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Nervous system The nervous system is the part of an animal's body that coordinates its actions and transmits signals to and from different parts of its body. The nervous system consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). 1-The central nervous system includes the brain and spinal cord. 2-The peripheral nervous system (PNS) consists mainly of nerves, which are enclosed bundles of the long fibers (axons) that connect the CNS to every other part of the body. It includes 1- Spinal nerves, 2- Cranial nerves Nerves coming out from the spinal cord are called spinal nerves. Nerves coming out directly from the brain are called cranial nerves Nerves that transmit signals from the brain to parts of the body are called motor or efferent nerves, Nerves that transmit information from the body to the CNS are called sensory or afferent. Most nerves serve both functions (afferent and efferent) and are called mixed nerves. Autonomic nervous system overlaps the central and peripheral nervous systems (the higher central autonomic Centre is located in the Hypothalamus) The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, prepairing the animal for fight-or-flight response The parasympathetic nervous system is activated when animals are in a relaxed state. Autonomic nervous systems function involuntarily. The Peripheral Nervous System includes a- somatic and bautonomic nerves. Somatic nerves mediate voluntary movement. The autonomic nerves serve the involuntary action. At the cellular level The nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. The brain is an organ of the human central nervous system. It is located in the head, protected by the skull. The brain includes the following parts 1- Cerebrum: has two cerebral hemispheres. 2- Brain stem: (Midbrain, Pons and Medulla) 3- Cerebellum Cerebrum The cerebrum (or forebrain), which makes up 75% of the brain size and 85% by weight, is divided by a large groove, known as the longitudinal fissure, into two distinct hemispheres. The left and right hemispheres are convoluted and joined by a large bundle of nerve fibres called the corpus callosum, and also by other smaller connections called commissures Cerebral hemisphere has outer layer called Cortex which is brown in colour called Grey matter containing cell bodies The inside of the hemisphere is whitish in colour and called White Matter, it contains axons only The forebrain is subdivided into two parts A- cerebral hemispheres laterally and B- central part composed mainly of two thalami and Hypothalamus (diencephalon) Each hemisphere is divided in lobes 1-Frontal lobe 2-Parietal lobe 3-Occipital lobe 4-Temporal lobe The surface of the cerebral hemisphere is thrown into elevations called Gyri (Gyrus) and depressions called Sulci (Sulcus). It’s for increasing the surface area of the brain Lateral Sulcus The most important sulci on cerebral cortex are 1- Lateral sulcus 2- Central sulcus The most important Gyri on cerebral cortex are 1-Precentral gyrus: containing the Motor Centre of the whole body 2-Post central gyrus: containing the Sensory Centre of the whole body The brain has cavities called Ventricles, They are called 1- Lateral ventricles, they are two 2- Third Ventricle: one ventricle 3- Fourth Ventricle: one ventricle These ventricles are interconnected and contained cerebrospinal fluid The brain and spinal cord is covered by three membranes called meninges, they are 1- Dura Mater: Extends down to the level of 2nd Sacral vertebra 2- Arachnoid: is adherent to the dura mater and extends down to the level of 2nd Sacral vertebra 3- Pia Mater: it ends with the spinal cord at the level of the lower end of the 1st lumbar vertebra The dura mater and arachnoid are adherent to each other and lining the skull and the vertebral canal The Pia mater clothing the brain, spinal cord and part of the spinal nerves inside the spinal canal up till the intervertebral foramen Circulation of the cerebrospinal fluid (CSF) Between the arachnoid and pia mater there is a space called the subarachnoid space. It contains the cerebrospinal fluid (CSF) The CSF is secreted from the blood through arachnoid granulation inside the ventricles and return back to the blood through arachnoid Villi and granulation. Functions of Cerebrospinal Fluid 1-Delivers nutrients from blood to the brain and spinal cord 2-Protects the brain and spinal cord 3-Removes the wastes from the brain and spinal cord 4-Shock absorber that reduces trauma. Function of the cerebrum It controls voluntary movements, and mental activities. It is the higher centre for sensation It is the centre for all consciousness. It is also the higher centre for autonomic nervous system (located in the lateral wall of the hypothalamus) Brain Stem Composed of the following parts 1-Midbrain: Is involved in Visual reflexes, movements of eye ball and dilatation of pupil. It is also involved in auditory reflexes 2-Pons: Controls respiration 3-Medulla oblongata: Contains the centres that regulate Heart beat, Respiratory rate, Blood pressure, Swallowing, Vomiting, Sneezing and cough Cerebellum Regulates muscular movements, Balance and body position. Spinal Cord It extends from the medulla oblongata at Foramen Magnum of the skull and ends at the level of the lower end of the first lumbar vertebra. The spinal cord and its meninges are located in the vertebral canal. It measures about 45 Cm in men. The spinal cord contains spinal canal. The spinal cord ends as a tapering end called conus medullaris. From apex of conus medullaris, filum terminale, is a delicate strand of fibrous tissue, about 20 cm in length, proceeding downward. It is one of the modifications of pia mater.It is attached downward to the dura mater. It gives longitudinal support to the spinal cord 31 pairs of spinal nerves originate from the spinal cord: 8 pairs are called Cervical spinal nerves, 12 pairs are Thoracic nerves, 5 pairs are Lumbar nerves, 5 pairs are Sacral nerves and I pair are coccygeal nerves Spinal cord is covered with meninges and CSF. It has central canal. Peripheral Nervous System It connects the central nervous system to the body. It includes 1- 31 pairs of spinal nerves (arise from the spinal cord) 2- 12 pairs of cranial nerves (arise directly from the brain) The cranial nerves