Table of Contents
Conduction of excitation describes how an electrical signal, once generated at an excitable cell (for example, a neuron or a muscle cell), travels along that cell or from cell to cell. In this chapter, the focus is on how the signal spreads, not on how it is first generated or how it is interpreted by the nervous system.
Passive Versus Active Spread of Excitation
When a small region of a cell membrane becomes depolarized (for instance, by a local synaptic input), the change in voltage does not remain confined to a single point:
- Passive spread (electrotonic conduction)
Electrical charges move through the cytoplasm and along the membrane, causing neighboring membrane regions to change their voltage slightly. - This spread is:
- Graded: the size of the voltage change decreases with distance.
- Decremental: the signal decays as it moves away from the source.
- Useful for:
- Short-distance communication within dendrites and cell bodies.
- Summation of many small inputs.
- Active spread (propagation of the action potential)
If passive spread depolarizes the next membrane segment enough to reach threshold, voltage-gated ion channels open there and a new action potential is generated. - This makes the signal:
- Regenerative: each segment actively “refreshes” the signal.
- Non-decremental: the amplitude of the action potential stays essentially constant along the axon.
Passive spread and active regeneration combine to allow rapid, long-distance conduction along nerve fibers.
Length Constant and Time Constant (Basic Idea)
Two simple physical properties help describe passive spread:
- Length constant ($\lambda$)
Distance over which a voltage change falls to about 37% of its original value. - Large $\lambda$: signals spread farther with less decay.
- Increased by:
- Larger fiber diameter (lower internal resistance).
- Better membrane insulation (higher membrane resistance, e.g. myelin).
- Time constant ($\tau$)
Time it takes the membrane potential to reach about 63% of its final value in response to a step of current. - Small $\tau$: membrane can change voltage quickly.
- Determined by membrane resistance and capacitance.
These parameters explain why some axons conduct faster than others, even without changing the basic action potential mechanism.
Conduction in Unmyelinated Axons
In unmyelinated nerve fibers, voltage-gated sodium and potassium channels are distributed relatively evenly along the membrane.
Key features:
- Local circuit currents
When an action potential occurs at one segment: - Sodium ions entering during depolarization spread along the inside of the axon.
- This depolarizes the neighboring segment to threshold.
- That segment then generates its own action potential.
- Continuous conduction
The action potential appears to move smoothly along the membrane as each adjacent segment becomes active in turn. - Speed factors
Conduction velocity in unmyelinated axons depends mainly on: - Axon diameter: larger diameter → lower internal resistance → faster spread.
- Temperature: higher temperature (within physiological limits) → faster channel kinetics.
Typical conduction velocities in small unmyelinated fibers are relatively low (on the order of 0.5–2 m/s in vertebrates), which is sufficient for some functions (e.g. slow pain).
Myelin and Saltatory Conduction
Many vertebrate axons (and some invertebrate ones, in a different structural form) are wrapped by myelin, a multilayered insulating sheath formed by glial cells.
- In the peripheral nervous system: Schwann cells.
- In the central nervous system: oligodendrocytes.
Myelin alters conduction in several crucial ways:
Nodes of Ranvier
Myelin does not cover the axon continuously; small gaps remain:
- Nodes of Ranvier:
- Short, unmyelinated segments.
- Very high density of voltage-gated sodium and potassium channels.
- Sites where action potentials are actively generated.
Between nodes:
- Internodes:
- Myelinated segments.
- Very few ion channels, very low membrane capacitance, and very high membrane resistance.
Saltatory (“Jumping”) Conduction
Because myelin prevents current leakage across the membrane:
- The depolarizing current produced at one node flows rapidly along the interior of the axon to the next node.
- The membrane in the internodal region depolarizes passively (electrotonically), but:
- The signal does not need to regenerate at every micrometer.
- It only needs to remain large enough to trigger an action potential at the next node.
The action potential thus appears to “jump” from node to node:
- This is called saltatory conduction (from Latin saltare = “to jump”).
- In reality, the action potential is regenerated only at the nodes:
- Nodes: active, energy-consuming.
- Internodes: mainly passive, rapid spread.
Advantages of Myelination
- Greatly increased conduction velocity
Speeds of 50–120 m/s are common in large myelinated vertebrate axons. - Energy efficiency
Ion flux (primarily Na⁺ entry) is concentrated at the nodes: - Fewer ions need to be pumped back by the Na⁺/K⁺-pump.
- Lower ATP consumption per unit length of axon.
- Space saving
Myelinated axons can be thinner than unmyelinated axons that would achieve similar speeds simply by increasing diameter.
