Development of the Central Nervous System-1.docx

**DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM** Following fertilisation, the** nervous system **begins to form in the 3^rd^ week of development. It continues after birth and for many years into the future. Structurally, the nervous system is divided into two parts: - **Central nervous system** --...

**DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM** Following fertilisation, the** nervous system **begins to form in the 3^rd^ week of development. It continues after birth and for many years into the future. Structurally, the nervous system is divided into two parts: - **Central nervous system** -- consists of the brain and the spinal cord. - **Peripheral nervous system** -- consists of cranial and spinal nerves, ganglia, plexuses, and sensory receptors. **Early Stages** At the end of week two, a structure called the **primitive streak** appears as a groove in the epiblast layer of the bilaminar disk. Cells within the epiblast migrate downward through the primitive streak, giving rise to three layers from the initial two. These three germinal layers form the **trilaminar embryonic disk**: - **Endoderm** -- innermost layer - **Mesoderm** -- middle layer - **Ectoderm** -- outermost layer The nervous system is derived from the **ectoderm**, which is the outermost layer of the embryonic disc.  https://teachmeanatomy.info/wp-content/uploads/Embryonic-Development-Gastrulation-Bilminar-to-Trilaminar-Disk.jpg Fig 1 -- Formation of the three primary germ layers occurs during the third week of development. The embryo at this stage is only a few millimeters in length. (teachmeanatomy.info) **NEURULATION** In the third week of development, the **notochord **appears in the mesoderm. The notochord secretes growth factors which stimulate the differentiation of the overlying ectoderm into neuroectoderm -- forming a thickened structure known as the neural plate. The lateral edges of the neural plate then rise to form neural folds. The neural folds move towards each other and meet in the midline, fusing to form the** neural tube** (precusor to the brain and spinal cord). During fusion of the neural folds, some cells within the folds migrate to form a distinct cell population -- known as the **neural crest**. They give rise to a diverse cell lineage -- including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia The formation of neural tube is known as **neurulation**, and is achieved by the end of the fourth week of development.  Fig 2 -- Neurulation: the formation of the neural tube from neuroectoderm (teachmeanatomy.info) **Later Development** **Brain and Cerebellum** In the fifth week of development, swellings appear at the cranial end of the neural tube. Three **primitive vesicles** appear first, and subsequently these develop into five secondary vesicles. These vesicles will give rise to all the structures of the brain and cerebellum as shown in the table below: **Primary Vesicles** **Secondary Vesicles** **Neural Derivatives** ---------------------- ------------------------ ------------------------------------------ Prosencephalon Telencephalon Cerebral hemispheres and globus pallidus Diencephalon Thalamus, hypothalamus, and epithalamus Mesencephalon Mesencephalon Midbrain Rhombencephalon Metencephalon Pons and cerebellum Myelencephalon Medulla Meanwhile, neuroderm cells begin to differentiate into neurones and glial cells. **Neurones **migrate throughout the brain, and once they have reached their final destination they develop axons and dendrites, forming synapses. https://teachmeanatomy.info/wp-content/uploads/The-Five-Secondary-Brain-Vesicles.jpg Fig 3 -- The five secondary brain vesicles and their derivatives. (teachmeanatomy.info) **\ ** **Spinal Cord** Whilst the cranial end of the neural tube forms the brain and cerebellum, the caudal end develops to form the **spinal cord**. Cells on the dorsal side form the alar plate, which subsequently becomes the **dorsal horn** (posterior). Cells at the ventral end form the basal plate, which then becomes the ventral horn (anterior). ORGANIZATION OF THE CENTRAL NERVOUS SYSTEM  Figure 4: The Organization of the Central Nervous system\ (https://www.pinterest.com/) - The central nervous system consists of the brain and [spinal cord](https://www.emedicinehealth.com/slideshow_pictures_low_back_pain/article_em.htm). The brain or the Encephalon is possibly the most complex organ to examine within the human body. With an average weight of about 1.3 kg in the average adult, the brain plays a central role in the control of most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. - The spinal cord is connected to a section of the brain called the brainstem and runs through the spinal canal. Some reflex movements can occur via spinal cord pathways without the participation of brain structures. - Cranial nerves exit the brainstem. - Nerve roots exit the spinal cord to both sides of the body. - The spinal cord carries signals (messages) back and forth between the brain and the peripheral nerves. - Cerebrospinal fluid surrounds the brain and the spinal cord and also circulates within the cavities (called ventricles) of the central nervous system. - The leptomeninges surround the brain and the spinal cord. - The cerebrospinal fluid circulates between 2 meningeal layers called the - pia matter and - the arachnoid - (or pia-arachnoid membranes). - The outer, thicker layer serves the role of a protective