Neurons

The basic unit of nervous tissue is the cell called the Neuron.

Neuron

Neurons consist of a main part called the cell body, which contains the nucleus and various organelles.

Cell Body

Nucleus

Nucleolus

Neurons also contain cell body “extensions” called processes, which carry impulses to and from the cell body.

axon

dendrites

Two different types of processes can come from the cell body. The first is called the dendrite and the other the axon.

Dendrites, also called afferent processes, carry impulses TOWARDS the cell body.

Axons, also called efferent processes, carry impulses AWAY from the cell body.

There are actually three different types of neurons in the human body. They can be classified either according to their function or structure:Functional Classification:

Structural Classification:

1. Sensory Neurons2. Motor Neurons3. Interneurons (Association Neurons)

1. Unipolar Neurons2. Bipolar Neurons3. Multipolar Neurons

Sensory neurons or Afferent neurons – These neurons transmit impulses from the periphery of the body to the central nervous system. They are described as unipolar (optic neurons are often bipolar), meaning they have only a single process. This process is actually an axon which branches into two parts and spread in opposite directions. In this way, one branch acts as the dendrite, while the other acts as the axon. These neurons are usually concentrated in areas called ganglia and their dendrite branches extend to the skin or to sensory organs and act as sensory receptors (either directly or indirectly).

Motor neurons or Efferent neurons – These neurons carry impulses AWAY from the cell body and thus the central nervous system to muscles, gland, or some other “effector” to produce a certain action. This action can be the contraction of a muscle or the secretion of a gland. Motor neurons are classified as multipolar, which means they have numerous branching dendrites leading into the cell body and a single long axon leading out. Most of the neurons in the spinal cord and many of those in the brain are motor neurons.

Interneurons or Association neurons- This type of neuron is restricted to the central nervous system. They are also called connector neurons. These neurons act as bridges between sensory and motor neurons or relay impulses to various functional centers of the brain or spinal cord. They resemble motor neurons in that they have one axon with multiple dendrites, however, their function is much different.

Sensory Neuron

Interneuron

Motor Neuron