Final answer:
Monopolar neurons, predominantly found in invertebrates, have a single process that extends from the soma. They are key in stimulating muscles or glands and are exclusively sensory neurons. Humans have pseudounipolar neurons, which are similar to unipolar but with a split process interacting with the CNS for sensory transmission.
Step-by-step explanation:
Neurons are the fundamental building blocks of the nervous system and are crucial for processing and transmitting information throughout the body. There are four prevalent types of neurons: unipolar, bipolar, multipolar, and pseudounipolar.
Unipolar neurons, also known as unipolar, are characterized by a single process extending from the soma (cell body). Contrary to their designation, true unipolar neurons are not present in vertebrates; instead, they predominantly exist in invertebrates, such as insects, where they activate muscles or glands. These neurons are distinct because they do not possess dendrites. The axon emanating from the unipolar cell body splits, enabling the neuron to cover extended distances; dendrites are usually found on one end, while the other end is equipped to form synaptic connections with target cells.
It's important to note that what we sometimes refer to as unipolar neurons in humans are more accurately described as pseudounipolar neurons. These neurons exhibit characteristics akin to both unipolar and bipolar neurons. A pseudounipolar neuron originates with a single process like a unipolar neuron but later divides into two pathways like a bipolar neuron. These are predominantly sensory neurons, with one branch associated with dendrites that intercept sensory stimuli and another transmitting this sensory data to the spinal cord.
Found exclusively within sensory pathways, unipolar neurons have two principal features. Firstly, their dendrites receive sensory information, sometimes directly interacting with stimuli. Secondly, the cell bodies of unipolar neurons are consistently located within ganglia, which are clusters of nerve cells found outside the central nervous system (CNS).
Bipolar neurons, which exhibit one axon and one dendrite, serve as communication relays in regions such as the retina. Meanwhile, multipolar neurons, the most widespread neuronal type, showcase one axon and numerous dendrites and predominantly populate the CNS.