MODULE 24 CENTRAL NERVOUS SYSTEM - PDF document

MODULE 24: CENTRAL NERVOUS SYSTEM (CNS) AND THE PERIPHERAL NERVOUS SYSTEM (PNS) Learning objectives: The nervous system in humans is a complex integration of organs like brain, spinal cord and sensory organs along with nerves. Nerve cells are required for exchange of information between brain and spinal cord to rest of body parts for coordinated control. The present module primarily focuses on the nervous system and its basic components and their precise function in coordination of body.  Introduction and role of nervous system.  Differentiation of central nervous system and peripheral nervous system.  Central nervous system and its role in maintaining homeostasis.  Peripheral nervous system, its different kinds of pathways and their function. Introduction: The nervous system is the integral organ system of humans/animals which coordinates the homeostasis from external environment as well as internal coordination of v arious body parts with brain using signal transmission . Nervous system is composed of complex network of billions of nerve cells /neurons which communicated with each o ther in a highly controlled fashion . Coordination of nerve cells result in a quick response to each reaction from external world center of the body. The basic function of nervous system is sensation, integration and reaction for monitor changes in internal and external environment of the body, processing of the sensory information and activation of glands to release neurotransmitters respectively. The nervous system acts as integrating center for getting the information through our senses, processing them and triggers responses. For example, if we accidentl

y touch a hot plate, immediately our reflex action makes our hand to pull in seconds. It is only because of our nervous system in which nerves cells immediately send signals to our brain and we feel pain and hence pull back our hand . I n vertebrates , n ervous system consists of two major parts a) the central neural system (CNS) and b) the peripheral neural system (PNS) (Figure 1) .The coordination of both CNS and PNS allow human beings to make equilibrium with in their internal system and as well as external environment. CENTRAL NERVOUS SYSTEM (CNS) The c entral nervous system (CNS) consists of brain and spinal cord. C entral nervous system functions to integrate information and coordinate majority of physiological activit ies in bilaterally symmetric animals . Several sensory system like retina , optic nerve , the olfactory nerves (1st) and olfactory epithelium are also considered as component of CNS because of their direct synapsing with brain without intermediate ganglia . Due to its direct proximity with brain tissue , olfactory epithelium is also the site for therapeutic treatments. The part of central nervous system within brain lies in the cranial cavity. T he spinal cord component of CNS is protected and enveloped by the vertebrae, and both brain and spinal cord are also enveloped within the meninges . The brain and the spinal cord are surrounded by Cerebrospinal fluid which circulates within the ventricles cavities of central nervous system. Figure1 - Diagrammatic representation of human nervous system : Pink colour (brain and spinal cord) represents centr al nervous system, while blue colour (gan

glia and nerves) represents peripheral nervous system. ( Source - http://bio1520.biology.gatech.edu/chemical - and - electrical - signals/nervous - systems/ ) The brain and spinal cord are two major components of central nervous system and these coordinately control and process information. BRAIN Brain is the main component of CNS and the main function of brain is to coordinate all physiological processes occurring within the body. The function of brain is to integrate the major sensory information and coordinate function, consciously and unconsciously. Different parts of the brain also control complex functions such as thinking, feeling and regulation of homeostasis. Differentiation between human and o ther animal brains can be made out via studying anatomy of this organ and it is estimated that human brain is the largest brain (1.5 kilograms; about 2% of a human's body weight) among all vertebrates relative to their body size. The brain of vertebrates i s enclosed in a protective skeletal framework which is made up of bones or cartilage. Brain has three components namely cerebrum, cerebellum, and brainstem (Figure 2) . During development, human brain is built from three fluid filled sections; forebrain, midbrain and hindbrain. Cerebrum develops from forebrain, brainstem from midbrain and cerebellum and pats of brainstem develop from hindbrain region. Figure 2. D iagram matic representation of different parts of a human brain. (Source/ Credit: udaix Shutter stock , creative commons ) Cerebrum is the largest part of brain and makes up 85% of the brain's weight and this region controls voluntary actions, speech, memory and thought process (Figuer2) . It consists of

