Unit 3 Neurobiology (1).pdf

Unit 3 Neurobiology ORGANIZATION OF THE NERVOUS SYSTEM Central & Peripheral Nervous System; Structure and functions of different brain structures; Lobe Functions; Neuroplasticity of Brain - neural degeneration, neural regeneration a...

Unit 3 Neurobiology ORGANIZATION OF THE NERVOUS SYSTEM Central & Peripheral Nervous System; Structure and functions of different brain structures; Lobe Functions; Neuroplasticity of Brain - neural degeneration, neural regeneration and neural reorganization; hemispheric specialization  Nervous System  Your nervous system plays a role in everything you do. The three main parts of your nervous system are your brain, spinal cord and nerves. It helps you move, think and feel. It even regulates the things you do but don’t think about like digestion. It contains the central nervous system and the peripheral nervous system. The nervous system, the electrical information highway of the body, is made up of nerves, which are bundles of interconnected neurons that fire in synchrony to carry messages. The nervous system has two major divisions. The central nervous system (CNS), made up of the brain and spinal cord, is the major controller of the body’s functions, charged with interpreting sensory information, and responding to it with its own directives. The CNS interprets information coming in from the senses, formulates an appropriate reaction, and sends responses to the appropriate system to respond accordingly. Everything that we see, hear, smell, touch, and taste is conveyed to us from our sensory organs as neural impulses, and each of the commands that the brain sends to the body, both consciously and unconsciously, travels through this system as well. The peripheral nervous system (PNS) links the CNS to the body’s sense receptors, muscles, and glands.  Nerves are differentiated according to their function. 1. A sensory or afferent neuron carries information from the sensory receptors, 2. whereas a motor or efferent neuron transmits information to the muscles and glands.  Both of these neurons are located in the peripheral nervous system.  An interneuron, responsible for communicating among the neurons, is by far the most common type of neuron, and is located primarily within the central nervous system. Interneurons allow the brain to combine the multiple sources of available information to create a coherent picture of the sensory information being conveyed.  The CNS includes the brain and spinal cord.  The brain is the body’s “control center.” The brain is an organ of nervous tissue responsible for responses, sensation, movement, emotions, communication, thought processing, and memory. The skull, meninges, and cerebrospinal fluids protect the human brain. The nervous tissue is extremely delicate and can be damaged by the smallest amount of force. In addition, the brain has a blood-brain barrier that prevents the brain from any harmful substance floating in the blood.  The spinal cord is the long, thin, tubular bundle of nerves and supporting cells that extends down from the brain. The spinal cord is a vital aspect of the CNS found within the vertebral column. Its purpose is to send motor commands from the brain to the peripheral body and relay sensory information from the sensory organs to the brain. Bone, meninges, and cerebrospinal fluids provide spinal cord protection.  It is the central throughway of information for the body.  Within the spinal cord, ascending tracts of sensory neurons relay sensory information from the sense organs to the brain while descending tracts of motor neurons relay motor commands back to the body.  When a quicker-than-usual response is required, the spinal cord can do its own processing, bypassing the brain altogether.  A reflex is an involuntary and nearly instantaneous movement in response to a stimulus.  Reflexes are triggered when sensory information is powerful enough to reach a given threshold and the interneurons in the spinal cord act to send a message back through the motor neurons without relaying the information to the brain (see Figure 3.18).  When you touch a hot stove and immediately pull your hand back, or when you fumble your cell phone and instinctively reach to catch it before it falls, reflexes in your spinal cord order the appropriate responses before your brain even knows what is happening. THE PERIPHERALNERVOUS SYSTEM  The Peripheral Nervous System (PNS) consists of the nerves that lie in the peripheral region, outside the central nervous system. That is, it consists of all the nerves that branch out from the brain and spinal cord (not contained in the brain and spinal cord).  From there it extends to other parts of the body as various muscles or organs. It is divided into somatic nervous system and the autonomic nervous system.  The somatic nervous system (SNS) is the division of the PNS that controls the external aspects of the body, including the skeletal muscles, skin, and sense organs.  The somatic nervous system consists primarily of motor nerves responsible for sending brain signals for muscle contraction. We become aware of the world through the sensory division of the somatic nervous system, and we act on the world through the motor division of the somatic nervous system.  Somatic Nervous System  The somatic nervous system conducts all sensory and motor information to and from the CNS. It is responsible for voluntary movement. This system consists of two major types of neurons such as sensory neurons and motor neurons.  