16.6:
Nervous Tissue: Neuron Types
Neurons can be categorized either by their structure or their function.
Structurally, neurons are classified by the number of processes emerging from the cell body.
Multipolar neurons, most abundant in the CNS, have one axon and multiple dendrites emerging from the cell body.
Bipolar neurons have two processes, an axon and a main dendrite that branches distally. They are rare and are found in the retina, inner ear, and olfactory mucosa.
Unipolar or pseudounipolar neurons have a single process consisting of an axon partially fused with dendrites. They are primarily sensory neurons in the PNS with axons extending over long distances and cell bodies in the spinal or cranial ganglia.
Functionally, neurons are categorized as sensory neurons that deliver information from sensory receptors to the CNS, motor neurons that relay instructions from the CNS to peripheral effectors, and interneurons that transmit information between sensory and motor neurons.
Sensory neurons are mostly unipolar and sometimes bipolar, while motor neurons and interneurons are multipolar.
16.6:
Nervous Tissue: Neuron Types
Neurons, the fundamental units of the nervous system, can be classified based on both their structural and functional characteristics.
Structurally, neurons are categorized into three main types: multipolar, bipolar, and unipolar (or pseudounipolar). Multipolar neurons, which are the most common type in the brain and spinal cord, as well as all motor neurons, possess multiple dendrites and a single axon.
Bipolar neurons, on the other hand, have one primary dendrite and one axon. They are typically located in specialized sensory areas such as the retina of the eye, the inner ear, and the olfactory region of the brain. These neurons play crucial roles in sensory perception, processing information about vision, hearing, and smell.
Unipolar or pseudounipolar neurons are unique in that their dendrites and axons fuse together into a single process extending from the cell body. This structure results from neuronal development, where the neuron begins as a bipolar neuron and later merges its dendrites and axon. The dendrites of unipolar neurons often function as sensory receptors, detecting stimuli like touch, pressure, pain, or temperature changes.
Functionally, neurons can be divided into sensory neurons, motor neurons, and interneurons. Sensory neurons, which are primarily unipolar and occasionally bipolar, carry information from sensory receptors to the central nervous system (CNS). Multipolar motor neurons convey commands from the CNS to peripheral effectors such as muscles. Interneurons, also multipolar, serve as intermediaries, transmitting information between sensory and motor neurons.