Consequences of Demyelination
When myelin is damaged or lost:
- Passive spread between nodes becomes weak and slow.
- Depolarization may not reach threshold at the next node.
- Conduction becomes:
- Slowed.
- Unreliable.
- Incomplete or blocked.
Neurological diseases characterized by demyelination therefore lead to conduction problems, such as delayed or failed signal transmission in motor and sensory pathways.
Direction of Conduction and Refractory Period
Although an action potential could, in principle, propagate in both directions along an excitable membrane, in neurons conduction is usually unidirectional (from cell body toward axon terminal). This is largely due to:
- Refractory periods of voltage-gated sodium channels:
- Absolute refractory period:
- Shortly after opening, sodium channels inactivate.
- No new action potential can be generated at that membrane region, regardless of stimulus strength.
- Relative refractory period:
- Some channels have recovered, but potassium conductance is still elevated.
- A stronger-than-usual stimulus is required to reach threshold.
During propagation:
- The segment behind the advancing action potential is refractory.
- The segment ahead is excitable.
- Result: the action potential spreads forward but not backward.
Additionally, in many neurons, excitation is initiated close to the cell body (axon hillock), so the “forward” direction is anatomically defined.
Conduction in Different Types of Nerve Fibers
Nerve fibers can be classified by:
- Diameter (thin vs. thick).
- Myelination (unmyelinated vs. myelinated).
Functionally:
- Slow, thin, unmyelinated fibers:
- Used where speed is less critical (e.g. slow pain, some autonomic functions).
- Fast, thick, myelinated fibers:
- Used where rapid responses are crucial (e.g. motor control of skeletal muscles, acute touch).
This division allows the nervous system to allocate resources (space, energy, speed) according to functional needs.
Conduction in Non-Neuronal Excitable Tissues
Excitation is also conducted in tissues other than neurons, often using different structural arrangements:
Skeletal Muscle Fibers
- Each muscle fiber is a long excitable cell.
- The action potential travels along the surface membrane and into:
- Transverse tubules (T-tubules), membrane invaginations that carry the depolarization deep into the fiber.
- This ensures nearly simultaneous activation of the contractile machinery across the muscle fiber.
Cardiac Muscle
Cardiac muscle has two key organizational features:
- Gap junctions:
- Direct electrical connections between neighboring cells.
- Ions flow from cell to cell, spreading excitation without chemical synapses.
- The heart muscle behaves largely as a functional syncytium (electrically unified tissue).
- Specialized conduction system:
- Certain cardiac cells (e.g. in the atrioventricular bundle, bundle branches, Purkinje fibers) are specialized for rapid conduction.
- They coordinate the timing of contraction between atria and ventricles.
Smooth Muscle
- Cells are often connected by gap junctions.
- Excitation spreads relatively slowly from cell to cell.
- Conduction properties are adapted to functions such as peristalsis and sustained contractions.
Electrical Synapses and Direct Cell-to-Cell Conduction
While chemical synapses use neurotransmitters and synaptic clefts, electrical synapses conduct excitation directly between cells:
- Based on gap junctions:
- Channels connecting the cytoplasms of two cells.
- Allow rapid and bidirectional flow of ions and small molecules.
- Conduction features:
- Very fast transmission (almost no synaptic delay).
- Often used where synchronization is crucial (e.g. some brain circuits, escape behaviors in invertebrates, parts of the vertebrate retina).
Electrical synapses therefore extend conduction beyond a single cell, creating larger electrically coupled networks.
Conduction in Invertebrate Nervous Systems
Although detailed organization differs from vertebrates, some principles are shared:
- Many invertebrates rely on:
- Giant axons (very large diameter) to achieve high conduction velocities without myelin.
- Example: cephalopods (e.g. squids), some crustaceans, and annelids.
- The same basic mechanisms apply:
- Passive spread of depolarization.
- Regenerative action potentials with voltage-gated channels.
- Conduction speed increased by:
- Larger diameter.
- Shorter distances between excitable segments.
This shows that high-speed conduction can be achieved either by increasing fiber diameter (invertebrates’ giant axons) or by myelination (vertebrates).
Summary of Key Principles
- Excitation spreads within cells by passive (electrotonic) and active (regenerative) processes.
- Unmyelinated axons conduct via continuous conduction, with action potentials regenerated at each segment.
- Myelinated axons conduct via saltatory conduction, with action potentials generated mainly at nodes of Ranvier, resulting in fast and energy-efficient transmission.
- The refractory period of sodium channels enforces a preferred direction of propagation.
- Structural adaptations (axon diameter, myelin, gap junctions, specialized conduction systems) tailor conduction speed and pattern to the functional demands of different tissues and organisms.