The sensory neurons are afferent neurons as they conduct impulses from the sensory organs to the central nervous system. The motor neurons are efferent neurons that convey information from the brain and spinal cord to muscle throughout the body.  This is responsible for making voluntary movements. Thus, the somatic nervous system receives sensory information from the sensory organs and controls the movements of the skeletal muscles.  Autonomic Nervous System  The Autonomic Nervous System (ANS) is that part of the peripheral nervous system that helps to transmit the efferent neurons to various autonomic or visceral effectors.  As the name autonomic suggests that the functions are more or less automatic. It helps to regulate the effectors, like the cardiac muscles in the heart, smooth muscles on the skin, blood vessels and epithelial tissue in the glands. Thus, the somatic nervous system controls the senses and voluntary muscles, while as, the autonomic nervous system controls the organs, glands and involuntary muscles.  The ANS functions involve regulating the heart rate, contraction of smooth muscles in the gall bladder and urinary bladder and maintain a state of homeostasis by regulating the glandular secretions. Hence, the ANS regulates the autonomic effectors that not only help to maintain homeostasis but also restores it.  The autonomic nervous system itself can be further subdivided into the sympathetic and parasympathetic systems.  The sympathetic division of the ANS is involved in preparing the body for behavior, particularly in response to stress, by activating the organs and the glands in the endocrine system.  The parasympathetic division of the ANS tends to calm the body by slowing the heart and breathing and by allowing the body to recover from the activities that the sympathetic system causes.  The sympathetic and the parasympathetic divisions normally function in opposition to each other, such that the sympathetic division acts a bit like the accelerator pedal on a car and the parasympathetic division acts like the brake.  Our everyday activities are controlled by the interaction between the sympathetic and parasympathetic nervous systems.  For example, when we get out of bed in the morning, we would experience a sharp drop in blood pressure if it were not for the action of the sympathetic system, which automatically increases blood flow through the body.  Similarly, after we eat a big meal, the parasympathetic system automatically sends more blood to the stomach and intestines, allowing us to efficiently digest the food.  Perhaps you have had the experience of not being at all hungry before a stressful event, such as a sports game or an exam when the sympathetic division was primarily in action, but suddenly finding yourself starved afterward, as the parasympathetic takes over. The two systems work together to maintain vital bodily functions, resulting in homeostasis, the natural balance in the body’s systems.  Functions of the Sympathetic Division  Sympathetic division is located primarily on the middle of the spinal column (top of the ribcage to the waist area that is thoracic and lumbar areas). The sympathetic division is responsible for "fight-or-flight" mechanism (fight : anger; flight : fear). You must have also experienced this kind of a moment at some point of time. Thus, it helps to maintain the normal functioning of the body under resting conditions.  This means that when the parasympathetic division slows down the working of the autonomic effectors, it counteracts its functioning by regulating the heartbeat and keeping it at a normal pace.  It helps to maintain normal muscle tone and blood pressure under usual circumstances. However, when there is a change in the external environment, it serves as an emergency response to cope with the changes in the external environment and maintaining homeostasis. It helps the person or animal to deal with stressful situation (sympathy with one's emotions).  In stress, the sympathetic division becomes very active, and starts sending its impulses very rapidly to defend the body. It also stimulates the adrenal gland to produce epinephrine and norepinephrine which help to enhance heart rate, blood sugar level, and increase the blood flow to the skeletal muscles to deal with stress. Most of the sympathetic nervous system use norepinephrine.  But not all organs are stimulated by sympathetic division. For example, digestion of food and eliminating waste products (excretion) from the body are not active during stressful situation. Infact, these systems tend to be stopped or inhibited in such a situation. But when there is excessive anxiety, then there is an urge to empty the bladder or bowels. When the arousal ends, the activities of sympathetic system are replaced by the activities of parasympathetic system. The sweat glands, the adrenal glands, the muscles that erect the hairs of the skin, and the muscles that constrict the blood vessels are only stimulated by sympathetic division.  Functions of the Parasympathetic Division  The neurons of this system are located top and bottom of the spinal column on both the sides of neurons of sympathetic division (para means 'beyond' or 'next to’).  It is also known as craniosacral system because it consists of cranial nerves and nerves from sacral spinal cord.  