Classes of Neurons: Structure and Function

Classes of Neurons

Neurons are specialized cells of the nervous system responsible for transmitting information throughout the body. They can be classified based on their structure and function, which is essential for understanding how the nervous system processes and relays information.

Structural Classes of Neurons

Neurons are categorized structurally by the number and arrangement of their processes (axons and dendrites). The main structural types are:

Multipolar Neurons

• Definition: Neurons with multiple dendrites and a single axon extending from the cell body.

• Prevalence: This is the most common type of neuron in the human body.

• Examples: All motor neurons and most interneurons are multipolar.

• Function: Well-suited for integrating information from multiple sources and transmitting it to other neurons or effector cells.

Bipolar Neurons

• Definition: Neurons with two processes extending from the cell body: one axon and one dendrite.

• Location: Found in specialized sensory organs.

• Examples: Neurons involved in vision (retina) and sense of smell (olfactory epithelium).

Unipolar (Pseudounipolar) Neurons

• Definition: Neurons with a single process that branches into two: one part acts as a dendrite, the other as an axon.

• Location: Most commonly found in the peripheral nervous system (PNS).

• Function: Typically serve as sensory neurons.

Anaxonic Neurons

• Definition: Neurons with many dendrites but no distinct axon.

• Location: Rare, mostly found in the brain and some special sense organs.

• Function: Their precise function is not fully understood, but they are thought to be involved in local processing within neural circuits.

Functional Classes of Neurons

Functionally, neurons are classified by the direction in which they transmit impulses relative to the central nervous system (CNS):

Sensory (Afferent) Neurons

• Definition: Neurons that carry information from sensory receptors toward the CNS.

• Types:

  • Somatic Sensory Neurons: Transmit information from receptors in the skin, muscles, and joints.

  • Visceral Sensory Neurons: Transmit information from receptors in internal organs (organ systems).

• Example: Sensory neurons in the skin detect touch and send signals to the spinal cord and brain.

Motor (Efferent) Neurons

• Definition: Neurons that carry commands from the CNS to effectors (muscles or glands).

• Types:

  • Somatic Motor Neurons: Control voluntary movements by innervating skeletal muscle.

  • Visceral Motor Neurons: Control involuntary responses by innervating cardiac muscle, smooth muscle, and glands (part of the autonomic nervous system).

• Example: Motor neurons stimulate muscle contraction to produce movement.

Interneurons (Association Neurons)

• Definition: Neurons located entirely within the CNS that process information and coordinate responses between sensory and motor neurons.

• Prevalence: The most numerous type of neuron in the CNS.

• Function: Responsible for higher functions such as learning, memory, and decision-making.

Neuronal Circuits and Processing Patterns

Neurons are organized into functional groups called neuronal pools, which process information in specific patterns:

Diverging Circuit

• Definition: Information spreads from one neuron to several others, allowing a single input to be distributed to multiple outputs.

• Example: Sensory input from the eye is distributed to different areas of the brain for processing.

Converging Circuit

• Definition: Multiple neurons synapse on a single postsynaptic neuron, allowing integration of information from various sources.

• Example: Control of breathing involves both voluntary and involuntary inputs converging on the same motor neurons.

Parallel Processing

• Definition: Several neurons process the same information simultaneously, enabling multiple responses to occur at once.

• Example: Touching a hot object triggers both a withdrawal reflex and a conscious perception of pain.

Serial Processing

• Definition: Information is relayed in a stepwise fashion from one neuron to the next in a linear sequence.

• Example: Transmission of sensory information from the spinal cord to the brain.

Reverberation

• Definition: A feedback loop where information is sent back to the original source, maintaining activity within the circuit.

• Function: Important for processes that require continuous stimulation, such as breathing.

Parallel-After-Discharge Circuit

• Definition: Neurons stimulate other neurons in parallel pathways, all converging on a common output neuron, allowing for complex processing and prolonged responses.

• Example: Involved in higher-order thinking and problem-solving.

Summary Table: Structural and Functional Classes of Neurons

Structural Class	Key Features	Location/Function
Multipolar	Many dendrites, one axon	Most common; motor neurons, interneurons
Bipolar	One dendrite, one axon	Sensory organs (retina, olfactory epithelium)
Unipolar	Single process splits into two branches	Sensory neurons in PNS
Anaxonic	Many dendrites, no axon	Brain, special sense organs

Functional Class	Direction of Impulse	Example
Sensory (Afferent)	From receptors to CNS	Touch receptors in skin
Motor (Efferent)	From CNS to effectors	Motor neurons to muscles
Interneurons	Within CNS	Processing in brain and spinal cord