Friday, 3 April 2015

Human nervous system


We comprehend, perceive and respond to the world around us because of the nervous system. Other unconscious functions of the body such as digestion, respiration etc are also controlled by the nervous system. The human immune system controls and unites the different body functions. It also keeps the internal body environment normal despite any kind of changes that may occur in the external environment. The nervous system acts because of certain regulatory reactions. All the actions and behaviors that are done by a human being depend on the nervous system as it receives, stores and releases sensory and  motor information.
The resultant actions can be as small as cellular in nature and as large as involving the whole body.The 2 main parts of nervous system are:The central nervous system, and The peripheral nervous system

Development of nervous system

The human nervous system develops from the primitive layer of ectoderm. Some areas of nervous system also originate from medullary plate or neural plate. The brain and its ventricles are formed of cephalic tube while the spinal cord and its central canal are formed of caudal end of the neural plate. The nerve cells or the neurons originate from neuroblasts and the neuroglia develops from spongioblasts.

Neural cells or neurons

As stated earlier, the neural cells are formed of neuroblasts.  During maturation of the neuroblasts, at first the neurofibrils appear in the cytoplasm of the cell. The neuroblasts then migrate away from the central canal. An axon appears at the periphery of the neuroblast. The axon containing neuroblast is bipolar in nature. It cuts off from the central canal. Dendrites then appear and multipolar neuroblasts or mature neurons are formed.

Glial cells or spongioblasts

Primitive spongioblasts get differentiated into astroblasts. The astroblasts connect with blood through protoplasmic extensions called end feets. They then branch further to form protoplasmic astrocyte. Some astroblasts have glial fibrils in cytoplasm. These glial fibrils containing spongioblasts form fibrous astrocytes.

The undifferentiated medulloblasts may proliferate and differentiate to form spongioblasts, neuroblasts and oligodendroglia.

Central nervous system or CNS

The central nervous system consists of brain and spinal cord. It is also called somatic nervous system. The CNS can be divided into 2 halves, 1 half mirroring the other. The spinal cord is located inside the vertebral column and the brain is located inside the cranial cavity. The brain and the spinal cord are divided by foramen magnum or the limiting line.

Spinal cord

The spinal cord lies in the vertebral column. The length of the spinal cord in males is 45 cm and in females is 43 cm. An adult spinal cord weighs approximately 35 gm. It is located between the foramen magnum and the lower border of lumbar vertebra adjacent to the 12th thoracic segment. The spinal cord is cylindrical in shape. There are 2 swellings in it. 1st is at the 6th cervical and 2nd is at the 3rd lumbar. The superimposed segments of spinal cord are arranged in series and form these segments from where the nerve roots originate. 31 pairs of nerves originate from the spinal cord. The 31pairs can be divided into 12pairs of thoracic, 8 pairs of cervical, 5 pairs of sacral, 5 pairs of lumbar and 1 pair of coccygeal nerves.
Brain

The brain can be divided into forebrain or prosencicephalon, midbrain or mesencephalon and hindbrain or rhombencephalon.
Hindbrain or rhombencephalon

The hindbrain or rhombencephalon

Cephalin connects the spinal cord with the rest of the brain. The hindbrain is composed of medulla oblongata, pons and 3rd to 12th cranial nerves. This portion of the brain governs the sub consciously done essential functions of the body through reflexes. The hind brain can be divided into myelencephalon or medulla oblongata and metencephalon which is comprised of pons and cerebellum Medulla oblongata or myelencephalon

It is situated in the posterior section of the cranial fossa. The cervical spinal cord continues from the foramen magnum to form medulla which extends upto the pons. The autonomic functional control of the body for lungs, heart and circulation lies in medulla. Medulla also governs the autonomic reflex actions like coughing, sneezing, swallowing, vomiting and gagging.

Pons- a part of metencephalon

Pons is situated above and in front of medulla. The transverse fibers collect here to form a bulge. The 4th ventricle separates the pons and the cerebellum. The inferior pontine sulcus separate the medulla and the pons. Superior pontine sulcus is present between the pons and cerebellum. Pons regulates respiration and some other vital functions of the body.

Cerebellum- a part of metencephalon

It is located in the caudal part of cranial cavity and is locate behind and above the medulla. Three peduncles or stalks join the cerebellum to other parts of the brain. The superior or rostral pair joins it to the midbrain, the middle pair of peduncles joins it to the pons while caudal or inferior pairs of peduncle join it to the medulla oblongata.The central part of cerebellum is called vermis which is surrounded by 2 lateral hemispheres. Cerebellum is concerned with smooth movements and therefore governs posture.

Midbrain or mesencephalon
The brain stem continues upward to form midbrain. The aqueduct of sylvius passes through it. Above the acqueduct (tectum) are located collection of nerve cells called superior colliculus and inferior colliculus. The third nerve nucleus and the red nucleus lie below and lateral to the aqueduct.