The parasympathetic division is active most of the time and controls various functions of the body in non-stressful situations or when everything is all right, that is in day-to-day functioning.  The activities of sympathetic division are replaced by the parasympathetic division when the stress is over. It helps in repairing the body systems and bringing them back to a resting state and restoring the body to normal functioning after arousal.  It produces acetylcholine that reduces the heart rate and brings it to a normal level and also tends to improve the digestion by stimulating the digestive glands. It enhances the activity of the gastric and intestinal system for smooth functioning of the body.  Parasympathetic division restores the energy that is burned by the sympathetic division. Thus, it is also known as "eatdrink-and-rest system". Summarizing Central Nervous System (CNS)  The CNS consists of the brain and spinal cord. The brain, enclosed within the skull, is responsible for higher cognitive functions, sensory perception, and motor control. The spinal cord is a long, tubular structure encased in the vertebral column, and it serves as a communication pathway between the brain and the rest of the body.  Summarizing Peripheral Nervous System (PNS)  The PNS includes all nerve structures outside of the CNS. It can be further divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary muscle movements and sensory perception. The autonomic nervous system regulates involuntary functions such as heart rate, digestion, and respiratory rate. It is further divided into the sympathetic and parasympathetic divisions, which often have opposing effects on physiological processes.  It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery— meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal. Brain and structure and functions of different brain structures) The brain is a complex organ that controls  thought,  memory,  emotion,  touch,  motor skills,  vision,  breathing,  temperature,  hunger and every process that regulates our body. Together, the brain and spinal cord that extends from it make up the central nervous system, or CNS.  What is the brain made of?  Weighing about 3 pounds in the average adult, the brain is about 60% fat. The remaining 40% is a combination of water, protein, carbohydrates and salts. The brain itself is a not a muscle. It contains blood vessels and nerves, including neurons and glial cells.  What is the gray matter and white matter?  Gray and white matter are two different regions of the central nervous system.  In the brain, gray matter refers to the darker, outer portion, while white matter describes the lighter, inner section underneath.  In the spinal cord, this order is reversed: The white matter is on the outside, and the gray matter sits within.  Gray matter is primarily composed of neuron somas (the round central cell bodies), and white matter is mostly made of axons (the long stems that connects neurons together) wrapped in myelin (a protective coating). The different composition of neuron parts is why the two appear as separate shades on certain scans.  Each region serves a different role. Gray matter is primarily responsible for processing and interpreting information, while white matter transmits that information to other parts of the nervous system.  How does the brain work?  The brain sends and receives chemical and electrical signals throughout the body. Different signals control different processes, and your brain interprets each. Some make you feel tired, for example, while others make you feel pain.  Some messages are kept within the brain, while others are relayed through the spine and across the body’s vast network of nerves to distant extremities. To do this, the central nervous system relies on billions of neurons (nerve cells). Main Parts of the Brain and Their Functions  At a high level, the brain can be divided into the cerebrum, brainstem and cerebellum. Cerebrum  The cerebrum (front of brain) comprises gray matter  (the cerebral cortex) and  white matter at its center.  The largest part of the brain, the cerebrum initiates and coordinates movement and regulates temperature.  Other areas of the cerebrum enable speech, judgment, thinking and reasoning, problem-solving, emotions and learning.  Other functions relate to vision, hearing, touch and other senses. Cerebral Cortex  Cortex is Latin for “bark,” and describes the outer gray matter covering of the cerebrum with billions of neurons to conduct high-level executive functions. The cortex divides into 4 lobes: frontal, parietal, occipital, and temporal, by different sulci  The cortex has a large surface area due to its folds, and comprises about half of the brain’s weight.  The cerebral cortex is divided into two halves, or hemispheres.  It is covered with ridges (gyri) and folds (sulci).  The two halves join at a large, deep sulcus (the interhemispheric fissure, AKA the medial longitudinal fissure) that runs from the front of the head to the back.  The right hemisphere controls the left side of the body, and the left half controls the right side of the body.  The two halves communicate with one another through a large, C-shaped structure of white matter and nerve pathways called the corpus callosum. The corpus callosum is in the center of the cerebrum.  