The nervous system with its intricate mechanisms of control over all bodily functions makes it indeed a fascinating study. It is supported by the endocrine system in its various functions. While the nervous system is vested with the control of rapidly changing events, the endocrine system is concerned with slower metabolic and other vegetative functions.
The central nervous system (CNS) is unique in the complexity of its functions. Anatomically, this system is a collection of cells specialized to convey signals at a rapid rate with great precision.
The system receives information concerning:
- The internal state of the body.
- The immediate external environment.
- More distant objects and events in the external world; and
- Responds by sending appropriate signals to muscles and glands and initiates action on the environment.
The brain:
- Processes information
- Stores data, makes final decisions for initiating action after comparing it with previous information.
The 2 main functions of the brain are:
- Homeostatic regulation, and
- Initiation of action
Homeostatic Regulation is Concerned with Mechanisms that:
- Control body temperature
- Maintain appropriate levels of blood pressure and other parameters.
- Maintain the body posture against the collapsing force of gravity.
The mechanisms, in general, are automatic and occur without conscious effort.
Initiation of action is concerned with those actions that are performed without any external stimulus. Though these may serve regulatory ends, they appear to be spontaneous and are seen in animals with large brains. Lastly, we have complex actions that are part of learning, thinking and remembering-understood as part of general intelligence.
Evolution
If one makes a study of phylogeny, it is found that in the primitive unicellular organisms which originated in the sea, biological exchange occurred through simple diffusion and locomotion took place by diapedesis which required no coordination.
During evolution, there was more and more elaboration in the organisation of the cardiovascular, respiratory and other systems. Coordinated movement needed the development of complex motor pathways and maintaining the erect posture called for appropriate mechanisms. Expansion of association areas in the cerebral cortex became necessary with the evolution of the art of communication and social interaction.
Process of Evolution makes an interesting study where, in the unicellular organism, we find a single cell differentiates from one region to the other, e.g., where the cilia around the mouth is excitable, acting as receptors and also being motile. The differentiation being due to the modification of the structure of a single cell.
Next we see as in the metazoa, special types of cells appearing for conducting the effect of a stimulus from one region to another. Later, definite sensory cells with prolongations making connections with a muscle cell are seen. Thus, we see a more complex arrangement, namely, the appearance of a ganglion cell placed between the sensory and muscle cell. A further development become necessary for efficient coordination between various parts and this called for the development of some control over the transmission of impulses from one stage to another. This became possible with the multiplication of units of conduction, introducing at the same time discontinuity in the pathway 3between the sensory and effector cells. It is at this junctional region, the so-called synapse, that control can be exercised.
The striking differences in large and small brains are:
- the increasing number of neuronal units
- extensive elaboration of the number and complexity of these inter-connections, and
- the introduction of interconnective systems and sub-systems.
Development
All parts of the nervous system of mammals including man originate either from the neural tube or from the neural crest of the embryonic ectoderm. The neural crest gives rise to:
- Neurons whose cell bodies are located in the dorsal root ganglion.
- Homologous ganglion of cranial nerves.
- Post-ganglionic neurons of the autonomic nervous system.
The Neural Tube
The lumen of the cranial end of the neural tube becomes dilated and produces the large ventricular system in the brain. The lumen at caudal end remains small, recognizable in the adult as the tiny central canal of the spinal cord. In a human embryo, about 5 weeks old, 3 main dilatations of the neural tube can be made out:
- Prosencephalon or fore-brain vesicle.
- Mesencephalon or mid-brain vesicle.
- Rhombencephalon or hind brain vesicle.
Embryologically, the brain is divided into the Rhombencephalon and the cerebrum joined together by an isthmus:
General Outline of the CNS
The CNS consists of the brain contained within the
skull and the spinal cord lying in the vertebral canal. The peripheral portion consists of 43 pairs of nerves:
The nervous system with its intricate mechanisms of control over all bodily functions makes it indeed a fascinating study. It is supported by the endocrine system in its various functions. While the nervous system is vested with the control of rapidly changing events, the endocrine system is concerned with slower metabolic and other vegetative functions.
The central nervous system (CNS) is unique in the complexity of its functions. Anatomically, this system is a collection of cells specialized to convey signals at a rapid rate with great precision.
The system receives information concerning:
- The internal state of the body.
- The immediate external environment.
- More distant objects and events in the external world; and
- Responds by sending appropriate signals to muscles and glands and initiates action on the environment.