The underside of the aqueduct contains cerebral peduncle. The dorsal and ventral pats of this peduncle are divided by nerve cells (containing melanin) that are called substantia niagra. The ventral area is called pis and the dorsal area is called tegmentum. Nerve fibers pass through these areas. The midbrain governs vision and hearing. Substantia nigra produces dopamine that is vital for motivation.

Forebrain or procencephalon

The forebrain or procencephalon can be divided into 2 parts viz. the telencephalon and diencephalon. The telencephalon is composed of cerebrum, cerebral cortex, corpus striatum and rhinencephalon.  The diencephalon is composed of thalamus, hypothalamus, epithalamus and metathalamus. Speech, thought, sensory integration, control of voluntary movements and higher intellectual functions are controlled by forebrain. The forebrain can be divided into telencephalon and diencephalon.
Telencephalon

It consists of cerebrum, cerebral cortex, corpus striatum & rhinencephalon.
Cerebrum

It is the largest part of the human brain and accounts for 4/5 of its weight. The longitudinal divide splits it into 2 prominent hemispheres. The deep median fissure or the dividing line is also called cerebral fissure. Nerve fibers called corpus callosum connect these two hemispheres. Each of the 2 hemispheres is divided into frontal, parietal, occipital and temporal lobes. The fissures dividing them are central, parieto-occipital and sylvian. The right hemisphere receives sensory inputs from the left side of the body and provides motor function to it and vice versa. Thus each hemisphere controls the opposite side of the body. This arrangement is called contra lateral arrangement. Each hemisphere receives touch and vision input contra laterally while auditory input is common for both hemispheres.  In each individual one hemisphere is more dominant than the other. This dominant hemisphere controls analytical, mathematical, language and handedness functions. That is why there are right handed and left handed individuals. The non dominant hemisphere deals with facial recognition, emotions, auditory senses and spatial concepts.
Grey matter and white matter

The human brain has many levels of neurons that are connected to each other. These neuron collections are differentiated into grey and white matter. The grey matter is also called substantia griesa. The grey matter has no myelin blanket and is controlled by dendrites (short and branching filaments that carry the nerve impulses from the brain to rest of the body) and nerve cell bodies. The nerve cell bodies are those areas of the neuron that contain nucleus. The grey matter is grey because of the grey colored nuclei of the cells. Real information processing happens in the grey matter of the brain. It is approximately 40% of the whole brain. It also consumes 90% of the oxygen that reaches brain. The neurons of the grey matter do not have axons. The neurons of grey matter are connected by neurotransmitters. All the senses of the human body like hearing, speech, seeing, memory and feelings are controlled by the grey matter. The grey matter also controls the muscles and their movements.

The other name for white matter is substantia alba. The white matter neurons have axons or myelinated nerve fibers. The white matter is found between the cerebellum and the brain stem. The hypothalamus and the thalamus are composed of white matter neurons. The main function of the white matter is to relay all the information it receives from different parts of the body towards the cerebral cortex. It also controls sub-conscious functions of the body like blood pressure, temperature and heart rate regulation. Apart from these functions the white matter also controls food and water digestion, dispensing of hormones in different parts of the body and expression of emotions. Axons of the white matter neurons are covered and protected by myelin sheath. The myelin sheath gives the white matter its white color. The white matter composes around 60% of the total brain.

Cerebral cortex

The surface layer of cerebrum is called cerebral cortex. It is called the grey matter of the cerebrum. The cerebral cortex is made up of nerve fibers that are highly convoluted. The crest or the ascending area of the convolution is called gyrus (singular gyri). Two gyri are separated by a fissure called Sulcus (singular sulci). The sulci and gyri form patterns on the basis of which the cererbral cortex is divided into 4 parts viz. frontal, temporal, parietal and occipital lobes. These lobes are distinguished by 2 major sulci that are located on the sides of hemispheres. The frontal and parietal lobes are separated by fissure of ronaldo or central sulcus. The fissure of sylvius or lateral sulcus separates temporal, frontal and parietal lobes.

Frontal lobes- this is the largest among all the 4 lobes. The frontal lobe lies in front of the central Sulcus. The primary motor region of brain lies in this lobe and is formed of the precentral gyrus. An injury to this part of the brain may result in paralysis of the opposite side or contra-lateral part of the body. The bronco area, the region involved in speech is located in the inferior frontal lobe.

Parietal lobe- it lies at the back of the central Sulcus. It is divided in 3 parts viz. the post central gyrus, the superior parietal lobe and the inferior parietal lobe. All the sensory input from the opposite half of the body is received in the post central gyrus. The superior parietal lobe is located below and behind the postcentral gyrus and above the intraparietal sulcus. This area is not involved in sensory input processing and does not provide motor function. It is associative in function. Supramarginal and angular gyri compose the inferior parietal lobe. The inferior parietal lobe is concerned with the integration of sensory signals.

