Chapter 2.1 Nervous System PDF

## 2.1 Nervous System Whenever a person gets injured on his foot while walking, he feels pain and his hand immediately reaches the injured site. There is an organ system in our body which carries messages from one part of the body to another and coordinates body functions. This system is called th...

## 2.1 Nervous System Whenever a person gets injured on his foot while walking, he feels pain and his hand immediately reaches the injured site. There is an organ system in our body which carries messages from one part of the body to another and coordinates body functions. This system is called the nervous system. Human nervous system consists of: * **Central nervous system (CNS)**: Composed of brain & spinal cord. * **Peripheral nervous system (PNS)**: A network of nerves which connect the central nervous system to all body parts. ### 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 (brain, spinal cord and nerves) are made up of neurons. Neurons transmit messages in the form of electrochemical waves called nerve impulses. **Structure of a Neuron:** * **Cell body**: Contains nucleus and cytoplasm * **Dendrites**: Fine projections that receive messages * **Axon**: Long projection that conducts messages away from the cell body * **Terminal Ends of axon**: Transmit messages to the next cells ### Nerve A nerve is a cable-like bundle of axons enclosed in a common sheath. Nerves transmit messages from one part of the body to the other. ### Types of neurons Neurons can be classified into 3 types based on their function: * **Sensory neurons**: Carry nerve impulses from sense organs to the central nervous system. * **Motor neurons**: Carry nerve impulses from the central nervous system to effectors (muscles and glands). * **Interneurons**: Present in the central nervous system (brain and spinal cord). They form a link between sensory and motor neurons. **Interesting Information**: Impulses may travel as fast as 150 meters per second or as slow as 0.2 meters per second ### 2.1.1 Central Nervous System (CNS) The central nervous system acts as a control center of the whole nervous system. It comprises brain and spinal cord. #### Brain The human brain is enclosed in a bony skull called cranium and consists of billions of interneurons. It is divided into the following parts: **1. Forebrain**: The largest part of the brain. It consists of three main parts: * **Cerebrum**: Topmost and largest part. It is divided into right and left hemispheres. The left hemisphere controls speech, comprehension, arithmetic and writing. The right hemisphere controls creativity, spatial ability, artistic and musical skills. * **Thalamus**: A small structure inside the cerebrum. It controls many sensory functions. * **Hypothalamus**: Located at the base of the thalamus. Controls body temperature, hunger, and thirst. **2. Midbrain**: A small part of the brain present below the cerebrum. It receives information from sense organs, which is then passed to the appropriate part of the forebrain. **3. Hindbrain**: Consists of three parts: * **Cerebellum**: Lies under the back part of the cerebrum. It acts as a controller for maintaining body balance and making precise and accurate movements. * **Pons**: An oval structure present beneath the midbrain. Controls functions like sleep, swallowing, equilibrium and taste. * **Medulla oblongata**: Posterior part of the brain, connected to the spinal cord. Controls heartbeat, breathing, and digestion. ### Spinal Cord An extension of the medulla oblongata. It runs backwards inside the backbone up to its lower end. Made up of interneuons. The 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. ### 2.1.2 Peripheral Nervous System Peripheral nervous system consists of a network of nerves which spread in the body to connect all the body parts to the central nervous system (brain and spinal cord). * **Cranial nerves**: Arise from the brain * **Spinal nerves**: Arise from the spinal cord The human body has 12 pairs of cranial nerves and 31 pairs of spinal nerves. ### 2.1.3 Working Model of the Nervous System The nervous system coordinates all body functions. It also detects the changes in the environment and produces a 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.** Heat, cold, pressure, sound waves, etc. are examples of stimuli. The special organs, tissues or cells that detect these stimuli are called **receptors**. * **Sensory neurons**: Carry messages regarding stimuli in the form of nerve impulses from receptors to the central nervous system. * **Central nervous system**: Processes messages and transmits nerve impulses to motor neurons. * **Motor neurons**: Carry nerve impulses to parts of the body which produce responses. These parts are called **effectors**. Muscles and glands in the body act as effectors. ### 2.1.4 Actions Controlled by the Nervous System * **Voluntary actions:** Performed under conscious control, i.e., done after thinking over them. Examples include speaking, eating, reading, walking, running, and clapping. * **Involuntary actions:** Performed without the involvement of the thinking process. These actions are not performed under conscious control. Examples include heartbeat, breathing, blinking of the eyes, and movement of the small intestine. ### 2.2 Reflex Action An immediate and involuntary response to a stimulus is called a reflex action. A common example of this is quickly pulling one's hand back after touching a hot object. This reflex arc involves the following steps: 1. The hot object acts as a stimulus to the **thermoreceptors** in your skin. 2. **Sensory neuron**: Carries a nerve impulse to the spinal cord. 3. **Inter-neuron**: Transmits the impulse to the motor neuron in the spinal cord. 4. **Motor neuron**: Carries the nerve impulse to the arm muscles (effectors). 5. The arm muscles contract, causing the hand to be pulled back. This pathway of nerve impulses which complete a reflex action is called a **reflex arc**. The reflex arc consists of: * **Receptor:** Detects the stimulus. * **Sensory neuron:** Carries the nerve impulse to the central nervous system. * **Inter-neuron:** Relays the nerve impulse to the motor neuron. * **Motor neuron:** Carries the nerve impulse to the effector. * **Effector:** The muscles that cause a response to the stimulus.