While in constant communication, the left and right hemispheres are responsible for different behaviors, known as brain lateralization. The left hemisphere is more dominant in language, logic, and math abilities. The right hemisphere is more creative and dominant in artistic and musical situations and intuition. Brainstem  The brainstem (middle of brain) connects the cerebrum with the spinal cord. The brainstem includes the midbrain, the pons and the medulla. 1. Midbrain. The midbrain (or mesencephalon) is a very complex structure with a range of different neuron clusters (nuclei and colliculi), neural pathways and other structures.  These features facilitate various functions, from hearing and movement to calculating responses and environmental changes.  The midbrain also contains the substantia nigra, an area affected by Parkinson’s disease that is rich in dopamine neurons and part of the basal ganglia, which enables movement and coordination. 2. Pons. The pons is the origin for four of the 12 cranial nerves, which enable a range of activities such as tear production, chewing, blinking, focusing vision, balance, hearing and facial expression. Named for the Latin word for “bridge,” the pons is the connection between the midbrain and the medulla. 3. Medulla. At the bottom of the brainstem, the medulla is where the brain meets the spinal cord. The medulla is essential to survival. Functions of the medulla regulate many bodily activities, including heart rhythm, breathing, blood flow, and oxygen and carbon dioxide levels. The medulla produces reflexive activities such as sneezing, vomiting, coughing and swallowing. The spinal cord extends from the bottom of the medulla and through a large opening in the bottom of the skull. Supported by the vertebrae, the spinal cord carries messages to and from the brain and the rest of the body. Cerebellum  The cerebellum (“little brain”) is a fist-sized portion of the brain located at the back of the head, below the temporal and occipital lobes and above the brainstem. Like the cerebral cortex, it has two hemispheres. The outer portion contains neurons, and the inner area communicates with the cerebral cortex.  Its function is to coordinate voluntary muscle movements and to maintain posture, balance and equilibrium. New studies are exploring the cerebellum’s roles in thought, emotions and social behavior, as well as its possible involvement in addiction, autism and schizophrenia.  Apart from this, the cerebellum has the cerebellar peduncles, cerebellar nuclei, anterior and posterior lobes. The cerebellum consists of two hemispheres, the outer grey cortex and the inner white medulla. It is mainly responsible for coordinating and maintaining the body balance during walking, running, riding, swimming, and precision control of the voluntary movements. The main functions of the cerebellum include:  It senses equilibrium.  Transfers information.  Coordinates eye movement.  It enables precision control of the voluntary body movements.  Predicts the future position of the body during a particular movement.  Both anterior and posterior lobes are concerned with the skeletal movements.  The cerebellum is also essential for making fine adjustments to motor actions.  Coordinates and maintains body balance and posture during walking, running, riding, swimming. Brain Coverings: Meninges  Three layers of protective covering called meninges surround the brain and the spinal cord.  The outermost layer, the dura mater, is thick and tough. It includes two layers: The periosteal layer of the dura mater lines the inner dome of the skull (cranium) and the meningeal layer is below that. Spaces between the layers allow for the passage of veins and arteries that supply blood flow to the brain.  The arachnoid mater is a thin, weblike layer of connective tissue that does not contain nerves or blood vessels. Below the arachnoid mater is the cerebrospinal fluid, or CSF. This fluid cushions the entire central nervous system (brain and spinal cord) and continually circulates around these structures to remove impurities.  The pia mater is a thin membrane that hugs the surface of the brain and follows its contours. The pia mater is rich with veins and arteries. Deeper Structures Within the Brain Pituitary Gland  Sometimes called the “master gland,” the pituitary gland is a pea-sized structure found deep in the brain behind the bridge of the nose. The pituitary gland governs the function of other glands in the body, regulating the flow of hormones from the thyroid, adrenals, ovaries and testicles. It receives chemical signals from the hypothalamus through its stalk and blood supply. Hypothalamus  While the hypothalamus is one of the smallest parts of the brain, it is vital to maintaining homeostasis. The hypothalamus is located above the pituitary gland and sends it chemical messages that control its function. It regulates body temperature, synchronizes sleep patterns, the release of various hormones, controls hunger and thirst and also plays a role in some aspects of memory and emotion.  