0708-Biological Miracle III: Command and Communication

Background reading material for Year 1 Semester 1 medical student PPSD session on Wednesday 29th August 2007 by Professor Omar Hasan Kasule, Sr.


Centralized control removes contradictions and ensures harmony. The centralised master control of physiological functions is mediated by the nervous system, the endocrine system, and the immune system. Underlying all these systems is the DNA molecule.


2.0 DNA

DNA has 6x10E9 bits of information called codons that represent 10E7 genes. Each gene codes for one protein molecule and it is these molecules that catalyze or mediate the biochemical functions of the body. The functions of DNA are control of all aspects of cell functioning primarily through protein synthesis and transfer hereditary information to the next generation. DNA within one organism is quite stable. The processes of mitosis and meiosis occur in millions of cells daily with few variations in DNA structure. To ensure stability DNA replication is semi-conservative. Mutations are rare if we consider the billions of DNA-related processes that occur


The nervous system sends sensory impulses from the periphery to the brain and the spinal cord. It sends motor impulses from the brain and spinal cord to the periphery. It controls, co-ordinates, and integrates body activity in response to internal and external environmental stimuli. It correlates sensory information, motor co-ordination, memory, and intellectual function. Most human activities are not under conscious control. The existence of the autonomic nervous system enables many background life-sustaining activities to continue without sensory input into the cerebrum.



Hormones secreted in minute amounts and transported by the blood profoundly influence target organs and tissues. Target tissues have specific receptors for hormones. The hypothalamus-pituitary-target organ axis is one of the miracles of human biology. It enables the endocrine system to respond to environmental signals, emotions, and thoughts mediated through the cerebral cortex. The hypothalamus, the pituitary and individual endocrine gland secretions are controlled by negative feedback. As the product mediated by the hormone accumulates and reaches a certain level, further hormone production is blocked. Hormones from the hypothalamus and the pituitary affect secretions by other glands. The endocrine system has several functions: maintaining homeostasis (fluid and end electrolytes), growth and development (tissue growth and sex differentiation), and adaptation to physiological stresses (fight or flight reaction), reproduction (menstruation and other reproductive processes). The specific functions of hormones are morphogenesis, metabolic regulation, and functional integration.



Immune defense is recognition and elimination of non-self (foreign) material. Non-specific immunity is innate while specific immunity is acquired. Non-specific immunity may be mechanical barriers like skin or mucous membranes, chemical barriers such as transferrin or interferon, cells such as natural killer cells, granulocytes, and leukocytes. Specific acquired immunity is based on lymphocytes. B lymphocytes produce antibodies (humoral immunity) whereas T lymphocytes attack the bacteria directly (cell-mediated immunity). Cellular immunity is more important than humoral immunity.

ŠProfessor Omar Hasan Kasule, Sr. August 2007