How does information travel along the axon of a sensory neurone?
In sensory neurons, information is transmitted along the axon to the central nervous system (CNS) in the form of electrical signals called action potentials. This process is known as signal transduction, and it involves a series of molecular and electrical events that can be summarized as follows:
1. Stimulus Detection: Sensory neurons have specialized endings that detect specific types of stimuli, such as touch, temperature, pain, and chemical substances. When a sensory receptor detects a stimulus, it converts it into an electrical signal.
2. Generation of Receptor Potential: The stimulus causes a change in the membrane potential of the sensory neuron, resulting in a graded potential called the receptor potential. This potential is a localized change in the electrical potential of the membrane and does not propagate along the axon.
3. Depolarization: If the receptor potential reaches a certain threshold, it initiates the generation of an action potential. This occurs through the opening of voltage-gated sodium (Na+) channels in the neuronal membrane, allowing an influx of positively charged sodium ions into the neuron.
4. Propagation of Action Potential: As a result of sodium influx, the inside of the membrane becomes more positive, depolarizing the cell. This depolarization spreads rapidly along the axon, regeneratively opening more voltage-gated sodium channels and causing a chain reaction of action potentials.
5. Saltatory Conduction (in myelinated neurons): In myelinated axons, the myelin sheath insulates the neuron except at regular intervals called nodes of Ranvier. Action potentials "jump" from node to node, which speeds up the transmission of the signal.
6. Repolarization and Hyperpolarization: After the action potential has passed, the voltage-gated sodium channels close, and voltage-gated potassium (K+) channels open. Potassium ions move out of the neuron, restoring the negative charge inside the membrane. This process is called repolarization. In some cases, the membrane potential can briefly become more negative than its resting potential, a state known as hyperpolarization.
7. Refractory Periods: Following an action potential, there is a brief period during which the neuron cannot generate another action potential. This is called the refractory period and prevents the backward propagation of signals.
8. Neurotransmitter Release: When the action potential reaches the axon terminal (the end of the axon located in the CNS), it causes the release of neurotransmitters into the synaptic cleft, which is the space between the neuron and its target cell (usually another neuron).
9. Synaptic Transmission: The neurotransmitters released from the sensory neuron bind to receptors on the target cell, influencing its electrical properties and potentially generating a new action potential in the next neuron, continuing the transmission of the sensory information toward the brain or spinal cord.
This sequence of events allows sensory neurons to convert sensory stimuli into electrical signals, transmit them along their axons, and release neurotransmitters to communicate with other neurons in the CNS.