Neurons

Structural Diversity of Neurons

A number of anatomically neuron types have evolved to participate in different organismal functions.

Learning Objectives

Describe the structural diversity of neurons

Key Takeaways

Key Points

  • Neurons are electrically excitable cells that are the structural unit of the nervous system. 
  • A typical neuron consists of a cell body and neuronal processes such as dendrites and axon. 
  • Neurons can generally be anatomically characterized as unipolar, bipolar, or multipolar. 
  • A number of anatomically distinct neuron types, such as sensory, motor, and interneurons, have evolved to participate in different organismal functions.

Key Terms

  • Neurons: Electrically excitable cells that are the structural unit of the nervous system.
  • dendrites: Short, tapering extensions that convey incoming messages toward the body of the neuron.
  • Axons: The conducting region of the neuron.

Neurons are electrically excitable cells that are the structural unit of the nervous system. A typical neuron consists of a cell body and neuronal processes such as dendrites and axon. The dendrites are short, tapering extensions that are the receptive regions and help in conveying incoming messages towards the cell body. Axons arise from a cone-shaped area of the cell body called axon hillock. These extensions are the conducting region of the neuron. Nerve impulses are generated in the axon and transmitted away from the cell body towards the synapse. The cell body is the major biosynthetic center of the neuron. It contains neurotransmitters and other organelles needed to synthesize proteins and chemicals. The cell body is the focal point for the outgrowth of neuronal process during embryonic development. 

A number of anatomically distinct types of neurons have evolved to participate in different organismal functions. For example, sensory neurons respond to touch, sound, light, and other sensory inputs. Motor neurons receive signals from the brain and spinal cord to initiate muscle contractions and affect glands. Interneurons act as relays between neurons in close proximity to one another.

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Neurotransmission at a Chemical Synapse: A signal propagating down an axon to the cell body and dendrites of the next cell

Neurons can generally be grouped according to the number of processes extending from their cell bodies. Three major neuron groups make up this classification: multipolar, bipolar, and unipolar. Unipolar neurons have a single short process that emerges from the cell body and divides T-like into proximal and distal branches. Bipolar neurons have two processes, an axon and a dendrite, that extend from opposite ends of the soma. Multipolar neurons, the most common type, have one axon and two or more dendrites.

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types of neurons: 1: Unipolar neuron, 2: Bipolar neuron, 3: Multipolar neuron, 4: Pseudounipolar neuron

Classification of Neurons

Neurons can be classified by direction of travel, neurotransmitter utilized, or electrophysiological properties.

Learning Objectives

Describe the various methods used to classify neurons

Key Takeaways

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.

This diagram depicts neuron classification with terms including brain (cortex), pyramidal neuron of motor cortex, sensory neuron of cortex, spinal cord, motor neuron of ventral horn, sensory neuron in dorsal root ganglion, PNS, neuromuscular junction, muscle fiber or myocyte, myelin sheath, ruffinian corpuscle, and paccinian corpuscle

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.

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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.