Short essay on The Autonomic Nervous System

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The term ‘autos’ means ‘self’ and ‘nomos’ means ‘governing’, so ‘autonomic nervous system’ means ‘self governing nervous system’. The autonomic nervous system is that part of the peripheral nervous system which controls the activities of the organs inside our body automatically even without our thinking about them.

The autonomic nervous system is a specific network of nerves in the body which controls the processes like breathing, heart beat, digestion, sweating, etc., that maintain our life and keep us alive.

The nerves of the autonomic nervous system are attached to the smooth muscles of the various internal organs of the human body like head, heart, blood vessels, alimentary canal, lungs, kidneys, urinary bladder, glands and skin, etc. Thus, the autonomic nervous system controls and regulates the functions of the internal organs of our body involuntarily (on its own).

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Voluntary Nervous System :

Those actions which need thinking and which are performed by us knowingly are called voluntary actions. For example, speaking to a friend, writing a letter, dancing, cycling, kicking a football, standing in a room or sitting on a chair, are all voluntary actions. The voluntary nervous system helps us take voluntary actions which are under the conscious control of the brain. We will now give an example to understand the working of voluntary nervous system.

Suppose we are walking down to school at a slow pace. After covering some distance, we look at our watch and find that we are getting late. So, we start walking very fast. We can do this because of our voluntary nervous system as follows :

(i) When our eyes see time on the watch, they send this information to the brain through the sensory nerves.

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(ii) The brain analyses this information and decides that since there is risk of being late to school, so we should walk faster.

(iii) The brain sends the instructions to walk faster to the muscles of our legs through the motor nerves.

(iv) The muscles of the legs act accordingly and make us walk faster.

This is an example of voluntary action and the decision to take this voluntary action has been made by the voluntary nervous system.

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Central Nervous System :

The central nervous system (CNS) consists of the brain and the spinal cord. Like a telephone exchange with ingoing and outgoing wires, it is responsible for coordination and control of the activity of the nervous system. The work of central nervous system is to direct incoming messages to the motor neurons that are connected to the part of the body which will respond to a stimulus.

In complicated responses, the brain and spinal cord are both involved. That is, in complicated responses, central nervous system is involved. The central nervous system enables a person to give a more appropriate and more intelligent response to various situations. By using the central nervous system, a person can vary his behaviour according to the changing situations. This point will become clearer from the following example.

If we pick up a very hot plate in the kitchen (without knowing that it is very hot), then our reflex action produced by the spinal cord alone says that we should pull away our hand (so that our hand is saved from burns).

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But if we pull away our hand, then the plate would drop and break into pieces (and our mother will definitely scold us for breaking the plate!). Now, it is here that the central nervous system involving brain steps in. When the message from our fingers saying that the ‘plate is too hot’ arrives at our central nervous system, there is already another message saying ‘but don’t drop it’ (This is due to the intelligence of the brain). The central nervous system will consider the two messages together. It may then decide to send a message to our muscles to tell them to put down the plate gently and not drop it. This intelligent response has been made possible only due to the central nervous system.

The job of the central nervous system is to collect all the information from all the receptors in our body. This information is added together before messages are sent out to the effectors. In this way, the best action can be taken in a particular set of circumstances. We will now describe the two organs of the central nervous system, brain and spinal cord in detail.

Brain :

Brain is the highest coordinating centre in the body. The brain is located inside the skull of our body (at the top of the spinal cord). It is protected by a bony box in the skull called cranium. The brain is surrounded by three membranes called meninges, which help to protect it. The space between the membranes (or meninges) is filled with a cerebro spinal fluid which protects the brain from mechanical shocks. Pairs of cranial nerves arise from the brain.

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The brain is broadly divided into three regions: forebrain, midbrain and hindbrain. The forebrain consists mainly of cerebrum. The midbrain does not have any further divisions. The hindbrain consists of three centres called pons, cerebellum and medulla. We will now discuss the functions of the forebrain, midbrain and hindbrain. Let us start with cerebrum which is in the forebrain.

The cerebrum (or forebrain) is the main thinking part of the brain. It is the site of our faculties such as learning, reasoning, intelligence, personality and memory. All our thoughts, sensations, actions and movements are controlled by the cerebrum. The cerebrum has different areas for performing different functions.

There are association areas in cerebrum which control thinking and memory. These association areas also store information and experiences. There are sensory areas where information is received from the sense organs like eyes, ears, nose, tongue and skin, and give us the ‘sensation’ or ‘feeling’. Similarly, cerebrum has motor areas from which instructions are sent to muscles to do various types of jobs.

All the voluntary actions of the body are coordinated by the cerebrum. This happens as follows: The cerebrum receives sensory information through the receptors of sense organs. The cerebrum interprets this information in the light of previous experiences and takes a decision which it thinks is right. It then sends out instructions to the motor area (which controls the movement of voluntary muscles) so as to make voluntary muscles move to bring about the appropriate responses.

