10.3B: Classification of Neurons
Neurons can be classified by direction of travel, neurotransmitter utilized, or electrophysiological properties.
- Describe the various methods used to classify neurons
Key Points
- Neurons can be classified by the direction of the action potential or route by which information travels. Afferent neurons convey information from tissues and organs to the brain and efferent signals transmit information from the brain to effector cells in the body.
- Neurons can have excitatory, inhibitory, or modulatory effects on target neurons depending on the neurotransmitter they release.
- Afferent neurons convey information from tissues and organs into the central nervous system.
- Interneurons connect neurons within specific regions of the central nervous system.
- Efferent neurons carry information away from a brain region.
Key Terms
- efferent : Efferent neurons transmit signals from the central nervous system to the effector cells (e.g. motor neurons).
- afferent : Afferent neurons convey information from tissues and organs into the central nervous system (e.g. sensory neurons).
- tonic or regular spiking : Neurons that are typically constantly (or tonically) active are called tonic or regular spiking.
Direction of Nerve Impulse
The functional classification of neurons is based on the direction the action potential (i.e. information) travel relative to the central nervous system. Afferent neurons convey information from tissues and organs into the central nervous system (e.g. sensory neurons). Efferent neurons transmit signals from the central nervous system (CNS) to the effector cells (e.g. motor neurons ). Afferent and efferent also refer generally to neurons that bring information to or send information from a brain region. Interneurons connect neurons within specific regions of the central nervous system.
The organization of the nervous system : Gross organization of the nervous system, with the peripheral nervous system, the spinal, and the cortical levels.
Neurotransmitter Type
Neurons are also classified by their effect on target neurons. A neuron releases a neurotransmitter that binds to chemical receptors on the target neuron. The combination of neurotransmitter and receptor properties results in an excitatory, inhibitory, or modulatory change to the target neuron. For example, the two most common neurotransmitters in the brain (released by 90% of neurons), glutamate and GABA, have opposing actions. Glutamate acts on several different types of receptors with largely excitatory effects. GABA acts on several different classes of receptors to exert inhibitory effects. Other types of neurons include excitatory motor neurons in the spinal cord that release acetylcholine and inhibitory spinal neurons that release glycine.
Major elements in neuron-to-neuron communication : Electrical impulses travel along the axon of a neuron. When this signal reaches a synapse, it provokes release of neurotransmitter molecules, which bind to receptor molecules located in the the target cell.
Firing Properties
A third, less common method of neuron classification is according to their electrophysiological characteristics. Neurons that are typically constantly (or tonically) active are called tonic or regular spiking. Neurons that are intermittently active are called phasic or bursting. Neurons with high activity rates are classified as fast spiking.
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