The brain:
- Processes information
- Stores data, makes final decisions for initiating action after comparing it with previous information.
The 2 main functions of the brain are:
- Homeostatic regulation, and
- Initiation of action
Homeostatic Regulation is Concerned with Mechanisms that:
- Control body temperature
- Maintain appropriate levels of blood pressure and other parameters.
- Maintain the body posture against the collapsing force of gravity.
The mechanisms, in general, are automatic and occur without conscious effort.
Initiation of action is concerned with those actions that are performed without any external stimulus. Though these may serve regulatory ends, they appear to be spontaneous and are seen in animals with large brains. Lastly, we have complex actions that are part of learning, thinking and remembering-understood as part of general intelligence.
Evolution
If one makes a study of phylogeny, it is found that in the primitive unicellular organisms which originated in the sea, biological exchange occurred through simple diffusion and locomotion took place by diapedesis which required no coordination.
During evolution, there was more and more elaboration in the organisation of the cardiovascular, respiratory and other systems. Coordinated movement needed the development of complex motor pathways and maintaining the erect posture called for appropriate mechanisms. Expansion of association areas in the cerebral cortex became necessary with the evolution of the art of communication and social interaction.
Process of Evolution makes an interesting study where, in the unicellular organism, we find a single cell differentiates from one region to the other, e.g., where the cilia around the mouth is excitable, acting as receptors and also being motile. The differentiation being due to the modification of the structure of a single cell.
Next we see as in the metazoa, special types of cells appearing for conducting the effect of a stimulus from one region to another. Later, definite sensory cells with prolongations making connections with a muscle cell are seen. Thus, we see a more complex arrangement, namely, the appearance of a ganglion cell placed between the sensory and muscle cell. A further development become necessary for efficient coordination between various parts and this called for the development of some control over the transmission of impulses from one stage to another. This became possible with the multiplication of units of conduction, introducing at the same time discontinuity in the pathway 3between the sensory and effector cells. It is at this junctional region, the so-called synapse, that control can be exercised.
The striking differences in large and small brains are:
- the increasing number of neuronal units
- extensive elaboration of the number and complexity of these inter-connections, and
- the introduction of interconnective systems and sub-systems.
Development
All parts of the nervous system of mammals including man originate either from the neural tube or from the neural crest of the embryonic ectoderm. The neural crest gives rise to:
- Neurons whose cell bodies are located in the dorsal root ganglion.
- Homologous ganglion of cranial nerves.
- Post-ganglionic neurons of the autonomic nervous system.
The Neural Tube
The lumen of the cranial end of the neural tube becomes dilated and produces the large ventricular system in the brain. The lumen at caudal end remains small, recognizable in the adult as the tiny central canal of the spinal cord. In a human embryo, about 5 weeks old, 3 main dilatations of the neural tube can be made out:
- Prosencephalon or fore-brain vesicle.
- Mesencephalon or mid-brain vesicle.
- Rhombencephalon or hind brain vesicle.
Embryologically, the brain is divided into the Rhombencephalon and the cerebrum joined together by an isthmus:
General Outline of the CNS
The CNS consists of the brain contained within the skull and the spinal cord lying in the vertebral canal. The peripheral portion consists of 43 pairs of nerves:
- 12 pairs called cranial nerves
- 31 pairs of spinal nerves that emerge from the spinal cord.
The peripheral nerves contain 2 kinds of fibres - one group called afferent or sensory carrying impulses to the CNS.
Second group efferent or motor fibres carrying impulses from the CNS to the muscles, glands and other organs.
Each spinal nerve arises from the cord by 2 roots-posterior or dorsal carries afferent fibres while anterior or ventral carries efferent fibres.5
The nervous system can also be divided into two anatomical and functional components:
- The somatic
- The autonomic or visceral.
The somatic component is vested with control mechanisms in response to information arising from the surface of the body; from the muscle and joints and from the surrounding environment. The autonomic component controls the blood vessels and viscera.
The afferent nerves bring information from the external environment from receptors which are sensitive to sound, light, temperature and pressure and also information about the internal state of the body. As a result of this, the nervous system sends impulses along with efferent nerves to produce appropriate movements of muscles and secretion of glands. Sometimes the process rises to conscious levels but many of the activities of the body are carried out without one being aware of them. The activities are called reflex actions and the pathways are reflex arcs.
