Neuron Structure & Nerve Impulse Transmission - Tenth Grade Science
To survive in a dynamic and unpredictable environment, multicellular animals must detect environmental stimuli, process sensory information, and coordinate rapid, integrated physiological and behavioral responses across diverse tissues. In the human body, this rapid communication is orchestrated by the nervous system, a network of billions of excitable cells that transmit information through electrochemical signals. The fundamental structural and functional unit of the nervous system is the neuron, a cell adapted for electrical excitability and directional signal transmission. Supported by glial cells, neurons generate action potentials through the movement of sodium and potassium ions across plasma membranes, transmitting impulses along axons at velocities exceeding one hundred meters per second, and communicating across microscopic intercellular junctions known as synapses via chemical neurotransmitters. In this chapter, you will investigate the anatomical ultrastructure of the neuron, analyze resting membrane potential and action potential dynamics, explore the mechanics of chemical synaptic transmission, and evaluate saltatory conduction along myelinated nerve fibers.
Anatomical Ultrastructure of the Neuron
A neuron is a specialized animal cell consisting of three primary structural regions: the cell body (soma), receptive dendrites, and an elongated conducting axon.
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| THE ARCHITECTURE OF A NEURON |
+-----------------------------------------------------------------------------------+
| |
| Dendrites (Receptive Arbors) |
| \ | / |
| v v v |
| +---------------+ |
| | CELL BODY | <- Nucleus & Nissl Granules (Rough ER) |
| | (SOMA) | |
| +---------------+ |
| | |
| [Axon Hillock] (Impulse Trigger Zone) |
| | |
| v |
| +---------------+ |
| | Myelin Sheath| <- Schwann Cell (PNS) / Oligodendrocyte (CNS) |
| +---------------+ |
| | |
| [Node of Ranvier] <- Unmyelinated gap (High Na+ channel density) |
| | |
| +---------------+ |
| | Myelin Sheath| |
| +---------------+ |
| | |
| v |
| [Axon Terminals] <- Synaptic Knobs / Boutons |
| / | \ (Synaptic vesicles containing neurotransmitter) |
| v v v |
+-----------------------------------------------------------------------------------+
The Cyton (Cell Body or Soma)
The cyton, also termed the soma or perikaryon, is the metabolic center of the neuron. It contains a large, spherical, centrally located nucleus with a prominent nucleolus, surrounded by granular cytoplasm containing mitochondria, Golgi complexes, and neurofibrils.
A distinguishing cytological feature of the neuronal soma is the presence of Nissl granules (Nissl bodies). Nissl granules are dense, basophilic clusters of rough endoplasmic reticulum and free polyribosomes that synthesize structural proteins, peptide neurotransmitters, and membrane channels required to sustain the neuron. Mature mammalian neurons lack functional centrosomes and centrioles, rendering them incapable of mitotic cell division following embryonic differentiation.
Dendrites: The Receptive Input Arbors
Dendrites are short, tapered, branched cytoplasmic extensions that project from the soma like tree branches. The dendritic membrane contains chemical receptor proteins (ligand-gated ion channels) that detect neurotransmitter molecules released from the axon terminals of upstream neurons. Dendrites receive incoming stimuli and conduct graded electrical potentials centripetally (inward) toward the cell body.
The Axon: The Conduction Highway
The axon is a single, elongated, cylindrical cytoplasmic extension that conducts action potentials centrifugally (away from the soma) toward downstream effector cells or neighboring neurons. The axon originates from a funnel-shaped region of the cell body called the axon hillock, which functions as the physiological trigger zone where incoming graded potentials are integrated to initiate action potentials.
The axon is bounded by a plasma membrane called the axolemma, enclosing specialized cytoplasm called axoplasm. While axons branch minimally along their length, they terminate distally in multiple fine branches called telodendria. Each telodendron ends in an expanded bulbous terminal known as a synaptic knob (terminal bouton), which stores membrane-bound synaptic vesicles containing chemical neurotransmitters like acetylcholine.
Myelination: Schwann Cells and Nodes of Ranvier
In the peripheral nervous system (PNS), the axons of many neurons are enveloped by a lipid-rich insulating sheath called the myelin sheath, synthesized by supporting glial cells known as Schwann cells (in the central nervous system, myelin is synthesized by oligodendrocytes):
Schwann cells wrap around the axon in concentric layers. The compact lipid layers of the Schwann cell plasma membrane form the myelin sheath, which acts as an electrical insulator, preventing ion leakage across the axolemma.
