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Control and Coordination in Humans – Overview of the Nervous System
There are two systems of coordination of activities in humans. These are the Nervous system, and the Endocrine system (or Hormonal system).
In human beings, the nervous system and endocrine system work together to control and coordinate all our activities such as our physical actions, our thinking processes, and our emotional behaviour. Both the systems of coordination, nervous system, and endocrine system, consist of a number of organs working together in a systematic way.
Human Nervous System
The function of the nervous system is to coordinate the activities of our body. It is the control system for all our actions, thinking, and behaviour. The nervous system helps all other systems of our body to work together. The nervous system is like a manager inside our body. Its job is to control and coordinate the parts of our body so that they work together, doing their job at the right time. Our nervous system coordinates muscles so that we can do things that need thinking like reading, writing, cycling, or dancing.
The nervous system also coordinates things that we don’t have to think about, like heartbeat and breathing. The human nervous system receives information from the surroundings, processes it, interprets it, and then responds accordingly. The nervous system also passes information from one internal system to another. For example, as soon as we put food in our mouth, it immediately causes the release of saliva from the salivary glands.
The Unit of Nervous System: Neuron
The units which make up the nervous system are called nerve cells or neurons. So, the neuron is the structural and functional unit of the nervous system. We can now say that the nervous system is made of special cells called neurons. The neuron is the largest cell in the body (which looks like an electric wire). Neurons contain the same basic parts as any other animal cell but their structure is specially adapted to be able to carry messages over large distances in the body quickly. The neurons carry messages in the form of electrical signals called electrical impulses or nerve impulses. A neuron is shown in Figure.
A neuron (or nerve cell) has three components Cell body, Dendrites, and Axon. The cell-body of a neuron is like a typical animal cell which contains cytoplasm and a nucleus (see Figure). A number of long and thin fibres are stretching out from the cell body of a neuron. They are called nerve fibres. The shorter fibres on the body of a neuron are called dendrites. The longest fibre on the cell body of a neuron is called an axon. The axon has an insulating and protective sheath (or cover) of myelin around it (Myelin is made of fat and protein). It is clear that both dendrites and axons arise from the cell body of a neuron.
The messages which the neurons transmit in the nervous system are in the form of electrical impulses called nerve impulses (or just impulses). The dendrites pick up the nerve impulses (or messages) from receptors. They pass the impulses to the cell body and then along the axon. The axon passes the impulse (or message) to another neuron through a junction called a synapse. Neurons are of three types sensory neurons, motor neurons, and relay neurons.
- Sensory neurons transmit impulses from the sensory cells (or receptors) toward the central nervous system (spinal cord and brain).
- Motor neurons transmit impulses from the central nervous system (spinal cord and brain) toward the muscle cells (or effectors).
- Relay neurons occur in the central nervous system (brain and spinal cord) where they serve as links between other neurons.
Nerve cells have long thin fibres which carry ‘messages’ around our body.
This picture shows a highly enlarged photograph of motor nerve endings (or motor neuron endings) attached to the muscle.
We will now explain how nerve impulses (or messages) are transferred from one neuron to another in the nervous system. Any two neurons in the nervous system do not join with one another completely. There is always a very, very small gap between the two neurons (where they join). This gap is called a synapse. The nerve impulses are carried over this small gap between a pair of neurons by means of a chemical substance called neurotransmitter substance. We can now say that A microscopic gap between a pair of adjacent neurons over which nerve impulses pass when going from one neuron to the next is called a synapse. Thus, synapses connect neurons (though it looks surprising that even gaps can connect two things!). We will now understand the conduction of electrical nerve impulses through synapses with the help of a diagram.
Suppose there are two neurons (or nerve cells) A and B near each other (see Figure). Let A be a sensory neuron that is directly connected to the receptor. There is an extremely small, microscopic gap between the end of the axon of neuron A and the dendrite of the next neuron B which is called a synapse (see Figure). We will now explain how the electrical impulse travels through the gap (synapse) between the two neurons.
The diagram shows how electrical impulses (or nerve impulses) are conducted from one neuron to another across synapses (gaps) between them by the release of a chemical substance (called a neurotransmitter substance).
The receptor in a sense organ is in touch with the dendrites of sensory neurons. When a stimulus acts on the receptor, a chemical reaction is set off which produces an electrical impulse in it. This impulse travels from the dendrite of sensory neuron A to its cell body and then along its axon. At the end of the axon of sensory neuron A, the electrical impulse releases a tiny amount of a chemical substance into the synapse (or gap). This chemical substance crosses the gap (or synapse) and starts a similar electrical impulse in the dendrite of the next neuron B (see Figure).
From the dendrite, this electrical impulse is carried to the cell body and then to the end of the axon of the second neuron. It can then be transferred to a third neuron in a similar way. This process goes on till the electrical impulse reaches the relay neurons in the spinal cord and brain. The relay neurons and motor neurons connect in a similar way to bring electrical impulses from the brain and spinal cord to the effectors like muscles and glands.
Synapses actually act like one-way valves. This is because the chemical substance is present on only one side of the gap. Due to this, the nerve impulses (or messages) through a particular set of neurons can go across only from one side (which contains the chemical substance). In this way, synapses ensure that nerve impulses travel in only one direction (through a particular set of neurons).
The Organs of the Human Nervous System
The main organs of the nervous system are Brain, Spinal cord, and Nerves. The sense organs like eyes, ears, tongue, nose, and skin can be considered to be other organs of the nervous system because they help in the functioning of the nervous system. The main organs of the nervous system are shown in Figure. The brain is located inside the skull of our head. The spinal cord is a very thick nerve that runs inside the cavity of the backbone of our body (see Figure). The upper end of the spinal cord is attached to the brain. The nerves are kind of wires which are distributed all over our body.
The brain and spinal cord are connected to all the sense organs and other parts of our body by millions of nerves. There are mainly two types of nerves in our body: cranial nerves and spinal nerves. The cranial nerves connect all the parts of the head directly to the brain. The spinal nerves connect all the remaining parts of the body (like muscles and skin, etc.) to the spinal cord (see Figure). There is also a third type of nerves called visceral nerves.
Most of the visceral nerves connect the internal organs of the body to the spinal cord though some also connect to the brain (Visceral nerves have not been shown in the Figure to keep the diagram simple and avoid confusion). The cranial nerves, spinal nerves, and visceral nerves are also of two types sensory nerves and motor nerves. The nerves which carry messages ‘from the body parts to the brain (or spinal cord) are called sensory nerves. And the nerves which carry messages from the brain (or spinal cord) to the body parts for action are called motor nerves.
This is how the nervous system works: When the sense organ (like eyes, ears, tongue, nose, or skin) in our body is affected, it sends the message to the brain in the form of electrical impulses (called nerve impulses) through the sensory neurons. The brain analyses this message and decides the action to be taken. The brain then sends out instructions to the muscles of the concerned body part (for taking necessary action) through motor nerves. The concerned body part then acts according to the instructions sent by the brain. Please note that in the processing of complicated responses (which require thinking) both, the brain and spinal cord are involved, but in the simple responses (which do not require thinking), the spinal cord alone is involved.
The Parts of the Nervous System
The nervous system can be divided into two main parts: The central nervous system (consisting of the brain and spinal cord), and the Peripheral nervous system (consisting of all the nerves of the body like cranial nerves, spinal nerves, and visceral nerves).
The peripheral nervous system can be further divided into two parts: The voluntary nervous system (which is under voluntary control from the brain), and the Autonomic nervous system (which operates automatically or involuntarily).
The classification of the nervous system into various parts is given in the following chart: