the gray matter of the spinal cord. It may not have any symptoms or may cause paralysis of the lower limbs.

  • Degenerative disorders
    • One degenerative disorder is Huntington’s chorea. This leads to progressive deterioration of nervous system leading to insanity and death.  It is thought to be due to GABA abnormalities and is a genetic condition that runs in families. There is no cure.
    • Another degenerative disorder is Dementia. This involves loss of memory with age
    • Alzheimer’s disease if another degenerative disease. It is a severe form of senility marked by advanced memory loss.  It affects 5 to 10% of people over 65.  In this disease there may be formation of protein plaques in the brain and destroy brain cells.  There is below normal acetylcholine secretion in brain.
    • Parkinson’s disease involves tremors of the limbs and difficulty in maintaining balance along with muscle rigidity. This is due to lack of dopamine. 
    • Amyotrophic lateral sclerosis (ALS) affects motor neurons leading to muscle degeneration and loss of function.
    • Multiple Sclerosis is a chronic, progressive disease that leads to loss of the myelin sheath over the nerves.
  • Epilepsy

This is caused by derangements of normal connections in the brain. Episodes of convulsions are known as seizures.  There are "grand mal" and "petite mal" seizures.  In a grand mal seizure, the cerebrum becomes extremely excited, the individual may lose consciousness. 

  • Cerebral palsy

This is a disorder of childhood and occurs from birth in many. There is weakness of arms and legs. It is caused by lack of oxygen during birth which damages motor areas of cerebral cortex.

  • Mental disorders
    • These include depression. Depression is caused by a lack of serotonin and/or norepinephrine in the brain. Depression is a serious medical disorder that affects more one person in 10 during their lifetime.
    • Schizophrenia is a severe mental illness is probably linked, in part, to a surplus of dopamine.
    • Phobias are excessive and abnormal fears.
  • Stroke or cerebrovascular accident

This is caused by rupture of a blood vessel within the brain leading to pressure over vital areas of the brain and this may cause paralysis and weakness of the limbs.

Other problems with the central nervous system include:

  • Headaches and migraines
  • Head injuries

Diseases of the Peripheral Nervous System

Many conditions may also affect the peripheral nerves of the body leading to loss of sensation and paralysis.

Physical injury to the peripheral nerves is a common condition that affects their functioning. Often the peripheral nerves have the capacity to regenerate if they are injured. But this process of regeneration may take years of exercise and physiotherapy.

Damage to nerves may also be caused by swellings at places or channels where the median nerve passes through. This is called carpal tunnel syndrome.

Some conditions affect many of the terminal nerves. This is called peripheral neuropathy. Peripheral neuropathy may begin as a tingling numbness of the fingers and toes and extend along the limbs. It is commonly caused in diabetic individuals and those with certain genetic diseases of the nerves, vitamin deficiencies of vitamin B12 etc., infections such as herpes infection or leprosy, poisoning with Mercury, lead and other heavy metals, inflammatory conditions such as Guillain-Barré syndrome etc.

Neuropathy may also result from prolonged pressure to the nerve, leading to numbness and stiffness (pins and needles), sharp drops in temperature and prolonged action of local anesthetic drugs like lignocaine. An actual cause of peripheral neuropathy may also be elusive. This is called idiopathic neuropathy.

Reviewed by , BA Hons (Cantab)

Further Reading

Last Updated: Feb 25, 2013

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