Thalamus. The thalamus is a small structure, its is the brain's relay center. Its located right above the brain stem responsible for relaying sensory information from the sense organs. It is also responsible for transmitting motor information for movement and coordination. It receives afferent impulses from sensory receptors throughout the body and processes the information for distribution to the appropriate cortical area. It is also responsible for regulating consciousness and sleep. Thalamus is found in the limbic system within the cerebrum. This limbic system is mainly responsible for the formation of new memories and storing past experiences. Amygdala  Small, almond-shaped structures, an amygdala is located under each half (hemisphere) of the brain. Included in the limbic system, the amygdalae regulate emotion and memory and are associated with the brain’s reward system, stress, and the “fight or flight” response when someone perceives a threat. Hippocampus  A curved seahorse-shaped organ on the underside of each temporal lobe, the hippocampus is part of a larger structure called the hippocampal formation. It supports memory, learning, navigation and perception of space. It is particularly important in forming new memories, and connecting emotions and senses, such as smell and sound, to memories. It receives information from the cerebral cortex and may play a role in Alzheimer’s disease. Pineal Gland  The pineal gland is located deep in the brain and attached by a stalk to the top of the third ventricle. The pineal gland responds to light and dark and secretes melatonin, which regulates circadian rhythms and the sleep-wake cycle. Ventricles and Cerebrospinal Fluid  Deep in the brain are four open areas with passageways between them. They also open into the central spinal canal and the area beneath arachnoid layer of the meninges.  The ventricles manufacture cerebrospinal fluid, or CSF, a watery fluid that circulates in and around the ventricles and the spinal cord, and between the meninges. CSF surrounds and cushions the spinal cord and brain, washes out waste and impurities, and delivers nutrients. Lobes of the Brain and What They Control  Each brain hemisphere (parts of the cerebrum) has four sections, called lobes: frontal, parietal, temporal and occipital. Each lobe controls specific functions.  Frontal lobe. The largest lobe of the brain, located in the front of the head, the frontal lobe is involved in personality characteristics, decision-making, problem- solving, attention, memory, language, and voluntary motor function. Recognition of smell usually involves parts of the frontal lobe. Frontal lobe. It also contains the motor cortex and the Broca area. The motor cortex allows for the precise voluntary movements of our skeletal muscles, while the Broca area controls motor functions responsible for producing language  Parietal lobe. The middle part of the brain, the parietal lobe helps a person identify objects and understand spatial relationships (where one’s body is compared with objects around the person). The parietal lobe is also involved in interpreting pain and touch in the body. It is responsible for processing sensory information and contains the somatosensory cortex. Neurons in the parietal lobe receive information from sensory and proprioceptors throughout the body, process the can, and form an understanding of what is being touched based on previous knowledge.The parietal lobe houses Wernicke’s area, which helps the brain understand spoken language.  Occipital lobe. The occipital lobe is the back part of the brain that is involved with vision. It contains the visual cortex. Like the parietal lobe, it receives information from the retina and then uses past visual experiences to interpret and recognize stimuli.  Temporal lobe. The sides of the brain, temporal lobes are involved in short- term memory, speech, musical rhythm and some degree of smell recognition.  The temporal lobe processes auditory stimuli through the auditory cortex. Sound energy activates mechanoreceptors located in the hair cells lining the cochlea, sending impulses to the auditory cortex. The impulse is processed and stored based on previous experiences. The Wernicke area is in the temporal lobe and functions in speech comprehension. Cranial Nerves  Inside the cranium (the dome of the skull), there are 12 nerves, called cranial nerves:  Cranial nerve 1: The first is the olfactory nerve, which allows for your sense of smell.  Cranial nerve 2: The optic nerve governs eyesight.  Cranial nerve 3: The oculomotor nerve controls pupil response and other motions of the eye, and branches out from the area in the brainstem where the midbrain meets the pons.  Cranial nerve 4: The trochlear nerve controls muscles in the eye. It emerges from the back of the midbrain part of the brainstem.  Cranial nerve 5: The trigeminal nerve is the largest and most complex of the cranial nerves, with both sensory and motor function. It originates from the pons and conveys sensation from the scalp, teeth, jaw, sinuses, parts of the mouth and face to the brain, allows the function of chewing muscles, and much more.  Cranial nerve 6: The abducens nerve innervates some of the muscles in the eye.  Cranial nerve 7: The facial nerve supports face movement, taste, glandular and other functions.  Cranial nerve 8: The vestibulocochlear nerve facilitates balance and hearing.  Cranial nerve 9: The glossopharyngeal nerve allows taste, ear and throat movement, and has many more functions.  Cranial nerve 10: The vagus nerve allows sensation around the ear and the digestive system and controls motor activity in the heart, throat and digestive system.  Cranial nerve 11: The accessory nerve innervates specific muscles in the head, neck and shoulder.  Cranial nerve 12: The hypoglossal nerve supplies motor activity to the tongue.  