The myelin sheath is not continuous along the axon; it is interrupted at regular intervals of approximately one millimeter by uninsulated gaps called Nodes of Ranvier. At these nodes, the axolemma is exposed to extracellular interstitial fluid and contains a high concentration of voltage-gated sodium channels. The outermost layer of the Schwann cell, containing its nucleus and cytoplasm, forms a thin outer sheath called the neurilemma, which aids in axonal regeneration following nerve injury.
Electrochemical Mechanics of Nerve Impulse Transmission
A nerve impulse is an electrical signal propagated along a neuronal membrane, driven by the regulated movement of sodium ($\text{Na}^+$) and potassium ($\text{K}^+$) ions across voltage-gated ion channels.
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| ACTION POTENTIAL VOLTAGE-TIME PROFILE |
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| |
| Membrane Potential (mV) |
| |
| +40 + - - - - - - - - - - - - - - . (Peak Depolarization) |
| | / \ |
| 0 + - - - - - - - - - - - - - / - \ - - - - - - - - - - - - - - - - |
| | / \ |
| -55 + - - - - - - - - - - - - . \ (Repolarization: K+ exits) |
| | (Threshold Potential) \ |
| -70 +-----------------------. \ .------------------ |
| | (Resting Potential) \ / (Resting Potential) |
| -80 + - - - - - - - - - - - - - - - - - - '-------' - - - - - - - - - |
| | (Hyperpolarization) |
| +------------------------------------------------------------------ |
| 0 1 2 3 4 5 |
| Time (Milliseconds) |
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The Resting Membrane Potential ($-70\text{ mV}$)
When a neuron is not transmitting an impulse, its axolemma is in a polarized resting state, maintaining an electrical potential difference across the membrane of approximately $-70\text{ millivolts}$ (inside negative relative to outside).
The resting membrane potential is generated by two factors: Unequal Ion Distribution: Active sodium-potassium ATPase pumps ($\text{Na}^+/\text{K}^+$ pumps) in the axolemma pump three sodium ions ($\text{Na}^+$) out of the axoplasm into the extracellular fluid for every two potassium ions ($\text{K}^+$) pumped into the cell, consuming one molecule of ATP. This creates an extracellular concentration of $\text{Na}^+$ roughly ten times higher than inside, and an intracellular concentration of $\text{K}^+$ roughly thirty times higher than outside. Differential Membrane Permeability: At rest, the axolemma possesses non-gated potassium leak channels that make it roughly fifty times more permeable to $\text{K}^+$ than to $\text{Na}^+$. Potassium ions diffuse down their chemical concentration gradient out of the cell. Concurrently, large negatively charged organic anions (proteins, organic phosphates, sulfates) inside the axoplasm cannot cross the membrane. The outward movement of positive potassium ions combined with the trapped intracellular anions establishes a negative resting electrical potential of $-70\text{ mV}$.
Depolarization and the Action Potential ($+40\text{ mV}$)
When a stimulus strikes the receptive region of a neuron, it alters the local membrane potential:
If the stimulus is strong enough to depolarize the membrane to the critical threshold potential (approximately $-55\text{ mV}$), voltage-gated sodium channels open rapidly. Sodium ions rush down both their concentration and electrical gradients into the axoplasm. This influx of positive charge neutralizes the internal negativity and reverses the membrane polarity to approximately $+30\text{ to }+40\text{ millivolts}$, a state termed depolarization.
The generation of an action potential follows the All-or-None Law: if a stimulus fails to depolarize the membrane to the $-55\text{ mV}$ threshold, no action potential is generated; if the stimulus reaches or exceeds the threshold, an action potential of maximal, uniform amplitude is triggered. The magnitude of the action potential does not increase with stronger stimuli; stimulus intensity is encoded by the frequency of action potentials per second, not by their voltage amplitude.
Repolarization and Hyperpolarization
Within one millisecond after opening, the voltage-gated sodium channels inactivate, halting further sodium entry. Concurrently, voltage-gated potassium channels open in response to depolarization.
Potassium ions rush out of the axoplasm into the extracellular fluid down their electrochemical gradient. The loss of positive charges restores the negative internal potential, a process termed repolarization.
Because voltage-gated potassium channels close slowly, an excess of potassium exits, causing the membrane potential to drop below the resting baseline to approximately $-80\text{ mV}$, a state called hyperpolarization. The resting potential of $-70\text{ mV}$ is subsequently restored by the sodium-potassium pump.
