HUMAN ORGAN SYSTEMS: Nervous System Whenever a person gets injury on his foot while walking, he feels pain and his hand immediately reaches the injured site. Who asked the hand to reach the site? In fact, there is an organ system in our body which carries messages form one part of the body to another and coordinates body functions. This system is called nervous system.

Human nervous system consists of central nervous system (CNS) and peripheral nervous system (PNS) (Figure 1.1). The central nervous system is composed of brain and spinal cord. Peripheral nervous system consists of a network of nerves which connect the central nervous system to all parts of the body.

Neuron or Nerve Cell Neuron or nerve cell is the basic structural and functional unit of the nervous system. All parts of the nervous system, i.e., brain, spinal cord and nerves are made up of neurons. Neurons transmit messages in the form of electrochemical waves called nerve impulses.

The part of a neuron which contains nucleus and most of the cytoplasm is called cell body. The fine projections of the cell body which receive messages are called dendrites. A long projection of the cell body which conducts messages away from the cell body is called axon. Terminal ends of the axons transmit the messages to the next cells.
Nerve A nerve is cable-like bundle of axons enclosed in a common sheath. Nerve transmits messages from one part of body to another. Types of Neurons On the basis of their functions, neurons are of three types, i.e. sensory neurons, motor neurons and inter-neurons. Sensory neurons carry nerve impulses from sense organs (ears, eyes, skin, tongue, nose, etc.) to the central nervous system. Motor neurons carry nerve impulses from central nervous system to effectors (muscles and glands), i.e., the parts which respond. Inter-neurons are present in central nervous system (brain and spinal cord). They form a link between sensory and motor neurons.

Central Nervous System(CNS) Central nervous system acts as a control centre of the whole nervous system. It comprises brain and spinal cord. Brain Human brain (Figure 1.4) is enclosed in a bony skull called cranium, and consists of billions of inter-neurons. It is divided into the following parts. 1. Forebrain Forebrain is the largest part of the brain. It consists of three main parts, i.e., cerebrum, thalamus and hypothalamus. Cerebrum is the topmost and the largest part of the brain. It is divided into right and left cerebral hemispheres. Cerebrum controls many actions like thinking, feelings, emotions, seeing, hearing, perceptions, memory, speech, decision making, etc. Inside cerebrum there is small structure called thalamus. It controls many sensory functions. Hypothalamus lies at the base of thalamus. It controls body temperature, hunger and thirst.

2 Midbrain Midbrain is a small part of the brain which is present below the cerebrum. It receives information from sense organs which is then passed on appropriate part of the forebrain.

3. Hindbrain Hindbrain consists of three parts, i.e., cerebellum, pons and medulla oblongata. Cerebellum lies under the back part of the cerebrum. It acts as a controller for maintaining the body balance and making precise and accurate movements. Pons is an oval structure present beneath midbrain. It controls many functions like sleep, swallowing, equilibrium and taste, etc. Medulla oblongata forms the posterior part of the brain where it is connected with the spinal cord. Medulla oblongata controls heartbeat, breathing and digestion, etc. Medulla oblongata keeps on working when rest of the brain goes to sleep.

Spinal Cord Spinal cord is an extension of medulla oblongata (Figure 1.5). It runs backwards inside the backbone up to its lower end. It is also made up of inter-neurons. Spinal cord creates a link between brain and different body parts. It also controls some reflex actions (immediate and involuntary actions) and some other involuntary actions. 1.1.2 Peripheral Nervous System Peripheral nervous system (PNS) consists of a network of nerves which are spread in the body to connect all the body parts to the central nervous system (brain and spinal cord) (Figure 1.1). The nerves which arise from brain are called cranial nerves. The nerves which arise from spinal cord are called spinal nerves. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in human body. 1.1.3 Working Model of the Nervous System Nervous system coordinates all body functions. It also detects the changes in environment and produces response to the changes.

Any change in the environment (external or internal) that can be detected by a receptor to initiate a nerve impulse is called stimulus (Plural: stimuli). Heat, cold, pressure, sound waves, etc. are the examples of stimuli. The special organs, tissues or cells which detect stimuli are called receptors. § The sensory neurons carry the messages regarding stimuli in the form of nerve impulses from receptors to central nervous system. § The central nervous system processes the messages and transmits the nerve impulses to motor neurons. § The motor neurons carry the nerve impulses to the parts of the body which produce responses. Such parts are called effectors. Muscles and glands in the body act as effectors.

Actions Controlled by the Nervous System Voluntary Actions The body actions which are performed under conscious control, i.e., which are done after thinking over them are called voluntary actions. For example; speaking, eating, reading, walking, running, clapping, etc., are voluntary actions. Involuntary Actions The body actions which are performed without involvement of thinking process are called involuntary actions. Involuntary actions are not performed under conscious control. Heartbeat, breathing, blinking of eyes, movement of small intestine, etc., are the examples of involuntary actions. 1.2 Reflex Action An immediate and involuntary response to a stimulus is called reflex action. Quick pulling of hand just after touching the hot object is a common example of reflex action. In this example of reflex action shown in Figure 1.7, temperature of hot object is a stimulus which is received by the cells (receptors) of the skin. A nerve impulse is created in the sensory neuron present in skin. The nerve impulse is carried by the sensory neuron to the spinal cord. The inter-neuron of the spinal cord transmits the impulse to the motor neuron. The motor neuron carries the impulse to the arm muscles (effectors). The arm muscles contract and the hand is pulled back. The pathway of nerve impulses which complete a reflex action is called reflex arc. It consists of receptor, a sensory neuron, an inter-neuron, a motor neuron and effectors.