The first two nerves originate in the cerebrum, and the remaining 10 cranial nerves emerge from the brainstem, which has three parts: the midbrain, the pons and the medulla. SPINAL NERVES  Spinal nerves are thirty-one in all. They do not have special names like the cranial nerves.  These nerves emerge from the spinal cavity and are known according to the level of the vertebral column from which they emerge.  Those that arise from the cervical vertebrae are known as cervical nerve pairs. These are 8 in number and labeled as C1 through C8.  The nerves arising from the vertebrae in the thoracic region are known as thoracic nerve pairs. These are 12 in number and are known as T1 through T12.  There are 5 nerves that emerge from the vertebrae in the lumbar area, known as lumbar nerve pairs, L1 through L5. The nerves arising from the sacral vertebrae are 5 and known as sacral nerve pairs, S1 through S5.  The last one is known as the coccygeal pair of spinal nerves in the coccyx region or tip of the spinal cord.  NEUROPLASTICITY OF BRAIN  Plasticity refers to the quality of an object to transform to any shape and size  Neuroplasticity can be defined as “the ability of the nervous system to change its activity in response to intrinsic or extrinsic stimuli by reorganizing its structure, functions or connections”  It is the capacity of neural networks and neurons in the brain to change (strengthening or weakening) their connections and behaviour in response to,  Gaining new information  Sensory stimulation  Development  Damage or Dysfunction  Neuroplasticity is also referred as neural plasticity, brain plasticity or cortical plasticity  Earlier Neuroscientists believed that the brain’s structure and functioning was essentially fixed through out the adulthood  William James first opposed this notion in his book “The Principles of Psychology” and put forward the idea that the brain and its function are not fixed throughout adulthood. However, this idea did not received much attention that time  The term “neuroplasticity” was first used by Polish neuroscientist Jerzy Konorski in 1948  However, it is Santiago Ramon y Cajal, revered as “father of neuroscience”, first described this concept paving way to continuous research resulting in deeper insights.  Neuroplasticity of the brain has been believed to be limited to infancy  But studies in the latter half of the twentieth century revealed that many facets of the brain can be changed even in adulthood  However, the developing brain has high neuroplasticity then that of developed brain.  Neuroplasticity is often confused with neurogenesis. Although related these two are different concepts. While neuroplasticity is the ability of the brain to form new connections and pathways and change how its circuits are wired, neurogenesis the ability of the brain to grow new neurons. THE CONCEPTS NEURAL DEGENERATION, NEURAL REGENERATION, NEURAL REORGANIZATION  Neural Degeneration  Neural degeneration is a result of neural deterioration with age and/or disease that triggers degeneration. It is of two types  When the axon of the neuron is cut, it causes two kinds of degeneration or deterioration.  Anterograde degeneration: When the axon breaks from the point of cut towards the terminal button, it is known as anterograde degeneration and the distal end of the axon degenerates.  Retrograde degeneration: When the neuron breaks from the center of axon including the cell body, it is known as retrograde degeneration. It is the degeneration of the segment that is proximal cut between the cut on the axon and the cell body  Neural Regeneration  Neural regeneration the regrowth of damaged neurons.  Once the neurons are destroyed in the CNS in adult mammals, they do not recover.  However, in the peripheral nervous system (PNS), they do try to regenerate, but the normal functions may not be possible.  If the recovery process does take place then there are different ways.  If the myelin sheath is intact, then the regenerating axons may grow through them to their desired target areas.  If the nerve is severed and the ends of the myelin sheath moves apart, then no meaningful regeneration will take place.  If the nerve is severed and the myelin sheath ends slightly gets separated from one another, then incorrect myelin sheaths develop that reaches out to undesired target areas.  The PNS neurons have the inherent capability to regenerate, while as, CNS neurons cannot regenerate.  Some CNS neurons are capable of regeneration if transplanted to the PNS, while some PNS neurons transplanted to CNS are not capable of regeneration.  This clearly indicates the environment of PNS that promotes regeneration.  When an axon degenerates, new axons branch out from adjacent healthy axons and synapse at the place vacated by degenerating axon. This is known as collateral sprouting. Collateral sprouts may grow from axon terminal branches or the nodes of Ranvier on adjacent neurons  Neural Reorganization  Studies conducted on laboratory animals to study neural reorganization after brain damage have primarily focused on sensory and motor cortex areas of the brain.  The results of the studies by Kaas and Colleagues (1990), Pons and Colleagues (1991) and Sanes, Suner, and Donaghue (1990) clearly indicate cortical reorganization following damage in laboratory animals.  