Following an action potential, the axonal segment enters a brief refractory period during which sodium channels cannot reopen, ensuring that the nerve impulse propagates unidirectionally away from the soma toward the axon terminals without reversing direction.
Saltatory Conduction in Myelinated Axons
In unmyelinated axons, action potentials must depolarize every adjacent patch of the axolemma sequentially along the entire length of the fiber (continuous conduction), resulting in slow velocities ($0.5\text{ to }2\text{ m/s}$).
In myelinated axons, the myelin sheath prevents ion flow across the internodal membrane. Depolarization can occur only at the uninsulated Nodes of Ranvier, where voltage-gated sodium channels are concentrated. When an action potential fires at one node, the incoming positive electrical charge flows rapidly through the internal axoplasm to the next node, bringing it to threshold. The action potential appears to jump from node to node in a process termed saltatory conduction (from Latin *saltare*, to leap). Saltatory conduction increases nerve impulse velocity up to $100\text{ to }120\text{ m/s}$ ($250\text{ mph}$), while conserving metabolic ATP by minimizing the membrane area requiring sodium-potassium pumping.
Chemical Synaptic Transmission
Neurons do not physically touch; they communicate across specialized intercellular junctions called synapses.
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| CHEMICAL SYNAPTIC TRANSMISSION MECHANISM |
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| |
| PRESYNAPTIC AXON TERMINAL |
| +---------------------------------------------------------------+ |
| | Action potential arrives ----> Voltage-gated Ca2+ channels open| |
| | Ca2+ influx triggers exocytosis of Synaptic Vesicles | |
| | | |
| | ( * ) ( * ) ( * ) <- Synaptic Vesicles | |
| | \ | / (Acetylcholine) | |
| +----------\--------|--------/----------------------------------+ |
| v v v |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <- Synaptic Cleft (20 nm) |
| | | | |
| +-----------v-------v-------v-----------------------------------+ |
| | Receptor Proteins (Ligand-Gated Ion Channels) | |
| | Na+ enters post-synaptic cell; generates EPSP (Depolarization)| |
| +---------------------------------------------------------------+ |
| POSTSYNAPTIC NEURON DENDRITE |
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The Synaptic Junction and Calcium Influx
A chemical synapse consists of three components: the presynaptic membrane (the axon terminal of the transmitting neuron), the synaptic cleft (a fluid-filled extracellular space approximately 20 nanometers wide), and the postsynaptic membrane (the dendrite, soma, or muscle endplate of the receiving cell).
When an action potential arrives at the presynaptic terminal, depolarization triggers the opening of voltage-gated calcium channels ($\text{Ca}^{2+}$). Calcium ions, present in higher concentrations in the extracellular fluid, rush into the presynaptic terminal down their concentration gradient.
Neurotransmitter Exocytosis and Receptor Binding
The influx of calcium ions activates proteins that cause synaptic vesicles to mobilize, dock, and fuse with the presynaptic plasma membrane, releasing chemical neurotransmitters (such as acetylcholine, noradrenaline, dopamine, or GABA) into the synaptic cleft via exocytosis.
Neurotransmitter molecules diffuse across the 20-nanometer synaptic cleft and bind to specific receptor proteins located on the postsynaptic membrane: In an excitatory synapse, neurotransmitter binding (e.g., acetylcholine) opens ligand-gated sodium channels, allowing $\text{Na}^+$ to enter the postsynaptic cell. This generates an Excitatory Postsynaptic Potential (EPSP), depolarizing the postsynaptic membrane toward threshold. In an inhibitory synapse, neurotransmitter binding (e.g., GABA or glycine) opens ligand-gated chloride ($\text{Cl}^-$) or potassium ($\text{K}^+$) channels, causing hyperpolarization (Inhibitory Postsynaptic Potential, IPSP), making the postsynaptic cell less likely to fire.
Termination of the Signal
To prevent continuous, uncontrolled stimulation of the postsynaptic cell, the neurotransmitter must be rapidly cleared from the synaptic cleft within milliseconds of its release: Enzymatic Degradation: Enzymes anchored within the cleft break down the neurotransmitter. For example, acetylcholinesterase cleaves acetylcholine into inactive acetate and choline, which is reabsorbed by the presynaptic terminal for recycling. Reuptake: Membrane transport proteins actively pump the neurotransmitter back into the presynaptic neuron or surrounding glial astrocytes (e.g., the reuptake of serotonin and dopamine).
