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ISLAMIC MEDICAL EDUCATION RESOURCES 04

0102-THE NERVOUS SYSTEM: OVERVIEW

Lecture to 1st year medical students on 16th February 2001 by Professor Omar Hasan Kasule Sr.

A. ANATOMY and PHYSIOLOGY

QUR'ANIC CONCEPTS RELATING TO FUNCTIONS OF THE NERVOUS SYSTEM

Functional description: The Qur’an did not describe the anatomy of the nervous system in a direct and explicit way but it dwelt on the functions of the nervous system. The functions of thought, memory, forgetting, and emotions were described in association with 4 Qur’anic phenomena: naasiyat, lubb, qalb/nafs, fuaad, and dhihn. These four cannot be located anatomically.

 

Naasiyat: The Qur’anic concept of nasiyat:  (p. 1190 11:56, 55:41, 96:15-16).

 

Lubb: Lubb & ayaat al llaah 3:19. Lubb  & ‘ibrat (12:111). Lubb & tadhkirat (13:19).

Lubb & tadabbur (38:29).

 

Qalb: Qalb is very important. The human has only one qalb, ‘adam ta’addud al qalb (p 961 33:4), an indication that it is a master organ that directs others. If a human had more than one qalb then centralized control and coordination would not have been possible. Qalb is involved in intentional/purposive action (p 956 33:5). Qalb is involved in responsibility, qalb & mas uliyat (p 963 2:225, … 33:5). The qalb has the following emotional functions: ulfat al qalb (p 959 3:10, 8:63, 9:60), ra afat al qalb (p 959 57:27),  rahmat al qalb (p 959 3:159, 57:27), tanafur al quluub (p 959 14:59), hasrat al qalb (p 959 3:156), khawf al qalb (p 959 3:151, 8:2, 8:12, 23:60, 28:10, 33:10, 40:18, 59:2, 59:13), riibat al qalb (p 959 9:45, 9:110), zaygh al qalb (p 959 3:7), and ghaldhat al qalb (p 959 3:159). Qalb has the following Intellectual functions: tadabbur al qalb (p 960 6:113, 40:80, 47:24, 59:9), tadhakkur al qalb (p 960 50:37), hijaab al qalb (p 960 6:25, 17:46, 18:57, 41:5), ru uyat al qalb (p 960 22:48, 53:11-12), dhann al qalb al sayyei (p 961 48:12), ‘aql al qalb (p 961 22:46), ‘ilm al qalb (p 962-963 2:97, 2:118, 9:93, 16:78, 26:192-195, 29:49, 30:59, 47:24), and fiqh al qalb (p 963 7:179, 9:87, 9:127, 17:46, 18:57, 63:3). The qalb has sensory functions, qalb & hawaas (p 962 2:7, 6:46, 6:110, 7:10, 7:179, 16:78, 16:108, 17:36, 17:46, 18:57, 22:46, 23:78, 45:2, 46:26, 50:37). The eyes sleep but the qalb does not (     ).

 

Fuad: fuad is important because of accountability, suaal al fuaad (17:36). It is related to bounty, ni’imat al fuaad (16:78). It may be empty, faragh al fuaad (28:10). It has no mentioned intellectual functions. It however has the following sensory functions: isighaau al fuaad (6:113), samau al fuad (23:78), basar al fuad (23:78, 32:9, 46:26, 67:23). Fuad is a center for moral guidance/misguidance. It can tell lies, kadhb al fuaad (53:11). It can be firm, thabaat al fuaad (11:120, 25:32). It has passions and desires, hawa al fuaad (14:37). It can change and transform, taqallub al fuaad (6:110).

 

Dhihn:

 

 

THE BRAIN:

The central nervous system (CNS): CNS consists of the brain and the spinal cord. There are 100 billion neurones in the brain. The CNS controls and co-ordinates body activity in response to internal and external environmental stimuli. It correlates sensory information, motor co-ordination, memory, and intellectual function. The modus operandi is sensory input followed by processing and then action. The command, coordinating, and integrating functions of the CNS are in conformity with the concept of tauhid. Every system must have a unifying center to operate well. Just as the body has the CNS the cosmos must have one creator who created it to be well coordinated harmonious and integrated. 

 

Size and area of the brain: The adult human brain is 2.5% of body weight. Male have a bigger brain because of bigger muscle mass. There is no relation between brain size and intelligence. The total cortical surface area is 2200 square meters. It is increased by the gyri and sulci. The human brain is superior to that of other mammals because it has the highest brain to body weight ratio. The ratio of the brain to the weight of the body is highest in infancy when the brain is the most rapidly growing organ. It falls progressively after the brain has completed its growth. In old age there is atrophy of brain cells and decrease of brain weight.

