The Nervous System PDF

# The Nervous System ## The Spinal Cord The spinal cord is a bundle of nerves arranged in cylindrical fashion, with a narrow canal called the central canal, at the centre. Arising from the medulla oblongata, the spinal cord runs down the entire length of the vertebral column and is protected by th...

# The Nervous System ## The Spinal Cord The spinal cord is a bundle of nerves arranged in cylindrical fashion, with a narrow canal called the central canal, at the centre. Arising from the medulla oblongata, the spinal cord runs down the entire length of the vertebral column and is protected by the vertebrae (singular: vertebra), or the bones of the vertebral column. Like the brain, the spinal cord is covered by meninges that have cerebrospinal fluid between them. The central canal too is filled with cerebrospinal fluid. The spinal cord relays messages from the brain to different parts of the body and vice versa. It is connected to all parts of the body by nerves. ## The Peripheral Nervous System The peripheral nervous system consists of the nerves that connect the central nervous system to the rest of the body. These nerves arise from either the brain or the spinal cord. Each nerve has innumerable branches that connect to a particular area of the body. The peripheral nervous system carries impulses from the sense organs (such as the skin) and other organs to the CNS. It also conveys messages from the CNS to effector organs (organs that act in response to a stimulus) such as glands and muscles. The peripheral nervous system is involved in both voluntary and involuntary activities. The somatic, or voluntary nervous system is the part of the peripheral nervous system that is concerned with the movement of the skeletal muscles. It also carries information from the sense organs. The autonomic, or involuntary nervous system is involved in involuntary activities such as heartbeat, digestion, respiration, secretion from glands and excretion. The autonomic nervous system is controlled by the medulla oblongata. ### Points to Remember - The nervous system is made up of nerve cells, or neurons. These carry messages to and from the brain and spinal cord. The nervous system consists of the central nervous system and the peripheral nervous system. - A neuron consists of a cyton, dendrites and an axon. Neurons are of two types-myelinated and unmyelinated. Myelinated neurons have a myelin sheath covering their axons, while unmyelinated neurons do not have a myelin sheath covering their axons. Myelinated neurons conduct impulses faster than unmyelinated neurons. Synapses are communication links between neurons. - Sensory neurons carry messages from various parts of the body to the central nervous system, while motor neurons carry messages from the central nervous system to the various muscles and organs of the body. - Nerves are cablelike structures made up of bundles of axons. Nerves can be of three types-sensory, motor and mixed. - The central nervous system consists of the brain and the spinal cord. The brain processes all kinds of impulses and sends out appropriate instructions to the concerned parts of the body. The spinal cord relays messages from the brain to different parts of the body and vice versa. - The brain has three parts-cerebrum, cerebellum and brainstem. The medulla oblongata is a part of the brainstem. The cerebrum controls learning, speech, memory, emotion, reasoning and voluntary activities. It also controls response to pain and temperature, and the senses. The cerebellum ## The Nervous System ### The Cerebrum The cerebrum is the anterior (frontal) part of the brain. It is the largest part of the brain and is divided into two halves-the right and left cerebral hemispheres. The two hemispheres are partially separated by a deep groove. The outer surface of the cerebrum is full of folds, which increase its surface area. This allows a large number of neurons to be packed into the folds. The cerebrum is the centre of intelligence and is concerned with learning, memory, emotion and reasoning. It also coordinates our voluntary activities and makes us feel sensations like pain and heat. It controls speech, and the senses of touch, smell, sight, hearing and taste. ### The Cerebellum The cerebellum (means 'small brain' in Latin) is smaller than the cerebrum and is located below it towards the back of the skull. The surface of the cerebellum has a large number of thinly spaced parallel grooves. The cerebellum coordinates both voluntary and involuntary movements. It maintains the body's posture and balance and helps us learn new movements, such as a dance or playing a musical instrument. ### The Medulla Oblongata The medulla oblongata is the lowest part of the brainstem. It lies in front of the cerebellum, at a slightly lower level and is connected to the spinal cord. It has a cone-shaped appearance. It controls involuntary functions such as heartbeat, breathing, sneezing, coughing, swallowing, vomiting, movements of the alimentary canal and secretion by glands. | Part of the brain | Functions | |---|---| | Cerebrum | Controls learning, voluntary activities, speech, memory, emotion, reasoning, response to pain and temperature, and the senses | | Cerebellum | Coordinates body movement posture and balance; helps learn new movements | | Medulla oblongata | Controls involuntary functions | ## Nerve cells are joined end to end. The junctions are called synapses. body to the brain and the spinal cord. Impulses can also travel from the brain and the spinal cord to the rest of the body. The junction of the dendrites of one neuron and the axon of another is called a synapse. The neurons that carry impulses to the brain and the spinal cord from the rest of the body are called sensory neurons. Those that carry impulses from the brain and the spinal cord to the various muscles and organs of the body are called motor neurons. ## Nerves Nerves are cablelike structures made up of bundles of axons. The neurons, or rather their cytons, from which the axons arise are either located in the brain or the spinal cord. Nerves can be of three types - sensory, motor and mixed. Sensory nerves carry impulses from the sense organs to the CNS, while motor nerves carry impulses away from the CNS to the muscles and organs. Mixed nerves can conduct impulses to and from the CNS. ## The Central Nervous System The central nervous system consists of the brain and the spinal cord. It receives information about the various parts of the body and about the surroundings through the nerves that connect every part of the body. It processes the information and directs the body to act accordingly. The brain sorts out all the information it receives and directs all our activities in accordance with this information. It stores some of the information as memory and discards the rest. It is the centre of what we call consciousness. It helps us think, learn, evaluate, analyse, make decisions, and so on. The spinal cord acts mostly according to the instructions given by the brain. However, it can act on its own sometimes. You will learn more about the spinal cord's independent actions in higher classes. ## The Brain The brain lies within the cranium, a set of bones commonly known as the skull. It is covered by membranes called meninges, and cushioned against shocks by a fluid called cerebrospinal fluid (CSF). This fluid is found between the meninges and inside the brain. It helps distribute nutrients and other chemicals to the neurons of the brain, and collects waste ## The Nervous System By now you know about several of the life processes that go on within our body without our even being aware of them. What keeps these processes going and coordinates the functioning of the various organ systems? What makes us perceive the world around us? What makes us act in response to the things around us, for example, smile when we see a familiar person or move away when we see an approaching vehicle? It is the nervous system. The nervous system is a network of specialised cells called nerve cells, or neurons, that acts as a communication link between various parts of the body. It has two major components - the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is the primary control centre. It consists of the brain and the spinal cord. The peripheral nervous system is a network of nerves that connects the rest of the body to the CNS. Let us learn more about neurons, nerves and the nervous system. ### Neurons Neurons are the basic units of the nervous system (including the brain and the spinal cord). Each neuron has a cell body that has a nucleus. The cell body is called cyton, or soma. It has fibrelike projections called dendrons, or dendrites. The dendrites, which have many fine branches, help carry impulses (messages encoded as electrical signals) to the cyton. Another long tubelike projection, called the axon, extends outwards from the cyton and ends in many branches. The axons of some neurons have a covering, or sheath, made of a fatty white substance called myelin. Such neurons are called myelinated neurons. Neurons whose axons do not have a myelin sheath are called unmyelinated neurons. Myelinated neurons conduct impulses faster than unmyelinated neurons. An axon carries impulses away from the cyton. Its branches make connections with the dendrites of other neurons, forming a continuous pathway. Thus, an impulse can travel uninterruptedly from any part of the