shield and is called the dura matter. - The basic unit of the central nervous system is the neuron (nerve cell). - Billions of neurons allow the different parts of the body to communicate with each other via the brain and the spinal cord. - A fatty material called myelin coats nerve cells to insulate them and to allow nerves to communicate quickly. ### ### ### The Cerebrum The cerebrum is the largest part of the brain and controls voluntary actions, speech, senses, thought, and memory. The surface of the cerebral [cortex](https://www.emedicinehealth.com/human_body_quiz_iq/quiz.htm) has grooves or infoldings (called sulci), the largest of which are termed fissures. Some fissures separate lobes. The convolutions of the cortex give it a wormy appearance. Each convolution is delimited by two sulci and is also called a gyrus (gyri in plural). The cerebrum is divided into two halves, known as the right and left hemispheres. A mass of fibers called the corpus callosum links the hemispheres. The right hemisphere controls voluntary limb movements on the left side of the body, and the left hemisphere controls voluntary limb movements on the right side of the body. Almost every person has one dominant hemisphere. Each hemisphere is divided into four lobes, or areas, which are interconnected. - The frontal lobes are located in the front of the brain and are responsible for voluntary movement and, via their connections with other lobes, participate in the execution of sequential tasks; speech output; organizational skills; and certain aspects of behavior, mood, and memory. Located in the front of both cerebral hemispheres, the frontal lobes are the largest lobes of the brain. Anterior to the central sulcus (the groove that separates the frontal lobe and parietal lobe) is the [primary motor cortex] (homunculi), the centre for movement. (http://www.nidcd.nih.gov/health/voice/aphasia.asp) - Located above the occipital lobes and behind the frontal lobes are the parietal lobes. Posterior to the motor cortex ( of the frontal lobes) is the [somato-sensory corte]x ( which receives general sensory information and initial reception of tactile (touch, pain, temperature) and proprioceptive( sense of position) information. They process sensory information such as temperature, pain, taste, and touch. In addition, the processing includes information about numbers, attentiveness to the position of one\'s body parts, the space around one\'s body, and one\'s relationship to this space. - The temporal lobes are located on each side of the brain. They process memory and auditory ([hearing](https://www.emedicinehealth.com/hearing_loss/article_em.htm)) information and speech and language functions. They are also responsible for higher order visual information, complex aspects of memory, language and comprehension of language, abstract thought and judgement and control of written and verbal language skills.\ [Wernike's area] is primarily responsible for comprehension of speech and closely linked with Broca's area to produce speech. - Located at the rearmost portion of the brain, the occipital lobe is the main visual processing area of the cortex. The primary visual cortex, which receives raw sensory information from the retina processes information on colour, objects and facial recognition and is also involved in the perceiving motion.. **[Corpus Callosum]** The Corpus Callosum is the largest fiber bundle in the brain and connects the two hemispheres together. Allows for the transmission of information between the two hemispheres. The cortex, also called gray matter, is the most external layer of the brain and predominantly contains neuronal bodies (the part of the neurons where the DNA-containing cell nucleus is located). The gray matter participates actively in the storage and processing of information. An isolated clump of nerve cell bodies in the gray matter is termed a nucleus (to be differentiated from a cell nucleus). The cells in the gray matter extend their projections, called axons, to other areas of the brain. Fibers that leave the cortex to conduct impulses toward other areas are termed efferent fibers, and fibers that approach the cortex from other areas of the nervous system are termed afferent (nerves or pathways). Fibers that go from the motor cortex to the brainstem (for example, the pons) or the spinal cord receive a name that generally reflects the connections (that is, corticopontine tract for the former and corticospinal tract for the latter). Axons are surrounded in their course outside the gray matter by myelin, which has a glistening whitish appearance and thus gives rise to the term white matter. Cortical areas receive their names according to their general function or lobe name. If in charge of motor function, the area is called the motor cortex. If in charge of sensory function, the area is called a sensory or somesthetic cortex. The calcarine or visual cortex is located in the occipital lobe (also termed occipital cortex) and receives visual input. The auditory cortex, localized in the temporal lobe, processes sounds or verbal input. Knowledge of the anatomical projection of fibers of the different tracts and the relative representation of body regions in the cortex often enables doctors to correctly locate an injury and its relative size, sometimes with great precision. **[Thalamus]** The Thalamus is the main part of the Diencephalon and is located between the midbrain and cerebellum. All sensory pathways