Excretory System As a result of breakdown of various food items and other chemical components of the body, nitrogenous waste matter is produced, which must be immediately removed from the body. Waste products in the body also include nitrogenous materials and other salts. Accumulation of waste materials in the body is dangerous and therefore must be removed from the body. The removal of nitrogenous waste materials from the body is called excretion. Nitrogenous materials, extra water and salts are removed by the excretory system. Some extra salts are also removed through skin during perspiration. Human excretory system consists of one pair of kidneys and associated structures, i.e. two ureters, a urinary bladder and a urethra  Kidneys and Associated Structures Human body has two dark brown, bean-shaped kidneys in the abdominal region, one on either side of the vertebral column. The right kidney is a little lower than the left one. The outer surface of kidney is convex while the inner surface is concave. The following structures are attached with kidneys.
A tube which arises from each kidney and enters the urinary bladder is called ureter. It transports urine from kidneys to urinary bladder. Urinary bladder is a muscular sac which collects urine from both ureters. A fine tube through which urine is released from urinary bladder to the outside is called Urethra. Internal Structure of Kidney Internally, each kidney is divided into three regions, i.e., renal cortex, renal medulla and renal pelvis (Figure 1.9). Renal cortex is the outermost region. Renal medulla is the middle region which is divided into conical masses called renal pyramids. Renal pelvis is the inner area where urine is drained. The urine from renal pelvis moves into ureter. Nephron Nephrons are the functional units of the kidney. They are the tubules where urine is formed. There are over one million nephrons in each kidney. Each nephron has two parts, i.e., renal corpuscle and renal tubule.

Renal Corpuscle It is the first part of nephron. It consists of two structures, i.e., glomerulus and Bowman’s capsule (Figure 1.10). Glomerulus is a tuft of blood capillaries formed by the division and sub-division of small arteries and veins. Bowman’s capsule is a cup-shaped structure enclosing glomerulus. Renal Tubule This part of nephron starts after Bowman’s capsule. The first coiled part of renal tubule is called proximal tubule. The next part is U-shaped and is called Loop of Henle. The last part of the renal tubule is again coiled and is called distal tubule. The distal tubules of many nephrons open in a collecting duct (Figure 1.10). Many collecting ducts join and drain into renal pelvis.

Function of Kidneys Blood carries nitrogenous waste materials from the body to the kidneys. Inside the kidneys, blood containing nitrogenous waste reaches the glomerulus. Here, most of the water and waste materials are filtered from the blood into the Bowman’s capsule (Figure 1.11). The blood after losing waste material is collected in arterioles, which ultimately form renal artery. The “clean” blood is brought back to the main circulatory system. This filtrate which moves into the renal tubule of nephron also contains some useful substances. During its passage towards the collecting duct, 99% of the filtrate (containing useful substances) is reabsorbed into the blood in capillaries around renal tubule. During this reabsorption, more waste materials are absorbed from blood capillaries into the renal tubule filtrate. Now, the filtrate in renal tubule is called urine which moves into the collecting ducts and then into the renal pelvis.

Malfunctioning of Kidneys 1.4.1 Formation of Stones in Kidneys Sometimes kidneys cannot work efficiently, i.e. to remove nitrogenous waste or salts from the blood. In such situation, the salts accumulate in kidneys and form stones (Figure 1.12). Formation of stones disturbs the normal functioning of kidneys and causes severe pain. Kidney stones may travel to ureter or urinary bladder (Figure 1.12). The common causes of stones in kidneys are excessive calcium salts in the food and uric acid, etc.
Small sized stones can be removed through urinary system by drinking more water. Medium sized stones are removed by lithotripsy. Lithotripsy involves bombardment of shockwaves on the stones from outside. Shockwaves break the stones into small pieces which are passed out of the body through urine. Still larger stones need surgery for their removal. 1.4.2 Renal Failure Renal failure is the complete or partial failure of kidneys to work. The main causes of renal failure are long-term infections, diabetes mellitus and hypertension. Diabetes mellitus is a disease in which sugar level increases in the blood. Hypertension is a state of high blood pressure in the body. Sudden blockage of blood supply to the kidneys may also result in renal failure. Dialysis and kidney transplant are the treatments of renal failure. Treatment of Malfunctional Kidneys Dialysis Cleaning of blood by artificial methods is called dialysis. It is done by a machine called dialyzer. The blood of the patient is passed through the dialyzer which contains dialysis fluid. Blood flows through the tubes of the dialyzer and dialysis fluid flows around these tubes (Figure 1.13). The waste materials move from blood to the dialysis fluid. The cleansed blood is returned to the body.Kidney Transplant This method is used at the last stage of kidney failure. In this method, a kidney donated by some healthy person is grafted in the body of the patient (Figure 1.14). The donor of kidney may be blood relative or any other close relative.

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