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How Nerves Carry Messages
Nerves Carry Messages
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Nerves carry messages that let the body sense, move, and respond quickly to changes. A nerve message starts when dendrites receive signals from other cells or from sensory receptors. The cell body combines these inputs, and if the signal is strong enough, an electrical impulse travels down the axon. This one-way flow is essential for coordinated actions such as pulling your hand away from heat or keeping your heart rhythm steady.
The electrical impulse is called an action potential, and it depends on charged ions moving across the neuron membrane. Myelin, a fatty insulating layer around many axons, helps impulses travel faster by forcing the signal to jump between gaps called nodes of Ranvier. At the axon terminals, the electrical message is converted into a chemical message using neurotransmitters. These chemicals cross the synapse and can start a new signal in the next neuron, muscle cell, or gland cell.
Key Facts
- Main pathway: dendrites receive signals, cell body processes input, axon carries the impulse, axon terminals pass the message onward.
- A nerve impulse usually travels one way because synapses release neurotransmitters from the axon terminal side.
- Resting membrane potential is about -70 mV in many neurons.
- An action potential begins when the membrane reaches threshold, often about -55 mV.
- Myelinated axons conduct faster because impulses jump from node to node in saltatory conduction.
- Signal speed can range from less than 1 m/s in some unmyelinated fibers to over 100 m/s in large myelinated fibers.
Vocabulary
- Neuron
- A specialized cell that carries electrical and chemical messages through the nervous system.
- Dendrite
- A branched part of a neuron that receives signals from other cells.
- Axon
- A long fiber of a neuron that carries an action potential away from the cell body.
- Myelin
- A fatty insulating layer around some axons that increases the speed of nerve impulses.
- Synapse
- The small gap where a neuron passes a signal to another neuron, muscle cell, or gland cell.
Common Mistakes to Avoid
- Thinking nerve messages travel both directions along the same pathway, which is wrong because normal signaling is organized from dendrites to cell body to axon terminals.
- Confusing electrical signals with neurotransmitters, which is wrong because the action potential travels along the neuron while neurotransmitters carry the message across the synapse.
- Assuming myelin creates the nerve impulse, which is wrong because myelin mainly speeds conduction by insulating the axon and exposing only the nodes of Ranvier.
- Forgetting the threshold step, which is wrong because a neuron does not fire a full action potential unless the membrane reaches a critical voltage.
Practice Questions
- 1 A myelinated nerve impulse travels at 80 m/s. How long does it take to travel along a 1.2 m axon?
- 2 An unmyelinated axon carries a signal at 2 m/s, while a myelinated axon carries a signal at 100 m/s. How many times faster is the myelinated axon?
- 3 Explain why damage to myelin can slow or disrupt nerve messages even if the axon itself is still present.