two hemispheres which end in brainstem and behind sits cerebellum. Cerebral cortex is the outmost covering of cerebrum which is further classified into frontal, parietal, temporal and occipital lobes (Figure 3) . Size of cerebral cortex is enlarged in case of the human brain and generally regarded as seat of complex thoughts Frontal lobe: It is situated in area around forehead region. This lobe can be further categorized into prefrontal cortex, pre - motor area and motor area. It functions to control pro blem analysis and solution, planning, creativity, judgment, movement and emotions. Higher systematic level functions like logical and self control, thoughts which are essence of human race are regulated by frontal lobe . Parietal lobe: This lies behind the frontal and top back of brain (Figure 3) . It is known to control some languages and all the senses of body regarding taste, touch, pressure and ache. Navigation and spatial memory is also regulated by parietal lobe . Figure 3 : Diagrammatic representation of different parts of a human brain along with the representation of different lobes. source - https://commons.wikimedia.org/wiki/File:Brain_parts.jpg Temporal lobe: Temporal lobe is comprised of amygdala and hippocampus regions which play important roles i n sound and language, memory & emotions processing. It lies on both sides of brain and predominantly known to have language and auditory roles. Moreover it also functions as seat of emotions and learning. Occipital lobe: Occipital lobe lies near the back of skull (Figure 3) and is mainly involved in the recognition and visual identification of objects. The visual perceptions received by retina are processed here. The ce

rebellum is associated with balance and coordination and is also tho ught to be involved in cognitive functions. Cerebellum is located underneath cerebrum (Figure 3) . Although, cerebellum constitutes only 10% of human brain but it is involved in maintaining balance , posture, reflexes, sequence learnin g, timing and motor learning. Brainstem connects to the spinal cord. The brainstem is comprised of three different parts; medulla oblongata, pons and midbrain (Figure 2 and 3) . Its function is to transmit information between brain and other parts of body. It is also associat ed with many critical functions like heart beat, breathing rate and perception. Thalamus and hypothalamus lie between cerebrum part and brainstem. Hypothalamus is the seat of hormone production, following signals from pituitary gland. It works as link betw een endocrine and nervous system. Thalamus functions to control consciousness, perception and sleep. It broadcasts sensory and motor signals to cerebrum. SPINAL CORD It is a long bundle of nervous tissue which originates from the medulla oblongata in the brainstem a nd extends to the lumbar part of vertebral column (Figure 3) . The spinal cord may be divided into five se gments as the cervical, thoracic, lumbar, sacral, and coccygeal. The spinal cord serves as a medium for signals to connect the brain and the rest of the organs. The motor cortex neurons of the brain are associated with motor neurons of the spinal cord through the connections of their axons in cortico - spinal tract. Each segment of the spinal cord specifically collects sensory input ( pain , temperature, touch, vibration etc.) from the skin, muscles, joints, and internal organs to the sp

inal cord through neurons . These input s are then relayed to the brain using the spinothalmic tract and the lemniscal pathway. Cells of central nervous system T he basic cell of the central nervous system is known as neuron/nerve cell. Each neuron is made up of a cell body which have nucleus, axons and dendrites form extensions from the cell body (Figure 4) . These neurons are inter - connected through the connections called synapses. Billions of neurons are present at different parts of the body and these communicate with each other and coordinated by brain and the spinal cord. Neurons working together as network to make decision, control emotion or feel s ensation. There are approximately 100 billion neurons in the brain and spinal cord combined together and about 10,000 different subtypes of neurons have been identified. Each subtype is specialized to send and receive particular types of information. Fi gure 4: Neurons are the basic functional unit of nervous system. ( Source - https://simple.wikipedia.org/wiki/Neuron ) Glial cells (neuroglia) are non - neuronal cells which provide physical protection to neurons in the central nervous system as well as in peripheral nervous systems. These cells are required to form myelin sheath . In general, there are following functions of glial cells: a) Surround and protect the neurons b) Supply of nutrient material and oxygen to neurons c) Insulation of neuron for better conductivity d) Removal of dead neurons and protection form pathogens. Glial cells have different forms as astrocytes , oligodendrocytes , microglia and Ependymal cells (Figure 5) . Astrocytes are the type of a glial cell which primaril