We will now describe the functions of midbrain. The midbrain controls reflex movements of the head neck and trunk in response to visual and auditory stimuli. It also controls the reflex movements of the eye muscles, changes in pupil size and shape of the eye lens.

We will now describe the functions of the parts of the hindbrain which are pons, cerebellum and medulla. The pons takes part in regulating respiration. The cerebellum helps in maintaining posture and balance of the body. It also enables us to make precise and accurate movements. The cerebellum coordinates smooth body movements such as walking, dancing, riding a bicycle and picking up a pencil, etc. Medulla controls various involuntary actions such as heart beat (blood circulation), breathing, blood pressure and peristaltic movements of alimentary canal. Medulla is also the controlling centre for reflexes such as swallowing, coughing, sneezing, secretion of saliva and vomiting.

Spinal Cord:

Spinal cord is a cylindrical structure. The spinal cord begins in continuation with medulla and extends downwards. It is enclosed in a bony cage called vertebral column. Spinal cord is also surrounded by membranes called meninges. As many as 31 pairs of nerves arise from the spinal cord. The spinal cord is concerned with spinal reflex actions and the conduction of nerve impulses to and from the brain.

Before we end this discussion, we would like to give the various functions of brain. The various functions of brain are as follows:

1. The brain receives information-carrying nerve impulses from all the sensory organs of the body

2. The brain responds to the impulses brought in by sensory organs by sending its own instructions (through motor nerves) to the muscles and glands causing them to function accordingly.

3. The brain correlates the various stimuli from different sense organs and produces the most appropriate and intelligent response.

4. The brain coordinates the body activities so that the mechanisms and chemical reactions of the body work together efficiently.

5. The brain stores ‘information’ so that behaviour can be modified according to the past experience. This function makes the brain the organ of thought and intelligence.

Let us answer some questions now.

Sample Problem 1:

The gap between two neurons is called a:

(a) dendrite (b) synapse

(c) axon (d) impulse

Answer:

(b) synapse.

Sample Problem 2:

The brain is responsible for:

(a) thinking (b) regulating the heart beat

(c) balancing the body (d) all of the above

Answer:

(d) all of the above.

Before we describe the hormonal system or endocrine system for the coordination in human beings, we should know the meanings of two terms: hormones and endocrine glands. So, let us first discuss hormones and endocrine glands.

Classification of Hormones :

Hormones are chemical substances secreted in very small amounts by specialised tissues in the body called endocrine glands. These hormones coordinate the activities of living organisms and also their growth. So, we can now say that: Hormones are the chemical substances which coordinate the activities of living organisms and also their growth. Hormones are made inside the body of an organism in very small amounts. The various characteristics of hormones are given below:

1. The hormones are secreted in small amounts by the endocrine glands.

2. The hormones are poured directly into the blood and carried throughout the body by blood circulatory system.

3. The hormones have their effect at the sites different from the sites where they are made. So, they are also called chemical messengers.

4. The hormones act on specific tissues or organs (called target organs).

5. The hormones coordinate the activities of the body and also its growth.

Endocrine Glands:

A gland is a structure which secretes a specific substance (or substances) in the body. A gland is made up of a group of cells or tissue. There are two types of glands in the body:

(i) exocrine glands, and

(ii) endocrine glands.

A gland which secretes its product into a duct (or tube) is called an exocrine gland. For example, the salivary gland secretes the saliva into a duct called salivary duct, therefore, salivary gland is an exocrine gland. Thus, exocrine glands are the glands having ducts. A gland which does not have a duct and secretes its product directly into the blood stream is called an endocrine gland.

Thus, endocrine glands are ductless glands. An endocrine gland secretes a chemical substance called hormone. We can now say that: A structure (group of cells or tissue) which makes hormones in the body is called an endocrine gland. The various endocrine glands present in the human body.

The endocrine glands do not have ducts to secrete their hormones, so they are also called ductless glands. The endocrine glands release hormones directly into the blood of a person. These hormones reach the concerned body part through the blood and act on it.

Hormones are a kind of chemical messengers. A hormone is produced in one part of the body but it acts on some other part of the body. The hormones are of different types and perform different functions.

Some of the glands in our body have both exocrine and endocrine functions. The pancreas, testes and ovary are such glands. For example, pancreas acts as an endocrine gland and secretes the hormone insulin, It also acts as an exocrine gland and secretes pancreatic juice containing digestive enzymes into the pancreatic duct that leads to the alimentary canal. The testes are glands which act as endocrine glands and secrete the hormone called testosterone. They act as exocrine glands and release sperms (male sex cells) into the duct. Similarly, ovaries are glands which act as endocrine glands and secrete the hormones oestrogen (read as ‘estrogen’) and progesterone. They act as exocrine glands and release ova or eggs (female sex cells) into the duct.

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