A neuron becomes activated when it receives sufficient stimulation to reach its threshold, triggering an action potential. This activation process depends on the type of neuron and the nature of the stimulus.
Sensory Neuron Activation
Sensory neurons are activated by sensory input via specific receptors. These receptors can be categorized as:
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External Receptors: Respond to stimuli from outside the body.
- Olfactory receptors: Detect smells.
- Taste receptors: Detect tastes.
- Photoreceptors: Detect light (e.g., in the eyes).
- Cochlear hair receptors: Detect sound (e.g., in the ears).
- Thermoreceptors: Detect temperature changes.
- Mechanoreceptors: Detect physical touch, pressure, or stretching.
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Internal Receptors: Respond to stimuli from inside the body.
The Activation Process: A Detailed View
Here’s a breakdown of how a neuron gets activated:
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Resting Potential: A neuron at rest maintains a negative electrical charge inside relative to the outside. This is known as the resting membrane potential (typically around -70mV).
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Reception of Stimuli: A neuron receives signals from other neurons or sensory receptors. These signals can be either excitatory (depolarizing) or inhibitory (hyperpolarizing).
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Graded Potentials: Excitatory signals cause the membrane potential to become less negative (depolarization), while inhibitory signals cause it to become more negative (hyperpolarization). These changes in membrane potential are called graded potentials because their magnitude varies with the strength of the stimulus.
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Threshold: If the sum of the excitatory signals is strong enough to depolarize the membrane potential to a certain level (the threshold, usually around -55mV), an action potential is triggered.
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Action Potential: An action potential is a rapid, transient reversal of the membrane potential. It involves the opening of voltage-gated sodium channels, allowing sodium ions to rush into the cell and causing the inside to become positive. After a brief period, voltage-gated potassium channels open, allowing potassium ions to flow out of the cell, restoring the negative resting potential.
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Propagation: The action potential propagates down the axon to the axon terminals, where it triggers the release of neurotransmitters.
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Neurotransmitter Release: Neurotransmitters bind to receptors on the postsynaptic neuron, initiating a new round of graded potentials.
Factors Influencing Neuron Activation
- Strength of the Stimulus: Stronger stimuli lead to larger graded potentials and a greater likelihood of reaching the threshold.
- Number of Simultaneous Signals: A neuron integrates multiple signals arriving at the same time. If enough excitatory signals arrive together, they can trigger an action potential.
- Spatial Summation: Signals arriving at different locations on the neuron can sum together to reach the threshold.
- Temporal Summation: Signals arriving in quick succession can sum together to reach the threshold.
Summary
A neuron is activated when it receives sufficient excitatory input to depolarize its membrane potential to the threshold level. This triggers an action potential, allowing the neuron to transmit information to other neurons. For sensory neurons, activation is initiated by specific sensory receptors responding to external or internal stimuli.