y supports cells of the brain and spinal cord. They are responsible for the synthesis and secretion of proteins called neurotrophic factors and also important for the clearance of proteins or chemicals / neurotransmitters which might be harmful to neurons in excess (for example, glutamate, a neurotransmitter that in excess causes cells to become overexcited and leads to cell death). Oligodendrocytes are glial cells that secrete a lipid rich myelin sheath which wraps around axon s in neurons. Morphologically, these cells are round and have lymphocyte - like nuclei. Axon fibers insulated by myelin can carry action potential at a speed of 100 meters per second, whereas without myelin can only carry action potential at very slow speed that is one meter per second. The high speed of action potential in myelinated axons i s due to the phenomenon of saltatory conduction . Figure 5 : F our different types of glial cells of central nervous system: Astrocytes (green), Microglial cells (red), Ependymal cells (light pink) and Oligodendrocytes (light blue). Source: (Artwork by Holly Fischer - http://open.umich.edu/education/med/resources/second - look - series/materials - CNS Slide 4, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=24367125) Microglia cells are specialized immune cells for the brain and function as antigen - presenting monocytes . These cells are mitotically stable and evenly present in grey and white matter of brain. They migrate to the site of injury to assist removal of dead and dying cells. They can also secrete cytokines that activate cells of the immune system to respond to the injury site. Ependymal cells line the spinal cord and the ventricu

lar system of the brain. These cells are required for formation and secretion of cerebrospinal fluid (CSF). These cells have a continuous layer of single cuboidal/ columnar cells with cilia which beat and help to circu late the CSF and make up the blood - CSF barrier. White and gray matter: The central nervous system especially brain may be divided into two morphologically distinct region as white and gray matter. The region which has high amount of axons and oligodendr ocytes is termed as white matter, while the region which consists of neurons and unmyelinated fibers is termed as gray matter. Myelin sheath around axon (myelinated axon) is major reasons of glistening white colour in white matter. While, grey matter may h ave a very light grey colour with yellowish hues due to the presence of blood capillary and neuronal cell bodies. Grey matter is present in the brain , brainstem, in the spinal cord (as grey column ) and is involved in thinking, overall personality of individual and management of movements. W hite matter has more contribution in the deeper parts of the brain and the superficial parts of the spinal cord. PERIPHERAL NERVOUS SYSTEM Peripheral nervous system (PNS) is the second part nervous system which is made up of the nerves and ganglia , radiating outside of the brain and spinal cord. It works to connect central nervous system to the limbs and organs . Peripheral nervous system basically acts as a tra nsmitter between the brain and spinal cord and the rest of the body. PNS does not contain any protective coverings ( skull , blood – brain barrier ) which are present in central nervous system ( brain and spinal cord ) therefore, it is more susceptible for

toxins and mechanical injures. PNS allow sensing of the external environment and actions of muscular movement and for this PNS consists of two types of nerve fibres a) afferent fibres which transmit impulses from tissues/organs to the CNS called sensory division and b) efferent fibres that transmit regulatory impulses from the CNS to the peripheral organs called motor division (Figure 6 ) . Figure 6 - Working network of afferent and efferent neurons. Afferent neurons are responsible for sensation of external factors and process that information to CNS where brain commands efferent neurons to carry information to protect body parts from external stress. ( Source: https://oli.cmu.edu/jcourse/workbook/activity/page?context=21af2e8380020ca600f14ec0c 80b79d4 ) PNS may also be segregated on the basis of type of action as voluntary actions and involuntary actions . Somatic nervous system is under voluntary control and composed of all cranial nerves except for optic nerves along with retina . While, a utonomic nervous system regulates involuntary actions related to smooth muscle and glands. The somatic neural system includes nerves beneath the skin and responsible for the connect ion of external environment with the CNS. Although, t hese nerves are in conscious control, but also automatic component control their function for example; they function even in the coma state. The C ranial nerves ( 12 pairs ) and the spinal nerves ( 31 pairs ) control the somatic nervous system in humans. Some pairs are completely sensory and are involved in functions like vis ion, smell, hearing, and equilibrium ; while other are sternly motor neurons a

nd control the eyeball movement , swallowing, and movement of the head . S everal pairs have both sensory and motor neurons working together , e.g. the sense of taste. All spinal neu rons have both motor and sensory neurons. The autonomic nervous system works independent of conscious control, responsible for the function of involuntary actions such as heart muscles , endocrine and exocrine glands. Peripheral nervous system Autonomic neural system ( from the CNS to the involuntary organs and smooth muscles) Sympathetic neural system ( Rest and digest) Parasympathetic neural system ( Fight - or - flight) Somatic neural system ( from the CNS to skeletal muscles) Relationship between CNS and other body organs is regulated by two functional states; sympathetic and parasympathetic. Neuron cells are key player for sensing and transmitting signals in between target tissue and CNS, to control all voluntary and involuntary functions of body. Sympathetic a nd Parasympathetic nervous systems are not communally exclusive, but both are negatively associated and function in a “symbiotic” fashion (Figure 7 ) . Figure 7. Functions of parasympathetic and sympathetic nervous system to control different functions of the body. ( Source - https://www.online - sciences.com/the - living - organisms/the - structure - and - function - of - the - peripheral - nervous - system/ ) As both system controls d iffe rent internal systems, when sympathetic system in active, the parasympathetic will be inactive. The analogy of the two stress systems is a see - saw, where they both are in motion, but one is usually higher (more activated) than the other, and they are inversely proportional to each other. (Fig

