neuron

Neurons are also known as nerve cells. Nearly 86 billion nerve cells work together within the nervous system to communicate with the rest of the body.

LEARNING OBJECTIVE

In this lesson, you’ll learn about

• What are neurons?
• Structure of neuron
• Different classes of neurons – motor, sensory and interneurons
• Understanding the mechanism of how neuron respond to stimuli such as touch, sound, or light

WHAT ARE NEURONS?

Neurons are the specialized cells that transmit chemical and electrical signals in the brain; they are the basic building blocks of the central nervous system. They send and receive signals with allow us to move our muscles, feel our surroundings, remember things and much more.

STRUCTURE OF A NEURON

A neuron has four main parts:

• Cell body – Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, and other organelles.
• Dendrites – Dendrites are branch-like structures extending away from the cell body, and their job is to receive nerve impulses from other neurons and allow those messages to travel to the cell body.
• Axon – An axon is a tube-like structure that passes the nerve impulses on to other cells.
• Synapse – The synapse is the chemical junction between the axon terminals of one neuron and the dendrites of the next. It is a gap where specialized chemical interactions can occur.

A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells but only one axon.

Myelin sheath

The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The Node of Ranvier is any gap within the myelin sheath exposing the neuron, and it allows even faster transmission of a signal.

Glial cells

Myelin is produced by glial cells which are non-neuronal cells that provide support for the nervous system. Glia function to hold neurons in place, supply them with nutrients, provide insulation, and remove pathogens and dead neurons. In the central nervous system, the glial cells that form the myelin sheath are called oligodendrocytes; in the peripheral nervous system, they are called Schwann cells.

1. Dendrite
2. Cell Body
3. Node of Ranvier
4. Axon Terminal
5. Schwann Cell
6. Myelin Sheath
7. Axon
8. Nucleus

CLASSES OF NEURONS

Based on their roles, the neurons can be divided into three classes:

• Sensory neurons – They get information about what’s going on inside and outside of the body and bring that information into the CNS so it can be processed. Sensory neurons carry electrical signals (impulses) from receptors or sense organs to the CNS, and they are also called afferent neurons.
• Motor neurons – They get information from other neurons and convey commands to your muscles, organs, and glands. They carry impulses from the CNS to effector organs; and are also called efferent neurons.
• Interneurons – They are found only in the CNS, connect one neuron to another. They receive information from other neurons (either sensory neurons or interneurons) and transmit information to other neurons (either motor neurons or interneurons).

Interneurons are the most numerous class of neurons and are involved in processing information, both in simple reflex circuits and in more complex circuits in the brain.

THE RESPONSE MECHANISM

When a stimulus is received by a sensory neuron, the impulse is carried through dendrites to the cell body. The impulse travels through the cell body and is carried through the axon to the end brush, a collection of fibers that extend off the axon. Here, the impulse triggers a release of chemicals that allow the impulse to travel through the synapse. An impulse travels along the neuron pathways as electrical charges move across each neural cell membrane. Ions moving across the membrane cause the impulse to move along the nerve cells.

The difference in the number of positively and negatively charged ions causes electrical charge on each side of the cell membrane – this produces a resting potential. Neurons have a resting potential of approximately 70 millivolts (mV).

Specifically, the cell membrane proteins pump sodium ions (Na+) out of the neuron and pump potassium ions (K+) into the neuron. This movement of both the Na+ and K+ ions produces a negative charge on the inside of the neuron’s cell membrane.

When a neuron is stimulated by another neuron or by an environmental stimulus, an impulse begins. The cell membranes begin to change the flow of ions and a reversal of charges result in the ‘action potential’. An impulse that changes one neuron, changes the next. This is how the impulse moves along the pathway.