Neural signals are the basis of communication within the nervous system, facilitating everything from simple reflexes to complex cognitive processes. Transmitted through a combination of electrical and chemical events, these signals allow the nervous system to interpret and respond to the environment, controlling bodily functions and enabling thought and emotion.
Fundamentals of Neuronal Electrical Activity
A resting neuron has a negative electrical charge inside the cell relative to the outside, a state known as the resting membrane potential. This potential is established by an uneven distribution of ions, such as sodium (Na+), potassium (K+), and chloride (Cl-), across the cell membrane. The sodium-potassium pump actively transports Na+ ions out of the cell and K+ ions in, contributing to this charge difference.
The cell membrane contains specialized proteins called ion channels that control the flow of these ions. Some channels are always open, but many are “gated,” meaning they open or close in response to specific triggers. Voltage-gated ion channels, for example, respond to changes in the membrane’s electrical potential. These channels are central to generating an action potential, the electrical impulse that is the neural signal.
An action potential is initiated when a stimulus causes the neuron’s membrane potential to become less negative, a process called depolarization. If this depolarization reaches a certain threshold, voltage-gated Na+ channels open. This allows a swift influx of positive Na+ ions, causing a rapid reversal of the membrane’s charge.
Following this peak, repolarization begins. The voltage-gated Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ ions to flow out of the cell. This outflow of positive charge restores the negative membrane potential. The potential briefly becomes more negative than the resting state, a phase called hyperpolarization, before returning to the resting potential.
Synaptic Transmission: Neuron-to-Neuron Communication
When an action potential reaches the end of a neuron, the signal passes to the next cell at a junction called a chemical synapse. A synapse consists of the presynaptic terminal of the sending neuron, a narrow gap called the synaptic cleft, and the postsynaptic membrane of the receiving cell. The presynaptic terminal contains synaptic vesicles filled with chemical messengers called neurotransmitters.
The action potential’s arrival opens voltage-gated calcium (Ca2+) channels. This influx of Ca2+ causes vesicles to release neurotransmitters into the synaptic cleft. This process, known as exocytosis, converts the electrical signal into a chemical one.
Neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic membrane. This binding opens or closes ion channels, causing a change in the receiving neuron. The effect can be either excitatory, making the neuron more likely to fire, or inhibitory, making it less likely.
After the signal is transmitted, the neurotransmitter is cleared from the synaptic cleft. This happens through reuptake into the presynaptic neuron, enzymatic degradation, or diffusion.
Key Properties of Neural Signal Conduction
A primary property of signal transmission is the all-or-none principle. This principle states that if a stimulus reaches the threshold, the neuron fires an action potential of a fixed, maximum strength. A weaker stimulus produces no action potential at all. Sensation intensity is conveyed by the frequency of firing, not the strength of individual signals.
Signal propagation is unidirectional, traveling from the axon hillock toward the axon terminals. The refractory period after an action potential prevents the signal from moving backward. This is because the sodium channels are temporarily inactivated.
The speed of conduction is increased by an insulating layer called the myelin sheath. This sheath is produced by glial cells and is wrapped around the axon in segments, with small gaps known as nodes of Ranvier. In myelinated axons, the action potential “jumps” from one node to the next. This process, called saltatory conduction, significantly accelerates transmission and is more energy-efficient.
Neuronal Signal Integration
A neuron integrates thousands of simultaneous inputs to determine if it will fire an action potential. This process, called summation, involves adding up all excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). This calculation occurs at the axon hillock, which acts as the decision-making region for the neuron.
Excitatory postsynaptic potentials (EPSPs) are depolarizing events that bring the membrane potential closer to the firing threshold. In contrast, inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing events that move the potential further from the threshold. The neuron’s response depends on the net effect of these inputs.
Two types of summation exist. Temporal summation occurs when one presynaptic neuron fires repeatedly, causing potentials to build. Spatial summation happens when multiple presynaptic neurons fire simultaneously at different locations on the receiving neuron.
If the combined effect of all EPSPs and IPSPs at the axon hillock reaches the threshold potential, an action potential is generated. If the threshold is not reached, no action potential occurs.