Spinal Cord
The spinal cord begins as a continuation of the medulla oblongata, the inferior part of the brain-stem and extends from the foramen magnum of the occipital bone to the level of the second lumbar vertebra. In cross section, a segment of the cord forms a central core of “H” shaped grey matter consisting mainly of the cell bodies of neurons organized in functional groups. The surrounding white matter consists of myelinated nerve fibres constituting of ascending and descending tracts. Activity at segmental levels is possible by interneurons that connect the processes of the sensory to the motor neurons of the same side, while commissural interneurons connect the symmetrical half segments making coordination between the two sides of the body possible. The axons of the sensory nerve splits into ascending and descending branches, synapsing with interneurons at each level allowing the direct spread of impulses.
Medulla
The ascending branch reaches a relay station in the medulla which may be considered as part of the conducting pathway to the higher regions of the brain. The medulla may be considered as an upward 6continuation of the spinal cord. The medulla contains all ascending and descending tracts that communicate between the spinal cord and various parts of the brain. The medulla contains various vital centres that control heart, respiration and so on. Spinal cord, medulla and pons roughly constitute that sensory-motor division of the CNS to and from which pass nerves from every part of the body.
The reticular formation: An area of dispersed grey matter and dense network of communicating fibres extends from the spinal cord to diencephalon - mainly functions in consciousness and arousal in addition to control of muscle tone, visceral functions and modulation of pain.
Thalamus
From the relay station in the medulla, information is passed on to the diencephalon by way of mesencephalon or mid-brain to terminate in a body of grey matter called the thalamus. The thalamus is a centre for the reception and coordination of the vast number of impulses derived from the sensory nerves entering the spinal cord and brainstem.
Brainstem - is the name given to those parts of the brain that lie on the main conducting pathway and is thus made up of the medulla, the pons, the isthmus and mid-brain (structures remaining after removal of the cerebral hemispheres, cerebellum and basal ganglia).
Basal Ganglia: Immediately beneath the hemispheres and belonging to the telencephalon, are the basal ganglia concerned with the control of bodily movements. They function as a relay and programming station for impulses on the way to and from the cerebral cortex.
Cerebral Hemisphere
The thalamus is connected with a still higher part of the brain, the telencephalon comprising mainly of the cerebral hemispheres. The cerebral hemispheres may be regarded as the highest region to which sensory impulses may be relayed. Mainly, by virtue of their interaction in the cells here, they give rise to conscious sensations which are brought into relation with past experience. It is again in the cerebral hemisphere that a considerable degree of control over the motor side of the nervous system is exerted. Nerves arising from the motor region of the cortex passing down as pyramidal tracts terminate in the cord as somatic nerves in the ventral roots.7
The cerebrum consists of an external mantle or cortex made up of grey matter, a base and an interior with ventricles and bundles of white fibres. Convoluted masses of grey matter form the gyri which are separated by fissures. Each hemisphere is divided into lobes by these fissures, e.g., frontal, parietal, temporal and occipital lobes. The brain is covered by 3 layers of meninges. They are from without inwards - the dura, the arachnoid and the pia mater. The ventricles of the brain are 2 lateral, 3rd and 4th ventricles. Between the 2 lateral ventricles is the interventricular foramen which is connected to the 3rd ventricle by foramen of Munro. The third ventricle opens into the fourth through the aqueduct of Sylvius. This is continuous with the central canal of spinal cord.
Intermingled with neurons in the brain and spinal cord are other types of cells classified together as glia:
- One type, the neuroglia originate from the ectoderm and found in:
- The other known as microglia are of mesodermal origin and enters the embryonic brain when it is penetrated by blood vessels. The neuroglia carry out a variety of functions.
- They are thought to play a role in providing some structural support to the neurons.
- Microglia have a phagocytic action against bacteria.
- Astrocytes form a component of the blood brain barrier and protect the neurons from harmful substances in the vascular system.
- 12 pairs called cranial nerves
- 31 pairs of spinal nerves that emerge from the spinal cord.
The peripheral nerves contain 2 kinds of fibres - one group called afferent or sensory carrying impulses to the CNS.
Second group efferent or motor fibres carrying impulses from the CNS to the muscles, glands and other organs.
Each spinal nerve arises from the cord by 2 roots-posterior or dorsal carries afferent fibres while anterior or ventral carries efferent fibres.5
The nervous system can also be divided into two anatomical and functional components:
- The somatic
- The autonomic or visceral.