Experiments conducted on adult mammalian brain also conclude that adult brain can reorganize its primary motor and sensory functions after gaining sufficient experience.  Mechanisms like strengthening of existing connections, collateral sprouting, adult neurogenesis, etc. have a role to play in neural reorganization TYPES OF NUEROPLASTICITY  Neuroplasticity is broadly distinguished into two types – Structural Plasticity and Functional Neuroplasticity  Structural Neuroplasticity – In this type, the strength of the connections between neurons (or synapses) changes (either becomes strong or weak). In this form, the brain actually change its physical structure as a result of learning  Functional neuroplasticity – In this type, the permanent changes occur in synapses due to learning and development. In this form the functions are moved from a damaged area of the brain to other undamaged areas. MECHANISMS OF NUEROPLASTICITY  There are a variety of mechanisms by which neural plasticity can occur. The most common of these mechanisms are  Axonal sprouting – In this mechanism, healthy axons sprout new nerve ending that connect to other pathways in the nervous system. This can be used to strengthen existing connections or to repair damaged parts of the nervous system by repairing damaged neural pathways and restoring them to full functionality  Synaptic pruning– In this mechanism, the brain removes neurons and synapses that it does not need. Synaptic pruning eliminates weaker synaptic contacts while stronger connection are kept and strengthened. By getting rid of the synapses that are no longer used, the brain becomes more efficient as one ages. ENHANCING NUEROPLASTICITY Research supports the following methods to boost neuroplasticity  Intermittent fasting: increases synaptic adaptation, promotes neuron growth, improve overall cognitive function and decreases the risk of neurodegenerative disease  Traveling: exposes brain to novel stimuli and new environments, opening up new pathways and activity in the brain  Using mnemonic devices: memory training can enhance connectivity in the prefrontal parietal network and prevent some age-related memory loss  Learning a musical instrument: may increase connectivity between brain regions and help form new neural networks;  Non-dominant hand exercises: can form new neural pathways and strengthen the connectivity between neurons;  Reading fiction: increases and enhances connectivity in the brain;  Expanding your vocabulary: activates the visual and auditory processes as well as memory processing  Creating artwork: enhances the connectivity of the brain at rest (the “default mode network” or DMN), which can boost introspection, memory, empathy, attention, and focus Dancing: reduces the risk of Alzheimer’s and increases neural connectivity  Sleeping: encourages learning retention through the growth of the dendritic spines that act as connections between neurons and help transfer information across cells HEMESPHERIC SPECIALIZATION  Hemispheric specialization  Cerebrum is largest area of brain occupying ¾ of the brain.  Cerebrum divides brain into two halves called cerebral hemispheres  These hemispheres are attached by a bundle of nerve fibers called the corpus callosum which allows two hemisphere to communicate with each other.  The cerebral hemispheres are covered by a sheet of neural tissue called the cerebral cortex which is 2 to 4 millimeters thick  It is a highly developed structure in human brain It is composed of 14-16 billion neurons arranged in six layers.  The outer layer of the cerebral cortex are composed of neuronal cell bodies and unmyelinated neurons giving it a grey appearance because of which it referred to as ‘grey matter’.  The inner layer of the cerebral cortex is composed of myelinated axons. These myelinated axons, due to the relatively high lipid fat content of the myelin that sheathes them, give the inner layer a white appearance because of which it referred to as ‘white matter’.  In humans, the cerebral cortex is deeply convoluted/furrowed  Its distinctive shape arose during evolution as the volume of the cortex increased more rapidly than the cranial volume resulting in the convolution of the surface and the folding of the total structure of the cortex.  90% of the cerebral cortex is called neocortex. Neo meaning new, it is so named because its appearance is thought to be relatively new in vertebrate evolution.  The large furrows in a convoluted cortex are called fissures. There are primarily three types of fissures. The deeper one is known as ‘Longitudinal fissure’ and separates both the hemispheres. The ‘Central fissure’ also known as ‘Fissure of Rolando’ separates frontal and parietal lobes. ‘Lateral fissure’, also known as ‘Fissure of Silvius’, separates frontal lobe from temporal lobe.t  The small furrows are called sulci (singular sulcus)  The ridges between fissures and sulci are called gyri (singular gyrus) Longitudinal Fissure Central Fissure Lateral Fissure Frontal Lobe Parietal Lobe Temporal Lobe Occipital Lobe  Cerebral cortex is divided into four lobes by fissures – frontal, parietal, temporal and occipital.  Each of these lobes is responsible for processing different types of information.  The frontal lobe primarily controls motor movement of the legs, arms, face etc.  The Parietal lobe primary contains the somesthetic area which receives cutaneous impulses coming from various parts of the body.  