What is a Neuron? or What is a Nerve Cell?
The basic unit of the nervous system is the nerve cell or neuron. Neurons send and receive messages to and from one another in the brain.
The human brain is composed about of 150 billion nerve cells. Neurons come in a variety of sizes and shapes. It ranges in length from less than a millimeter to more than a meter. Depending upon the fiber the neural impulse travels at speeds from 2 miles per hour to 200 or more miles per hour.
The neuron is one of the important parts of the nervous system, which receives the information from the outside world, send it to our body parts, and helps to make understandable the received information. The greater the number of neurons in the area, the more complex the interconnection among them. Similarly, the fewer neurons in the area, the interconnection is faster and safe.
Types of Neurons
Information travels in the nervous system through three types of neurons: sensory, motor, and interneurons.
Sensory neurons also called afferent neurons to send information from the body’s tissues and sensory organs inward to the brain and spinal cord, which process the information. It helps us to see, hear, taste, feel, smell the things around us.
Motor neurons also called efferent neurons to carry messages going out from the central nervous system to the organs and muscles. Motor neurons usually have longer distances to travel, so they tend to be large.
As its name suggests, inter-neurons enables internal communication within the nervous system. It helps to facilitate the work of sensory and motor neurons by mediating between them so it is also called association neuron.
For example, if you touch a hot object, the impulses travel up a sensory nerve to the spinal cord. Here the association neurons transmit the information to a motor neuron. That in turn, stimulates the muscles of the limb to contract. Thus, inter-neurons usually help to process the behavior, whether to communicate or intervene between sensory inputs and motor outputs.
Functions of the Neuron
- A neuron receives the change that occurs around the environment of an organism.
- It circulates the changed information all over the body.
- Finally, coordinate to adapt to that changed situation.
The Structure of a Neuron
The basic part of a neuron consists of the cell membrane, dendrite, cell body, axon, terminal buttons, myelin sheath, neurotransmitters, and synapse.
Describing the components of a neuron,
The Cell Membrane
The cell membrane is like a fence that surrounds the entire neuron, giving it shape, the cell’s internal fluids inside. It has small pores to allow some substances to pass through it so it is semi-permeable. It excites when gets outer stimulus or irritation and conducts the response from one part of the cell body to the other part called conductivity.
The short branch-like structures of neurons are called dendrites. They receive signals or information from receptors (for example nose, eyes, ears, skin) or from other neurons. Some of these signals are excitatory (pushing like an accelerator). Other signals are inhibitory (like pushing brake), which makes it possible for a single neuron to receive signals from many other neurons. Most neurons have many (even thousands) dendrites.
The Cell Body
The cell body also called soma, is the central part of the neuron. Once a signal has been received by the dendrites, it passes through the cell body.
The cell body contains the cell’s control center the nucleus, which is the career of the genetic influence. The nucleus keeps cells alive and helps in reproduction and divisions. The cell body also has cytoplasm, which shapes the cell, preserves, and provides nourishment. The cytoplasm is made up of different chemical substances. They help in contraction, secretion, and excretion.
The axons are the long fibers on the neuron that transmit electrical signals to other neurons and to muscles and glands. Each neuron has only one axon extending from the cell body. Unlike short dendrites, axons may be very long, projecting several feet long through the body, depending upon the location of the neuron.
For example, neurons of the brain have microscopic lengths while leg muscles are long.
Myelin sheaths are the whitish, fatty protein covering some axons. It helps to speed the neural transmission. Myelin sheaths are composed of glial cells. The sheathe covers the axon except at small, regularly spaced gaps called nodes of Ranvier. A spinal cors has long myelin covered axons, while the brain has fewer myelin sheaths covered axon, which appears grayish, like the color of the rest neurons or brain.
One neuron has only one axon extending the cell body, but before it reaches the end, it may have several branches. The branches make the impulses travel to different neurons. At the end of the axon branch, most axons have several knobs called terminal buttons. The terminal buttons store neurotransmitters in a sac called vesicles. The neurotransmitters are released from vesicles when a chemical and electrical signal reaches there.
When a neuron receives a signal (heat, pressure, light) from adjacent neurons or from sensory receptors it fires or becomes active. This neural impulse is called an action potential. It is a brief electrical charge that travels down the axon. When the action potential reaches the knob-like terminal buttons at an axon’s end, it triggers the release of chemical messengers, called neurotransmitters. A stimulus travels to the dendrite, cell body, axon, terminal buttons neurotransmitters synapse to the dendrite of the neuron, and so on.
Synapse is the microscopic gap between the axon of one neuron and the dendrite of another neuron. Neurons never touch each other, it is the synapse that plays the intermediating, coordinating part between the neurons.
The gap of the synapse helps in controlling the flow of nerve impulses or neurotransmitters. It may be inhibiting or excite the electro-chemical stimulation. Synapses allow adjustment possible with the environment according to the nature of the stimulus.