Synaptic transmission is strictly unidirectional: synaptic vesicles are stored exclusively in presynaptic axon terminals, and chemical receptors are located exclusively on postsynaptic membranes. Consequently, nerve impulses can cross a synapse in only one direction.
Chapter Exercises
Exercise 1: Describe the electrical and ionic events that occur along an unmyelinated axon as an action potential passes a given point, detailing the changes in membrane potential (in millivolts), ion channel states, and ionic movements during resting, depolarization, repolarization, and hyperpolarization phases.
Exercise 2: Contrast continuous conduction along an unmyelinated nerve fiber with saltatory conduction along a myelinated nerve fiber across three criteria: propagation mechanism, conduction velocity, and metabolic ATP expenditure by the sodium-potassium pump.
Exercise 3: Explain why nerve impulse transmission across a chemical synapse is strictly unidirectional, and describe the physiological mechanism by which the nerve poison curare, which blocks acetylcholine receptors on muscle endplates, causes flaccid paralysis.
Solutions and Explanations
Solution 1: At the resting phase, the axolemma maintains a resting membrane potential of $-70\text{ mV}$ (interior negative). Sodium-potassium pumps maintain ion gradients ($\text{Na}^+$ high outside, $\text{K}^+$ high inside), and non-gated potassium leak channels allow outward potassium diffusion, while voltage-gated sodium and potassium channels remain closed. When a stimulus depolarizes the membrane to the threshold of $-55\text{ mV}$, voltage-gated sodium channels open rapidly. Sodium ions rush into the axoplasm down their electrochemical gradient, causing depolarization: the internal potential reverses from $-55\text{ mV}$ to $+30\text{ to }+40\text{ mV}$. At this peak, voltage-gated sodium channels inactivate, halting sodium entry, while voltage-gated potassium channels open. Potassium ions rush out of the cell down their electrochemical gradient, causing repolarization, which restores negative internal voltage. Because the potassium channels close slowly, potassium efflux continues briefly past the resting potential, causing transient hyperpolarization to approximately $-80\text{ mV}$. Once the potassium channels close, active sodium-potassium ATPase pumps restore the resting concentration gradients and the resting potential of $-70\text{ mV}$.
Solution 2: Across the three criteria: First, propagation mechanism: In an unmyelinated fiber, continuous conduction requires every adjacent micro-patch of axolemma along the entire axon to undergo depolarization and repolarization sequentially; in a myelinated fiber, the lipid myelin sheath insulates the internodal axolemma, forcing action potentials to jump from one unmyelinated Node of Ranvier to the next via saltatory conduction. Second, conduction velocity: Continuous conduction in unmyelinated fibers is slow, with velocities typically ranging from $0.5\text{ to }2\text{ meters per second}$; saltatory conduction in myelinated fibers is significantly faster, achieving velocities of $100\text{ to }120\text{ meters per second}$ because the depolarization jumps across insulated internodes. Third, metabolic ATP expenditure: Continuous conduction requires large numbers of sodium-potassium ATPase pumps along the entire length of the axon, consuming large amounts of ATP to restore ion gradients; saltatory conduction restricts ion movement exclusively to the Nodes of Ranvier, reducing the total number of ions transferred and lowering ATP consumption by the sodium-potassium pumps.
Solution 3: Nerve impulse transmission across a chemical synapse is strictly unidirectional due to the asymmetrical localization of synaptic components: synaptic vesicles containing chemical neurotransmitters (such as acetylcholine) are localized exclusively within the presynaptic axon terminals, while the corresponding chemical receptor proteins (ligand-gated ion channels) are localized exclusively on the postsynaptic membrane. When an impulse arrives at an axon terminal, neurotransmitter is released into the synaptic cleft and diffuses to the postsynaptic receptors. If an impulse were artificially initiated in the postsynaptic cell, it could not transmit backward across the synapse because the postsynaptic membrane has no neurotransmitter vesicles to release, and the presynaptic terminal has no receptors to detect them. The botanical toxin curare binds competitively to nicotinic acetylcholine receptors located on the postsynaptic motor endplates of skeletal muscle fibers. Curare binds to the receptor without opening the ligand-gated sodium channels, preventing endogenous acetylcholine from binding. Because sodium influx is blocked, no endplate potential or muscle action potential can be generated. Muscular contraction ceases, resulting in flaccid paralysis; if the intercostal muscles and diaphragm are affected, death ensues from respiratory arrest.