The Nervous System: Let see how it work

The nervous system controls all of the activities of the body. It is made up of the brain, spinal cord, and nerves that are found throughout the body. The nervous system also allows you to stimuli (singular: stimulus). A stimulus is a change in the environment, or surroundings of an organism that produces a response.

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Stimuli may be external or internal. Some stimuli come from outside the organism’s body. For example, smells and noise are stimuli to which you respond. So is tickling. Light and water are stimuli and to which plants respond. Such stimuli are external. Other stimuli come from inside an organism’s body. For example, a lack of oxygen in your body is a stimulus that often causes you to yawn. Such stimuli are internal. Some responses to stimuli are involuntary, such as blinking your eye when an insect zooms towards your eye. Many response of the nervous system are far more complex. For example, leaving a football game because it begins to rain is a voluntary reaction.

Central Nervous System (CNG) and Autonomic Nervous System (ANS):
Attempts at understanding the human nervous system usually begin by dividing it into two parts. One part is the central system. It is made up of the brain and the spinal cord. The central nervous system is the control centre of the body. The other part of the human nervous system is a network of nerves and sense organs, which is called the peripheral nervous system. A subsystem of the peripheral nervous system controls all involuntary body processes, such as heartbeat. This subsystem is known as the autonomic nervous system.

The Nervous System – Working:
As already stated, the nervous system is made up of the brain, spinal cord and nerves that are found throughout the body. Let’s known how these constituents work.

The Brain : More than 10 billion nervous (nerve cells) make up the human brain. Despite its many neurons, the brain weighs only about 1.5 kilograms. It is made up of spongy nerves tissue, which is surrounded by three membranes. These membranes nourish and protect the brain. Between the inner membrane and the middle membrane is a watery fluid. The brain floats in this fluid, which cushions it against shock. The outer membrane is thicker and tougher than the other two, and makes contract with the inside of the bony skull. As you will discover, the brain is divided into three parts.

The Cerebrum: Most of the mental activities that make human beings different from other animals take place in the cerebrum. The cerebrum is the largest part of the human brain. Located here are the nerve impulses that allow you to think, remember and speak. The cerebrum also controls most voluntary muscle contractions. It is here that your attitudes, your emotions and even your personalities are shaped. The cerebrum actually is composed of two halves. Each of these halves controls different kinds of activities. The right part of the cerebrum controls the left part of the body. The left part controls the right part of the body.

The Cerebellum: Below and to the rear of the cerebrum is the cerebellums is the cerebrum, the second largest part of the human brain. Motor nerve impulses beginning in the cerebrum pass through the cerebellum. The cerebellum then adjusts the motor impulses so that the body movements they produce are smooth rather than jerky. It also sends out impulses that help to control balance and posture.

The Brain Stem: Nerves passing from the cerebrum and cerebellum from a thick stalk called the brain stem at the base of the brain. The lowest part of this stem is called medulla which joins the brain to the spinal cord. Nerve impulses from the automatic body processes such as heartbeat, breathing and blood pressure. They also control swallowing, sneezing, coughing and blinking.

The Spinal Cord: If you bend forward slightly and run your thumb down the centre of your back, you can feel the vertebrae through your skin. The vertebrae are a series of interlocking bones that protect the nerves running through the spinal cord. The spinal cord runs the entire length of the neck and back. It connects the brain with rest of the nervous system through a series of thirty one pair of nervous.

Reflexes: Hopefully, you may not notice a fly circling in the air near your face. Perhaps even if it landed on your hair and started to crawl down on the side of your face, you would automatically blink shut. Such an automatic reaction to the environment is called a reflex. In this case, the reflex begins as soon as the fly’s legs bend on an eyelash. Such reflexes are controlled by the spinal cord. There are many different reflexes your spinal cord can control without first notifying the brain. For example, if you touch a hot iron, you will pull away from it in a quick reflex response. Another example is of knee-jerk reflex. It occurs when a doctor checks your reflexes by striking your knee in a certain place. Besides controlling reflex, your spinal cord relays information between your brain and various parts of your body.

Nerves: A nerve is actually a bundle of neurons, or nerve cells. Neurons perform one basic task – to carry messages throughout your body. All neurons have a cell body, called cyton that contains a nucleus. You can think of the cell body as a switchboard of the message-carrying neuron. Running into this switchboard are one or more branches of short fibres called dendrons o dendrites, which carry fibre, called an axon, carries messages away from the cell body, when the nerve impulse reaches the end of an axon, it can go no further. The reason it ends is because the axon of one neuron is not directly attached to the dendrite of the next neuron. There is a tiny gap a dendrite. An impulse cannot leap across that gap. But it does not stop either. The tip of the axon produces certain chemicals in tiny bubbles. When an impulse reaches the end of an axon, a bubble bursts and the chemicals move across the synapse and trigger a new impulse in a dendrite. The dendrite sends the impulse to its cell body, which leads to its axon and then to a synapse, and so forth.

Types of neurons: There are three main types of neurons in the human nervous system – sensory neurons, motor neurons and inter neurons.

• Sensory neurons carry messages from special receptors to the central nervous system. Receptors are the parts of the nervous system that respond to stimuli. For example, your eyes and ears are receptors that respond to glaring light and an alarm bell respectively.
• Motor neurons carry messages from your central system to effectors. Effectors are the parts of the body that carry out the instructions of the nervous system. Obviously, if the activities of sensory and motor neurons are to be coordinated, a third kind of neuron must connect them.
• Inter neurons connect sensory and motor neurons.

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