Nervous Tissue: Neurons and Neuroglia

The study of nervous tissue ranks among the most transformative chapters in biomedical science. For centuries, the brain and nerves were understood only in the broadest strokes—Galen proposed that animal spirits flowed through hollow nerves, and Renaissance anatomists could trace nerve trunks with their scalpels yet had no means of seeing the cells within them. It was not until optical microscopy matured in the nineteenth century that scientists began to resolve the fine structure of the nervous system, setting the stage for the neuron doctrine—the foundational principle that discrete, individual cells called neurons are the structural and functional units of the nervous system. Understanding this history is essential because it reveals why we categorize nervous tissue into two broad populations: the signal-carrying neurons and the supportive neuroglia, a distinction that continues to guide modern neuroscience research and clinical neurology.

The central question that this lesson addresses is deceptively simple: what are the cells that compose nervous tissue, and how do their structures relate to their functions? Answering this question requires an appreciation of two distinct but interdependent cell populations—neurons and neuroglia—whose architecture is among the most specialized in the human body.

Nervous tissue is one of the four primary tissue types in the body, distinguished by its capacity for excitability (the ability to generate electrical signals in response to stimuli) and conductivity (the ability to propagate those signals over distance). The tissue is found in the brain, spinal cord, and peripheral nerves, forming the structural basis of the central nervous system (CNS) and the peripheral nervous system (PNS). Two broad categories of cells populate nervous tissue: neurons, which are responsible for transmitting electrical impulses, and neuroglia (also called glial cells), which support, protect, insulate, and nourish the neurons. Although neurons receive the lion's share of attention, glial cells outnumber them in many regions and are indispensable for normal nervous system function.

The diagram above captures the essential architecture of a multipolar neuron, the most common structural type in the CNS. The dendrites are highly branched cytoplasmic extensions that dramatically increase the surface area available for receiving signals from other neurons. They funnel graded (local) potentials toward the cell body (soma), which houses the nucleus and the major biosynthetic machinery of the neuron, including abundant rough endoplasmic reticulum aggregates called Nissl bodies. At the junction of the soma and the axon lies the axon hillock, a region with a high density of voltage-gated sodium channels that functions as the neuron's 'trigger zone'—it is here that the decision to fire an action potential is made when the threshold voltage is reached. The axon then propagates the action potential away from the soma toward the axon terminals (synaptic boutons), where vesicles filled with neurotransmitter fuse with the membrane and release their contents into the synaptic cleft.

The functional significance of neuronal architecture becomes clear when we examine the mechanism by which signals are generated and propagated. At rest, a neuron maintains a resting membrane potential of approximately −70 mV, established primarily by the differential distribution of Na⁺ and K⁺ ions across the plasma membrane and the activity of the Na⁺/K⁺-ATPase pump. When a stimulus depolarizes the membrane at the axon hillock to the threshold potential (approximately −55 mV), voltage-gated Na⁺ channels open rapidly, producing the upstroke of the action potential. This all-or-nothing event propagates along the axon and, in myelinated fibers, 'jumps' from one node of Ranvier to the next in a rapid mode called saltatory conduction.

Once the action potential reaches the axon terminal, synaptic transmission converts the electrical signal into a chemical one. Voltage-gated Ca²⁺ channels open, calcium influx triggers fusion of synaptic vesicles with the presynaptic membrane, and neurotransmitter molecules (e.g., acetylcholine, glutamate, GABA) diffuse across the synaptic cleft (a gap of approximately 20–40 nm) to bind receptors on the postsynaptic membrane. This binding opens or closes ion channels on the postsynaptic cell, producing either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). The integration of thousands of EPSPs and IPSPs at the axon hillock determines whether the postsynaptic neuron will fire its own action potential, a process known as summation.

Structural Classification of Neurons

Functional Classification of Neurons

Functionally, neurons fall into three categories. Sensory (afferent) neurons carry information from receptors toward the CNS; most are pseudounipolar. Motor (efferent) neurons transmit commands from the CNS to effectors (muscles and glands); they are typically multipolar. Interneurons (association neurons) reside entirely within the CNS and integrate information between sensory and motor pathways; over 99% of all neurons belong to this category, reflecting the enormous processing power concentrated in the brain and spinal cord.

Types of Neuroglia

Among the CNS glia, astrocytes deserve particular attention because of their diverse functions. Their end-feet wrap around capillaries to form the blood-brain barrier (BBB), a selective permeability barrier that protects the delicate neuronal environment from fluctuations in blood composition. Astrocytes also buffer extracellular K⁺ concentrations after neuronal firing, recycle glutamate and GABA from the synaptic cleft, and provide metabolic fuel (lactate) to neurons. Recent research has revealed that astrocytes can release 'gliotransmitters' and participate in synaptic modulation, elevating them from passive support cells to active players in neural circuit function.

To synthesize the structural and functional concepts introduced above, let us trace the cellular events that occur when you accidentally touch a hot stove. This scenario involves a simple reflex arc, the most basic functional circuit of the nervous system, and illustrates how each cell type contributes to the response.

Although neurons and neuroglia coexist within nervous tissue, they differ profoundly in structure, function, and life cycle. The table below highlights the most clinically and conceptually significant distinctions between these two cell populations. Appreciating these differences is essential for understanding nervous system pathology, since many diseases preferentially target one population over the other.

The foundational concepts of neurons and neuroglia presented in this lesson serve as the gateway to several advanced topics you will encounter in upper-division neuroscience, neurophysiology, and pathology courses. Understanding where these introductory ideas connect to cutting-edge research helps frame their lasting significance.

The emerging field of glial biology is one of the most active frontiers in neuroscience. Once considered mere scaffolding, neuroglia are now recognized as integral to synaptic plasticity, circadian rhythm regulation, pain processing, and even psychiatric disorders such as depression and schizophrenia. Future coursework in neurophysiology, neuropathology, and pharmacology will repeatedly build upon the neuron-and-glia framework established here, so a thorough mastery of these cell types and their interactions will pay dividends throughout your career in the health sciences.