2.3.1: Neurons

Structure of a Neuron

The structural characteristics of neurons play a pivotal role in determining their functional importance. The intricate morphology of these cells is crucial for establishing the network of interconnections inherent in the complex architecture of the nervous system.

The cardinal component of a neuron is the cell body, termed the soma (soma = "body"). This nucleus-encasing structure encompasses a majority of pivotal organelles. The soma emits numerous projections of its cell membrane, collectively denoted as processes. Neurons are conventionally characterized by a singular axon—an extension originating from the cell body and extending toward target cells. This solitary axon can undergo extensive branching, facilitating communication with multiple target cells. The nerve impulse propagation, crucial for intercellular communication, transpires along the axon to reach one or more recipient cells. The neuron's supplementary processes are dendrites, which adeptly intercept information from other neurons at specialized contact points called synapses. Dendrites exhibit intricate branching patterns, affording prime locations for reciprocal neural communication with the cell body. Information follows a sequential trajectory through the neuron: from dendrites, traversing the cell body, and subsequently coursing along the axon. This established pattern endows the neuron with polarity, signifying the unidirectional flow of information. A visual representation of the spatial interrelation of these components is portrayed in Figure 2.

A distinct region, designated as the axon hillock, marks the emergence of the axon from the cell body. The axon hillock manifests as a tapered extension of the cell body directed toward the axon. Enclosed within the axon hillock, the cytoplasm transitions, forming a composition of limited constituents known as axoplasm. This pivotal juncture signifies the inception of the axon and is concurrently termed the initial segment. Remarkably sensitive to the initiation of nerve impulses, the axon hillock boasts an elevated abundance of voltage-gated sodium and potassium ion channels.

Most axons are enveloped by an insulating myelin sheath, an entity synthesized by glial cells analogous to the insulation encasing electrical wires. A distinguishing feature between myelin and wire insulation is the presence of intervals or voids within the myelin sheath covering the axon. These intervals, known as nodes of Ranvier, bear significance in transmitting electrical signals down the axon.

The terminal phase of the axon culminates in the axon terminal. Typically, multiple branches extend toward the target cell from the axon terminal, ending in enlarged structures called synaptic end bulbs or synaptic knobs. These bulbs establish the junction with the target cell at the synapse.

Types of Neurons

The human body contains billions of neurons, each falling into different classifications. One type involves examining processes attached to the cell body.

Structural Classification of Neurons

Unipolar neurons, also known as pseudounipolar neurons, are a type of sensory neuron with a single process emerging from the cell body (Figure 3). This single process splits into two branches, one acting as a dendrite to receive sensory input and the other functioning as an axon to transmit the information to the CNS. Ganglia frequently house the cell bodies of unipolar neurons, especially those involved in transmitting sensory signals from peripheral receptors to the CNS.

Bipolar neurons encompass two processes emanating from opposing poles of the cell body (Figure 3). One process is designated as the axon, while the other functions as a dendrite. Examples of bipolar neurons include specialized sensory receptors, such as olfactory receptors within the olfactory epithelium, photoreceptors (rods and cones) within the retina, and hair cells within the cochlea, vestibule, and semicircular canals.

Multipolar neurons possess a single axon and two or more dendrites (Figure 3). Multipolar neurons primarily serve motor functions.

Functional Classification of Neurons

Neurons can be classified based on their functions into sensory (afferent), motor (efferent), and interneurons (association neurons). As the name suggests, sensory neurons transmit sensory signals from the PNS to the CNS. Motor neurons, conversely, facilitate the conveyance of motor signals from the CNS to the PNS. Interneurons, exclusively confined to the CNS, contribute to processing received signals and the modulation of impulses slated for release via motor neurons (Figure 4).

One of the fundamental functions of the nervous system is to enable the communication of information between the CNS and the PNS. An illustration of a sensory and a motor pathway is utilized here to elucidate how a sensory signal is communicated (Figure 5 ).

Located within the skin portion of the fingers are thermoreceptors adept at discerning alterations in thermal conditions. By subjecting the hand to the influence of heat, for example, the cell membrane of these thermoreceptors undergoes a transition in its electrochemical state, characterized by shifts in voltage. Should the stimulus exhibit considerable strength, the voltage alteration in the cellular membrane surpasses a critical threshold, thus eliciting the initiation of an electrical impulse, referred to as an action potential, which undertakes a process termed propagation, spreading along the axon to its ultimate destination of the cerebral cortex via the spinal cord, brainstem, and the thalamus, wherein conscious perception of the temperature occurs. Within the confines of the cerebral cortex, intricate processing occurs across a complex network of neurons, integrating the temperature stimulus. Eventually, a decision is reached concerning whether to augment the temperature or withdraw the hand. Enacting any of these choices mandates the cerebral cortex to disseminate directives that initiate muscular movement, as indicated in Figure 5.