Neurons-Reflex action and Reflex arc-Types of Neurons-Motor neurons-Sensory neurons-Effectors-Receptors

Neurons:

The central Nervous system (which includes the brain and the spinal cord) is made up of cells called Neurons.Neurons are designed to transmit information to other nerve cells,muscles, or glands cells.Neurons send message in the form of nerve impulse. Nerve impulses are electrical signals that travel along a neuron.It is responsible for communication between different types of neurons and to give orders to effectors. Speed of nerve impulse is about 100m/s.

Parts of Neurons:---------------------------------------------------------------------------------------

Cell Body: The cell body has a nucleus and most of the organelles of the cell.

Dendrites:  The small extensions of the cell body are called dendrites.They receive messages from other cell and conduct them towards the cell body.

Axon:       Axon is also called the nerve fibre. It is a portion of a neuron that carries nerve impulses away from the cell body/ The axon receive message from the cell body and conducts it away  to other end of cell.

Myelin   Sheath:     It consists of fat-containing cells that insulate the axon. It does not allow  messages to pass into the neighbouring neuron.

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Types of Neurons:

There are three types of neurons:  sensory neuron,motor neuron and interneuron. The sensory neurons  carry impulses (message)from the sense organs like the eye, nose or skin to the central nervous system. The motor neurons carry impulses (orders)from the central nervous system to the muscles or glands. The interneuron make  connections with the sensory neuron and motor neurons. In this way, they enable the impulses to be transmitted from sensory neurons to motor neurons.

---------------Reflex action and reflex arc----------------------------------------

Introduction:

A reflex action is an automatic, involuntary and rapid response to a stimulus, which minimizes any damage to the body from potentially harmful conditions,such as touching something hot. Reflex actions are therefore essential to the survival of many organisms. The path taken by the nerve impulses in a reflex is called a reflex arc.

Examples:

Let's understand Reflex action with examples. The pupil of the eye changes its size when light act as a stimulus; when a pin pricks our hands or legs or we touch something very hot, we suddenly withdraw them; we cough or sneeze when foreign irritants go to our nasal passages.

Reflex arcs have five basic parts: receptors, sensory neurons, interneurons, motor neurons,and effectors. Lets discuss with the help of pin prick example. 

1. Receptors  located in the skin detect pin prick as a stimulus  and create a nerve impulse.

2.Sensory Neurons conduct nerve impulses towards spinal cord.

3. Interneurons in the spinal cord  connect sensory neurons to motor neurons. One or more interneurons in the spinal cord carry messages from a sensory neuron to a motor neuron. 

4. Motor neurons carries messages from the spinal cord to an effector.

5. Effectors respond to the impulses by contracting (if the effector is a muscle fiber) or secreting enzymes or hormones (if the effector is a gland). In this case the muscles in  arms contract to suddenly withdraw hand from pin.