The somatic component is vested with control mechanisms in response to information arising from the surface of the body; from the muscle and joints and from the surrounding environment. The autonomic component controls the blood vessels and viscera.
The afferent nerves bring information from the external environment from receptors which are sensitive to sound, light, temperature and pressure and also information about the internal state of the body. As a result of this, the nervous system sends impulses along with efferent nerves to produce appropriate movements of muscles and secretion of glands. Sometimes the process rises to conscious levels but many of the activities of the body are carried out without one being aware of them. The activities are called reflex actions and the pathways are reflex arcs.
Spinal Cord
The spinal cord begins as a continuation of the medulla oblongata, the inferior part of the brain-stem and extends from the foramen magnum of the occipital bone to the level of the second lumbar vertebra. In cross section, a segment of the cord forms a central core of “H” shaped grey matter consisting mainly of the cell bodies of neurons organized in functional groups. The surrounding white matter consists of myelinated nerve fibres constituting of ascending and descending tracts. Activity at segmental levels is possible by interneurons that connect the processes of the sensory to the motor neurons of the same side, while commissural interneurons connect the symmetrical half segments making coordination between the two sides of the body possible. The axons of the sensory nerve splits into ascending and descending branches, synapsing with interneurons at each level allowing the direct spread of impulses.
Medulla
The ascending branch reaches a relay station in the medulla which may be considered as part of the conducting pathway to the higher regions of the brain. The medulla may be considered as an upward 6continuation of the spinal cord. The medulla contains all ascending and descending tracts that communicate between the spinal cord and various parts of the brain. The medulla contains various vital centres that control heart, respiration and so on. Spinal cord, medulla and pons roughly constitute that sensory-motor division of the CNS to and from which pass nerves from every part of the body.
The reticular formation: An area of dispersed grey matter and dense network of communicating fibres extends from the spinal cord to diencephalon - mainly functions in consciousness and arousal in addition to control of muscle tone, visceral functions and modulation of pain.
Thalamus
From the relay station in the medulla, information is passed on to the diencephalon by way of mesencephalon or mid-brain to terminate in a body of grey matter called the thalamus. The thalamus is a centre for the reception and coordination of the vast number of impulses derived from the sensory nerves entering the spinal cord and brainstem.
Brainstem - is the name given to those parts of the brain that lie on the main conducting pathway and is thus made up of the medulla, the pons, the isthmus and mid-brain (structures remaining after removal of the cerebral hemispheres, cerebellum and basal ganglia).
Basal Ganglia: Immediately beneath the hemispheres and belonging to the telencephalon, are the basal ganglia concerned with the control of bodily movements. They function as a relay and programming station for impulses on the way to and from the cerebral cortex.
Cerebral Hemisphere
The thalamus is connected with a still higher part of the brain, the telencephalon comprising mainly of the cerebral hemispheres. The cerebral hemispheres may be regarded as the highest region to which sensory impulses may be relayed. Mainly, by virtue of their interaction in the cells here, they give rise to conscious sensations which are brought into relation with past experience. It is again in the cerebral hemisphere that a considerable degree of control over the motor side of the nervous system is exerted. Nerves arising from the motor region of the cortex passing down as pyramidal tracts terminate in the cord as somatic nerves in the ventral roots.7
The cerebrum consists of an external mantle or cortex made up of grey matter, a base and an interior with ventricles and bundles of white fibres. Convoluted masses of grey matter form the gyri which are separated by fissures. Each hemisphere is divided into lobes by these fissures, e.g., frontal, parietal, temporal and occipital lobes. The brain is covered by 3 layers of meninges. They are from without inwards - the dura, the arachnoid and the pia mater. The ventricles of the brain are 2 lateral, 3rd and 4th ventricles. Between the 2 lateral ventricles is the interventricular foramen which is connected to the 3rd ventricle by foramen of Munro. The third ventricle opens into the fourth through the aqueduct of Sylvius. This is continuous with the central canal of spinal cord.
Intermingled with neurons in the brain and spinal cord are other types of cells classified together as glia:
- One type, the neuroglia originate from the ectoderm and found in:
- The other known as microglia are of mesodermal origin and enters the embryonic brain when it is penetrated by blood vessels. The neuroglia carry out a variety of functions.
- They are thought to play a role in providing some structural support to the neurons.
- Microglia have a phagocytic action against bacteria.
- Astrocytes form a component of the blood brain barrier and protect the neurons from harmful substances in the vascular system.