 

Parts of the brain:  The brain has 3 main areas: cerebrum, cerebellum and the medulla oblongata. The cerebrum is responsible for recognition, memory, imagination, creative thoughts, and abstract reasoning. The main functional areas of the cerebrum are: olfactory, optical, auditory, sensory input, motor. There is also somatotopic representation of parts of the body. The cerebellum coordinates activity of voluntary muscles as well as posture and equilibrium. The cerebellum is mid-way between the cerebral cortex whose actions are very voluntary and the medulla oblongata whose actions are mostly involuntary. The medulla oblongata has several important centers: vasomotor (BP control), cardiac (heart rate and heart force), respiratory, vomiting, temperature, posture and balance, chemoreceptor. The hypothalamus controls homeostatic mechanisms. The pituitary, directly under hypothalamic control, is the master gland of the body.

 

Histology: The cerebrum has gray and white matter. A normal individual has 10E11 - 10E13 neurones in the brain. These provide a large ability to store and process a lot of information. Brain cells are either neurones or glial cells. Glial cells are supportive or connective cells. The following are the commonest types of glial cells: schwann cells, oligodendrocytes, microglia, epindyma, and astrocytes. Brain cells cannot regenerate. This may be a secret of Allah the almighty in view of the memory function of these cells. They must be permanent in order to keep the life-long record of human experiences and learned skills. Such permanency would not have been possible if brain tissue could be destroyed easily and be regenerated.

 

Hierarchy: The 3 parts of the brain parallel the hierarchy of sophistication. The least sophisticated is the medulla, the most sophisticated is the cerebrum, and the cerebellum is in between. The medulla controls automatic largely unconscious but essential functions. The cerebrum deals with intellectual functions. The human cortex is more sophisticated than that of animals. The human also relies less on the cerebellum and the medulla than other animals. The human cortex is able to override the autonomic functions of the medulla and cerebellum.

 

Parity: The brain follows the principal of parity in having two hemispheres. In any one individual only one of the hemispheres is dominant. One hemisphere can take over the functions of the other in cases of damage.

 

Specialization and mapping: Each part of the brain has specific sensory, motor, and informational processing functions. The science of brain mapping has enabled us to recognize and localize those areas. The high level of specialization of the brain is because of the sophisticated human intellect. Allah in His wisdom made this specialization flexible. Functions of a damaged part of the brain can be taken over by another part. Sometimes one hemisphere can take over some functions of a damaged opposite hemisphere.

 

Vulnerability of the brain

The brain is very important and therefore has special circulatory and metabolic systems. These must be very reliable because any malfunction of the brain especially the cortex has far more reaching effects than malfunction of other parts of the body. The brain is 2% of body-weight but receives 15% of cardiac output and 20% of oxygen consumption.. The brain uses 103 g of glucose per day that is equivalent 3.4 liters of oxygen a day. The brain is very sensitive to hypoxemia. Of all tissues the brain is the most dependent on glucose as a source of energy. Lipids have no metabolic role in the brain. The brain is thus very sensitive to hypoglycemia.

 

THE SPINAL CORD

The Qur’an mentions the backbone, sulb (86:7) but does not refer to the spinal cord directly. The spinal cord is the continuation of the brain into the vertebral canal. It anchors the peripheral nervous system that supplies the peripheral organs and tissues of the body.

 

PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system (PNS): The PNS is an extension of the CNS in the organs and tissues. The PNS is largely under the control and direction of the CNS. PNS consists of 31 pairs of spinal nerves from the spinal cord and 12 pairs of cranial nerves. It collects information that is transmitted eventually to CNS. It conveys CNS commands to the periphery. Some parts of the PNS like intestinal plexuses are autonomic and are not under the influence of CNS.

 

NEURONES AND SYNAPSES

Types of neurones: Neurones vary in structure and function. Nerve fibers can be divided into three categories depending on their speed of conduction: alphas are the fastest and are myelinated. Betas are myelinated but not as fast as aplha. Thetas are the slowest and are non-myelinated.

Synapses and neuro-transmitters: Neurotransmitters help transfer impulses across synapses. Among the neurotransmitters are: acetylcholine, dopamine, nor-epinephrine, 5HT, adenosine, glutamate, GABA, segothionine, glycine, aspartate.

 

Energy transduction: Neurones and synapses illustrate the phenomenon of transduction of energy. Neurones are excited by electrical, chemical, and mechanical stimuli. The action potential is a propagated electrical charge. Nerve conduction is slower than the electrical current (why?). The action potential is all-or-none. It requires a threshold stimulus. The speed of conduction and excitability of nerve fibers increase with fiber diameter. Myelinated fibers conduct faster than non-myelinated ones.