pass through the Thalamus and are relayed to various areas throughout the brain. The thalamus accomplishes this by filtering incoming information and deciding what to pass on or not to pass on to cortex, preventing the overload of sensory information. The Thalamus plays a role in mood and body movement associated with strong emotive responses such as fear or rage and also has some influence in prefrontal functions such as foresight and affect therefore its dysfunction has been implicated in abnormal behaviour. **[Hypothalamus]** The Hypothalamus can be viewed as the central control for the brain. It is located just below the thalamus, above the brain stem and is what keeps our body in homeostasis. Functions as the main control centre for the pituitary gland, regulating autonomic, emotional, endocrine and somatic function (body temp, arterial blood pressure, thirst, fluid balance, gastric motility and secretions), plays a part in 'primitive' states directly involved in stress related and psychosomatic illnesses, controls emotional and mood relationships, physical drive such as hunger and sex and co-ordinates our sleep/wake cycle. **[Midbrain (mesencephalon)]** Middle part of the brain, located at the upper part of the brain stem and connects to several major structures including the basal ganglia, substanitia nigra, pons and motor cortex making its exact function difficult to define. **[Hindbrain (brainstem)]** **Metencephalon** **[Cerebellum]** The cerebellum or 'little brain' is located posterior to the brain stem and plays an important role in sensory perception and fine motor control. The Cerebellum has two main functions; 1\) Receive input from all sensory sites and project this information to other parts of the brain such as the brainstem and thalamus. 2\) Act as part of the motor system regulating equilibrium, muscle tone, postural control, and coordination of voluntary movement. The Cerebellum is the part of the brain allows for fine movement, damage to the area results in poor coordination, poor motor learning, and a loss of equilibrium. **[Pons]** The pons is the main relay station between the cerebrum and the cerebellum. Majority of the brains norepinephrine is produced in the locus cerculeus located within the pons and aids in regulating arousal and respiration. **\ ** **Myelencephalon** **[Medulla oblongata]** 250px-Illu\_pituitary\_pineal\_glands (http://training.seer.cancer.gov/module\_anatomy/unit6\_3\_endo\_glnds1\_pituitary.html) The Medulla acts as a conduction pathway for ascending and descending nerve tracks for the conscious control of skeletal muscles, balance, co-ordination, regulating sound impulses in the inner ear, regulating autonomic responses such as heart rate, breathing, swallowing, vomiting, coughing and sneezing. **[Reticular formation]** Functions as the central core of the brainstem. Most important in arousal and maintaining consciousness, alertness attention and our reticular activating system (RAS) which controls all cyclic functions such as Circadian Rhythm, cardiac rhythms and respiration. Damage to the RAS system can result in Coma, and this is the main area of the brain the anaesthetics suppressed to put someone to sleep. Stimulating this area results in arousal. **[Basal Ganglia]**  (Barlow and Durand , 2005) The Basal Ganglia consists of several gray matter structures located in both sides of the brain in the cerebrum, diencephalon and midbrain. Main roles are to control muscle tone, activity, posture, large muscle movements and inhibit unwanted muscle movements. This is the main centre of the brain where EPSE's and Parkinson's symptoms originate due to loss of dopamine in the Substantia Nigra( the main dopamine producer in the brain) that is connected to basal ganglia. **The** **Limbic system** Located just above the thalamus and hypothalamus at the base of the forebrain is the limbic system. The limbic system is the 'emotional centre' of the brain. Components of the limbic system help to regulate our emotions, expression of emotion and ability to learn and control impulses. Limbicsystem (Barlow and Durand , 2005) **[Amygdala]** Almond shaped structure located deep in the temporal lobe, connected to other parts of limbic system including the hippocampus and thalamus. Through these connections the amygdala mediates and controls major affective and mood states such as friendship, love, affection, fear, rage and aggression and generates these emotions from perceptions and thoughts. The amygdala is also the centre for which there are many opiate receptors. Studies of damage to this part of the brain in animals has a taming effect (deprivation of emotion, indifference), however stimulation elicits violent aggression. **[Hippocampus ]** The Hippocampus is a part of the limbic system, and is located deep in the temporal lobe. The Hippocampus contains a large amount of neurotransmitters and its main functions appear to be memory, particularly turning short term memory into long term memory. Although the exact function of the hippocampus is unknown in Alzhiemers disease the Hippocampus is one of the major affected areas further suggesting its role in memory and orientation. **Further reading & References** - Guyton and Hall. Textbook of Medical Physiology. - Barlow and Durand (2005). *Abnormal Psychology, and intergrated approach.* Thompson/Wadsworth, Australia. - http://www.nidcd.nih.gov/health/voice/aphasia.asp - https://www.pinterest.com/ - www.quizlet.com