Temporal lobe-it is located behind the lateral Sulcus. The temporal lobe fills the middle hollow fosaa of the brain. The superior, middle and inferior gyri compose the outer area of the lobe. 2 transverse gyri are present at the margin of the lateral Sulcus which from the basic or primary auditory area of the brain. Different parts of this area represent different auditory frequencies. Near the ventral surface of temporal an inner protrusion called uncus is present. The uncus forms the larger part of the olfactory area.

Occipital lobe- it lies posterior to the parieto- occipital Sulcus that is joined by the calcarine sulcus. It is primary visual area of the brain and receives vision sensory input from the contra lateral or opposite part of the body. The rear areas of this portion of the brain represent central vision (the gaze sensory input) and the front areas represent peripheral vision (outside direct vision or gaze sensory input).

Limbic lobe- this lobe is composed of adjacent portions of temporal, parietal and frontal lobes which surround the corpus collasum.  Thus limbic lob is also called the synthetic lobe. This lobe relates with the autonomic and sensory activities. It receives sensory input from the thalamic nuclei that are connected with the hypothalamus.

Corpus striatum

The corpus striatum or striatum receives sensory input from the cerebral cortex. This part of the brain deals with motivation. It is composed of medium spiny neurons. This area of the brain releases acetylcholine.

Rhinencephalon

This is olfactory lobe of the brain. The olfactory nerves arise out of these lobes. Rhinencephalon is a paired structure and consists of 2 halves.

Diencephalon

The diencephalon section of for brain consists of thalamus, hypothalamus, epithalamus and metathalamus.

Thalamus-A pair of large and ovoid organs occupy majority of 3rd ventricle walls. Each singular part is called thalamus while the pair is termed as thalami. Various receptors provide neural impulses to the thalamus. The thalamus translates these impulses and transfers them to the cerebral cortex. Touch, temperature, pain and synaptic transmissions of resting stage form these sensations.

Hypothalamus

This region of the brain lies on the floor of 3rd cerebral ventricle and is located below the thalamus. This small cone shaped structure projects downwards from the brain and ends in the pituitary stalk which is tubular connection to the pituitary gland. The hypothalamus controls the majority of autonomous functions of the body and also affects the flow of endocrine hormones. The endocrine effect of the hypothalamus is because of its complex relationship with the pituitary gland.

Epithalamus- it consists of pineal body, habenular nucleus and  habenular striae.

Pineal body-pineal body is also called the pineal gland. It is a small endocrine gland situated in brain that produces melatonin. Melatonin is responsible for cincardian rhythm or sleep cycle of the body. The gland forms the root of diencephalon. The pineal gland is located between the 2 cerebral hemispheres and behind the 3rd cerebral ventricle. It is around 0.8cm long and has a weight of 0.1gm.adrenergic nerves join pineal gland. Pinealocytes and other supportive cells constitute the pineal gland.

Habenular nucleus- the habenular nucleus is formed of small group of nuclei. This nuclei group is located behind the thalamus. This region links the pineal gland with the rest of the brain. 

Habenular striae- It is located at the medial surface of thalamus. Habenular striae are formed of afferent fibers which originate from the septal nuclei.  Read more..

The peripheral nervous system or PNS

The peripheral nervous system consists of the cranio-spinal nerve system (composed of 31 pairs of spinal nerves and 12 pairs of cranial nerves) and visceral nerve system consisting of sympathetic and parasympathetic nervous system.The essential function of the PNS is to connect the CNS to other parts (limbs and organs) of the body. It forms the communication link between the CNS and the other extremities of the body. PNS is not enclosed in bones and hence is open to attack to toxins and other harmful elements/pathogens that may circulate in blood. PNS is composed of somatic nervous system and autonomic nervous system. Acetylcholine and noradrenaline are the 2 main neurotransmitters of the PNS.

Somatic nervous system

It is also called the voluntary nervous system as it is concerned with the skeletal muscles that have a voluntary control over body movements. It consists of afferent and efferent nerves. The afferent nerves relay sensory inputs towards the CNS while the efferent nerves stimulate the muscle contraction in skin and skeletal muscles. The somatic nervous system is made up of spinal, cranial and association nerves. The spinal nerves carry sensory and motor information from the spinal cord. The cranial nerves carry sensory and motor information towards and away from the brain stem and are associated with sensory inputs like vision, smell, taste etc. the association nerves integrate the sensory and motor functions of the body.

Autonomous nervous system or ANS

It is also called as involuntary or visceral nervous system. It influences the functions of the internal organs of the body. Thus, heart rate, urination, sexual arousal, digestion and other subconscious movements of the body are governed by ANS. The ANS also governs the fight or flight response of the body. The hypothalamus governs the autonomic functions of the body within the brain. ANS is divided into sympathetic and parasympathetic nervous system. The sympathetic nervous system is the “fight or flight” system. The “fight or flight” system is the body’s physiological reaction in situations of survival, harmful attack and event. The parasympathetic nervous system deals with the” rest and digest” scenarios.


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