 

PROTECTION

Because of its importance, CNS has extra protection from mechanical, chemical, and microbial injury. Extra protection becomes necessary because the nervous system is very important repair mechanisms are limited. There is no regeneration of damaged neurones in the CNS. Only limited regeneration is possible in the PNS. Mechanical injury could be due to direct mechanical trauma. It could also be due to gravity or acceleration forces. Mechanical protection consists of the bony cranium, the vertebral column, the meninges, and fluids. The meninges provide additional mechanical protection. Three layers of meninges (dura mater, pia mater, and arachnoid) ensure protection from sudden movements. The CSF is protective by cushioning CNS from sudden or violent mechanical movements. Chemical injury is due to toxins. The blood-brain barrier protects the sensitive neurones from endogenous and exogenous toxins, ensures that the environment of the neurones is constant, and prevents escape of neurotransmitters into the general circulation. It also makes it difficult for infective organisms to enter the brain. The blood-brain barrier protects the brain cells from sudden changes in the chemical environment. Microbial injury could be due to infective organisms: The blood brain barrier provides some protection against entry of infective organisms.

 

B. INFORMATION INPUT, PROCESSING, STORAGE, and OUTPUT

SENSORY MODALITIES:

Principal sensory modalities: The principal sensory modalities in humans are: vision, hearing, smell, taste, acceleration (rotational & linear), touch-pressure, temperature, pain, joint position (propioception), muscles (tension, length), arterial blood pressure, central venous pressure, lung inflation, arterial P-O2, arterial P-CO2, glucose concentration, pH of CSF, and osmotic pressure of plasma.

 

Hierarchy of sensory modalities: Some senses are higher than others. The criterion used is the proportion of sensory input from the environment. Vision and hearing are in this sense are higher senses and are the most frequently mentioned by the Qur’an. Smell is not as important in humans as it is in other animals. Animals rely more on their sense of small than do humans and have a relatively larger olfactory area in the brain. Insects for example use pheromones to communicate. Studies of pheromones in humans are not yet conclusive.

SENSORY RECEPTORS:

Human sensory receptors can be classified as: exteroceptive, interoceptive, and propioceptive. Some receptors are in direct contact with the external environment (extero-ceptors) such as the 5 major senses: vision, hearing, taste, smell, and touch or pressure. Some receptors are in the internal environment (inter-ceptors) such as osomoreceptors, baroreceptors, and chemoreceptors. Information from both the internal and external environments is picked up by these receptors. Propioceptors in muscles and joints detect movements and provide information about position in space.

 

SENSORY STIMULI:

Forms of sensory stimuli: The sensory stimulus is in the form of energy (mechanical, thermal, photo, electromagnetic, or chemical). This energy is eventually transformed into electro-chemical action potentials in the sensory neurones. In the end all sensory stimuli have the same common pathway. The brain is able to distinguish them depending on the efferent neurones and the intensity of the message.

 

Concept of thresholds: It is part of Allah’s design to protect the CNS from unnecessary over stimulation that there is a threshold stimulation that results into an action potential. Sub-threshold stimuli are thus ignored. The threshold level varies by sensory modality and even within the same sensory modality by location. All these variations are determined by the relative importance of the sensory information.

 

Intensity and duration of stimulus: Detection of changes in intensity of sensory stimuli varies by sensory modality and duration. If a stimulus is continuous there is adaptation. Rate receptors incorporate time of events.

 

MAPPING OF ORIGIN OF THE SENSORY STIMULUS:

A neuron from a particular region of the body sends its stimulus to a known part of the brain that enables the brain to locate the source of the stimulus. However in cases of referred pain, this does not apply. The explanation is related to embryological dermatomes. Referred pain is a blessing from Allah. It can enable location of internal pain from the surface of the body.

 

PROCESSING CAPACITY:

The massive information input into the brain is processed rapidly with no mistakes. Study of the processing speeds of modern computers makes us appreciate the power of Allah. However sophisticated they may be they do not even approach the processing capacity of the human brain.

 

C. HUMAN ACTIONS

ENCEPHALISATION:

Human intellectual superiority is explained by the greater role of the cortex in many functions. The cortex controls higher functions of human thought: learning, memory, judgment, and language. In general anesthesia and other situations such as drug and alcohol intoxication in which the psyche is altered, the cortex is depressed first affecting the higher functions that distinguish humans from animals.

 

UNCONSCIOUS ACTION

The human organism is such a complicated system with millions of activities. If all these activities were part of the conscious will, the system would be overwhelmed. Humans must have the humility to know that they have very little conscious control over their bodies. Humans do not consciously control individual muscle movements, glandular secretions, or metabolic activities.

 

AUTONOMIC ACTION:

The existence of the autonomic nervous system enables many background life-sustaining activities to continue without sensory input into the cerebrum. Most neuronal sensation and activity is not under conscious control. It is part of the autonomic nervous system (ANS). ANS consists of sympathetic, parasympathetic, and the enteric NS. It is characterized by rapid, automatic, reflex response with no conscious control. The sympathetic and parasympathetic systems have opposite effects and are finely balanced. This is an illustration of the Qur’anic concept of tadaf’u without which human physiology and human civilization would collapse.