ure 8). This may be seen by various following function. a) Du ring stress or danger, sympathetic nervous system is responsible for the release of epinephrine that results in increased metabolic rates and activities of muscles tissues. Parasympathetic nervous system lowers the metabolic rates and hence responsible for rest and digest. b) D uring heavy exercise or hard work , the sympathetic system gets activated and it results in inacti vation of parasympathetic system therefore the conditions like rest and digestion get stop ped (Figure 8 ) . Figure 8 - Functioning of sympathetic and parasympathetic nervous system in analogous way. Source - https://nl.pinterest.com/pin/397090892129847176/ In the end, one can differentiate between the central nervous system and peripheral nervous system with the following points as in table 1. Table 1 : Difference between central nervous system and peripheral nervous system. S. No Central nervous system [CNS] Peripheral nervous system [PNS] 1. Comprises brain and spinal cord Comprises somatic nervous system and autonomic nervous system. 2. It processes the information carried by PNS from different sensory receptors. Have nerves which leave the brain and spinal cord towards different body parts. It has 2 main divisions; motor a nd sensory. Sensory collect information and motor relays the processed information by CNS. 3. It is protected by blood brain barrier and osteolithic cavity. It’s not protected by any covering or barrier. 4. Damage to CNS can lead to global Injury to PNS can lead to only localized damage. function loss. 5. Communicates all sorts of involuntary information Peripheral Nervous Sys

tem communicates all the voluntary information 6. Central Nervous System plays no role in regulation of blood pressure, thirst and body temperature. PNS plays a role in regulation of blood pressure, thirst and body temperature Summary  The nervous system in humans is a complex integration of organs like brain, spinal cord and sensory organs along with nerves. Nerve cells are required for exchange of information between brain and spinal cord to rest of body parts for coordinated control.  I n vertebrates, nervous system consists of two major parts a) the central neural system (CNS) and b) the peripheral neural system (PNS) .The coordination of both CNS and PNS allow human beings to make equilibrium within their internal system and as well as external environment.  Central nervous system functions to integrate information and coordinate majority of physiological activities in bilaterally symmetric animals .  The brain and spinal cord are two major components of central nervous system and these coordinately control and process information.  The brain of vertebrates is enclosed in a skeletal framework which is made up of bones or cartilage. Human brain is the largest organ among all other animals.  Brain has three components namely cerebrum, cerebellum, and brainstem. During development, human brain is built from three fluid filled sections; forebrain, midbrain and hindbrain. Cerebrum develops from forebrain, brainstem from midbrain and cerebellum and pats of brainstem develop from hindbrain region .  Cerebrum is the largest part of brain and mak es up 85% of the brain's weight and this region controls voluntary actions, speech, memory and th

ought process . The cerebellum is associated with balance and coordination and is also thought to be involved in cognitive functions.  Brainstem connects to the spinal cord. Its function is to transmit information between brain and other parts of body.  The spinal cord serves as a medium for signals to connect the brain and the rest of the organs.  Neuron cells are key player for sensing and transmitting signals in between target tissue and CNS, to control all voluntary and involuntary functions of body. Neurons interact with one another to pass information in the brain and spinal cord. Neurons working together as network to make decision, control emotion or feel sensation.  Glial cells (neuroglia) are non - neuronal cells which provide physical protection to neurons in the central nervous system as well as in peripheral nervous systems.  Peripheral nervous system (PNS) is the second part nervous system which is made u p of the nerves and ganglia , radiating outside of the brain and spinal cord.  PNS consists of two types of nerve fibres a) afferent fibres which transmit impulses from tissues/organs to the CNS called sensory division and b) efferent fibres that transmit regulatory impulses from the CNS to the peripheral organs called motor division .  Somatic nervous system is under voluntary control and composed of all cranial nerves except for optic nerves along with retina. While, autonomic nervous system regulates involuntary actions related to smooth muscle and glands.  Sympathetic and Parasympathetic nervous systems are not communally exclusive, but both are negatively associated and function in a “symbiotic” f