The Temporal lobe primarily contains the auditory area and the olfactory area.  The Occipital lobe has visual area  Collectively, cerebral cortex is responsible for the higher-level processes of the human brain, including language, memory, reasoning, thought, learning, decision-making, emotion, intelligence and personality.  However, research was long interested in understanding whether there is any difference in structural or functional aspects of the two hemispheres HEMISPHERIC SPECIALIZATION  Functional Asymmetry of cerebral hemispheres refers to the differences in the roles of right and left hemispheres in mediating behavior and higher cognitive processes  This phenomenon is also referred as also referred as ‘lateralization of brain function’ or ‘hemispheric specialization’  Earlier research on a number of nonhuman species and in the fossil record of human evolution indicated structural asymmetries. However, no behavioral or physiological asymmetries were reported  The finding by Paul Broca in 1861 that damage to a specific area on the left frontal lobe, later called as Broca’s area, produced a discrete language problem, or aphasia, was major breakthrough in the field of neurology at that time.  Carl Wernicke identified another type of aphasia resulting from damage to the superior portion of the left temporal lobe.  Until the 1940s, the functional significance of the connecting fiber tracts between the two hemispheres was largely a mystery. It is only known that corpus callosum, anterior commissure, optic chiasma etc. are extended from one hemisphere to the other structurally connecting them. It was widely believed that a severing of these fiber tracts would produce little or no behavioral effects.  In 1955 Ronald Myers through his experiments on cats by cutting the optic chiasma established that corpus callosum is transmitting information from hemisphere to hemisphere  Roger Sperry, who conducted ‘split-brain’ experiments on animals by severing both optic chiasma and corpus callosum concluded that bilateral transfer of learning is not possible when these structures are severed  These studies concluded that intact corpus callosum and other fiber tracts are critical for the two hemispheres of the neocortex to work as a unit.  Information is normally laid down in duplicate, through the transverse travel of activity from the hemisphere directly receiving the information to the hemisphere on the contralateral side  Once the transverse fibers are severed in the animals, however, the two hemispheres are functionally isolated.  Yet there is no evidence of functional asymmetry.  Each hemisphere seems to be equivalent in its capability to handle the existing behavioral repertoire of the opposite side of the animal’s body.  It was not until studies were made of humans with isolated hemispheres that there was evidence that functional asymmetry existed.  In 1960, Philip Vogel and Joseph Bogen, developed a surgical procedure in which the corpus callosum and anterior commissure is severed in order to control epileptic seizures.  Investigation of the cognitive functions of these individuals by Sperry and his associate Gazzaniga, provided a rare opportunity to understand the consequences of one hemisphere being isolated from the other in the humans  In contrast to the earlier animal studies, functional asymmetry in the two hemispheres was observed in their investigations. Most notably the left hemisphere of most split- brain patients is capable of speech, whereas the right hemisphere is not.  More recently, in late 1970s, neurosurgeon Donald Wilson has modified the surgical procedure of Vogel and Bogen by severing the corpus callosum but leaving the anterior commissure intact.  Rapid technological advancement led to invention of various brain imaging procedures and tools like MRI, fMRI etc. allowing the researchers to observe the brain of living organisms and thus giving them an opportunity to understand the specific and unique functions of each hemisphere  However, it is important to note that, for many functions there are no substantial differences between the hemispheres and when functional differences do exist, these tend to be slight biases in favor of one hemisphere or the other but not absolute differences  The research done in the past two decades suggests abilities that display some degree of cerebral lateralization. Most important areas in which difference in hemispheric dominance are reported include,  Vision– Left Hemisphere (LH) is dominant in reading and comprehending words and letters, where as Right Hemisphere (RH) is dominant in recognizing faces, geometric patterns and emotional expressions  Audition – LH is dominant in understanding the language sounds, where as RH is dominant in understanding Non-language sounds and Music  Touch – RH is dominant while understanding tactile patterns and Braille  Movement – LH is dominant during complex and ipsilateral movements, where as RH is dominant during movement in spatial patterns  Memory – LH is dominant for verbal memory and finding meaning in memories, where as RH is dominant for non- verbal memory and perceptual aspect of memories  Language – LH is dominant in speech, reading, writing and arithmetic, where as RH is dominant while understanding the emotion content  Spatial Ability– RH plays a dominant role in mental rotation of shapes, understand geometric, directional and distance concepts. Talents in the Left Brain