 

MOTOR FUNCTION:

Neuro-muscular unit: The functional unit of the neuromuscular unit is the motor unit which consists of: the lower motor neuron that arises from the spinal cord, the axon of the lower motor neuron, and the multiple muscle fibers that are innervated. Muscles with fine movements have a higher neuron to muscle ratio (1:10) than those with relatively coarse and stereotypes movements (1:2000). Each muscle has its neurones. The innervation is embryo logically derived (dermatomes). There are three types of muscles distinguishable on the basis of histology and function: somatic, visceral, and neuro-endocrine. The bulk of the musculature is striated skeletal muscles. Skeletal muscles are controlled at three levels: spinal cord in simple reflexes, brain stem, and the cerebral hemisphere. Reflexes are automatic muscular contractions that protect the body. They are at the spinal level without cerebral involvement. Examples are the knee jerk and the withdrawal reflex. The spinal reflexes are: flexor/withdrawal, Crosse extensor, stretch (myotatic), and tendon (inverse stretch reflex). There is mapping of muscles in the motor cortex. Fingers, lips, the tongue, and vocal cords have more neurones than the elbow, trunk and ankle. The cortex is aware of the autonomic or reflex activity of muscles and can intervene to override them.

 

HUMAN FREEDOM OF ACTION:

The Qur'an mentions that the human has freedom of action (p. 841 4:66, 17:18-19, 17:84, 41:40, 73:19, 74:37, 76:29, 78:34, 81:28, 92:4-10). Human freedom of action is limited. The observation that most mechanical and metabolic activities in the body are under autonomic, reflex, and endocrine control is further indication of the limitations of human will in actions. The human is not able to control many of the emotions. Human control is limited to a few cortical functions.

 

D. HUMAN BEHAVIOR

ELECTRO-CHEMICAL BASIS OF HUMAN BEHAVIOR:

Organic basis: Many phenomena in the nervous system related to behaviour can be explained by organic factors. More are likely to be explained in the future. The un-explained phenomena reflect the impact of nafs, the essence of the human. The nafs can over-ride the organic factors. Drugs react with CNS transmitters to produce behaviour or mental state changes. Thus proves that human behaviour has a chemical basis. Neurohormones also control behaviour.

 

Rage: Rage and aggression have an organic basis. Noradrenaline stimulates rage in the amygdala. Serotonin inhibits rage through the forebrain. Acetylcholine is involved in rage at the hypothalamic level. The directive of the prophet to sit down or get wudhu when angry is understood I this sense. Wudhu and sitting down may have a relaxing effect mediated either directly or through the cerebral cortex.

 

Mood: Normal mood is when the state of the mind is in tune with the environment. The affective state is a balance between amine-containing and actylcholine-containing neurones in the brain stem and the forebrain.  Depression and mania are imbalance disorders. The following chemicals play a role in mood: serotonin, catcholamines, cyclic AMP, and acetylcholine.

 

Differences among people: The observed differences in behavior and emotional reactions between men and women may be explainable on the basis of neuro-hormonal differences. Differences among ethnic groups could also be similarly explained.

 

GENETIC BASIS OF HUMAN BEHAVIOUR:

The anatomical and chemical factors that affect human behavior are genetically controlled. Genetic control of behavior cannot be perfect or over-riding because that would nullify any human responsibility for misbehavior. Processes originating in the nafs and qalb can override the genetic and neuro-humoral mechanisms to impose human will on behavior. This override usually acts through the cortex but other mediums are possible. A dangerous trend in genetic research is the discovery of specific genes for human sexual and addictive misbehavior. Genes have been defined for homosexuality and alcohol addiction. If the results of these studies are interpreted in a secular way, they will disempower humans and prevent them from struggling to improve by fighting evil promptings of their nafs.

 

CULTURAL BASIS OF HUMAN BEHAVIOUR:

Early learning and conditioning have an imprint on human behavior. Early learning experiences in the family may act at the cognitive level or may actually impact the architecture of neuronal networks in the brain during the plastic phase.

 

HUMAN RESPONSIBILITY FOR BEHAVIOR:

Although there are anatomical and chemical bases for human behavior, humans still have responsibility for their actions. This is because the nafs can over-ride the chemical and anatomical factors. Denial of this over-ride turns humans into animals with no responsibility for actions.

 

ARTIFICIAL PHARMACOLOGICAL EFFECT ON HUMAN BEHAVIOR:

Alcohol and other pharmacologically active substances affect human behavior. More research is needed on the impact of the ordinary diet that people take which may explain some culturally determined patterns of behavior. The diet or environmental pollutants may have an effect on the internal chemical milieu affecting behavior of individuals or groups in specific ways.

Professor Omar Hasan Kasule Sr. February 2001